diff --git a/CLAUDE.md b/CLAUDE.md
index ab961ee..3482438 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -15,7 +15,7 @@ Graph-based workflow orchestrator with unified state model for NullClaw AI bot a
 | File | Role |
 |------|------|
 | `main.zig` | CLI args (`--port`, `--db`, `--config`, `--version`, `--export-manifest`, `--from-json`), HTTP accept loop, engine thread, tracker thread |
-| `api.zig` | REST API routing and 30+ endpoint handlers (runs, workers, workflows, checkpoints, state, SSE stream, tracker) |
+| `api.zig` | REST API routing and 30+ endpoint handlers (runs, workers, workflows, checkpoints, state, stream snapshots, tracker) |
 | `store.zig` | SQLite layer, CRUD methods for all tables, schema migrations (4 migration files) |
 | `engine.zig` | Graph-based state scheduler: tick loop, 7 node type handlers, checkpoints, reducers, goto, breakpoints, deferred nodes, reconciliation |
 | `state.zig` | Unified state model: 7 reducer types (last_value, append, merge, add, min, max, add_messages), overwrite bypass, ephemeral keys, state path resolution |
@@ -46,7 +46,7 @@ Graph-based workflow orchestrator with unified state model for NullClaw AI bot a
 
 ```sh
 zig build              # build
-zig build test         # unit tests (320 tests)
+zig build test         # unit tests (355 tests)
 zig build && bash tests/test_e2e.sh   # e2e tests (requires Python 3 for mock workers)
 ./zig-out/bin/nullboiler --port 8080 --db nullboiler.db --config config.json
 ```
@@ -68,8 +68,8 @@ zig build && bash tests/test_e2e.sh   # e2e tests (requires Python 3 for mock wo
 | POST | `/workers` | Register worker |
 | GET | `/workers` | List workers |
 | DELETE | `/workers/{id}` | Remove worker |
-| POST | `/runs` | Create workflow run (legacy step-array or graph format) |
-| GET | `/runs` | List runs (supports ?status= filter) |
+| POST | `/runs` | Create workflow run from legacy step-array format |
+| GET | `/runs` | List runs (supports ?status= and ?workflow_id= filters) |
 | GET | `/runs/{id}` | Get run details |
 | POST | `/runs/{id}/cancel` | Cancel run |
 | POST | `/runs/{id}/retry` | Retry failed run |
@@ -82,7 +82,7 @@ zig build && bash tests/test_e2e.sh   # e2e tests (requires Python 3 for mock wo
 | GET | `/runs/{id}/events` | List run events |
 | GET | `/runs/{id}/checkpoints` | List checkpoints for run |
 | GET | `/runs/{id}/checkpoints/{cpId}` | Get checkpoint details |
-| GET | `/runs/{id}/stream` | SSE stream (supports ?mode=values\|updates\|tasks\|debug) |
+| GET | `/runs/{id}/stream` | JSON stream snapshot (supports ?mode=values\|updates\|tasks\|debug\|custom and ?after_seq=) |
 | POST | `/workflows` | Create workflow definition |
 | GET | `/workflows` | List workflow definitions |
 | GET | `/workflows/{id}` | Get workflow definition |
@@ -136,9 +136,12 @@ MQTT listener:     (conditional, for async MQTT workers)
 Redis listener:    (conditional, for async Redis workers)
 ```
 
-### SSE Streaming
+### Stream Snapshots
 
-5 modes for real-time consumption via `GET /runs/{id}/stream?mode=X`:
+`GET /runs/{id}/stream` currently returns a JSON snapshot containing persisted
+events and buffered in-memory stream events. It supports `mode=X` filtering and
+`after_seq=N` cursors for independent consumers. The internal stream hub uses 5
+modes:
 - `values` -- full state after each step
 - `updates` -- node name + partial state updates
 - `tasks` -- task start/finish with metadata
diff --git a/docs/api/README.md b/docs/api/README.md
new file mode 100644
index 0000000..39323fd
--- /dev/null
+++ b/docs/api/README.md
@@ -0,0 +1,134 @@
+# NullBoiler HTTP API
+
+This directory hosts the HTTP API contract for NullBoiler.
+
+- **Source of truth:** [`docs/openapi.yaml`](../openapi.yaml) — OpenAPI 3.1
+- **Maintainer roadmap reference:** [`reference/todo.md` P2-04](../../reference/todo.md)
+
+The spec covers all 36 HTTP operations exposed by `src/api.zig` and all
+domain types from `src/types.zig`.
