The subprocess seam (ctx.subprocess). The abstract SubprocessService exposes one method — spawn(spec): SubprocessHandle — plus the vocabulary shared by every consumer: the fully-explicit SubprocessSpawnSpec, SubprocessHandle with its non-consuming offset-based output readers, SubprocessOutcome, CollectedOutput, and the managed DSH_* environment namespace (DSH_ENV_PREFIX, DshEnvironment). The local implementation lives in dsh-subprocess-local.

Contract

spawn(spec) returns immediately with a live handle; done resolves at process close with exit facts (SubprocessOutcome carries no output and no cause classification) and rejects only for spawn-level failures.
The spec is fully explicit — argv, cwd, per-stream stdio dispositions, grace — because deployment-varying defaults belong to the calling seam's config, not to a hidden subprocess-service default (the dsh-bash request/spec split is the owning template). Grace must be positive, finite, and no greater than MAX_TIMER_DELAY_MS, so the implementation can represent it with one Node timer instead of accepting a value that Node collapses to one millisecond. argv is never shell-interpreted; a consumer that wants a shell passes ['bash', '-c', command] itself.
Stdio is Node-shaped per stream: 'pipe' hands the caller the raw stream for its own protocol framing (LSP JSON-RPC, ACP ndjson), 'inherit' passes the parent descriptor through for diagnostics, and collect mode ({ maxBytes, spill? }) buffers a bounded tail with an optional full-stream spill file. Collect readers take whole-stream byte offsets and never consume, so independent readers cannot steal one another's deltas; a read whose offset slid out of the in-memory tail is lossy and points at the spill file when one exists. Collected output stays readable after settlement.
Termination is tree-scoped on every platform (POSIX detached groups with direct-child fallback; Windows taskkill /T): terminate() — the only termination verb — escalates SIGTERM→grace→SIGKILL (idempotent, driven by the spec's abort signal too, a no-op once the tree is gone), and waitForExit(signal?) observes whole-tree liveness so a consumer-owned teardown ladder holds each tier on real quiescence — the manager reacts but never classifies why (callers own deadlines, teardown ladders, and cause classification).
scrubbedParentEnv() / SENSITIVE_ENV_PATTERN are the one shared scrub definition: ambient credential-shaped and DSH_* names are dropped, and the spec's explicit env merges after the scrub with no namespace validation — a string deliberately forwards or overrides a value, while an undefined tombstone removes an ordinary ambient entry. Spawners that cannot route through the service (node-pty backends, SDK-managed transports) import the scrub.
Disposal of the service terminates all still-running managed processes and awaits their exit.

See the subprocess data-structure catalog and the seam Agent Note.

Model Experience

Indirectly, through consumer seams (today the bash executor family behind dsh-tool-bash), which own all model-facing rendering of process output and lifecycle.

KV Cache effect

No direct invalidation; the named consumers own any request-prefix changes.

Known Limitations and Deferred Work

node-pty and SDK-managed spawns share only the scrub — the PTY backend's terminal fork and the MCP SDK's own stdio transport cannot route their spawns through this seam (the library owns the fork/spawn call); they import scrubbedParentEnv so the environment policy stays single-sourced.
Teardown ladders are consumer-owned — the seam ships signalling verbs and the tree-liveness wait, not a canned quiesce sequence; each out-of-process consumer encodes its child's cooperation shape itself (the ACP backend's stdin-EOF-first ladder is the in-repo template).