# Extension and integration APIs

Public seams for other Pi extensions and host integrations: the in-process RPC, the structured delegation API, launch preflight, capability ceilings, the background-work provider contract, and the Herdr integration.

## In-process event-bus RPC

Other Pi extensions can use the in-process event-bus RPC instead of scraping slash output or calling internal modules. Listen for `subagents:rpc:v1:ready`, send requests on `subagents:rpc:v1:request`, and read replies from `subagents:rpc:v1:reply:<requestId>`.

```typescript
const requestId = crypto.randomUUID();
pi.events.on(`subagents:rpc:v1:reply:${requestId}`, (reply) => {
  // { version: 1, requestId, success: true, data } or
  // { version: 1, requestId, success: false, error: { code, message } }
});
pi.events.emit("subagents:rpc:v1:request", {
  version: 1,
  requestId,
  method: "spawn",
  params: {
    workflowScript: `return runs.run("main", { agent: "reviewer", task: "Review the current diff" })`,
    context: "fresh"
  }
});
```

The RPC methods are `ping`, `status`, `manage`, `spawn`, `steer`, `interrupt`, `stop`, and `resume`. `status`, `manage`, `steer`, `interrupt`, and `resume` reuse normal package-owned actions.

Method notes:

- `manage` exposes a narrow schedule-only allowlist: `schedule.list`, `schedule.show`, `schedule.history`, `schedule.pause`, `schedule.resume`, `schedule.run`, and `schedule.delete`. All actions except `schedule.list` require `id`. Mission, agent, config, worktree, and arbitrary management actions are rejected before executor dispatch. `ping.capabilities.managementActions` advertises the exact allowlist.
- `spawn` accepts structured single-child execution (`agent`, `task?`) or `workflowScript` and is async-only: omit `async` or set `async: true`, omit `clarify`, and do not pass management `action` values. It goes through the same executor as the `subagent` tool, so agent discovery, validation, session attribution, configured spawn caps, child-safety depth, artifacts, and async status all behave the same.
- `steer` requires an async run `id` (plus optional child `index`) and a non-empty `message`; its reply preserves the normal acknowledged-delivery result. Optional `mode` values are `steer` (default), `follow_up`, and `auto`, and receipts include `deliveryStatus: "delivered" | "queued"`. RPC steering disables the direct tool's pause-and-revive recovery in every mode so an extension keeps authority over the exact child it spawned; `ping.capabilities.nonRecoveringSteer` advertises this guarantee.
- `resume` requires a run target and non-empty `message`. It delegates to the existing revival path, which validates current-session ownership, persisted session/recovery metadata, stopped/live state, capability ceilings, and the exclusive session lease before returning the new async run details. Callers may request a `file-only` output path for the revived result without overriding its model, tools, or budgets. `ping.capabilities.resume` advertises this seam.
- `stop` targets current-session top-level async runs through the stop control channel and records a `stopped` lifecycle instead of reporting a timeout.

Capability advertisements on `ping`:

- `events.asyncComplete` — exact process-local completion correlation after RPC `spawn`.
- `managementActions` — exact schedule management actions accepted by RPC `manage`.
- `launchResolvedExtensions` — the optional launch-resolved extension projection in status details.
- `runtimeAcknowledgedExtensions` — the optional child-runtime acknowledgement projection and event name.
- `processTerminalProof` — the process-terminal proof status (see [observability.md](observability.md#process-terminal-proof)).
- `nonRecoveringSteer` — RPC steering never pauses-and-revives.
- `resume` — the revival seam described above.
- `fleetStatus: { version: 1 }` — successful `status` replies additionally include `data.fleet`.

Structured delegation progress updates carry `runId` as soon as foreground execution allocates it, so a caller can retain the package-owned revival target even if its own tool turn is interrupted before the terminal response. Foreground `details.results[]` rows also include a numeric `index` that is unique within the run and stable across partial progress snapshots and the final result; use `(runId, index)` instead of row position to correlate single, counted parallel, and chain children.

### Fleet status DTO

When `ping.capabilities.fleetStatus` is `{ version: 1 }`, successful `status` replies include `data.fleet`: `{ version: 1, entries, totalActive, omitted }`.

