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The gc supervisor exposes a single, typed HTTP control plane described by an OpenAPI 3.1 document. Everything the CLI does, any third-party client can do too — there is no hidden surface.

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Get the spec

  • openapi.json — the authoritative contract. Drop it into Stoplight, Postman, Swagger UI, or any OpenAPI-aware tool to browse operations interactively.
  • events.json — the gc events JSONL line schema. It references DTO components in openapi.json, so the API remains the source of truth.

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Endpoint families

The spec is the full reference. A brief summary of the surfaces:
  • Cities. GET /v0/cities, POST /v0/city, GET /v0/city/{cityName}, GET /v0/city/{cityName}/status, GET /v0/city/{cityName}/readiness, POST /v0/city/{cityName}/stop.
  • Health & readiness. GET /health, GET /v0/readiness, GET /v0/provider-readiness.
  • Agents. GET/POST/DELETE under /v0/city/{cityName}/agents plus SSE /v0/city/{cityName}/agents/{agent}/output/stream.
  • Beads (work units). CRUD under /v0/city/{cityName}/beads, query + hook operations, dependencies, labels.
  • Sessions. CRUD under /v0/city/{cityName}/sessions, submit, prompt, resume, interaction response, transcript, SSE stream.
  • Mail, convoys, orders, formulas, molecules, participants, transcripts, adapters. External messaging and orchestration surfaces; see the spec for per-operation shapes.
  • Event bus. GET /v0/events + GET /v0/events/stream at supervisor scope, and GET /v0/city/{cityName}/events + GET /v0/city/{cityName}/events/stream at city scope.
  • Config & packs. Per-city config and pack metadata under /v0/city/{cityName}/config and /v0/city/{cityName}/packs.

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Request and response headers

Every operation’s header contract appears in the OpenAPI spec — if a request header is required or a response header is promised, the spec describes it. The two cross-cutting headers every API client should know about:
  • X-GC-Request (request header, required on all mutations). Anti-CSRF token required on every POST, PUT, PATCH, and DELETE. Any non-empty value is accepted; the header’s presence is what the server checks. Requests without it are rejected with 403 csrf: X-GC-Request header required on mutation endpoints. Leveraging the same-origin policy, a cross-origin attacker cannot set this header on a forged request. The generated Go and TypeScript clients set this header automatically; only raw HTTP clients need to remember it.
  • X-GC-Request-Id (response header, every response). Opaque per-response identifier the server assigns for log correlation. Every response — success or error — carries this header; the spec declares it via a $ref to components.headers.X-GC-Request-Id. Include its value in bug reports so the server’s logs can be traced.
SSE stream operations emit additional runtime-status headers before the first event frame:
  • stream-agent-output / stream-agent-output-qualified: GC-Agent-Status — set to stopped when the agent is not running and the stream is replaying transcript from the session log instead of live output.
  • stream-session: GC-Session-State (e.g. active, closed) and GC-Session-Status (stopped when the session’s underlying process is not running).
Each header’s schema is documented in the operation’s responses.200.headers in the spec.

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Errors

Every error response is an RFC 9457 Problem Details body (application/problem+json). Error types are documented in the spec under components.schemas.ErrorModel. The detail field carries a short code: prefix (e.g. pending_interaction: ..., conflict: ..., not_found: ..., read_only: ...) so clients can pattern-match on the semantic code without needing a typed error enum. Body-field validation errors (e.g. a required string posted empty) come back as 422 Unprocessable Entity or 400 Bad Request depending on the operation; the errors array of the Problem Details body pinpoints which fields failed.

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Streaming

SSE endpoints set Content-Type: text/event-stream and emit typed event: frames. The spec describes each event’s payload schema under the per-operation responses.200.content.text/event-stream entry. Clients should follow the standard SSE reconnection protocol (Last-Event-ID header) where the server supports it; the event bus stream (/v0/events/stream) replays from the last received index. When no cursor is supplied, event streams start at the current event head and deliver future events only. Async 202 Accepted responses include an event_cursor captured before the operation starts; pass that value as after_cursor or after_seq to wait for the operation’s request-result event without replaying unrelated historical backlog. Fatal setup errors are returned as normal Problem Details responses before the stream’s 200 headers commit, never as a 200 stream that closes immediately. For example, GET /v0/events/stream returns 503 application/problem+json with detail: "no_providers: ..." when no running city has an event provider registered.

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Creating a city (asynchronous)

POST /v0/city is an asynchronous operation. The response is 202 Accepted returned as soon as the city has been scaffolded on disk and registered with the supervisor. The slow finalize work (pack materialization, bead store startup, formula resolution, agent validation) runs on the supervisor reconciler’s next tick. Clients observe completion via the supervisor event stream — there is nothing to poll.

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Response

{
  "request_id": "req-...",
  "event_cursor": "__supervisor__:42,my-city:17"
}
Use request_id to correlate the completion event. Use event_cursor as the after_cursor value on the supervisor event stream.

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Completion events

On the same /v0/events/stream the client will see:
  • city.created (CityLifecyclePayload) — emitted by the scaffold step before POST returns. subject and payload name equal the resolved city name.
  • request.result.city.create (CityCreateSucceededPayload) — the reconciler finished prepareCityForSupervisor successfully.
  • request.failed (RequestFailedPayload) — the reconciler failed the async operation. Match payload.request_id to the 202 response.
Exactly one terminal event (request.result.city.create or request.failed) lands per successful POST. Clients wait for the returned request_id; no polling of GET /v0/cities or GET /v0/city/{cityName}/readiness is required.

