# Agent Note: Cross-family file sandbox — one policy home, a sandboxed fs provider, and fs escalation parity

Status: implemented

English | [中文](2026-07-14-cross-family-fs-sandbox.zh.md)

## Problem

`SandboxMode` claims file effects, but originally only `ctx.shell` enforced it. The fs tools (`write`/`edit`) mutate the host filesystem in-process through `ctx.fs`, where an OS argv wrapper is mechanically meaningless — [the sandbox Agent Note](2026-07-06-sandbox.md) § In-process tools records this and left cross-family enforcement as a deferred phase with an open question: whether in-process enforcement stays per-seam or becomes a uniform harness capability. This Agent Note is that phase, and answers it: one shared policy home, per-seam enforcement at each family's correct altitude.

The gap was not read-only-shaped. A confined coding agent's product mode is `workspace-write`: bash may already write under the workspace root while everything outside is denied, so an fs enforcement that could only deny-all would be strictly worse than disabling the fs tools — the model would attempt an in-workspace `write`, be denied, and learn to detour through `bash` heredocs. Cross-family enforcement therefore speaks the full mode ladder, including the path-containment judgment `workspace-write` requires (canonical targets; `..`/symlink/absolute-path escapes) and the same escalation lever bash carries.

A second enforcing family also exposed an ownership problem in the original layout. The deployment default (`mode` + `workspaceRoot`) was configured on `dsh-bash-sandbox`, and the per-session override event was `shell/sandbox-mode`, folded and written by `dsh-shell`'s session-mode kit. With fs enforcing the same policy, either fs reads bash's config and events (a capability family depending on a sibling's plugin config) or each family carries its own copy — and two copies of `workspaceRoot` drift into exactly the split world the sandbox RFC warns about: bash confined to one root while fs fences another.

## Decision

Three coordinated pieces, all composed from the leaf `cordis.yml`, none touching `agent-loop`.

### `ctx.sandboxPolicy` — one home for mode and workspace root

`packages/sandbox/sandbox-policy/` (`@deepseek-ai/dsh-sandbox-policy`) registers `ctx.sandboxPolicy`, the single owner of the deployment's sandbox policy:

- `Config`: `mode` (the closed `SandboxMode` union, default `read-only`) and `workspaceRoot` (default the process cwd, resolved absolute). Misconfiguration fails loud at load.
- The per-session override event `sandbox/mode`, with its pure fold (`effectiveSandboxMode(events)`), its write path (`setSandboxMode(session, mode)`), and `SANDBOX_MODES`. The event is policy state — consumed by two families — so it lives here, not in either capability's seam. Its shape and log-only semantics match the `approval/*` precedent.
- `resolve({ session?, mode? })`, which returns a complete per-call `SandboxExecutionPolicy`: explicit approved mode > the session fold > `defaultMode`, and the session's immutable cwd > configured `workspaceRoot` fallback.
- `defaultMode` / `workspaceRoot` accessors retained as deployment fallbacks and the capability-advertisement fact.

`dsh-bash-sandbox` carries no sandbox config of its own — it injects `sandboxPolicy` and uses its deployment fallback only for direct calls. `dsh-tool-bash` and `dsh-tool-fs` pass the active session to `ctx.sandboxPolicy.resolve()`, so both receive the same effective mode and cwd root on every call; `dsh-permission-presets` presets and the ACP bridge write through the relocated setter. The seams that own bash and fs execution remain session-free — the session dependency lives in the policy package and tool consumers.