Entries are bounded, current-session public display records with an opaque reconciliation `key`, resolved `agent`, optional `role`, `model`, `effort`, caller-facing `goal`, safe `startedAt`, and `{ input, output, total }` tokens. `totalActive` and `omitted` preserve overflow information beyond the bounded entry window.

The DTO intentionally never exposes run, async, or tool IDs. Clients must ignore unknown fields and fall back to status text when the capability is absent.

### Scope

`pi.events` is in-process only. It does not reach separate Pi processes or child subagents; use the file lifecycle artifacts or `pi-intercom` for cross-process coordination.

## External jobs in FleetView

Use `pi-subagents/external-runs` to publish display-only current-session jobs owned by another extension:

```ts
import {
  registerExternalRun,
  updateExternalRun,
  unregisterExternalRun,
} from "pi-subagents/external-runs";

registerExternalRun({
  id: "dependency-review",
  sessionId: ctx.sessionManager.getSessionId(),
  source: "interactive-shell",
  label: "Dependency review",
  state: "running",
  startedAt: Date.now(),
  currentAction: "Inspecting package metadata",
});

updateExternalRun(ctx.sessionManager.getSessionId(), "dependency-review", {
  state: "completed",
  updatedAt: Date.now(),
  endedAt: Date.now(),
  preview: "No dependency blockers found.",
  reportPath: "/tmp/dependency-review.md",
});

unregisterExternalRun(ctx.sessionManager.getSessionId(), "dependency-review");
```

The API validates and caches bounded display fields when the caller registers or updates a job. FleetView reads that cache only. It does not poll caller code. `snapshotExternalRuns(sessionId)` and `listExternalRuns(sessionId)` return bounded current-session snapshots. By default, malformed cached records throw with the validation error. Display-only Fleet callers can pass `{ ignoreMalformed: true, onMalformedRecord }` to remove bad records and keep rendering with a programmatic diagnostic.

External jobs are observational. The caller owns execution, persistence, cancellation, and result delivery. FleetView does not expose stop, steer, resume, cancel, or Herdr controls for them. Supplied report and transcript paths are shown as bounded text only; FleetView does not read arbitrary external paths.

## Launch contract preflight

Use `pi-subagents/preflight` when an extension needs to inspect the resolved child launch contract before deciding whether to run anything:

```ts
import { resolveSubagentLaunchContract } from "pi-subagents/preflight";

const result = await resolveSubagentLaunchContract({
  agent: "reviewer",
  task: "Review the current diff.",
  context: "fresh",
  cwd: ctx.cwd,
  sessionRoot: "/tmp/my-extension-preflight-session-root",
  availableModels: ctx.modelRegistry.getAvailable(),
});

if (!result.ok) {
  // missing_agent, ambiguous_agent, missing_skill, denied_required_tool,
  // invalid_artifact_dir, invalid_cwd, or unsupported_mode
  throw new Error(result.message);
}

console.log(result.contract.digest, result.contract.tools.effectiveAllowlist);
```

Preflight covers ordinary single-agent launch resolution:

- Selected agent identity and shadowed candidates.
- A parsed-definition digest, including system prompt and launch-affecting model, tool, skill, extension, output, and memory fields.
- Fresh/fork context, effective model and thinking, skill and tool resolution, direct MCP selections, runtime/configured extensions.
- Artifact/session paths, async lifecycle/status/result/event/process-terminal paths, package/lifecycle versions, capability-ceiling audit data, and stable digests.

`launchContractDigest` is the canonical digest of the caller task, effective system prompt (including the resolved `turnBudget` prompt augmentation when supplied), model candidates, effective tools/extensions/MCP (including inherited capability ceilings), output binding, and structured-output schema that ordinary foreground and async execution report in results/status/events and metadata.

Boundaries:

- Runtime acceptance prose and output-task annotations are intentionally excluded because side-effect-free preflight does not resolve those host/runtime augmentations; the launch and task digests make that boundary explicit.
- Raw prompts are not exposed in public contract output.
- It is side-effect-free for launch state: it does not create child sessions, temp prompt files, structured-output runtimes, tool-diagnostic files, or run artifacts.
- Some host-owned facts, such as exact fork snapshots, nested async roots, and live model registries, can only be proven by the Pi host; those appear as `host_required` diagnostics instead of silently pretending to be exact.
- Preflight reads the extension config, so `defaultSubagentContext: "fresh"` or `"fork"` affects omitted context in the same way as execution. Explicit `context` still wins.

