add prototype permission based fs access

Cargo.lock

@@ -20,10 +20,12 @@ dependencies = [
  "mlua",
  "reqwest",
  "rusqlite",
+ "rustix",
  "rustls",
  "rustyline",
  "serde",
  "serde_json",
+ "tempfile",
  "thiserror",
  "tokio",
 ]

Cargo.toml

@@ -32,9 +32,15 @@ mlua = { version = "0.11.6", features = ["lua55", "vendored", "serde", "async",
 reqwest = { version = "0.13.4", default-features = false, features = ["rustls-no-provider", "charset", "http2", "system-proxy"] }
 rustls = { version = "0.23", default-features = false, features = ["ring", "std", "tls12"] }
 rusqlite = { version = "0.32", features = ["bundled"] }
+# flock() guarding the fs-permission store (access.json) against concurrently
+# running scripts; already in the tree as a transitive dependency.
+rustix = { version = "1", features = ["fs"] }
 rustyline = "14"
 serde = { version = "1.0.228", features = ["derive"] }
 serde_json = "1.0.150"
 thiserror = "2.0.18"
 tokio = { version = "1.52.3", features = ["full"] }
 digest_auth = "0.3"
+
+[dev-dependencies]
+tempfile = "3"

README.md

@@ -42,12 +42,13 @@ The standard library is the main purpose of `a`. Available today:
 
 - **Networking** — HTTP client (WebSocket and MQTT are planned).
 - **SQLite** — access to embedded SQLite databases.
+- **Filesystem** — whole-file read/write and directory listing via [`fs`](docs/fs.md), guarded by a per-script folder permission system (interactive grants recorded in `access.json`; `--allow-fs`, `--no-fs` and `--sandbox` override it).
 - **Terminal UI** — interactive prompts (confirm, text, select, password, number, date), styled/colored output, and terminal control (cursor addressing, alternate screen, scroll regions) via [`tui`](docs/tui.md).
 - **Core helpers** — `math`, `table`, and `utils` extensions, structured `log`, `os` time helpers, and `task.join` concurrency.
 
 Planned:
 
-- **Filesystem** — read/write/glob/walk without stock Lua's `io` boilerplate.
+- **Filesystem ergonomics** — glob/walk on top of the existing `fs` core.
 - **TUI widgets & input** — progress bars, tables, and raw key/mouse events on top of the existing terminal control.
 
 The intent is for the common case to be a single call rather than a multi-step setup.

docs/README.md

@@ -15,6 +15,7 @@ also works and returns the same table).
 | [`log`](log.md) | Level-tagged diagnostic logging (`trace`…`error`), gated by `RUST_LOG`. |
 | [`sqlite`](sqlite.md) | Embedded SQLite: connect, query, parameters, transactions. |
 | [`http`](http.md) | HTTP/HTTPS client: requests, JSON, auth, cookie-jar sessions. |
+| [`fs`](fs.md) | Whole-file read/write and directory listing, behind a per-script folder permission system. |
 | [`tui`](tui.md) | Interactive terminal prompts (confirm/text/select/…), ANSI-styled output, message blocks. |
 
 ## Concepts

docs/fs.md

@@ -0,0 +1,157 @@
+# `fs`
+
+Whole-file filesystem access, guarded by a per-script folder permission
+system. The sandbox loads no stock `io`; this module is the supported way for
+a script to touch files, and every operation first passes a permission check
+(see [Permissions](#permissions) below).
+
+The v1 surface is deliberately small — whole files in and out, directory
+listing, two predicates, and an explicit permission probe:
+
+```lua
+local text = fs.readAll("/home/me/notes.txt")
+fs.writeAll("out/report.md", text .. "\n-- processed")
+
+for _, name in ipairs(fs.list(".")) do print(name) end
+
+if fs.isFile("config.json") then ... end
+if fs.access("/var/data", "w") then ... end
+```
+
+## Functions
+
+### `fs.readAll(path) -> string`
+
+Reads the whole file into a string. Lua strings are byte strings, so binary
+content round-trips exactly. Errors if `path` is not a regular file, the OS
+refuses, or the permission system denies **read** on the file's directory.
+
+### `fs.writeAll(path, content)`
+
+Writes the whole file: created if missing, truncated if it exists. The
+containing directory must already exist (there is no mkdir-p), and a dangling
+symlink as the target is refused. Errors if the file can't be created or the
+permission system denies **write** on the containing directory.
+
+### `fs.list(path) -> string[]`
+
+The names (not full paths) of a directory's entries, sorted. `.` and `..` are
+not included. Errors if `path` is not a directory or **read** on it is
+denied.
+
+### `fs.isDir(path) -> boolean`, `fs.isFile(path) -> boolean`
+
+`true` iff the path exists, has the right type, *and* is readable under the
+permission system. These never error: a nonexistent path is `false` without
+any permission prompt; an unknown permission prompts like any other read (a
+denial makes the result `false`).
+
+### `fs.getWorkdir() -> string`
+
+The process's current working directory, as an absolute path. Relative paths
+given to the other fs functions resolve against it, so it is the natural base
+for building paths to probe with `fs.access`:
+
+```lua
+local cwd = fs.getWorkdir()
+if fs.access(cwd, "w") then
+    fs.writeAll("report.txt", text) -- relative = cwd .. "/report.txt"
+end
+```
+
+Not permission-gated — it reports process state and touches no file content.
+
+### `fs.access(dir, mode) -> boolean`
+
+Runs the permission cycle for a directory explicitly — `mode` is `"r"` or
+`"w"`:
+
+1. a recorded grant/denial answers immediately;
+2. otherwise the interactive prompt asks;
+3. with no terminal to ask on, returns `false` **without** recording a
+   denial (a persisted "no" is always an explicit user decision).
+
+Use it to ask for permission up front at a natural moment instead of
+mid-operation. Errors if `dir` is not an existing directory.
+
+## Permissions
+
+Permissions attach to **directories**, not files, and are recorded per
+script:
+
+- The permission unit for `readAll`/`writeAll`/`isFile` is the file's
+  directory; for `list`/`isDir`/`access` it is the directory itself. Paths
+  are canonicalized first (symlinks resolved, relative paths against the
+  CWD), so two spellings of the same place share one grant.
+- Grants are **recursive**: an allow or deny on a directory covers
+  everything below it, and the *deepest* recorded ancestor wins — deny
+  `~/secrets` and it stays denied even though `~` is allowed.
+- Read and write are tracked separately; each is approved, denied, or not
+  decided yet.
+
+When an operation needs a decision that isn't recorded, `a` asks on the
+terminal (stderr, so it shows even with stdout piped) — one keypress, no
+Enter:
+
+```
+a: permission request
+  script: /home/me/bin/deploy.lua
+  wants:  read access to directory /home/me/project
+[y] allow once  [n] deny once  [A] allow always  [N] never allow:
+```
+
+- **`y` / `n`** apply for the rest of this run only.
+- **`A` / `N`** are recorded in `access.json` and answer silently from then
+  on. (A lowercase `a` deliberately does nothing — a slip of the finger must
+  not persist anything.)
+- **Esc / Ctrl-C** count as `n`.
+- No terminal (piped stdin, cron): the operation is denied for this run and
+  *nothing* is recorded.
+
+### `access.json`
+
+Recorded answers live in `$XDG_CONFIG_HOME/a/access.json` (usually
+`~/.config/a/access.json`), keyed by the script's canonical path — moving or
+renaming a script starts it with a clean slate. The file is plain JSON and
+safe to hand-edit; each flag is `true` (approved), `false` (denied), or
+`null` (not decided yet):
+
+```json
+{
+  "version": 1,
+  "scripts": {
+    "/home/me/bin/deploy.lua": {
+      "/home/me/project": { "read": true, "write": true },
+      "/etc": { "read": true, "write": false }
+    }
+  }
+}
+```
+
+Concurrent `a` processes update it safely (the write cycle holds a file
+lock and replaces the file atomically).
+
+The **REPL** prompts the same way, but its answers — including `A`/`N` —
+last only for the session; an interactive session has no script identity to
+record them under.
+
+### `sqlite` and `http` respect the same system
+
+- `sqlite.connect(path)` asks one combined **read/write** question for the
+  database's directory (SQLite keeps WAL/journal files next to it).
+  Memory-class databases — `":memory:"`, `""` and their `file:` URI
+  spellings — touch no file and are always allowed, even under `--no-fs` /
+  `--sandbox`.
+- A session's cookie-jar files (`http.session(path)`, `session:load`,
+  `session:save`) are read/write checked like any other file.
+
+## CLI flags
+
+| Flag | Effect |
+|------|--------|
+| `--allow-fs` | Skip the permission system: everything granted, nothing asked or recorded. |
+| `--no-fs` | Deny all filesystem access (fs module, on-disk sqlite databases, cookie-jar files). |
+| `--sandbox` | `--no-fs`, plus networking is cut (`http` errors); overrides `--allow-fs`. |
+
+Denials from a flag read `fs.readAll: filesystem access disabled by --no-fs`,
+interactive/recorded ones `fs.readAll: access to /some/dir denied`.

docs/http.md

@@ -249,6 +249,12 @@ Empty the in-memory jar.
 Cookies live in memory for the session's lifetime. Save them to reuse a logged-in
 session across script runs.
 
+Jar files hold live credentials, so both directions run the
+[fs permission system](fs.md#permissions) — write for `s:save`, read for
+`s:load` and `http.session(path)`. Note also that `--sandbox` disables
+networking entirely: every request errors with
+`http: networking disabled by --sandbox`.
+
 #### `s:save(path)`
 
 Write the jar to `path` as JSONL — one JSON object per line, one cookie per line:

docs/sqlite.md

@@ -26,6 +26,11 @@ local con = sqlite.connect(path)
 string `":memory:"` for a private in-memory database that vanishes when closed. The
 returned connection is an object you call methods on with `:` syntax.
 