### `dsh-fs-sandbox` — enforcement inside the provider

`packages/fs/fs-sandbox/` (`@deepseek-ai/dsh-fs-sandbox`) mirrors the `bash-local`/`bash-sandbox` split: `SandboxedFileSystem extends LocalFileSystem`, registered as `ctx.fs`, injecting `sandboxPolicy`. Reads (`resolve`/`stat`/`readText`/`streamText`/`listDir`) pass through untouched — every mode permits reading. The two mutations enforce by mode before delegating to the inherited atomic write:

- `read-only` denies `writeText`/`editText` outright.
- `workspace-write` fences the canonicalized target against the writable-root set — `writableRoots(policy)` in `dsh-sandbox`: the workspace root plus the platform temp areas (`/tmp`, `os.tmpdir()`), each realpathed — the SAME set the Seatbelt profile grants, so the fs fence is the fourth dialect of one mode meaning alongside the bwrap/Landlock/Seatbelt profiles, and "the write tool cannot write `/tmp` but bash can" asymmetries cannot arise. Canonical spellings take a lexical containment fast path; when Windows exposes one directory through different casing or long-name/8.3 spellings, an ancestor walk compares filesystem identity rather than weakening the boundary to textual prefix guesses. The target is re-canonicalized (`resolve` realpaths the deepest existing ancestor) immediately before delegating, so an ancestor symlink swapped since the tool resolved it is caught.
- `danger-full-access` delegates unfenced.

A denial is the structured `FS_SANDBOX_DENIED` carrying the effective mode — distinct from `FS_PERMISSION_DENIED` (a host EACCES is the world refusing; this is policy refusing). No text inference: an in-process fence knows exactly what it denied. The per-call carrier is a trailing optional `SandboxExecutionPolicy` on `writeText`/`editText` (the filesystem twin of `ShellExecRequest.sandboxPolicy`); the seam stays session-free, and the bare local backend ignores it. `FileSystem.sandboxMode` is the capability fact (`undefined` on the base and `fs-local`, the default on `SandboxedFileSystem`), so the tool layer advertises escalation from composition truth.

The threat model is stated in the package README: a policy fence in trusted code over model-controlled paths, not a kernel boundary — the operations are the seam's own, only the target path is untrusted, so canonicalize-then-contain is the complete answer to this surface (the `code-runtime` "containment, not a security boundary" precedent). Kernel-grade isolation of untrusted CODE stays `ctx.shell`'s job. The residual resolve-to-syscall race is narrowed by the in-place re-canonicalization and eliminated only by platform primitives (`openat2` `RESOLVE_BENEATH`) not worth their portability cost here.

### Tool parity — one denial marker, one escalation flow

`dsh-tool-fs` resolves the active session's complete policy onto each mutation and maps `FS_SANDBOX_DENIED` to the marker the model already knows from bash: `[sandbox: file access denied under <mode> mode]`. When `ctx.fs.sandboxMode` reports a confining mode at registration, `write` and `edit` advertise the same `sandbox_permissions` + `justification` fields, teach the same same-turn retry, and resolve the same `ctx.approval` request before executing — the four outcomes and their verbatim fail-closed texts carried over from [the sandbox Agent Note](2026-07-06-sandbox.md) § Escalation (strict widening checked at execution against the call's effective mode; a grant changes only that call's mode and retains its session root; no new session events).

The shared pieces live in `dsh-sandbox`, which owns the mode types: `WIDER_MODES`, the escalation-target enum, the argument-pairing validation, the denial/hint marker builders, and `approveEscalation` — the ordered fail-closed choreography. `approveEscalation` takes a minimal STRUCTURAL approver (`EscalationApprover`, generic over the agent and call-id types), not the approval service type, so `dsh-sandbox` gains no dependency on the approval or agent packages: each tool passes its own `ctx.approval`, agent, call id, and tool name as ingredients. `dsh-tool-bash` and `dsh-tool-fs` both use these; the cross-file duplication gate holds the single-sourcing honest.

The [`examples/acp-agent`](../../../../examples/acp-agent/cordis.yml) composition loads `dsh-sandbox-policy` and `dsh-fs-sandbox`, moves the `mode`/`workspaceRoot` config to the policy entry, and drops the old gating that disabled the fs stack under confined modes; `fs-observation-policy` (read-before-edit) composes orthogonally on top. The system prompt still states no sandbox mode — the marker teaches the boundary at the moment it matters, per the sandbox Agent Note's live evidence.