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Subscribe before or after POST

Either order works. The recommended flow is:
  1. POST /v0/city and wait for 202 {request_id, event_cursor}.
  2. GET /v0/events/stream?after_cursor=<event_cursor>.
  3. Read frames until payload.request_id == response.request_id and type ∈ {"request.result.city.create", "request.failed"}.
Empty supervisor is fine. The event stream works even when no cities existed before the POST. POST writes the city to the supervisor registry (cities.toml) and creates .gc/events.jsonl synchronously before returning 202, so the event multiplexer finds the new city on the very next buildMultiplexer call. Subscribers do not need to retry on 503 no_providers; if that error surfaces after a successful 202, it’s a bug.

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Errors

  • 409 conflict: city already initialized at <path> — the target directory already has a scaffolded city.
  • 422 — invalid provider, invalid bootstrap profile, or other body-validation failure.
  • 503 — a hard dependency is missing on the host, or a provider the city needs is not ready.
  • 500 — unexpected scaffold failure; consult the server logs via the X-GC-Request-Id correlation header.

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Unregistering a city (asynchronous)

POST /v0/city/{cityName}/unregister removes a city from the supervisor’s registry and signals the supervisor to stop the city’s controller. Like POST /v0/city, it is asynchronous: the response is 202 Accepted returned as soon as the registry entry is gone and the supervisor is notified. The supervisor reconciler stops the controller on its next tick and emits the completion event. The city directory on disk is not touched. This operation only detaches the city from the supervisor; reattaching it later is a simple gc register.

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Response

{
  "request_id": "req-...",
  "event_cursor": "__supervisor__:43,my-city:21"
}
Pass event_cursor as after_cursor on /v0/events/stream and wait for the terminal event whose payload contains the returned request_id.

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Completion events

On /v0/events/stream the client will see (in order):
  • city.unregister_requested (CityLifecyclePayload) — emitted by the handler before the registry write so subscribers see the teardown start.
  • request.result.city.unregister (CityUnregisterSucceededPayload) — emitted by the reconciler once the city’s controller has stopped.
  • request.failed (RequestFailedPayload) — emitted by the reconciler if the controller did not stop cleanly. Match payload.request_id.
Exactly one terminal event lands per successful unregister. Clients wait for the returned request_id.

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Errors

  • 404 not_found: city not registered with supervisor: <name> — no entry in the registry for that name.
  • 501 — supervisor has no Initializer wired (test-only configs).
  • 500 — unexpected registry write failure.

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Event Contract

The event APIs, the SSE streams, and gc events are the same contract at three different presentation layers. The API is the source of truth. For the explicit CLI output contract, including JSONL framing, empty-output behavior, heartbeat suppression, and the --seq plain-text cursor format, see gc events Formats.

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City Scope

Per-city routes are available only after the supervisor marks the city running=true in GET /v0/cities. During startup reconciliation, a city can appear in the city list with running=false and status=starting_agents; in that window typed /v0/city/{cityName}/... routes return 404 with not_found: city not found or not running: <cityName>. The raw /v0/city/{cityName}/svc/* workspace-service proxy is outside the Huma-typed API surface and returns the static readiness detail not_found: city not found or not running. Clients should use the supervisor city list or lifecycle events as the readiness boundary before issuing per-city requests.
  • GET /v0/city/{cityName}/events returns ListBodyWireEvent and includes X-GC-Index.
  • GET /v0/city/{cityName}/events/stream emits:
    • event: event with EventStreamEnvelope
    • event: heartbeat with HeartbeatEvent
  • Async session mutations in that city (session.create, session.message, session.submit) complete on this stream. Match terminal request.result.session.* or request.failed events by payload.request_id.
  • Resume:
    • Last-Event-ID or after_seq; omit both to start from the current city event head.
  • gc events in city scope outputs one TypedEventStreamEnvelope JSON object per line.
  • gc events --watch and gc events --follow in city scope output one EventStreamEnvelope JSON object per line.
  • gc events --seq in city scope prints the API’s X-GC-Index value.

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Supervisor Scope

  • GET /v0/events returns SupervisorEventListOutputBody with WireTaggedEvent items.
  • GET /v0/events/stream emits:
    • event: tagged_event with TaggedEventStreamEnvelope
    • event: heartbeat with HeartbeatEvent
  • Async supervisor mutations (city.create, city.unregister) complete on this stream. Match terminal request.result.city.* or request.failed events by payload.request_id.
  • Resume:
    • Last-Event-ID or after_cursor; omit both to start from the current supervisor event head.
  • gc events in supervisor scope outputs one TypedTaggedEventStreamEnvelope JSON object per line.
  • gc events --watch and gc events --follow in supervisor scope output one TaggedEventStreamEnvelope JSON object per line.
  • gc events --seq in supervisor scope prints the current composite supervisor cursor, suitable for --after-cursor.

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Transport vs Semantic Type

  • The SSE event: line is the transport envelope: event, tagged_event, or heartbeat.
  • The semantic event kind is the JSON payload’s type field: bead.created, mail.sent, session.woke, and so on.
  • The CLI does not define a separate event schema. It streams the same DTOs and envelopes as JSONL.

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Versioning

The API is versioned by URL prefix (/v0). Breaking changes ship as a new prefix; the current spec is the authoritative contract for v0.
Last modified on May 28, 2026