+Opening a file-backed database runs the [fs permission system](fs.md#permissions):
+one combined read/write question for the database's directory (SQLite keeps its
+WAL/journal files next to the file). In-memory databases touch no file and are
+always allowed, even under `--no-fs` / `--sandbox`.
+
 The connection closes automatically when it is garbage-collected. Call
 [`con:close()`](#conclose) to release it eagerly.
 

lua/stubs/fs.lua

@@ -0,0 +1,57 @@
+---@meta
+---@diagnostic disable: missing-return
+-- Type stubs for the `fs` module (fully native). Reference: docs/fs.md
+--
+-- Every operation runs under the folder permission system: recorded grants
+-- (per script, per directory, recursive) answer silently; unknown ones ask on
+-- the terminal — y/n for this run, A/N recorded in access.json. Denials and
+-- OS-level failures raise Lua errors; trap with pcall/utils.try where needed.
+-- `--allow-fs`, `--no-fs` and `--sandbox` override the whole system.
+
+fs = {}
+
+---Read a whole file into a string (byte-exact; no encoding assumed).
+---Errors if the path is not a regular file or not accessible.
+---@param path string
+---@return string content
+function fs.readAll(path) end
+
+---Write a whole file from a string: created if missing, truncated if not.
+---The containing directory must already exist. Errors if the file can't be
+---created or is not accessible.
+---@param path string
+---@param content string
+function fs.writeAll(path, content) end
+
+---Names of a directory's entries (files and subdirectories), sorted.
+---Errors if the path is not a directory or not accessible.
+---@param path string
+---@return string[] names
+function fs.list(path) end
+
+---true iff the path exists, is a directory, and is readable under the
+---permission system. Never errors; a nonexistent path is false without any
+---permission prompt.
+---@param path string
+---@return boolean
+function fs.isDir(path) end
+
+---true iff the path exists, is a regular file, and is readable under the
+---permission system. Never errors; a nonexistent path is false without any
+---permission prompt.
+---@param path string
+---@return boolean
+function fs.isFile(path) end
+
+---The process's current working directory (absolute path) — what relative
+---paths in the other fs functions resolve against. Not permission-gated.
+---@return string dir
+function fs.getWorkdir() end
+
+---Run the permission cycle for a directory explicitly: recorded answer, else
+---interactive prompt, else false (nothing recorded when there is no terminal
+---to ask on). Errors if the path is not an existing directory.
+---@param dir string
+---@param mode "r"|"w"
+---@return boolean granted
+function fs.access(dir, mode) end

lua/stubs/sqlite.lua

@@ -65,7 +65,8 @@ function Connection:rollback() end
 function Connection:transaction(fn) end
 
 ---Open a database file (created if missing), or ":memory:" for a private
----in-memory database.
+---in-memory database. A file-backed database runs the fs permission system
+---(one combined read/write ask for its directory); ":memory:" never does.
 ---@param path string
 ---@return sqlite.Connection
 function sqlite.connect(path) end

src/lua_tests/fs_tests.rs

@@ -0,0 +1,412 @@
+//! Tests for the `fs` module and its permission system, exercised through
+//! Lua with policies injected via app data (the same channel `main` uses).
+//!
+//! No test here can ever reach a real terminal prompt or the real
+//! access.json: in test builds the prompt path is compiled out (scripted
+//! answers only) and the default policy carries no script identity.
+
+use std::sync::Arc;
+
+use super::lua;
+use crate::stdlib::fs::{store, Choice, FsPolicy};
+
+/// The sandboxed state with an --allow-fs policy.
+fn lua_allow_all() -> mlua::Lua {
+    let lua = lua();
+    lua.set_app_data(Arc::new(FsPolicy::allow_all()));
+    lua
+}
+
+fn lua_with(policy: FsPolicy) -> mlua::Lua {
+    let lua = lua();
+    lua.set_app_data(Arc::new(policy));
+    lua
+}
+
+async fn lua_err(lua: &mlua::Lua, src: &str) -> String {
+    lua.load(src).exec_async().await.unwrap_err().to_string()
+}
+
+#[tokio::test]
+async fn write_read_round_trip_and_truncation() {
+    let lua = lua_allow_all();
+    let dir = tempfile::tempdir().unwrap();
+    let file = dir.path().join("data.bin");
+
+    // Non-UTF-8 bytes must survive; a rewrite must truncate, not append.
+    let got: mlua::String = lua
+        .load(format!(
+            r#"
+            fs.writeAll('{p}', "long original content \255\254\0tail")
+            fs.writeAll('{p}', "short\255")
+            return fs.readAll('{p}')
+            "#,
+            p = file.display()
+        ))
+        .eval_async()
+        .await
+        .unwrap();
+    assert_eq!(&*got.as_bytes(), b"short\xff");
+}
+
+#[tokio::test]
+async fn write_all_creates_missing_file_but_not_missing_parent() {
+    let lua = lua_allow_all();
+    let dir = tempfile::tempdir().unwrap();
+
+    let fresh = dir.path().join("new.txt");
+    lua.load(format!("fs.writeAll('{}', 'x')", fresh.display()))
+        .exec_async()
+        .await
+        .unwrap();
+    assert_eq!(std::fs::read_to_string(&fresh).unwrap(), "x");
+
+    let orphan = dir.path().join("no/such/dir/f.txt");
+    let err = lua_err(&lua, &format!("fs.writeAll('{}', 'x')", orphan.display())).await;
+    assert!(err.contains("fs.writeAll") && err.contains("parent directory"), "{err}");
+}
+
+#[tokio::test]
+async fn type_mismatches_error() {
+    let lua = lua_allow_all();
+    let dir = tempfile::tempdir().unwrap();
+    let file = dir.path().join("f.txt");
+    std::fs::write(&file, "x").unwrap();
+
+    let err = lua_err(&lua, &format!("fs.readAll('{}')", dir.path().display())).await;
+    assert!(err.contains("fs.readAll") && err.contains("not a regular file"), "{err}");
+
+    let err = lua_err(&lua, &format!("fs.writeAll('{}', 'x')", dir.path().display())).await;
+    assert!(err.contains("fs.writeAll") && err.contains("is a directory"), "{err}");
+
+    let err = lua_err(&lua, &format!("fs.list('{}')", file.display())).await;
+    assert!(err.contains("fs.list") && err.contains("not a directory"), "{err}");
+
+    let err = lua_err(&lua, "fs.readAll('/no/such/file/anywhere')").await;
+    assert!(err.contains("fs.readAll"), "{err}");
+}
+
+#[tokio::test]
+async fn list_returns_sorted_names() {
+    let lua = lua_allow_all();
+    let dir = tempfile::tempdir().unwrap();
+    std::fs::write(dir.path().join("b.txt"), "").unwrap();
+    std::fs::write(dir.path().join("a.txt"), "").unwrap();
+    std::fs::create_dir(dir.path().join("sub")).unwrap();
+
+    let joined: String = lua
+        .load(format!(
+            "return table.concat(fs.list('{}'), ',')",
+            dir.path().display()
+        ))
+        .eval_async()
+        .await
+        .unwrap();
+    assert_eq!(joined, "a.txt,b.txt,sub");
+}
+
+#[tokio::test]
+async fn predicates_check_existence_type_and_access() {
+    let lua = lua_allow_all();
+    let dir = tempfile::tempdir().unwrap();
+    let file = dir.path().join("f.txt");
+    std::fs::write(&file, "x").unwrap();
+
+    let (dir_is_dir, dir_is_file, file_is_dir, file_is_file, ghost_dir, ghost_file): (
+        bool,
+        bool,
+        bool,
+        bool,
+        bool,
+        bool,
+    ) = lua
+        .load(format!(
+            r#"
+            return fs.isDir('{d}'), fs.isFile('{d}'),
+                   fs.isDir('{f}'), fs.isFile('{f}'),
+                   fs.isDir('/no/such/path'), fs.isFile('/no/such/path')
+            "#,
+            d = dir.path().display(),
+            f = file.display()
+        ))
+        .eval_async()
+        .await
+        .unwrap();
+    assert!(dir_is_dir && !dir_is_file && !file_is_dir && file_is_file);
+    assert!(!ghost_dir && !ghost_file);
+}
+
+#[tokio::test]
+async fn access_validates_arguments() {
+    let lua = lua_allow_all();
+    let dir = tempfile::tempdir().unwrap();
+
+    let ok: bool = lua
+        .load(format!("return fs.access('{}', 'r')", dir.path().display()))
+        .eval_async()
+        .await
+        .unwrap();
+    assert!(ok);
+
+    let err = lua_err(&lua, &format!("fs.access('{}', 'rw')", dir.path().display())).await;
+    assert!(err.contains("fs.access") && err.contains("mode"), "{err}");
+
+    let err = lua_err(&lua, "fs.access('/no/such/dir', 'r')").await;
+    assert!(err.contains("fs.access"), "{err}");
+}
+
+#[tokio::test]
+async fn get_workdir_returns_the_cwd_ungated() {
+    // Works even under the strictest policy: it reveals process state, not
+    // filesystem content.
+    let lua = lua_with(FsPolicy::deny_all("--sandbox", false));
+    let cwd: String = lua.load("return fs.getWorkdir()").eval_async().await.unwrap();
+    assert_eq!(std::path::PathBuf::from(&cwd), std::env::current_dir().unwrap());
+    assert!(std::path::Path::new(&cwd).is_absolute());
+}
+
+#[tokio::test]
+async fn deny_all_blocks_everything_without_prompting() {
+    let lua = lua_with(FsPolicy::deny_all("--no-fs", true));
+    let dir = tempfile::tempdir().unwrap();
+    let file = dir.path().join("f.txt");
+    std::fs::write(&file, "secret").unwrap();
+
+    for op in [
+        format!("fs.readAll('{}')", file.display()),
+        format!("fs.writeAll('{}', 'x')", file.display()),
+        format!("fs.list('{}')", dir.path().display()),
+    ] {
+        let err = lua_err(&lua, &op).await;
+        assert!(err.contains("disabled by --no-fs"), "{op}: {err}");
+    }
+
+    let (acc, is_dir, is_file): (bool, bool, bool) = lua
+        .load(format!(
+            "return fs.access('{d}', 'r'), fs.isDir('{d}'), fs.isFile('{f}')",
+            d = dir.path().display(),
+            f = file.display()
+        ))
+        .eval_async()
+        .await
+        .unwrap();
+    assert!(!acc && !is_dir && !is_file);
+    // the file was not touched
+    assert_eq!(std::fs::read_to_string(&file).unwrap(), "secret");
+}
+
+#[tokio::test]
+async fn default_policy_denies_unknown_without_persisting() {
+    // No policy injected: the fs::install default (REPL semantics, no
+    // scripted answers = nobody to ask) must deny and record nothing.
+    let lua = lua();
+    let dir = tempfile::tempdir().unwrap();
+    let file = dir.path().join("f.txt");
+    std::fs::write(&file, "x").unwrap();
+
+    let err = lua_err(&lua, &format!("fs.readAll('{}')", file.display())).await;
+    assert!(err.contains("fs.readAll") && err.contains("denied"), "{err}");
+
+    let ok: bool = lua
+        .load(format!("return fs.access('{}', 'r')", dir.path().display()))
+        .eval_async()
+        .await
+        .unwrap();
+    assert!(!ok);
+}
+
+#[tokio::test]
+async fn interactive_allow_once_grants_operations() {
+    let dir = tempfile::tempdir().unwrap();
+    let file = dir.path().join("f.txt");
+    std::fs::write(&file, "content").unwrap();
+
+    // One AllowOnce answer covers the whole run for that directory: the
+    // second read and the fs.access probe consume no further answers.
+    let lua = lua_with(FsPolicy::interactive_scripted(
+        None,
+        Default::default(),
+        None,
+        [Choice::AllowOnce],
+    ));
+    let (a, b, acc): (String, String, bool) = lua
+        .load(format!(
+            r#"return fs.readAll('{f}'), fs.readAll('{f}'), fs.access('{d}', 'r')"#,
+            f = file.display(),
+            d = dir.path().display()
+        ))
+        .eval_async()
+        .await
+        .unwrap();
+    assert_eq!((a.as_str(), b.as_str(), acc), ("content", "content", true));
+}
+
+#[tokio::test]
+async fn interactive_deny_answer_blocks_and_read_grant_does_not_leak_to_write() {
+    let dir = tempfile::tempdir().unwrap();
+    let file = dir.path().join("f.txt");
+    std::fs::write(&file, "content").unwrap();
+
+    // First answer grants the read; the write is a separate (write-kind)
+    // question answered with DenyOnce.
+    let lua = lua_with(FsPolicy::interactive_scripted(
+        None,
+        Default::default(),
+        None,
+        [Choice::AllowOnce, Choice::DenyOnce],
+    ));
+    let got: String = lua
+        .load(format!("return fs.readAll('{}')", file.display()))
+        .eval_async()
+        .await
+        .unwrap();
+    assert_eq!(got, "content");
+
+    let err = lua_err(&lua, &format!("fs.writeAll('{}', 'nope')", file.display())).await;
+    assert!(err.contains("fs.writeAll") && err.contains("denied"), "{err}");
+    assert_eq!(std::fs::read_to_string(&file).unwrap(), "content");
+}
+
+#[tokio::test]
+async fn fs_access_allow_always_persists_and_is_honored_next_run() {
+    let data_dir = tempfile::tempdir().unwrap();
+    let cfg = tempfile::tempdir().unwrap();
+    let store_path = cfg.path().join("access.json");
+    let script_key = "/scripts/tool.lua";
+
+    // Run 1: fs.access asks, the user answers "A".
+    let lua = lua_with(FsPolicy::interactive_scripted(
+        Some(script_key.into()),
+        Default::default(),
+        Some(store_path.clone()),
+        [Choice::AllowAlways],
+    ));
+    let ok: bool = lua
+        .load(format!("return fs.access('{}', 'w')", data_dir.path().display()))
+        .eval_async()
+        .await
+        .unwrap();
+    assert!(ok);
+
+    // The grant is on disk, keyed by script and canonical dir…
+    let persisted = store::load_for_script(&store_path, script_key);
+    let dir_key = data_dir.path().canonicalize().unwrap();
+    assert_eq!(persisted[&dir_key.to_string_lossy().into_owned()].write, Some(true));
+
+    // …and a fresh "run" that loads it never needs a prompt, including for a
+    // file inside a subdirectory.
+    std::fs::create_dir(data_dir.path().join("sub")).unwrap();
+    let lua = lua_with(FsPolicy::interactive_scripted(
+        Some(script_key.into()),
+        store::load_for_script(&store_path, script_key),
+        Some(store_path.clone()),
+        [], // any prompt would deny
+    ));
+    lua.load(format!(
+        "fs.writeAll('{}', 'hello')",
+        data_dir.path().join("sub/out.txt").display()
+    ))
+    .exec_async()
+    .await
+    .unwrap();
+}
+
+#[tokio::test]
+async fn sqlite_memory_db_works_under_sandbox_but_file_db_is_blocked() {
+    let lua = lua_with(FsPolicy::deny_all("--sandbox", false));
+
+    let n: i64 = lua
+        .load(
+            r#"
+            local db = sqlite.connect(":memory:")
+            db:execute("CREATE TABLE t (x)")
+            db:execute("INSERT INTO t VALUES (41)")
+            return db:queryOne("SELECT x + 1 AS y FROM t").y
+            "#,
+        )
+        .eval_async()
+        .await
+        .unwrap();
+    assert_eq!(n, 42);
+
+    let dir = tempfile::tempdir().unwrap();
+    let err = lua_err(
+        &lua,
+        &format!("sqlite.connect('{}')", dir.path().join("x.db").display()),
+    )
+    .await;
+    assert!(err.contains("sqlite.connect") && err.contains("--sandbox"), "{err}");
+}
+
+#[tokio::test]
+async fn sqlite_file_db_asks_one_combined_question() {
+    let dir = tempfile::tempdir().unwrap();
+    // A single AllowOnce must cover the whole read/write open.
+    let lua = lua_with(FsPolicy::interactive_scripted(
+        None,
+        Default::default(),
+        None,
+        [Choice::AllowOnce],
+    ));
+    let n: i64 = lua
+        .load(format!(
+            r#"
+            local db = sqlite.connect('{}')
+            db:execute("CREATE TABLE t (x)")
+            db:execute("INSERT INTO t VALUES (1)")
+            return db:queryOne("SELECT COUNT(*) AS n FROM t").n
+            "#,
+            dir.path().join("x.db").display()
+        ))
+        .eval_async()
+        .await
+        .unwrap();
+    assert_eq!(n, 1);
+}
+
+#[tokio::test]
+async fn http_requests_are_blocked_under_sandbox_before_any_io() {
+    let lua = lua_with(FsPolicy::deny_all("--sandbox", false));
+    // Unroutable address: if the gate failed, this would time out / error
+    // differently; the sandbox message must come from our check.
+    let err = lua_err(&lua, "http.get('http://127.0.0.1:1/x')").await;
+    assert!(err.contains("networking disabled by --sandbox"), "{err}");
+
+    // --no-fs alone leaves networking on (the request fails at the socket,
+    // not at the gate).
+    let lua = lua_with(FsPolicy::deny_all("--no-fs", true));
+    let err = lua_err(&lua, "http.get('http://127.0.0.1:1/x')").await;
+    assert!(!err.contains("networking disabled"), "{err}");
+}
+
+#[tokio::test]
+async fn cookie_jar_files_go_through_the_permission_system() {
+    let dir = tempfile::tempdir().unwrap();
+    let jar_path = dir.path().join("jar.jsonl");
+
+    // Denied: save must not create the file.
+    let lua = lua_with(FsPolicy::deny_all("--no-fs", true));
+    let err = lua_err(
+        &lua,
+        &format!("local s = http.session() s:save('{}')", jar_path.display()),
+    )
+    .await;
+    assert!(err.contains("session:save") && err.contains("--no-fs"), "{err}");
+    assert!(!jar_path.exists());
+
+    // Allowed: save/load and the http.session(path) preload round-trip.
+    let lua = lua_allow_all();
+    lua.load(format!(
+        r#"
+        local s = http.session()
+        s:save('{p}')
+        s:load('{p}')
+        local s2 = http.session('{p}')
+        "#,
+        p = jar_path.display()
+    ))
+    .exec_async()
+    .await
+    .unwrap();
+    assert!(jar_path.exists());
+}

