fix(sandbox): resolve workspace roots per session
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# dsh-sandbox-policy — the sandbox policy home (`ctx.sandboxPolicy`)
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The single owner of the deployment's sandbox policy: the file-effect [`SandboxMode`](../sandbox/README.md) a session starts from, the `workspace-write` boundary root, and the per-session `sandbox/mode` override every enforcing capability family reads.
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The single owner of sandbox-policy resolution: the deployment's default [`SandboxMode`](../sandbox/README.md) and fallback root, plus each session's durable mode override and immutable workspace root. Every enforcing capability family receives one resolved mode-and-root policy per call.
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## Why a shared home
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Two families enforce the same mode vocabulary: the sandboxed bash executor (`@deepseek-ai/dsh-bash-sandbox`) and the sandboxed filesystem provider (`@deepseek-ai/dsh-fs-sandbox`). If each held its own `mode` + `workspaceRoot` config, the two could drift into a split world — bash confined to one root while fs fences another, exactly what [the sandbox RFC](../../../.agents/notes/implemented/feature/2026-07-06-sandbox.md) warns against. Both inject `ctx.sandboxPolicy` and read the SAME default instead. The [cross-family fs sandbox RFC](../../../.agents/notes/implemented/feature/2026-07-14-cross-family-fs-sandbox.md) records the decision.
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Two families enforce the same mode vocabulary: the sandboxed bash executor (`@deepseek-ai/dsh-bash-sandbox`) and the sandboxed filesystem provider (`@deepseek-ai/dsh-fs-sandbox`). If each resolved its own `mode` + `workspaceRoot`, the two could drift into a split world — bash confined to one root while fs fences another, exactly what [the sandbox RFC](../../../.agents/notes/implemented/feature/2026-07-06-sandbox.md) warns against. Both tool layers resolve policy through `ctx.sandboxPolicy`, and both enforcing backends consume that complete per-call result. The [cross-family fs sandbox RFC](../../../.agents/notes/implemented/feature/2026-07-14-cross-family-fs-sandbox.md) records the shared-policy decision.
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## Config
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- `mode` — the deployment default `SandboxMode` (`read-only` / `workspace-write` / `danger-full-access`), validated at load. Default `read-only` (fail-safe).
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- `workspaceRoot` — the absolute directory `workspace-write` may write under. Default `process.cwd()`, resolved absolute either way.
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- `workspaceRoot` — the fallback directory `workspace-write` may write under for agentless calls or sessions without a cwd. Default `process.cwd()`, resolved absolute either way. A normal agent call uses its session header's immutable `cwd` instead.
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## Surface
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- `ctx.sandboxPolicy.defaultMode` / `ctx.sandboxPolicy.workspaceRoot` — the deployment default the enforcing implementations read for their resolve fallback and boundary.
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- `effectiveSandboxMode(events)` — the pure fold of a session's `sandbox/mode` events (the last switch wins, or `undefined`). The tool layers apply it to stamp each call, so neither the executor nor the provider depends on session events.
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- `ctx.sandboxPolicy.resolve({ session?, mode? })` — resolves one complete per-call policy. An explicit approved mode outranks the session's last `sandbox/mode` event, which outranks `defaultMode`; the session's immutable `cwd` becomes `workspaceRoot`, otherwise the configured fallback applies.
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- `ctx.sandboxPolicy.defaultMode` / `ctx.sandboxPolicy.workspaceRoot` — the deployment default and fallback root used by `resolve()`.
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- `effectiveSandboxMode(events)` — the pure fold of a session's `sandbox/mode` events (the last switch wins, or `undefined`), used inside `resolve()`.
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- `setSandboxMode(session, mode)` — THE write path for a per-session override: appends exactly one `sandbox/mode` event. The switch IS its event; nothing mutates the mode out of band.
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- `SANDBOX_MODES` — every mode, for option advertisement and runtime validation.
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## The per-session store
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A runtime switch (an ACP `session/set_config_option`, a test scenario) is one log-only `sandbox/mode` event on the session it applies to. `effective = fold(events) ?? the deployment default`, so an override survives restart by replay, two sessions never see each other's state, and there is no external config store. The event is log-only (the `approval/*` precedent): the model learns the mode from the enforcing tools' denial markers, never from the event. Execution honors the fold in each tool layer, weakest-precedence beneath an escalation grant.
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A runtime switch (an ACP `session/set_config_option`, a test scenario) is one log-only `sandbox/mode` event on the session it applies to. `effective = explicit grant ?? fold(events) ?? deployment default`, so an override survives restart by replay and two sessions never see each other's state. Workspace identity does not need another event: the immutable `SessionHeader.cwd` recorded at creation is the root for every call in that session. The event is log-only (the `approval/*` precedent): the model learns the mode from the enforcing tools' denial markers, never from the event.
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## Model Experience
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@@ -32,5 +33,5 @@ No direct invalidation; the named consumers own any request-prefix changes, and
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## Known Limitations and Deferred Work
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- **`workspaceRoot` is process-wide and fixed for the service's lifetime** — a per-session workspace root is a deferred phase of the sandbox RFC; this package centralizing the root is its groundwork, not its design.
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- **One primary workspace root per session** — policy resolves `SessionHeader.cwd`; extra writable roots are not part of `SandboxExecutionPolicy`.
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- **File-effect modes only** — `SandboxMode` governs file effects; network and process policy are outside its vocabulary, so no knob here restricts them.
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