## Structured delegation API

Other Pi extensions can ask `pi-subagents` to run one configured foreground leaf agent through the structured delegation API. It uses the established `prompt-template:subagent:*` event family and the same executor as the `subagent` tool; it does not add another launcher.

```ts
import {
  SUBAGENT_DELEGATION_REQUEST_EVENT,
  SUBAGENT_DELEGATION_RESPONSE_EVENT,
  type SubagentDelegationRequest,
  type SubagentDelegationResponse,
} from "pi-subagents/delegation";

const request: SubagentDelegationRequest = {
  requestId: crypto.randomUUID(),
  ownerRunId: workflowRunId,
  nodeId: "review-accuracy",
  agent: "reviewer",
  task: "Review the supplied evidence.",
  context: "fresh",
  cwd: ctx.cwd,
  thinking: "high",
  result: {
    kind: "structured",
    schema: {
      type: "object",
      properties: { verdict: { type: "string" } },
      required: ["verdict"],
      additionalProperties: false,
    },
  },
};

const unsubscribe = pi.events.on(SUBAGENT_DELEGATION_RESPONSE_EVENT, (payload) => {
  const response = payload as SubagentDelegationResponse;
  if (response.requestId !== request.requestId) return;
  if (response.ownerRunId !== request.ownerRunId || response.nodeId !== request.nodeId) return;
  unsubscribe();
  // Inspect response.status, response.result, response.usage, model, and thinking.
});
pi.events.emit(SUBAGENT_DELEGATION_REQUEST_EVENT, request);
```

Identity:

- `ownerRunId` plus `nodeId` is the active logical identity; `requestId` identifies one attempt.
- A second active attempt for the same logical node receives `duplicate_node` without disturbing the original.
- Started, update, response, and cancellation payloads carry the full tuple. Cancellation affects only an exact tuple, including cancel-before-start races.
- Each attempt emits at most one terminal response.

Results:

- Result mode is explicit. Text remains literal even when it looks like JSON. Structured mode returns the separately captured, schema-validated JSON value.
- Terminal usage reports input, output, cache-read, cache-write, cost, turns, tool calls, and duration alongside the effective model and thinking level when known.

Bounds:

- Schemas are capped at 64 KiB; tasks and returned text/structured values are capped at 1 MiB, with smaller bounds on identity/configuration strings and a maximum `timeoutMs` of 2,147,483,647.
- Structured delegation accepts `toolBudget: { hard: 0, block: "*" }` to block the first tool call and run a zero-tool leaf; ordinary model-facing/configured budgets keep their existing minimum of one.
- The foreground bridge retains up to 8,192 exact pending-cancellation and settled-attempt identities per extension context. If either history fills, it fails closed with `unavailable_context` for later starts rather than evicting identity facts; lifecycle reset clears the bounded history.

Constraints:

- Delegation requires an active extension context. Emit requests from a supported event callback or queued application step, not by recursively invoking the `subagent` tool inside another tool's `tool_call` hook.
- The caller selects a configured agent, but agent discovery and effective tools remain package-owned. A request cannot grant arbitrary tools, and tool restrictions are not an operating-system sandbox.
- The detached RPC remains async-only; this API is foreground-only.

Unversioned prompt-template payloads with `requestId`, `agent`, `task`, `context`, `model`, and `cwd` are rejected as legacy direct delegation. New integrations must use the structured owned-leaf request above. `pi-subagents/delegation` is the canonical contract for extension integrations.