src/lua_tests/mod.rs

@@ -3,6 +3,7 @@
 
 mod async_tests;
 mod core_tests;
+mod fs_tests;
 mod math_tests;
 mod os_tests;
 mod require_tests;

src/main.rs

@@ -18,6 +18,43 @@ mod stdlib;
 struct Args {
     /// Path to the Lua script to execute. Omit to start an interactive REPL.
     filepath: Option<PathBuf>,
+
+    /// Grant all filesystem access without prompting or recording anything.
+    #[arg(long, conflicts_with = "no_fs")]
+    allow_fs: bool,
+
+    /// Deny all filesystem access (fs module, sqlite databases on disk,
+    /// cookie-jar files).
+    #[arg(long)]
+    no_fs: bool,
+
+    /// Deny filesystem access AND networking; overrides --allow-fs.
+    #[arg(long)]
+    sandbox: bool,
+}
+
+impl Args {
+    /// The fs/network permission policy the flags call for. In the default
+    /// interactive mode, grants recorded for this script (keyed by its
+    /// canonical path) are loaded from access.json; the REPL passes `None`
+    /// and never persists anything.
+    fn fs_policy(&self, script_key: Option<String>) -> stdlib::fs::FsPolicy {
+        use stdlib::fs::{store, FsPolicy};
+        if self.sandbox {
+            FsPolicy::deny_all("--sandbox", false)
+        } else if self.no_fs {
+            FsPolicy::deny_all("--no-fs", true)
+        } else if self.allow_fs {
+            FsPolicy::allow_all()
+        } else {
+            let store_path = store::default_store_path();
+            let persisted = match (&script_key, &store_path) {
+                (Some(key), Some(path)) => store::load_for_script(path, key),
+                _ => Default::default(),
+            };
+            FsPolicy::interactive(script_key, persisted, store_path)
+        }
+    }
 }
 
 #[tokio::main]
@@ -50,8 +87,11 @@ async fn run() -> Result<(), Box<dyn std::error::Error>> {
 
     let lua = sandbox::create_sandboxed_lua()?;
 
-    let Some(filepath) = args.filepath else {
+    let Some(filepath) = args.filepath.clone() else {
         // In the REPL, `.env` and modules are looked up from the CWD.
+        // Permissions are prompted for but never persisted (no script
+        // identity to record them under).
+        lua.set_app_data(std::sync::Arc::new(args.fs_policy(None)));
         setup_require(&lua, &std::env::current_dir()?)?;
         return repl::run(&lua).await;
     };
@@ -65,6 +105,11 @@ async fn run() -> Result<(), Box<dyn std::error::Error>> {
     let script_dir = realpath.parent().unwrap_or(Path::new("/"));
     setup_require(&lua, script_dir)?;
 
+    // The canonical path doubles as the script's identity in the permission
+    // store; this replaces the session-only default planted by fs::install.
+    let script_key = realpath.to_string_lossy().into_owned();
+    lua.set_app_data(std::sync::Arc::new(args.fs_policy(Some(script_key))));
+
     // Read as bytes, not a UTF-8 String: Lua source (and its string literals) are
     // byte strings, and stock Lua runs files that aren't valid UTF-8.
     let source = tokio::fs::read(&realpath)