### The enforcement point: provider, not intent gate

The sandbox Agent Note's original cross-family sketch put fs enforcement on the `fs/write-intent`/`fs/edit-intent` events. This Agent Note enforces in the provider instead, on two mechanical facts: the intent slots are single-decision first-wins (occupied by `dsh-fs-observation-policy`, whose contract names a second decider a misconfiguration), and the intent events are dispatched only by `dsh-tool-fs` — a direct `ctx.fs` caller (a cordis-mounted plugin, a custom tool) bypasses them, where provider-level enforcement covers every caller by construction.

### Out of scope

- **Network policy for `ctx.web`** — `SandboxMode` claims file effects only; a web-only network knob while bash `curl` runs free would be a false boundary. Revisit when a bash backend enforces network (bwrap `--unshare-net`, Landlock ABI v4+).
- **The `subagent-acp` consumer** — unchanged deferred phase of the sandbox RFC.
- **Additional writable roots inside one session** — the resolved policy carries one primary `SessionHeader.cwd`; ACP `additionalDirectories` remains a separate bridge and policy design.
- **A uniform per-tool sandbox runtime** — remains rejected for the reasons in the sandbox RFC.

## Alternatives considered

- **Enforce on the `fs/*` intent events (the sandbox Agent Note's original sketch)** — rejected on the two mechanical facts in § The enforcement point: single-slot first-wins already occupied, and a bypass for direct `ctx.fs` callers. Provider-level enforcement covers every caller and mirrors bash's swap-the-implementation shape.
- **Enforce in `tools/pre-execute`** — rejected: the listener sees the model's raw path string before `resolve()`, so it would re-implement cwd defaulting and symlink canonicalization and still race the real resolve. Disqualifying for `workspace-write`, a judgment over canonical paths.
- **Inline checks in `dsh-tool-fs`** — rejected: covers only the tool path (same bypass as the intent events) and duplicates resolve knowledge one layer above where the canonical target already exists.
- **A `mode` flag on `dsh-fs-local` instead of a sibling backend** — rejected: the capability fact must be composition truth the way `dsh-bash-local` vs `dsh-bash-sandbox` is; a config flag makes the tool's advertisement conditional on configuration, and the bash family already establishes the sibling-package shape.
- **Kernel-enforced fs mutations via a confined helper subprocess** — rejected: a process per write; `editText`'s read-match-write critical section would have to move wholesale into the child to stay atomic; and the threat surface (trusted operations, untrusted path argument) does not need a kernel — the fence in trusted code is the complete answer, while untrusted-code isolation stays on `ctx.shell`.
- **Per-family policy config with a load-time consistency check** — rejected: two homes for one fact, patched by a check that must enumerate every future enforcing family; the policy service makes drift inexpressible instead of detected.
- **Keep the override event in `dsh-shell` as `shell/sandbox-mode`** — rejected: the event is policy state consumed by two families; leaving it bash-named forces `dsh-fs-sandbox` to depend on bash vocabulary. Pre-release, the rename is a same-change move with snapshot re-records, no shims.
- **Escalation choreography imported from the approval/agent packages into `dsh-sandbox`** — rejected: it would invert the layering (a base vocabulary package depending on UI/agent packages). The structural approver keeps the logic single-sourced in `dsh-sandbox` while the dependencies stay in the tool layer that already holds them.
- **A consolidated mutation-options object on the fs seam** (the shape first sketched for the per-call carrier) — rejected on friction: it splits `signal` across an options bag for mutations while reads keep it positional. A trailing optional `SandboxExecutionPolicy` matches bash's carry-and-ignore pattern and keeps `signal` symmetric across the seam.
- **Extra writable-root grants on `SandboxPolicy` now** — deferred unchanged: `writableRoots()` derives from the mode meaning today; ad-hoc grants are an escalation-scope question the sandbox RFC left open.