## Capability ceilings

Parent extensions can enforce an out-of-band, session-scoped capability ceiling without adding a model-visible field to `subagent`:

```ts
import { registerSubagentCapabilityCeiling } from "pi-subagents/capability-ceiling";

const restriction = registerSubagentCapabilityCeiling({
  sessionId: ctx.sessionManager.getSessionId(),
  source: "plan-mode",
  ceiling: {
    allowedAgents: ["plan-scout", "plan-researcher", "plan-reviewer"],
    allowedTools: ["read", "grep", "find", "ls"],
    denyExtensions: true,
  },
});
// restriction.update(...) replaces this provider's policy atomically.
// restriction.dispose() removes only this provider's registration.
```

Semantics:

- Active registrations intersect their `allowedTools` and `allowedAgents` sets and OR `denyExtensions`.
- An explicit empty list means no caller-facing tools or launchable agents for that field; an omitted list does not restrict names.
- `allowedAgents` entries are canonical agent names and are case-sensitive.
- Launching a non-allowlisted agent fails before spawn, and `{ action: "list" }` keeps restricted agents visible in a separate non-executable section instead of silently hiding them.
- The resolved snapshot is propagated monotonically to nested and async children and is retained for recovery.
- `structured_output` may remain as a package-owned internal protocol tool when an output schema requires it; it is not a caller capability.
- A denied lazy-skill `read` requirement fails before spawn rather than widening the ceiling.

`denyExtensions` suppresses ambient, configured, and MCP provider extensions while retaining the package runtime needed for child protocol enforcement. This is a same-process policy boundary, not a sandbox against malicious code already running in the parent process.

Schedules created while a ceiling is active are rejected until durable schedule persistence is available; unrestricted schedules remain subject to any policy active when they fire. Public status exposes bounded audit counts and sources, never full extension paths.

## Background-work provider API

Other Pi extensions can make their current-session jobs visible to `subagent_wait` through the process-local provider contract:

```ts
import { registerBackgroundWorkProvider } from "pi-subagents/background-work";

const dispose = registerBackgroundWorkProvider({
  name: "my-background-extension",
  wakeChannels: ["my-extension:job-finished"],
  listActiveWork: () => jobs
    .filter((job) => job.status === "running")
    .map((job) => ({ id: job.id, sessionId: job.ownerSessionId })),
  reconcile: ({ sessionId, nowMs }) => reconcileJobs(sessionId, nowMs),
});
```

Semantics:

- Each item needs a stable provider-local ID and the exact Pi session ID that owns it. `subagent_wait` captures those identities rather than a count, so one job finishing while another starts still satisfies first-completion waits without losing the replacement.
- It filters snapshots to the active session, fails closed if a provider disappears while its work is tracked, and surfaces malformed snapshots or provider errors with provider context.
- Wake channels only shorten polling; validated snapshots remain authoritative.
- Providers share a registry through `Symbol.for("pi-subagents.background-work.v1")`, allowing independently loaded extension modules to meet in one Pi process.
- Registration is reload-safe: a new provider with the same name replaces the old callback, and the old disposer cannot remove the replacement. Call the disposer during extension shutdown when possible.

Child processes do not gain provider tools or extensions automatically. Add `subagent_wait` to the child agent's `tools` allowlist and load each provider through `extensions` or `subagentOnlyExtensions`. The parent's effective `waitTool` setting is serialized through foreground, async, resume, chain, parallel, and fanout launch paths; `SELESAI_SUBAGENT_WAIT_TOOL_ENABLED` keeps precedence.

## External job provider bridge

Extensions that own long-running advisor jobs can register a process-local provider for `runner.type: external-job` agents:

```ts
import { registerExternalJobProvider } from "pi-subagents/external-job-provider";

const dispose = registerExternalJobProvider({
  name: "surf-oracle",
  start: ({ prompt, promptDigest, cwd, runId, stepIndex, agent, options }) => startSurfJob({ prompt, promptDigest, cwd, runId, stepIndex, agent, options }),
  followUp: ({ prompt, parentProviderJobId, requestId, requestDigest, options }) => followUpSurfJob({ prompt, parentProviderJobId, requestId, requestDigest, options }),
  status: (providerJobId) => getSurfJobStatus(providerJobId),
  result: (providerJobId) => getSurfJobResult(providerJobId),
  reattach: (providerJobId) => reattachSurfJob(providerJobId),
});
```

The provider returns handles with `providerJobId`, `state`, optional `handleUrl`/`conversationUrl`, optional `failureCode`/`failureMessage`, and optional `blockingJobId` for capacity conflicts. `result` can also return `output` and/or `artifactPath`.