src/stdlib/fs/mod.rs

@@ -0,0 +1,294 @@
+//! `fs` — whole-file filesystem access behind a folder-based permission
+//! system.
+//!
+//! v1 keeps the API deliberately small: `fs.readAll` / `fs.writeAll` move a
+//! whole file to/from a Lua string, `fs.list` names a directory's entries,
+//! `fs.isDir` / `fs.isFile` are existence-and-type predicates, and
+//! `fs.access(dir, "r"|"w")` runs the permission cycle explicitly.
+//!
+//! Every operation is checked against a per-script, per-directory grant
+//! ([`policy`]): recorded answers live in `access.json` ([`store`]), unknown
+//! ones are asked on the terminal ([`prompt`]) — `y`/`n` for this run,
+//! `A`/`N` recorded. Grants are recursive (a granted parent covers all
+//! children) and keyed by the script's canonical path; the REPL prompts the
+//! same way but never persists. `--allow-fs`, `--no-fs` and `--sandbox`
+//! replace the interactive policy wholesale (see `main.rs`).
+//!
+//! `sqlite.connect` and http's cookie-jar files route through the same
+//! checks via [`ensure_access`]; `--sandbox` additionally cuts networking
+//! via [`ensure_net`].
+
+pub(crate) mod policy;
+// In test builds `policy::check` never reaches the real terminal prompt
+// (scripted answers only), leaving these functions deliberately unused.
+#[cfg_attr(test, allow(dead_code))]
+mod prompt;
+pub(crate) mod store;
+
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+
+use mlua::prelude::{LuaResult, LuaString};
+use mlua::Lua;
+
+pub(crate) use policy::{FsPolicy, Wants};
+#[cfg(test)]
+pub(crate) use prompt::Choice;
+
+/// `mlua::Error::external(format!(...))` — every Lua-facing error in this
+/// module goes through this, prefixed `fs.<fn>: …`.
+macro_rules! ext_err {
+    ($($arg:tt)*) => { mlua::Error::external(format!($($arg)*)) };
+}
+
+pub(super) fn install(lua: &Lua) -> LuaResult<()> {
+    // Default policy: REPL semantics — prompt if a terminal is there, never
+    // persist. `main` replaces it once the CLI flags and the script's
+    // canonical path are known; bare states (tests) can therefore never
+    // touch the real access.json.
+    lua.set_app_data(Arc::new(FsPolicy::interactive(None, Default::default(), None)));
+
+    let fs = lua.create_table()?;
+
+    fs.set(
+        "readAll",
+        lua.create_async_function(|lua, path: LuaString| async move {
+            const F: &str = "fs.readAll";
+            let path = to_path(&path);
+            let (dir, target) = read_target(F, &path).await?;
+            ensure_access(&lua, F, dir, Wants::READ).await?;
+            let bytes = tokio::fs::read(&target)
+                .await
+                .map_err(|e| ext_err!("{F}: {}: {e}", path.display()))?;
+            lua.create_string(&bytes)
+        })?,
+    )?;
+
+    fs.set(
+        "writeAll",
+        lua.create_async_function(|lua, (path, content): (LuaString, LuaString)| async move {
+            const F: &str = "fs.writeAll";
+            let path = to_path(&path);
+            let (dir, target) = write_target(F, &path).await?;
+            ensure_access(&lua, F, dir, Wants::WRITE).await?;
+            tokio::fs::write(&target, &content.as_bytes())
+                .await
+                .map_err(|e| ext_err!("{F}: {}: {e}", path.display()))
+        })?,
+    )?;
+
+    fs.set(
+        "list",
+        lua.create_async_function(|lua, path: LuaString| async move {
+            const F: &str = "fs.list";
+            let path = to_path(&path);
+            let canon = canonicalize(F, &path).await?;
+            let meta = metadata(F, &path, &canon).await?;
+            if !meta.is_dir() {
+                return Err(ext_err!("{F}: {}: not a directory", path.display()));
+            }
+            ensure_access(&lua, F, canon.clone(), Wants::READ).await?;
+
+            let mut entries = tokio::fs::read_dir(&canon)
+                .await
+                .map_err(|e| ext_err!("{F}: {}: {e}", path.display()))?;
+            let mut names: Vec<Vec<u8>> = Vec::new();
+            while let Some(entry) = entries
+                .next_entry()
+                .await
+                .map_err(|e| ext_err!("{F}: {}: {e}", path.display()))?
+            {
+                use std::os::unix::ffi::OsStrExt;
+                names.push(entry.file_name().as_bytes().to_vec());
+            }
+            // read_dir order is arbitrary; sorted output is deterministic.
+            names.sort_unstable();
+            let list = lua.create_table_with_capacity(names.len(), 0)?;
+            for name in &names {
+                list.raw_push(lua.create_string(name)?)?;
+            }
+            Ok(list)
+        })?,
+    )?;
+
+    fs.set(
+        "isDir",
+        lua.create_async_function(|lua, path: LuaString| async move {
+            let path = to_path(&path);
+            let Ok(canon) = tokio::fs::canonicalize(&path).await else {
+                return Ok(false); // nonexistent: false without prompting
+            };
+            match tokio::fs::metadata(&canon).await {
+                Ok(meta) if meta.is_dir() => check_access(&lua, canon, Wants::READ).await,
+                _ => Ok(false),
+            }
+        })?,
+    )?;
+
+    fs.set(
+        "isFile",
+        lua.create_async_function(|lua, path: LuaString| async move {
+            let path = to_path(&path);
+            let Ok(canon) = tokio::fs::canonicalize(&path).await else {
+                return Ok(false);
+            };
+            match tokio::fs::metadata(&canon).await {
+                Ok(meta) if meta.is_file() => {
+                    check_access(&lua, parent_dir(&canon), Wants::READ).await
+                }
+                _ => Ok(false),
+            }
+        })?,
+    )?;
+
+    fs.set(
+        "getWorkdir",
+        lua.create_function(|lua, ()| {
+            // Process state, not filesystem content — no permission check.
+            // This is what relative paths in the other fs functions (and in
+            // fs.access probes) resolve against.
+            use std::os::unix::ffi::OsStrExt;
+            let cwd = std::env::current_dir()
+                .map_err(|e| ext_err!("fs.getWorkdir: {e}"))?;
+            lua.create_string(cwd.as_os_str().as_bytes())
+        })?,
+    )?;
+
+    fs.set(
+        "access",
+        lua.create_async_function(|lua, (dir, mode): (LuaString, LuaString)| async move {
+            const F: &str = "fs.access";
+            let wants = match &*mode.as_bytes() {
+                b"r" => Wants::READ,
+                b"w" => Wants::WRITE,
+                _ => return Err(ext_err!("{F}: mode must be \"r\" or \"w\"")),
+            };
+            let dir = to_path(&dir);
+            let canon = canonicalize(F, &dir).await?;
+            let meta = metadata(F, &dir, &canon).await?;
+            if !meta.is_dir() {
+                return Err(ext_err!("{F}: {}: not a directory", dir.display()));
+            }
+            check_access(&lua, canon, wants).await
+        })?,
+    )?;
+
+    lua.globals().raw_set("fs", fs)?;
+    Ok(())
+}
+
+/// The active policy; fails closed if none was installed (cannot happen —
+/// `install` plants a default).
+fn active_policy(lua: &Lua) -> Arc<FsPolicy> {
+    lua.app_data_ref::<Arc<FsPolicy>>()
+        .map(|p| Arc::clone(&p))
+        .unwrap_or_else(|| Arc::new(FsPolicy::deny_all("(internal: no fs policy)", false)))
+}
+
+/// Run the permission cycle for the canonical directory `dir`; a denial is a
+/// Lua error prefixed with `fname`. Shared with sqlite (`Wants::READ_WRITE`
+/// on a database's directory) and http (cookie-jar files).
+pub(super) async fn ensure_access(
+    lua: &Lua,
+    fname: &'static str,
+    dir: PathBuf,
+    wants: Wants,
+) -> LuaResult<()> {
+    match active_policy(lua).check(dir.clone(), wants).await? {
+        policy::Outcome::Granted => Ok(()),
+        policy::Outcome::DeniedByFlag(flag) => {
+            Err(ext_err!("{fname}: filesystem access disabled by {flag}"))
+        }
+        policy::Outcome::Denied => Err(ext_err!("{fname}: access to {} denied", dir.display())),
+    }
+}
+
+/// Bool-returning permission cycle (`fs.access`, the predicates): denial is
+/// `false`, never an error.
+async fn check_access(lua: &Lua, dir: PathBuf, wants: Wants) -> LuaResult<bool> {
+    Ok(matches!(active_policy(lua).check(dir, wants).await?, policy::Outcome::Granted))
+}
+
+/// Networking gate for `--sandbox`; called by http before any request.
+pub(super) fn ensure_net(lua: &Lua) -> LuaResult<()> {
+    if active_policy(lua).net_allowed {
+        Ok(())
+    } else {
+        Err(ext_err!("http: networking disabled by --sandbox"))
+    }
+}
+
+/// Lua strings are byte strings; on unix so are paths.
+fn to_path(s: &LuaString) -> PathBuf {
+    use std::os::unix::ffi::OsStrExt;
+    PathBuf::from(std::ffi::OsStr::from_bytes(&s.as_bytes()))
+}
+
+/// The permission unit for operations on a file: its containing directory.
+fn parent_dir(canon: &Path) -> PathBuf {
+    canon.parent().unwrap_or(Path::new("/")).to_path_buf()
+}
+
+async fn canonicalize(fname: &str, path: &Path) -> LuaResult<PathBuf> {
+    tokio::fs::canonicalize(path)
+        .await
+        .map_err(|e| ext_err!("{fname}: {}: {e}", path.display()))
+}
+
+async fn metadata(fname: &str, path: &Path, canon: &Path) -> LuaResult<std::fs::Metadata> {
+    tokio::fs::metadata(canon)
+        .await
+        .map_err(|e| ext_err!("{fname}: {}: {e}", path.display()))
+}
+
+/// Resolve an existing regular file into (canonical containing dir = the
+/// permission unit, canonical file). Shared with http's cookie-jar load.
+pub(super) async fn read_target(fname: &'static str, path: &Path) -> LuaResult<(PathBuf, PathBuf)> {
+    let canon = canonicalize(fname, path).await?;
+    let meta = metadata(fname, path, &canon).await?;
+    if !meta.is_file() {
+        return Err(ext_err!("{fname}: {}: not a regular file", path.display()));
+    }
+    Ok((parent_dir(&canon), canon))
+}
+
+/// Resolve a write destination that may not exist yet into (canonical
+/// containing dir = the permission unit, write target). The containing
+/// directory must exist — v1 has no mkdir-p. Shared with http's cookie-jar
+/// save and sqlite's connect.
+pub(super) async fn write_target(
+    fname: &'static str,
+    path: &Path,
+) -> LuaResult<(PathBuf, PathBuf)> {
+    match tokio::fs::canonicalize(path).await {
+        Ok(canon) => {
+            let meta = metadata(fname, path, &canon).await?;
+            if meta.is_dir() {
+                return Err(ext_err!("{fname}: {}: is a directory", path.display()));
+            }
+            Ok((parent_dir(&canon), canon))
+        }
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
+            // A dangling symlink would make write() land at a location the
+            // permission check never saw; refuse it.
+            if let Ok(meta) = tokio::fs::symlink_metadata(path).await
+                && meta.file_type().is_symlink()
+            {
+                return Err(ext_err!("{fname}: {}: dangling symlink", path.display()));
+            }
+            let Some(name) = path.file_name() else {
+                return Err(ext_err!("{fname}: {}: not a file path", path.display()));
+            };
+            let parent = match path.parent() {
+                Some(p) if !p.as_os_str().is_empty() => p,
+                _ => Path::new("."),
+            };
+            let dir = tokio::fs::canonicalize(parent).await.map_err(|e| {
+                ext_err!("{fname}: {}: parent directory: {e}", path.display())
+            })?;
+            let target = dir.join(name);
+            Ok((dir, target))
+        }
+        Err(e) => Err(ext_err!("{fname}: {}: {e}", path.display())),
+    }
+}