## Consequences

What shipped — the tiers in § Testing hold each:

- Under `read-only`, `write`/`edit` return the `[sandbox: file access denied under read-only mode]` marker and the disk is untouched; `read`/`listDir` behave identically to `dsh-fs-local`.
- Under `workspace-write`, mutations land under the workspace root and the temp areas and are denied outside; the containment matrix — `..` traversal, absolute paths outside, a pre-existing symlinked directory inside pointing out, a new file created under such a symlink, and alias-equivalent root spellings — denies every escape while admitting the same directory identity on real disks.
- A denied fs mutation retried once with `sandbox_permissions` + `justification` prompts through the composed approval chain; a grant runs exactly that call under the wider mode and the write lands; rejected/cancelled/unavailable each produce their verbatim fail-closed text and mutate nothing.
- One `permission` preset switch governs both families: after a session switches modes, the next bash call and the next fs mutation both honor the new mode from the same `sandbox/mode` fold.
- Concurrent sessions with different cwd roots carry different policies through the same service instances; neither family caches one session's root for the next call.
- A direct `ctx.fs.writeText` with no per-call stamp is confined at the deployment default.
- The escalation fields on `write`/`edit` exist exactly when the mounted `ctx.fs` confines, absent under `dsh-fs-local`.
- `agent-loop` is untouched — everything rides `ctx.sandboxPolicy`, the `ctx.fs` seam, `SessionEventMap` merging, and the tool-execution pipeline.

Costs and accepted limits:

- **The fs fence is a policy boundary, not a kernel one.** Its threat surface is model-chosen paths, not adversarial host processes; the residual resolve-to-syscall TOCTOU is narrowed, not eliminated, and the README says so. Kernel boundaries remain bash's.
- **`dsh-bash-sandbox` gains a hard dependency on `ctx.sandboxPolicy`.** Every sandboxed composition adds one `cordis.yml` entry or fails loud at load — the intended pre-release foundation move; the examples update in the same change.
- **Fence-vs-runner parity is derived, not asserted.** The fs fence and the Seatbelt profile both take their writable set from `writableRoots`, and a parity unit test pins the sets; a runner profile changing its writable set without that function would drift.
- **The marker and escalation teaching now serve two families.** A wording change is a coordinated edit behind one builder in `dsh-sandbox`; the duplication gate and pinned snapshots hold it single-sourced, at the cost that fs and bash cannot deliberately diverge in phrasing without splitting the builder.

## Testing

- Unit: `dsh-sandbox` pins the escalation ladder, the marker builders, the argument-pairing validation, and `approveEscalation`'s ordered fail-closed sequence (non-widening, no-approval, no-agent, each outcome), plus `writableRoots`/`canonicalPath`. `dsh-sandbox-policy` pins deployment fallback, session mode/root resolution, explicit-mode precedence, the fold/setter, load-time mode rejection, and HMR safety. `dsh-fs-sandbox` pins the per-policy fence and containment matrix (inside, temp area, absolute-outside, `..`, symlinked-out directory, new file under one, path-equals-root, filesystem-root, root-ending-in-separator, and alias-equivalent spelling) on a real filesystem, plus per-call override and HMR safety. `dsh-tool-fs` pins advertisement gating, complete policy resolution, denial-marker mapping, and the full escalation matrix (grant, reject, no-service, no-agent, pairing, non-confining guard). `dsh-tool-bash`, `dsh-bash-sandbox`, and `dsh-permission-presets` consume the same policy kit.
- Keyless e2e: one real Cordis context creates two agents with different session cwd roots, runs the shipped bash and fs tools concurrently, and world-verifies that own-project writes land while both cross-project writes are denied.
- Snapshot: the acp-agent example composes `dsh-sandbox-policy` + `dsh-fs-sandbox`; the pinned header carries the fs escalation fields and the `sandbox/mode` event name, re-recorded once.