`followUp(input)` is optional. When it is present, a completed external-job run can be continued with `subagent({ action: "resume", id: "<run>", message: "..." })`. Pi sends the completed parent provider job id plus a stable `requestId` and `requestDigest`. The provider must continue that parent conversation or fail closed. It must not open a fresh thread when the parent conversation is missing.

The async runner process does not import provider internals. It writes operation requests into its async run directory. The parent Pi process services those requests against the registered provider and writes operation responses. If the provider is not registered, the bridge fails closed with an actionable error. If a run is recovered after provider job metadata exists, the runner calls `reattach` and `result`; it does not call `start` or `follow-up` again.

## Herdr integration

When Pi runs inside [Herdr](https://herdr.dev), pi-subagents automatically reports active async-run counts through Herdr pane metadata.

- The bridge is enabled only when Herdr supplies `HERDR_ENV=1` and `HERDR_PANE_ID`; outside Herdr it registers no listeners or timers.
- It restores current-session active runs after `/reload` or `/resume`, refreshes metadata while work is active, and clears it on completion or shutdown.
- The bridge uses Herdr's existing `herdr:blocked` sibling event when an async child needs attention, and emits `herdr:busy` while async work remains. Herdr versions that support the sibling event keep the pane's semantic state `working`; older versions ignore it safely and still display the metadata label while the Pi integration remains the lifecycle authority.
- The owning Pi session is the only publisher for its own pane metadata. While active subagents exist, it reports a compact `title-suffix` token: one active run uses that agent name, two or more use the active-run count, and attention adds `⚠`. The suffix is cleared when active work reaches zero.

To show the reported label in the expanded Agent sidebar, include `state_text` or `$summary` in its row layout:

```toml
[ui.sidebar.agents]
rows = [
  ["state_icon", "workspace", "tab"],
  ["agent", "state_text"],
]
```

### Inspector panes

Herdr 0.7.5+ can open an on-demand inspector for an existing async run:

```ts
subagent({ action: "inspector.open", id: "<run-id>", index: 0, focus: true })
subagent({ action: "inspector.status", id: "<run-id>", index: 0 })
subagent({ action: "inspector.close", id: "<run-id>", index: 0 })
```

The inspector is a raw dashboard pane, not the child process and not a literal attach. It reads lifecycle/status/output/mission artifacts and sends `steer` or `stop` through pi-subagents' existing control inbox. Closing it never stops the run.

Herdr remains optional. Ordinary launches stay headless, and missing/older Herdr versions affect only Herdr-specific inspector and project-pane actions. FleetView opens the selected active async child with `H`. Use `focus` only with `inspector.open`; Herdr 0.7.5 cannot focus an arbitrary existing raw pane id.

### Project panes

For substantial work in another codebase, Herdr 0.7.5+ can open a project-owned Pi pane rooted in that repository:

```ts
subagent({ action: "project.open", cwd: "/path/to/repo", message: "Own the auth refresh mission for this project." })
subagent({ action: "project.status", cwd: "/path/to/repo" })
subagent({ action: "project.close", cwd: "/path/to/repo" })
```

A project pane runs its own Pi session in the target directory, so subagents launched from that pane use that project's config, agents, skills, files, git state, and missions. The parent session keeps coordination authority, but it does not own or control the subagents inside the peer pane. Existing headless runs are not moved into the pane. Pane bindings live under `<projectRoot>/.pi-subagents/project-panes/herdr.json` and are only a local pointer to the Herdr pane.

Other Pi extensions should use the versioned public TypeScript surface instead of invoking the model-facing tool or importing inspector internals:

```ts
import {
  PROJECT_PANES_API_VERSION,
  openProjectPane,
  getProjectPaneStatus,
  focusProjectPane,
  closeProjectPane,
} from "pi-subagents/project-panes";

const opened = await openProjectPane({ cwd: "/path/to/repo", focus: false });
const status = await getProjectPaneStatus({ cwd: "/path/to/repo" });
const focused = await focusProjectPane({ cwd: "/path/to/repo" });
const closed = await closeProjectPane({ cwd: "/path/to/repo", requireIdle: true });
```

The API returns discriminated structured results with canonical project root, binding path, pane identity, bounded Herdr runtime fields, stable error codes, and `PROJECT_PANES_API_VERSION: 1`.