src/stdlib/fs/policy.rs

@@ -0,0 +1,519 @@
+//! The permission policy — who may touch which directory.
+//!
+//! One [`FsPolicy`] lives in mlua app data per Lua state (as `Arc<FsPolicy>`).
+//! `fs::install` plants a session-only default; `main` replaces it once the
+//! CLI flags and the script's canonical path are known.
+//!
+//! Grants are recorded per canonical directory and apply recursively: the
+//! deepest recorded ancestor decides, so a deny on `~/secrets` beats an allow
+//! on `~`. Session answers (y/n, and everything in the REPL) shadow the
+//! persisted store at every depth.
+
+use std::collections::BTreeMap;
+use std::path::{Path, PathBuf};
+use std::sync::{Arc, Mutex};
+
+use mlua::prelude::LuaResult;
+
+use super::prompt::Choice;
+use super::store::{self, DirAccess, DirGrants};
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub(crate) enum AccessKind {
+    Read,
+    Write,
+}
+
+/// Which directions an operation needs; `read && write` is sqlite's combined
+/// "open this database" request, answered by a single prompt.
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct Wants {
+    pub(crate) read: bool,
+    pub(crate) write: bool,
+}
+
+impl Wants {
+    pub(crate) const READ: Wants = Wants { read: true, write: false };
+    pub(crate) const WRITE: Wants = Wants { read: false, write: true };
+    pub(crate) const READ_WRITE: Wants = Wants { read: true, write: true };
+
+    fn kinds(self) -> impl Iterator<Item = AccessKind> {
+        [(self.read, AccessKind::Read), (self.write, AccessKind::Write)]
+            .into_iter()
+            .filter_map(|(wanted, kind)| wanted.then_some(kind))
+    }
+
+    fn describe(self) -> &'static str {
+        match (self.read, self.write) {
+            (true, true) => "read/write",
+            (true, false) => "read",
+            (false, true) => "write",
+            (false, false) => "no",
+        }
+    }
+}
+
+pub(crate) enum FsMode {
+    /// `--allow-fs`: everything granted, no prompts, no persistence.
+    AllowAll,
+    /// `--no-fs` / `--sandbox`: everything denied; `flag` names the culprit
+    /// for error messages.
+    DenyAll { flag: &'static str },
+    /// Default: consult the recorded grants, prompt on unknown.
+    Interactive {
+        /// Store key = the script's canonical path. `None` in the REPL (and
+        /// in bare test states): prompts work, nothing is ever persisted.
+        script_key: Option<String>,
+        /// This script's grants from access.json, snapshotted at startup.
+        persisted: Mutex<DirGrants>,
+        /// Session-only answers — y/n, plus A/N in the REPL. Shadows
+        /// `persisted`.
+        session: Mutex<DirGrants>,
+        /// Where A/N answers are persisted; `None` = no config dir found,
+        /// session-only operation.
+        store_path: Option<PathBuf>,
+        /// Scripted prompt answers for tests, consumed front to back; an
+        /// exhausted (or absent) queue behaves like "no terminal". In test
+        /// builds the real terminal prompt is never reached (see `check`).
+        #[cfg(test)]
+        scripted: Mutex<std::collections::VecDeque<Choice>>,
+    },
+}
+
+pub(crate) struct FsPolicy {
+    pub(crate) mode: FsMode,
+    /// `false` only under `--sandbox`: http refuses all requests.
+    pub(crate) net_allowed: bool,
+}
+
+/// Outcome of a permission check, kept apart from granted/denied booleans so
+/// error messages can name the CLI flag that caused a static denial.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(crate) enum Outcome {
+    Granted,
+    /// Denied by `--no-fs` / `--sandbox`.
+    DeniedByFlag(&'static str),
+    /// Denied by a recorded decision, an interactive answer, or the inability
+    /// to ask (no terminal).
+    Denied,
+}
+
+impl FsPolicy {
+    pub(crate) fn allow_all() -> Self {
+        FsPolicy { mode: FsMode::AllowAll, net_allowed: true }
+    }
+
+    pub(crate) fn deny_all(flag: &'static str, net_allowed: bool) -> Self {
+        FsPolicy { mode: FsMode::DenyAll { flag }, net_allowed }
+    }
+
+    pub(crate) fn interactive(
+        script_key: Option<String>,
+        persisted: DirGrants,
+        store_path: Option<PathBuf>,
+    ) -> Self {
+        FsPolicy {
+            mode: FsMode::Interactive {
+                script_key,
+                persisted: Mutex::new(persisted),
+                session: Mutex::new(BTreeMap::new()),
+                store_path,
+                #[cfg(test)]
+                scripted: Mutex::new(std::collections::VecDeque::new()),
+            },
+            net_allowed: true,
+        }
+    }
+
+    /// An interactive policy with a queue of pre-recorded prompt answers.
+    #[cfg(test)]
+    pub(crate) fn interactive_scripted(
+        script_key: Option<String>,
+        persisted: DirGrants,
+        store_path: Option<PathBuf>,
+        answers: impl IntoIterator<Item = Choice>,
+    ) -> Self {
+        let policy = Self::interactive(script_key, persisted, store_path);
+        if let FsMode::Interactive { scripted, .. } = &policy.mode {
+            scripted.lock().unwrap().extend(answers);
+        }
+        policy
+    }
+
+    /// The full permission cycle for the canonical directory `dir`: recorded
+    /// answer, else ask on the terminal, else deny (recording nothing — a
+    /// persisted denial must be an explicit user decision).
+    pub(crate) async fn check(self: Arc<Self>, dir: PathBuf, wants: Wants) -> LuaResult<Outcome> {
+        match &self.mode {
+            FsMode::AllowAll => return Ok(Outcome::Granted),
+            FsMode::DenyAll { flag } => return Ok(Outcome::DeniedByFlag(flag)),
+            FsMode::Interactive { .. } => {}
+        }
+
+        // Fast path — already decided, no prompt needed.
+        match self.resolve_wants(&dir, wants) {
+            Some(true) => return Ok(Outcome::Granted),
+            Some(false) => return Ok(Outcome::Denied),
+            None => {}
+        }
+
+        // Test builds never reach a real terminal prompt: scripted answers
+        // only. This is structural — an unwitting test cannot hang `cargo
+        // test` on a keypress.
+        #[cfg(test)]
+        {
+            let FsMode::Interactive { scripted, .. } = &self.mode else { unreachable!() };
+            let choice = {
+                let mut answers = scripted.lock().unwrap_or_else(|e| e.into_inner());
+                answers.pop_front().unwrap_or(Choice::NoTty)
+            };
+            Ok(self.decide(&dir, wants, |_, _, _| choice))
+        }
+
+        #[cfg(not(test))]
+        {
+            if !super::prompt::can_prompt() {
+                return Ok(Outcome::Denied);
+            }
+            let this = Arc::clone(&self);
+            tokio::task::spawn_blocking(move || {
+                // Serialize with tui prompts (and each other) so concurrent
+                // coroutines can't fight over the terminal.
+                let _guard = crate::stdlib::tui::PROMPT_LOCK
+                    .lock()
+                    .unwrap_or_else(|e| e.into_inner());
+                this.decide(&dir, wants, super::prompt::prompt_access)
+            })
+            .await
+            .map_err(|e| mlua::Error::external(format!("fs: prompt task failed: {e}")))
+        }
+    }
+
+    /// The ask-and-record step, run with the prompt lock held. Re-resolves
+    /// first — another coroutine may have answered for this directory while
+    /// we waited for the lock — then asks only for what is still unknown.
+    fn decide(&self, dir: &Path, wants: Wants, ask: impl FnOnce(&str, &str, &str) -> Choice) -> Outcome {
+        let FsMode::Interactive { script_key, session, store_path, .. } = &self.mode else {
+            unreachable!("decide() is only called in interactive mode");
+        };
+
+        match self.resolve_wants(dir, wants) {
+            Some(true) => return Outcome::Granted,
+            Some(false) => return Outcome::Denied,
+            None => {}
+        }
+        let missing = Wants {
+            read: wants.read && self.resolve_one(dir, AccessKind::Read).is_none(),
+            write: wants.write && self.resolve_one(dir, AccessKind::Write).is_none(),
+        };
+
+        let label = script_key.as_deref().unwrap_or("interactive session (REPL)");
+        let dir_key = dir.to_string_lossy().into_owned();
+        let choice = ask(label, &dir_key, missing.describe());
+        let value = match choice {
+            Choice::AllowOnce | Choice::AllowAlways => true,
+            Choice::DenyOnce | Choice::DenyAlways => false,
+            Choice::NoTty => return Outcome::Denied, // record nothing
+        };
+
+        let update = DirAccess {
+            read: missing.read.then_some(value),
+            write: missing.write.then_some(value),
+        };
+        {
+            let mut session = session.lock().unwrap_or_else(|e| e.into_inner());
+            let entry = session.entry(dir_key.clone()).or_default();
+            if update.read.is_some() {
+                entry.read = update.read;
+            }
+            if update.write.is_some() {
+                entry.write = update.write;
+            }
+        }
+        if matches!(choice, Choice::AllowAlways | Choice::DenyAlways)
+            && let (Some(key), Some(path)) = (script_key, store_path)
+            && let Err(e) = store::persist(path, key, &dir_key, update)
+        {
+            // The answer still holds for this run; failing to record it must
+            // not fail the script's operation.
+            eprintln!(
+                "{}: warning: cannot save permission to {}: {e}",
+                store::APP_NAME,
+                path.display()
+            );
+        }
+
+        // `missing` now carries the fresh answer; every other wanted kind
+        // already resolved to true (a false would have denied above).
+        if value { Outcome::Granted } else { Outcome::Denied }
+    }
+
+    /// `Some(true)` = every wanted kind granted, `Some(false)` = at least one
+    /// denied, `None` = at least one undecided (and none denied).
+    fn resolve_wants(&self, dir: &Path, wants: Wants) -> Option<bool> {
+        let mut all_known = true;
+        for kind in wants.kinds() {
+            match self.resolve_one(dir, kind) {
+                Some(false) => return Some(false),
+                Some(true) => {}
+                None => all_known = false,
+            }
+        }
+        all_known.then_some(true)
+    }
+
+    fn resolve_one(&self, dir: &Path, kind: AccessKind) -> Option<bool> {
+        let FsMode::Interactive { session, persisted, .. } = &self.mode else {
+            return None;
+        };
+        let session = session.lock().unwrap_or_else(|e| e.into_inner());
+        let persisted = persisted.lock().unwrap_or_else(|e| e.into_inner());
+        resolve(&session, &persisted, dir, kind)
+    }
+}
+
+/// Walk from `dir` up to and including `/`. At each depth the session layer
+/// shadows the persisted one; the first decided value wins — the deepest
+/// recorded ancestor decides.
+fn resolve(session: &DirGrants, persisted: &DirGrants, dir: &Path, kind: AccessKind) -> Option<bool> {
+    let mut cur = Some(dir);
+    while let Some(d) = cur {
+        let key = d.to_string_lossy();
+        for layer in [session, persisted] {
+            if let Some(v) = layer.get(key.as_ref()).and_then(|a| a.get(kind)) {
+                return Some(v);
+            }
+        }
+        cur = d.parent();
+    }
+    None
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn grants(entries: &[(&str, Option<bool>, Option<bool>)]) -> DirGrants {
+        entries
+            .iter()
+            .map(|(dir, read, write)| {
+                (dir.to_string(), DirAccess { read: *read, write: *write })
+            })
+            .collect()
+    }
+
+    #[test]
+    fn resolve_walks_ancestors() {
+        let persisted = grants(&[("/home/u/data", Some(true), None)]);
+        let session = DirGrants::new();
+        let r = |dir: &str| resolve(&session, &persisted, Path::new(dir), AccessKind::Read);
+        assert_eq!(r("/home/u/data"), Some(true), "exact hit");
+        assert_eq!(r("/home/u/data/sub/deep"), Some(true), "parent covers children");
+        assert_eq!(r("/home/u"), None, "no upward leakage");
+        assert_eq!(r("/elsewhere"), None, "unrelated dir unknown");
+    }
+
+    #[test]
+    fn resolve_deepest_ancestor_wins() {
+        let persisted = grants(&[
+            ("/home/u", Some(true), None),
+            ("/home/u/secrets", Some(false), None),
+        ]);
+        let session = DirGrants::new();
+        let r = |dir: &str| resolve(&session, &persisted, Path::new(dir), AccessKind::Read);
+        assert_eq!(r("/home/u/data"), Some(true));
+        assert_eq!(r("/home/u/secrets"), Some(false), "deny beats shallower allow");
+        assert_eq!(r("/home/u/secrets/inner"), Some(false));
+    }
+
+    #[test]
+    fn resolve_session_shadows_persisted() {
+        let persisted = grants(&[("/data", Some(false), None)]);
+        let session = grants(&[("/data", Some(true), None)]);
+        assert_eq!(
+            resolve(&session, &persisted, Path::new("/data/x"), AccessKind::Read),
+            Some(true)
+        );
+    }
+
+    #[test]
+    fn resolve_null_flags_are_skipped() {
+        // read is undecided at the deep entry; the shallow decided one wins.
+        let persisted = grants(&[("/a", Some(true), None), ("/a/b", None, Some(false))]);
+        let session = DirGrants::new();
+        assert_eq!(resolve(&session, &persisted, Path::new("/a/b"), AccessKind::Read), Some(true));
+        assert_eq!(resolve(&session, &persisted, Path::new("/a/b"), AccessKind::Write), Some(false));
+    }
+
+    #[test]
+    fn resolve_root_grant_covers_everything() {
+        let persisted = grants(&[("/", Some(true), Some(true))]);
+        let session = DirGrants::new();
+        assert_eq!(
+            resolve(&session, &persisted, Path::new("/any/where"), AccessKind::Write),
+            Some(true)
+        );
+    }
+
+    #[tokio::test]
+    async fn static_modes() {
+        let allow = Arc::new(FsPolicy::allow_all());
+        assert_eq!(allow.check("/x".into(), Wants::READ).await.unwrap(), Outcome::Granted);
+        let deny = Arc::new(FsPolicy::deny_all("--no-fs", true));
+        assert_eq!(
+            Arc::clone(&deny).check("/x".into(), Wants::WRITE).await.unwrap(),
+            Outcome::DeniedByFlag("--no-fs")
+        );
+        assert!(deny.net_allowed);
+        assert!(!FsPolicy::deny_all("--sandbox", false).net_allowed);
+    }
+
+    #[tokio::test]
+    async fn allow_once_grants_for_the_whole_run() {
+        let policy = Arc::new(FsPolicy::interactive_scripted(
+            None,
+            DirGrants::new(),
+            None,
+            [Choice::AllowOnce], // exactly one answer available
+        ));
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data".into(), Wants::READ).await.unwrap(), Outcome::Granted);
+        // second check hits the session cache — no second answer consumed
+        // (the queue is empty; consuming would mean NoTty => Denied)
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data/sub".into(), Wants::READ).await.unwrap(), Outcome::Granted);
+    }
+
+    #[tokio::test]
+    async fn deny_once_is_cached_for_the_run() {
+        let policy = Arc::new(FsPolicy::interactive_scripted(
+            None,
+            DirGrants::new(),
+            None,
+            [Choice::DenyOnce],
+        ));
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data".into(), Wants::READ).await.unwrap(), Outcome::Denied);
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data".into(), Wants::READ).await.unwrap(), Outcome::Denied);
+    }
+
+    #[tokio::test]
+    async fn allow_always_persists_and_covers_subdirs() {
+        let tmp = tempfile::tempdir().unwrap();
+        let store_path = tmp.path().join("access.json");
+        let policy = Arc::new(FsPolicy::interactive_scripted(
+            Some("/s/tool.lua".into()),
+            DirGrants::new(),
+            Some(store_path.clone()),
+            [Choice::AllowAlways],
+        ));
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data".into(), Wants::READ).await.unwrap(), Outcome::Granted);
+
+        let saved = crate::stdlib::fs::store::load_for_script(&store_path, "/s/tool.lua");
+        assert_eq!(saved["/data"], DirAccess { read: Some(true), write: None });
+
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data/deep".into(), Wants::READ).await.unwrap(), Outcome::Granted);
+    }
+
+    #[tokio::test]
+    async fn deny_always_persists_false() {
+        let tmp = tempfile::tempdir().unwrap();
+        let store_path = tmp.path().join("access.json");
+        let policy = Arc::new(FsPolicy::interactive_scripted(
+            Some("/s/tool.lua".into()),
+            DirGrants::new(),
+            Some(store_path.clone()),
+            [Choice::DenyAlways],
+        ));
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/secrets".into(), Wants::WRITE).await.unwrap(), Outcome::Denied);
+
+        let saved = crate::stdlib::fs::store::load_for_script(&store_path, "/s/tool.lua");
+        assert_eq!(saved["/secrets"], DirAccess { read: None, write: Some(false) });
+    }
+
+    #[tokio::test]
+    async fn repl_always_answers_never_persist() {
+        let tmp = tempfile::tempdir().unwrap();
+        let store_path = tmp.path().join("access.json");
+        // script_key = None (REPL): store_path present but must not be used
+        let policy = Arc::new(FsPolicy::interactive_scripted(
+            None,
+            DirGrants::new(),
+            Some(store_path.clone()),
+            [Choice::AllowAlways],
+        ));
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data".into(), Wants::READ).await.unwrap(), Outcome::Granted);
+        assert!(!store_path.exists(), "REPL grant must not create the store");
+        // ...but it holds for the session
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data".into(), Wants::READ).await.unwrap(), Outcome::Granted);
+    }
+
+    #[tokio::test]
+    async fn unknown_without_terminal_denies_and_records_nothing() {
+        let tmp = tempfile::tempdir().unwrap();
+        let store_path = tmp.path().join("access.json");
+        // empty answer queue = "no terminal"
+        let policy = Arc::new(FsPolicy::interactive_scripted(
+            Some("/s/tool.lua".into()),
+            DirGrants::new(),
+            Some(store_path.clone()),
+            [],
+        ));
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data".into(), Wants::READ).await.unwrap(), Outcome::Denied);
+        assert!(!store_path.exists());
+        // nothing was cached either: a persisted grant arriving later (other
+        // process) would be honored — verify session stayed empty
+        if let FsMode::Interactive { session, .. } = &policy.mode {
+            assert!(session.lock().unwrap().is_empty());
+        }
+    }
+
+    #[tokio::test]
+    async fn persisted_grants_answer_without_prompting() {
+        let persisted = grants(&[("/data", Some(true), Some(false))]);
+        let policy = Arc::new(FsPolicy::interactive_scripted(
+            Some("/s/tool.lua".into()),
+            persisted,
+            None,
+            [], // no answers available: any prompt would deny
+        ));
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data/x".into(), Wants::READ).await.unwrap(), Outcome::Granted);
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data/x".into(), Wants::WRITE).await.unwrap(), Outcome::Denied);
+        // combined read/write: the recorded write=false denies without asking
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/data/x".into(), Wants::READ_WRITE).await.unwrap(), Outcome::Denied);
+    }
+
+    #[tokio::test]
+    async fn combined_prompt_records_only_missing_kinds() {
+        let tmp = tempfile::tempdir().unwrap();
+        let store_path = tmp.path().join("access.json");
+        // read already granted; a READ_WRITE check must only ask (and record)
+        // the missing write flag
+        let persisted = grants(&[("/db", Some(true), None)]);
+        let policy = Arc::new(FsPolicy::interactive_scripted(
+            Some("/s/tool.lua".into()),
+            persisted,
+            Some(store_path.clone()),
+            [Choice::AllowAlways],
+        ));
+        let p = Arc::clone(&policy);
+        assert_eq!(p.check("/db".into(), Wants::READ_WRITE).await.unwrap(), Outcome::Granted);
+
+        let saved = crate::stdlib::fs::store::load_for_script(&store_path, "/s/tool.lua");
+        assert_eq!(
+            saved["/db"],
+            DirAccess { read: None, write: Some(true) },
+            "read was already known; only write gets recorded"
+        );
+    }
+}

