The process-sandbox seam of [dsh-sandbox](../../packages/sandbox/sandbox) wraps a same-world subprocess argv in a file-effect policy without coupling consumers to a platform runner. [dsh-sandbox-local](../../packages/sandbox/sandbox-local) supplies the Linux bwrap/Landlock and macOS Seatbelt backends; [dsh-bash-sandbox](../../packages/bash/bash-sandbox) is the first consumer. Containers, microVMs, and remote execution are sibling implementations of whole capability seams, not providers of `ctx.sandbox`.
`SandboxMode` governs filesystem effects only. `read-only` denies writes except the required `/dev/null` sink; `workspace-write` permits writes under the workspace root and the backend's promised temp area; `danger-full-access` bypasses confinement. Network and process visibility are outside this vocabulary.
type ConfinedSandboxMode = Exclude<SandboxMode, 'danger-full-access'>
```
Enforcement is a reported fact. `full` means the backend governs every file effect promised by the mode; `partial` means an active backend or older kernel ABI governs only a subset, so consumers that require the absolute promise must reject or surface that distinction.
The complete execution policy is resolved and carried per capability call. It includes `danger-full-access` so a consumer can resolve policy once before deciding whether to bypass confinement. Normal tool calls derive `workspaceRoot` from the calling session's immutable cwd; deployment configuration is the agentless fallback. The root is canonicalized with filesystem semantics before lexical normalization, so a cwd containing `symlink/..` identifies the directory where a spawned process actually runs.
`ctx.sandboxPolicy.resolve()` accepts the active session and, for an approved retry, an explicit mode. The service owns precedence and root fallback so bash and fs do not repeat it.
```ts type-equiv
/** Inputs that select the sandbox policy for one capability call. */
interface SandboxPolicyRequest {
/** Calling session; its immutable cwd becomes the workspace boundary. */
session?: Session
/** Explicit approved mode override, which outranks session policy. */
mode?: SandboxMode
}
```
Only a confined execution reaches `ctx.sandbox`; its provider policy narrows the mode while retaining the same root. This permits concurrent sessions, consumers, and one-shot escalated retries to ask the same provider for different boundaries without mutating provider state.
`RunnerFailureRule` combines evidence that a runner failed before executing the command. A consumer requires a nonzero exit, the optional allowed-exit-code gate, and a case-insensitive fatal signature within one remaining stderr line. Case-insensitive exact full-line informational exclusions are removed first, so a benign runner notice cannot prove failure by itself. The matched line remains available as error detail; classification does not rewrite stderr.
```ts type-equiv
/**
* Evidence that identifies a sandbox runner failing before it executes the
* wrapped command. A consumer first applies {@link allowedExitCodes} when
* present, removes {@link informationalLines} by case-insensitive exact line
* equality, then matches {@link fatalSignatures} case-insensitively within
* each remaining stderr line. Exit status alone never proves runner failure.
*/
interface RunnerFailureRule {
/** Nonzero process exit codes on which this rule may match; omitted permits any nonzero exit. */
allowedExitCodes?: readonly number[]
/** Non-empty substrings identifying a fatal runner diagnostic on one stderr line. */
fatalSignatures: readonly string[]
/** Benign stderr lines excluded by exact full-line equality before fatal matching. */
informationalLines?: readonly string[]
}
```
`ConfinedArgv` is what the consumer spawns. Besides the replacement argv, it carries the backend's enforcement fact and two orthogonal stderr classifiers. `denialSignatures` identify the confined command being blocked while the sandbox works correctly. `runnerFailureRules` identify the sandbox runner refusing or failing before it executes the command; consumers check these first and surface a sandbox infrastructure failure, never an ordinary task failure.
The operator-facing local-provider key remains `runnerFailureSignatures`: an operator-configured runner must supply at least one non-empty, single-line, case-insensitive substring for its own pre-exec refusal dialect. The provider maps those entries into one rule. Consumers directly spawn `ConfinedArgv.argv`, so a missing runner, a non-executable runner, or an executable script whose shebang interpreter is unavailable rejects through the spawn channel rather than a stderr rule when Node supplies attributable `ENOENT`/`EACCES` evidence; after a process starts, child exits such as 126 or 127 remain ordinary unless the selected runner's documented fatal signature matches.
`ctx.sandbox.confine(argv, policy)` returns a `ConfinedArgv` or throws `SandboxUnavailableError` with code `SANDBOX_UNAVAILABLE` when no usable backend exists. Any direct spawn rejection of the returned argv proves the confined launch never started, but only `ENOENT` or `EACCES` with positive Node provenance for provider argv[0] after the caller-owned workdir is independently verified usable carries infrastructure meaning and the original error as detail. A bare `syscall: 'spawn'` without an exact error path, any other code, an invalid or unusable workdir, a resource failure, an unrelated syscall, or an unstructured rejection retains the consumer's ordinary command-start semantics. After a process starts, a matching structured rule identifies a runner refusal. Silent unconfined passthrough is never legal for a confined policy.
Provider probing arbitrates between multiple candidates and is cached for the provider lifetime. A platform with one candidate may select it directly; execution-time refusal retains the safety property. The local provider reports bwrap and Seatbelt as full and preserves the Landlock launcher's full/partial kernel verdict.