- Close fails closed unless the saved pane id is still verified for that project and Herdr explicitly reports `agent_status: "idle"`. `requireIdle` is retained for callers that already pass it, but it cannot weaken that rule.
- Focus uses the saved pane id, asks Herdr for its `tab_id` or `workspace_id`, and then calls the matching Herdr focus command.
- The API reports `trust: "human-verification-required"`. It never bypasses or claims to attest Pi's project-trust prompt.

## Host session lifetime and completion wakes

A host that embeds this extension owns whether completion wakes can be delivered at all.

Ordinary async and foreground completion wakes use `registerSubagentNotify` and `sendCompletion`. They listen for completion events and deliver through `pi.sendMessage(..., { triggerTurn })`. Session shutdown stops the result watcher and disposes this completion notifier. `createWaitSubscriptionManager` is separate: it is the explicit non-blocking `subagent_wait` subscription path, not the ordinary completion wake path.

Detached children do not stop when the session does. They are the host process's children, not the session's, so the run keeps going, completes, and notifies nobody. What is lost is the notification, not the work.

This matters because "is the parent busy?" is the wrong idle signal. A parent that launches a detached run and hands control back — which is what the async launch output tells it to do — is not prompting, streaming, compacting, or running a shell command. A host that reaps sessions on those signals alone will dispose exactly the session that was waiting to be woken.

If your host reclaims idle sessions, keep a session alive while it still has live detached work:

- Read run state from the status files under the async run directory rather than from event traffic. A long, quiet workflow sends almost nothing to the parent, so recent-activity heuristics conclude the wrong thing.
- Treat `queued` and `running` as live, matching `isActiveAsyncState`. An interrupted run that is `paused` is finalized. A workflow that paused because a child used `contact_supervisor` still has a live child; keep that parent session until reconcile writes `complete` or `failed`.
- Do not treat `lastUpdate` as a heartbeat. The runner advances it in memory every second but only rewrites `status.json` when the activity classification changes, so a live run inside one long quiet tool call leaves a stale file behind. Judging liveness by file age will reap exactly the run you meant to protect.
- Prefer the recorded runner `pid`, which stays true through a silent tool call and goes false when the runner dies. Keep file age only as a fallback for runs that record no pid, and give it a wide window.
- Match `sessionId` in `status.json` against both forms. It is resolved as `getSessionFile() ?? getSessionId()`, so it is normally the parent's session *file path*, but a session that is not persisted records a bare session id instead.

The symptom when this is missed is quiet and easy to misattribute: subagents appear never to report back, which looks like a fault in this extension rather than in the host that disposed the listener.

## Runtime files

The main runtime files in this repository:

| File | Purpose |
|------|---------|
| `src/extension/index.ts` | Extension registration, tool registration, message/render wiring. |
| `src/agents/agents.ts` | Agent and chain discovery, frontmatter parsing. |
| `src/runs/foreground/subagent-executor.ts` | Main execution routing for single, parallel, chain, management, status, interrupt, and doctor actions. |
| `src/runs/foreground/execution.ts` | Core foreground `runSync` handling. |
| `src/runs/background/subagent-runner.ts` | Detached async runner. |
| `src/runs/background/async-execution.ts` | Background launch support. |
| `src/runs/background/async-status.ts` | Status discovery and formatting for async runs. |
| `src/workflows/scripted-workflow.ts` / `src/runs/foreground/subagent-executor.ts` | Scripted workflow orchestration and child launch routing. |
| `src/shared/settings.ts` | Chain behavior, instructions, and config helpers. |
| `src/runs/shared/worktree.ts` | Git worktree isolation. |
| `src/intercom/intercom-bridge.ts` | Runtime intercom bridge instructions and diagnostics. |
| `src/extension/schemas.ts` / `src/shared/types.ts` | Tool schemas, shared types, and event constants. |
| `test/unit/` / `test/integration/` / `test/e2e/` | Unit, loader-based integration, and real-session E2E tests. |