src/stdlib/fs/prompt.rs

@@ -0,0 +1,105 @@
+//! The single-keypress permission prompt.
+//!
+//! Runs on the blocking pool with the tui prompt lock held (see
+//! [`super::policy::FsPolicy::check`]), so it never fights an inquire prompt
+//! or a sibling permission prompt for the terminal. One keypress, no Enter:
+//! `y`/`n` answer for this run only, `A`/`N` are recorded in access.json.
+//! Deliberately, a lowercase `a` does nothing — a slip of the finger must not
+//! persist anything.
+
+use std::io::{IsTerminal, Write};
+
+use crossterm::event::{Event, KeyCode, KeyEventKind, KeyModifiers};
+
+use super::store::APP_NAME;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(crate) enum Choice {
+    AllowOnce,
+    DenyOnce,
+    AllowAlways,
+    DenyAlways,
+    /// No terminal to ask on (or raw mode unavailable): deny, record nothing.
+    NoTty,
+}
+
+/// Whether there is a terminal to ask on. The prompt itself goes to stderr so
+/// it shows even when the script's stdout is piped.
+pub(super) fn can_prompt() -> bool {
+    std::io::stdin().is_terminal() && std::io::stderr().is_terminal()
+}
+
+/// Ask the user; `what` is `"read"`, `"write"` or `"read/write"`.
+pub(super) fn prompt_access(script: &str, dir: &str, what: &str) -> Choice {
+    if !can_prompt() {
+        return Choice::NoTty;
+    }
+    eprint!(
+        "{APP_NAME}: permission request\n  \
+         script: {script}\n  \
+         wants:  {what} access to directory {dir}\n\
+         [y] allow once  [n] deny once  [A] allow always  [N] never allow: "
+    );
+    let _ = std::io::stderr().flush();
+
+    let Ok(raw) = RawMode::enable() else {
+        eprintln!();
+        return Choice::NoTty;
+    };
+    let choice = loop {
+        match crossterm::event::read() {
+            Ok(Event::Key(k)) if k.kind == KeyEventKind::Press => {
+                // In raw mode Ctrl-C is a plain key event (no SIGINT): a
+                // cautious "deny once", same as Esc.
+                if k.modifiers.contains(KeyModifiers::CONTROL) {
+                    if k.code == KeyCode::Char('c') {
+                        break Choice::DenyOnce;
+                    }
+                    continue;
+                }
+                match k.code {
+                    KeyCode::Char('y') => break Choice::AllowOnce,
+                    KeyCode::Char('n') => break Choice::DenyOnce,
+                    // Shifted letters arrive as the uppercase char (the SHIFT
+                    // modifier bit varies by terminal, so don't match on it).
+                    KeyCode::Char('A') => break Choice::AllowAlways,
+                    KeyCode::Char('N') => break Choice::DenyAlways,
+                    KeyCode::Esc => break Choice::DenyOnce,
+                    _ => {}
+                }
+            }
+            Ok(_) => {} // resize, mouse, focus: ignore
+            Err(_) => break Choice::DenyOnce, // fail closed
+        }
+    };
+    drop(raw);
+    // Echo the decision after raw mode is off (raw mode needs \r\n).
+    eprintln!(
+        "{}",
+        match choice {
+            Choice::AllowOnce => "allow once",
+            Choice::DenyOnce => "deny once",
+            Choice::AllowAlways => "allow always",
+            Choice::DenyAlways => "never allow",
+            Choice::NoTty => "",
+        }
+    );
+    choice
+}
+
+/// RAII raw-mode: a panic between enable and disable can't strand the
+/// terminal in raw mode.
+struct RawMode;
+
+impl RawMode {
+    fn enable() -> std::io::Result<RawMode> {
+        crossterm::terminal::enable_raw_mode()?;
+        Ok(RawMode)
+    }
+}
+
+impl Drop for RawMode {
+    fn drop(&mut self) {
+        let _ = crossterm::terminal::disable_raw_mode();
+    }
+}

src/stdlib/fs/store.rs

@@ -0,0 +1,321 @@
+//! The persistent grant store — `access.json` in the user's config directory.
+//!
+//! Grants are keyed by the script's canonical path, then by canonical
+//! directory, each carrying a tri-state per direction: `null` (not decided),
+//! `true` (approved), `false` (denied). The file is hand-editable; [`persist`]
+//! keeps concurrent `a` processes from losing each other's updates by holding
+//! an exclusive `flock` on a sidecar lock file across its read-merge-write
+//! cycle, and replaces the store with `rename()` so readers never see a
+//! half-written file.
+
+use std::collections::BTreeMap;
+use std::io::Write;
+use std::path::{Path, PathBuf};
+
+use serde::{Deserialize, Serialize};
+
+/// Config directory name under `$XDG_CONFIG_HOME`. A literal, not
+/// `CARGO_PKG_NAME`: renaming the binary silently abandoning every recorded
+/// grant should be an explicit decision made here.
+pub(crate) const APP_NAME: &str = "a";
+
+/// Tri-state access recorded for one directory. `None` = not decided yet,
+/// `Some(true)` = approved, `Some(false)` = denied — serialized as JSON
+/// `null` / `true` / `false`.
+#[derive(Serialize, Deserialize, Clone, Copy, Default, PartialEq, Eq, Debug)]
+pub(crate) struct DirAccess {
+    #[serde(default)]
+    pub(crate) read: Option<bool>,
+    #[serde(default)]
+    pub(crate) write: Option<bool>,
+}
+
+impl DirAccess {
+    pub(crate) fn get(&self, kind: super::policy::AccessKind) -> Option<bool> {
+        match kind {
+            super::policy::AccessKind::Read => self.read,
+            super::policy::AccessKind::Write => self.write,
+        }
+    }
+}
+
+/// Grants recorded for one script: canonical directory → tri-state pair.
+/// `BTreeMap` for deterministic serialization (stable diffs of access.json).
+pub(crate) type DirGrants = BTreeMap<String, DirAccess>;
+
+#[derive(Serialize, Deserialize)]
+pub(crate) struct AccessStore {
+    #[serde(default = "version_default")]
+    pub(crate) version: u32,
+    #[serde(default)]
+    pub(crate) scripts: BTreeMap<String, DirGrants>,
+}
+
+fn version_default() -> u32 {
+    1
+}
+
+impl Default for AccessStore {
+    fn default() -> Self {
+        AccessStore { version: 1, scripts: BTreeMap::new() }
+    }
+}
+
+/// `$XDG_CONFIG_HOME/a/access.json`, falling back to `~/.config/a/access.json`.
+/// Per the XDG spec a non-absolute `$XDG_CONFIG_HOME` is ignored. `None` when
+/// no absolute base can be found — permissions then live for the session only.
+pub(crate) fn default_store_path() -> Option<PathBuf> {
+    let base = std::env::var_os("XDG_CONFIG_HOME")
+        .map(PathBuf::from)
+        .filter(|p| p.is_absolute())
+        .or_else(|| {
+            std::env::var_os("HOME")
+                .map(|h| PathBuf::from(h).join(".config"))
+                .filter(|p| p.is_absolute())
+        })?;
+    Some(base.join(APP_NAME).join("access.json"))
+}
+
+/// Read the whole store. Missing file → empty store; unreadable or corrupt →
+/// warn and treat as empty (a later [`persist`] moves a corrupt file aside
+/// before overwriting, preserving whatever the user had).
+fn read_store(path: &Path) -> AccessStore {
+    let bytes = match std::fs::read(path) {
+        Ok(b) => b,
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => return AccessStore::default(),
+        Err(e) => {
+            log::warn!("cannot read {}: {e}", path.display());
+            return AccessStore::default();
+        }
+    };
+    match serde_json::from_slice(&bytes) {
+        Ok(store) => store,
+        Err(e) => {
+            log::warn!("{} is corrupt ({e}); ignoring it", path.display());
+            AccessStore::default()
+        }
+    }
+}
+
+/// The grants recorded for one script (empty if none). Startup snapshot; takes
+/// no lock — `persist`'s atomic rename guarantees a complete file.
+pub(crate) fn load_for_script(store_path: &Path, script_key: &str) -> DirGrants {
+    read_store(store_path).scripts.remove(script_key).unwrap_or_default()
+}
+
+/// Record a decision: merge `update`'s decided flags (only the `Some` ones)
+/// into the store entry for (`script_key`, `dir_key`) and rewrite the file.
+///
+/// Holds an exclusive `flock` on `access.json.lock` across the whole
+/// read-merge-write so a concurrently running script can't lose this update.
+/// The lock file is separate from the store on purpose: the final `rename`
+/// replaces the store's inode, so a lock on the store itself would not exclude
+/// a process that opened it just before the rename.
+pub(crate) fn persist(
+    store_path: &Path,
+    script_key: &str,
+    dir_key: &str,
+    update: DirAccess,
+) -> std::io::Result<()> {
+    let config_dir = store_path.parent().unwrap_or(Path::new("/"));
+    std::fs::create_dir_all(config_dir)?;
+
+    let lock_file = std::fs::File::options()
+        .create(true)
+        .truncate(false)
+        .read(true)
+        .write(true)
+        .open(store_path.with_extension("json.lock"))?;
+    rustix::fs::flock(&lock_file, rustix::fs::FlockOperation::LockExclusive)?;
+
+    // Re-read fresh under the lock: the startup snapshot (or a previous
+    // in-process copy) may be stale by now.
+    let mut store = match std::fs::read(store_path) {
+        Ok(bytes) => match serde_json::from_slice(&bytes) {
+            Ok(store) => store,
+            Err(e) => {
+                // Corrupt: move it aside instead of silently overwriting
+                // whatever the user had in there.
+                let aside = store_path.with_extension("json.corrupt");
+                log::warn!(
+                    "{} is corrupt ({e}); moving it to {}",
+                    store_path.display(),
+                    aside.display()
+                );
+                std::fs::rename(store_path, &aside)?;
+                AccessStore::default()
+            }
+        },
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => AccessStore::default(),
+        Err(e) => return Err(e),
+    };
+
+    let grants = store.scripts.entry(script_key.to_string()).or_default();
+    let entry = grants.entry(dir_key.to_string()).or_default();
+    if update.read.is_some() {
+        entry.read = update.read;
+    }
+    if update.write.is_some() {
+        entry.write = update.write;
+    }
+    // Keep the file tidy: drop entries that no longer record anything.
+    grants.retain(|_, a| a.read.is_some() || a.write.is_some());
+    store.scripts.retain(|_, g| !g.is_empty());
+
+    // Write-to-temp + rename: readers see the old or the new file, never a
+    // partial one, even across a crash mid-write.
+    let tmp = store_path.with_extension(format!("json.tmp.{}", std::process::id()));
+    let mut file = std::fs::File::create(&tmp)?;
+    file.write_all(&serde_json::to_vec_pretty(&store)?)?;
+    file.write_all(b"\n")?;
+    file.sync_all()?;
+    std::fs::rename(&tmp, store_path)?;
+    Ok(())
+    // lock_file drops here, releasing the flock.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::super::policy::AccessKind;
+    use super::*;
+
+    fn store_path(dir: &tempfile::TempDir) -> PathBuf {
+        dir.path().join("cfg").join("access.json")
+    }
+
+    #[test]
+    fn tri_state_round_trip() {
+        let json = r#"{
+            "version": 1,
+            "scripts": {
+                "/s/deploy.lua": {
+                    "/data": { "read": true, "write": true },
+                    "/etc": { "read": true, "write": false },
+                    "/secrets": { "read": null, "write": false }
+                }
+            }
+        }"#;
+        let store: AccessStore = serde_json::from_str(json).unwrap();
+        let grants = &store.scripts["/s/deploy.lua"];
+        assert_eq!(grants["/data"], DirAccess { read: Some(true), write: Some(true) });
+        assert_eq!(grants["/etc"], DirAccess { read: Some(true), write: Some(false) });
+        assert_eq!(grants["/secrets"], DirAccess { read: None, write: Some(false) });
+        // null and absent both mean "unknown"
+        let sparse: DirAccess = serde_json::from_str(r#"{ "write": true }"#).unwrap();
+        assert_eq!(sparse, DirAccess { read: None, write: Some(true) });
+        // and null survives serialization (the file must distinguish it)
+        let out = serde_json::to_string(&grants["/secrets"]).unwrap();
+        assert!(out.contains("\"read\":null"), "{out}");
+        // round trip
+        let again: AccessStore =
+            serde_json::from_str(&serde_json::to_string(&store).unwrap()).unwrap();
+        assert_eq!(again.scripts, store.scripts);
+    }
+
+    #[test]
+    fn dir_access_get() {
+        let a = DirAccess { read: Some(true), write: Some(false) };
+        assert_eq!(a.get(AccessKind::Read), Some(true));
+        assert_eq!(a.get(AccessKind::Write), Some(false));
+    }
+
+    #[test]
+    fn persist_creates_dir_and_merges_without_clobbering() {
+        let tmp = tempfile::tempdir().unwrap();
+        let path = store_path(&tmp); // parent "cfg" does not exist yet
+
+        persist(&path, "/s/a.lua", "/data", DirAccess { read: Some(true), write: None }).unwrap();
+        persist(&path, "/s/a.lua", "/data", DirAccess { read: None, write: Some(false) }).unwrap();
+
+        let grants = load_for_script(&path, "/s/a.lua");
+        assert_eq!(grants["/data"], DirAccess { read: Some(true), write: Some(false) });
+        // a second script's grants are separate
+        assert!(load_for_script(&path, "/s/b.lua").is_empty());
+    }
+
+    #[test]
+    fn persist_prunes_undecided_entries() {
+        let tmp = tempfile::tempdir().unwrap();
+        let path = store_path(&tmp);
+        std::fs::create_dir_all(path.parent().unwrap()).unwrap();
+        // Hand-written file with a null-null entry (as a user might leave it).
+        std::fs::write(
+            &path,
+            r#"{ "version": 1, "scripts": {
+                "/s/a.lua": { "/dead": { "read": null, "write": null } },
+                "/s/gone.lua": {}
+            } }"#,
+        )
+        .unwrap();
+
+        persist(&path, "/s/a.lua", "/data", DirAccess { read: Some(true), write: None }).unwrap();
+
+        let store = read_store(&path);
+        assert!(!store.scripts["/s/a.lua"].contains_key("/dead"));
+        assert!(!store.scripts.contains_key("/s/gone.lua"));
+        assert_eq!(store.scripts["/s/a.lua"]["/data"].read, Some(true));
+    }
+
+    #[test]
+    fn persist_moves_corrupt_file_aside() {
+        let tmp = tempfile::tempdir().unwrap();
+        let path = store_path(&tmp);
+        std::fs::create_dir_all(path.parent().unwrap()).unwrap();
+        std::fs::write(&path, "{ not json").unwrap();
+
+        persist(&path, "/s/a.lua", "/data", DirAccess { read: Some(true), write: None }).unwrap();
+
+        assert_eq!(
+            std::fs::read_to_string(path.with_extension("json.corrupt")).unwrap(),
+            "{ not json"
+        );
+        assert_eq!(load_for_script(&path, "/s/a.lua")["/data"].read, Some(true));
+    }
+
+    #[test]
+    fn missing_or_corrupt_reads_as_empty() {
+        let tmp = tempfile::tempdir().unwrap();
+        let path = store_path(&tmp);
+        assert!(load_for_script(&path, "/s/a.lua").is_empty());
+        std::fs::create_dir_all(path.parent().unwrap()).unwrap();
+        std::fs::write(&path, "]]]").unwrap();
+        assert!(load_for_script(&path, "/s/a.lua").is_empty());
+    }
+
+    #[test]
+    fn concurrent_persists_do_not_lose_updates() {
+        let tmp = tempfile::tempdir().unwrap();
+        let path = store_path(&tmp);
+        let threads: Vec<_> = (0..8)
+            .map(|i| {
+                let path = path.clone();
+                std::thread::spawn(move || {
+                    for j in 0..10 {
+                        persist(
+                            &path,
+                            "/s/a.lua",
+                            &format!("/dir-{i}-{j}"),
+                            DirAccess { read: Some(true), write: None },
+                        )
+                        .unwrap();
+                    }
+                })
+            })
+            .collect();
+        for t in threads {
+            t.join().unwrap();
+        }
+        // every one of the 80 grants survived the concurrent read-merge-writes
+        assert_eq!(load_for_script(&path, "/s/a.lua").len(), 80);
+    }
+
+    #[test]
+    fn default_store_path_respects_xdg_rules() {
+        // Can't mutate the process environment safely in a threaded test
+        // runner; assert only on the shape of whatever the real env yields.
+        if let Some(p) = default_store_path() {
+            assert!(p.is_absolute());
+            assert!(p.ends_with(format!("{APP_NAME}/access.json")));
+        }
+    }
+}

src/stdlib/http.rs

@@ -1,4 +1,5 @@
 use std::collections::HashMap;
+use std::path::Path;
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
@@ -173,6 +174,10 @@ async fn execute_request(
     url: String,
     opts: Option<LuaTable>,
 ) -> LuaResult<LuaTable> {
+    // Every request funnels through here (http.request, the shorthands, and
+    // all session methods) — the one choke point for --sandbox.
+    super::fs::ensure_net(&lua)?;
+
     let mut method = reqwest::Method::from_bytes(method.to_ascii_uppercase().as_bytes())
         .map_err(|e| mlua::Error::external(format!("http: invalid method '{method}': {e}")))?;
 
@@ -471,22 +476,40 @@ fn make_session(
         })?,
     )?;
 
+    // Cookie-jar files hold live credentials; both directions go through the
+    // fs permission system (async only because the permission cycle is).
     tbl.raw_set("save", {
         let jar = jar.clone();
-        lua.create_function(move |_, (_this, path): (LuaTable, String)| {
+        lua.create_async_function(move |lua, (_this, path): (LuaTable, String)| {
+            let jar = jar.clone();
+            async move {
+                let (dir, target) =
+                    super::fs::write_target("session:save", Path::new(&path)).await?;
+                super::fs::ensure_access(&lua, "session:save", dir, super::fs::Wants::WRITE)
+                    .await?;
                 let jsonl = jar.lock().unwrap_or_else(|e| e.into_inner()).to_jsonl();
-            std::fs::write(&path, jsonl)
+                tokio::fs::write(&target, jsonl)
+                    .await
                     .map_err(|e| mlua::Error::external(format!("session:save: {e}")))
+            }
         })?
     })?;
 
     tbl.raw_set("load", {
         let jar = jar.clone();
-        lua.create_function(move |_, (_this, path): (LuaTable, String)| {
+        lua.create_async_function(move |lua, (_this, path): (LuaTable, String)| {
-            let src = std::fs::read_to_string(&path)
+            let jar = jar.clone();
+            async move {
+                let (dir, target) =
+                    super::fs::read_target("session:load", Path::new(&path)).await?;
+                super::fs::ensure_access(&lua, "session:load", dir, super::fs::Wants::READ)
+                    .await?;
+                let src = tokio::fs::read_to_string(&target)
+                    .await
                     .map_err(|e| mlua::Error::external(format!("session:load: {e}")))?;
                 jar.lock().unwrap_or_else(|e| e.into_inner()).merge_jsonl(&src);
                 Ok(())
+            }
         })?
     })?;
 
@@ -564,7 +587,16 @@ pub(super) fn install(lua: &Lua) -> LuaResult<()> {
                 async move {
                     let mut jar = CookieJar::default();
                     if let Some(p) = path {
-                        let src = tokio::fs::read_to_string(&p)
+                        let (dir, target) =
+                            super::fs::read_target("http.session", Path::new(&p)).await?;
+                        super::fs::ensure_access(
+                            &lua,
+                            "http.session",
+                            dir,
+                            super::fs::Wants::READ,
+                        )
+                        .await?;
+                        let src = tokio::fs::read_to_string(&target)
                             .await
                             .map_err(|e| mlua::Error::external(format!("http.session: {e}")))?;
                         jar.merge_jsonl(&src);

src/stdlib/mod.rs

@@ -1,3 +1,6 @@
+// pub(crate) for FsPolicy & co. — `main` builds the permission policy from
+// the CLI flags and the script's canonical path.
+pub(crate) mod fs;
 mod http;
 mod logging;
 mod lua_dump;
@@ -22,6 +25,7 @@ pub(crate) fn install(lua: &mlua::Lua) -> mlua::Result<()> {
     logging::install(lua)?;
     sqlite::install(lua)?;
     http::install(lua)?;
+    fs::install(lua)?;
     task::install(lua)?;
     tui::install(lua)?;
     Ok(())

src/stdlib/require.rs

@@ -28,6 +28,7 @@ const BUILTINS: &[&str] = &[
     "log",
     "sqlite",
     "http",
+    "fs",
     "task",
     "tui",
     "coroutine",

src/stdlib/sqlite.rs

@@ -1,3 +1,4 @@
+use std::path::Path;
 use std::sync::{Arc, Mutex};
 
 use mlua::prelude::LuaResult;
@@ -404,6 +405,43 @@ impl UserData for SqliteConn {
     }
 }
 
+// ---------------------------------------------------------------------------
+// Path classification for the fs permission system
+// ---------------------------------------------------------------------------
+
+/// Databases that live in memory (or in an anonymous temp file SQLite manages
+/// itself): `""`, `":memory:"`, and their `file:` URI spellings. rusqlite's
+/// default open flags include `SQLITE_OPEN_URI`, so URIs are live syntax.
+fn is_memory_db(path: &str) -> bool {
+    if path.is_empty() || path == ":memory:" {
+        return true;
+    }
+    let Some(rest) = path.strip_prefix("file:") else {
+        return false;
+    };
+    let (file, query) = rest.split_once('?').unwrap_or((rest, ""));
+    file == ":memory:" || query.split('&').any(|kv| kv == "mode=memory")
+}
+
+/// The filesystem path of a file-backed database: strips the `file:` URI
+/// scheme, authority and query, leaving what SQLite would open.
+fn db_file_path(path: &str) -> &str {
+    let Some(rest) = path.strip_prefix("file:") else {
+        return path;
+    };
+    let rest = rest.split_once('?').map_or(rest, |(file, _query)| file);
+    // "file://localhost/p" and "file:///p" carry an (empty) authority;
+    // "file:/p" and "file:p" do not.
+    if let Some(after) = rest.strip_prefix("//") {
+        let (authority, p) = after.split_once('/').unwrap_or((after, ""));
+        match authority {
+            "" | "localhost" => return &rest[2 + authority.len()..],
+            _ => return p, // nonsensical host: best effort
+        }
+    }
+    rest
+}
+
 // ---------------------------------------------------------------------------
 // Installation
 // ---------------------------------------------------------------------------
@@ -413,7 +451,20 @@ pub(super) fn install(lua: &Lua) -> LuaResult<()> {
 
     sqlite.set(
         "connect",
-        lua.create_async_function(|_, path: String| async move {
+        lua.create_async_function(|lua, path: String| async move {
+            // File-backed databases fall under the fs permission system.
+            // One combined read/write ask on the database's directory: SQLite
+            // opens read-write-create and drops WAL/journal files next to the
+            // file, so a read-only grant would be a lie. Memory-class
+            // databases touch no user-reachable file and stay exempt (they
+            // keep working even under --no-fs / --sandbox).
+            if !is_memory_db(&path) {
+                let (dir, _target) =
+                    super::fs::write_target("sqlite.connect", Path::new(db_file_path(&path)))
+                        .await?;
+                super::fs::ensure_access(&lua, "sqlite.connect", dir, super::fs::Wants::READ_WRITE)
+                    .await?;
+            }
             // Opening a file-backed database touches disk, so do it on the
             // blocking pool rather than the tokio event loop.
             let conn = tokio::task::spawn_blocking(move || rusqlite::Connection::open(&path))
@@ -437,3 +488,29 @@ pub(super) fn install(lua: &Lua) -> LuaResult<()> {
     let methods: LuaTable = sample.metatable()?.get("__index")?;
     init.call::<()>(methods)
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn memory_db_detection() {
+        for mem in ["", ":memory:", "file::memory:", "file::memory:?cache=shared", "file:x.db?mode=memory", "file:x.db?cache=shared&mode=memory"] {
+            assert!(is_memory_db(mem), "{mem:?} should be memory-class");
+        }
+        for disk in [":memory:x", "x.db", "/tmp/x.db", "file:x.db", "file:/tmp/x.db?cache=shared", "file:x.db?mode=memoryz"] {
+            assert!(!is_memory_db(disk), "{disk:?} should be file-backed");
+        }
+    }
+
+    #[test]
+    fn db_file_path_strips_uri_wrapping() {
+        assert_eq!(db_file_path("/tmp/x.db"), "/tmp/x.db");
+        assert_eq!(db_file_path("x.db"), "x.db");
+        assert_eq!(db_file_path("file:x.db"), "x.db");
+        assert_eq!(db_file_path("file:/tmp/x.db"), "/tmp/x.db");
+        assert_eq!(db_file_path("file:///tmp/x.db"), "/tmp/x.db");
+        assert_eq!(db_file_path("file://localhost/tmp/x.db"), "/tmp/x.db");
+        assert_eq!(db_file_path("file:/tmp/x.db?cache=shared&vfs=unix"), "/tmp/x.db");
+    }
+}

src/stdlib/tui/mod.rs

@@ -26,6 +26,11 @@ mod output;
 mod prompts;
 mod term;
 
+// The fs module's permission prompt shares the terminal with tui's prompts
+// and must take the same lock. (In test builds fs never opens a real prompt,
+// leaving this re-export unused.)
+#[cfg_attr(test, allow(unused_imports))]
+pub(crate) use prompts::PROMPT_LOCK;
 pub(crate) use term::cleanup;
 
 use mlua::prelude::LuaResult;

src/stdlib/tui/prompts.rs

@@ -36,8 +36,9 @@ use super::opts::{
 /// Serializes terminal ownership across concurrent Lua coroutines. Locked
 /// inside the blocking closure, so a waiting prompt parks a blocking-pool
 /// thread, never the tokio event loop. Shared with `term`'s cursor-position
-/// query, which also reads from the terminal.
+/// query and the fs module's permission prompt, which also read from the
-pub(super) static PROMPT_LOCK: Mutex<()> = Mutex::new(());
+/// terminal (re-exported as `stdlib::tui::PROMPT_LOCK`).
+pub(crate) static PROMPT_LOCK: Mutex<()> = Mutex::new(());
 
 /// `Send`-safe error carried out of the blocking prompt worker; the Lua-side
 /// wrapper prepends the `tui.<fn>:` prefix.