+
+## At a glance
+
+| Group | Endpoints |
+|---|---|
+| Health, Metrics | `GET /health`, `GET /metrics` |
+| Runs | `POST /runs`, `GET /runs`, `GET /runs/{id}`, `POST /runs/{id}/{cancel,retry,resume,replay,state}`, `POST /runs/fork` |
+| Steps & Events | `GET /runs/{id}/steps`, `GET /runs/{id}/steps/{step_id}`, `GET /runs/{id}/events`, `GET /runs/{id}/stream` (JSON stream snapshot) |
+| Checkpoints | `GET /runs/{id}/checkpoints`, `GET /runs/{id}/checkpoints/{cp_id}` |
+| Workers | `POST /workers`, `GET /workers`, `DELETE /workers/{id}` |
+| Workflows | full CRUD on `/workflows`, plus `validate`, `mermaid`, `run` |
+| Tracker bridge | `GET /tracker/{status,tasks,stats,tasks/{id}}`, `POST /tracker/refresh` |
+| Admin | `POST /admin/drain`, `GET /rate-limits` |
+| Internal | `POST /internal/agent-events/{run_id}/{step_id}` (worker callback) |
+
+## Quick start
+
+### View the spec
+
+```bash
+# Redoc (no install — uses npx)
+npx @redocly/cli preview-docs docs/openapi.yaml
+
+# Swagger UI (Docker)
+docker run --rm -p 8088:8080 \
+  -e SWAGGER_JSON=/spec/openapi.yaml \
+  -v "$(pwd)/docs:/spec" \
+  swaggerapi/swagger-ui
+# then open http://localhost:8088
+```
+
+### Validate locally
+
+```bash
+# Python
+python -m pip install openapi-spec-validator
+python -m openapi_spec_validator docs/openapi.yaml
+
+# Node
+npx @apidevtools/swagger-cli validate docs/openapi.yaml
+
+# Redocly (also runs lint rules beyond bare OpenAPI)
+npx @redocly/cli lint docs/openapi.yaml
+```
+
+### Generate client SDKs
+
+The spec is suitable for `openapi-generator-cli`. Recommended targets and
+generators:
+
+```bash
+# TypeScript (fetch-based, browser & node)
+npx @openapitools/openapi-generator-cli generate \
+  -i docs/openapi.yaml \
+  -g typescript-fetch \
+  -o sdks/typescript-fetch \
+  --additional-properties=npmName=@nullboiler/client,supportsES6=true,typescriptThreePlus=true
+
+# Python (httpx async + sync)
+npx @openapitools/openapi-generator-cli generate \
+  -i docs/openapi.yaml \
+  -g python \
+  -o sdks/python \
+  --additional-properties=packageName=nullboiler_client,projectName=nullboiler-client
+
+# Go
+npx @openapitools/openapi-generator-cli generate \
+  -i docs/openapi.yaml \
+  -g go \
+  -o sdks/go \
+  --additional-properties=packageName=nullboiler,withGoMod=true
+```
+
+For first-class language coverage we recommend publishing each SDK from
+its own repository (e.g. `nullboiler/nullboiler-ts-sdk`) and pinning a
+spec version per release tag.
+
+## Conventions
+
+- **Ids** — opaque strings; do not parse (currently 22-char ULIDs but
+  this is not part of the contract).
+- **Timestamps** — `*_ms` fields are milliseconds since the Unix epoch
+  (UTC), `int64`.
+- **Errors** — every 4xx/5xx response uses the same envelope:
+  ```json
+  {"error": {"code": "<code>", "message": "<human readable>"}}
+  ```
+  See `ErrorDetail.code` for the closed enum of codes.
+- **Idempotency** — `POST /runs` honors `Idempotency-Key` (preferred) or
+  `idempotency_key` body field. Stored-workflow launches via
+  `POST /workflows/{id}/run` do not currently implement idempotency.
+- **Auth** — bearer token; `/health` and `/metrics` are public so that
+  load balancers and Prometheus scrapers can reach them without
+  provisioning a token.
+
+## Versioning the spec
+
+The spec carries the same `info.version` as `GET /health` returns. When
+the API surface changes:
+
+1. Update `src/api.zig` and the matching tests.
+2. Update `docs/openapi.yaml` and bump `info.version` in lockstep with
+   the next NullBoiler release.
+3. Re-run `python -m openapi_spec_validator docs/openapi.yaml` (or the
+   Node equivalent) before committing.
+4. Regenerate any vendored SDKs you ship.
+
+A future enhancement (P2-03 in `reference/todo.md`) is to validate the
+spec against a running orchestrator in CI by hitting every endpoint with
+a smoke client.
+
+## Provenance
+
+This spec was authored from the source of truth files on the `main`
+branch:
+
+- `src/api.zig` — route table (`handleRequest`) and per-handler bodies
+- `src/types.zig` — all enums and DB row types
+- `src/strategy.zig` — strategy expansion semantics
+- `src/workflow_validation.zig` and `src/engine.zig` — graph workflow shape
+  and validation rules
+- `src/metrics.zig` — Prometheus exposition (used in `/metrics` example)
+
+If you change one of those files, update this spec. CI does not yet
+diff them, so the discipline is currently social.
diff --git a/docs/openapi.yaml b/docs/openapi.yaml
new file mode 100644
index 0000000..75a9b3e
--- /dev/null
+++ b/docs/openapi.yaml
@@ -0,0 +1,1903 @@
+openapi: 3.1.0
+info:
+  title: NullBoiler Orchestrator API
+  summary: Workflow orchestration HTTP API for the Null ecosystem.
+  description: |
+    NullBoiler is the orchestration engine of the Null ecosystem. It pulls work
+    from a tracker (or accepts ad-hoc runs), applies scheduling/routing
+    strategies, and dispatches steps to compatible worker runtimes (NullClaw,
+    OpenClaw-compatible, ZeroClaw, PicoClaw bridges).
+
+    This document is the source of truth for the HTTP surface used by:
+
+    - dashboards and operators talking to a single NullBoiler instance,
+    - automation/SDKs driving runs and workflows,
+    - integration tests that need to assert request/response shapes,
+    - generated client SDKs (TypeScript, Python, Go) — see
+      `docs/api/README.md` for the generator command.
+
+    The spec was reverse-engineered from `src/api.zig` and `src/types.zig`
+    on the NullBoiler `main` branch and tracks the same version reported by
+    `GET /health`.
+
+    ## Endpoint availability
+
+    A few endpoints are conditional on runtime configuration:
+
+    - **`/tracker/*`** — only mounted when `tracker` is configured in
+      `nullboiler.config.json`. Otherwise the orchestrator runs in
+      tracker-less mode and these paths return `404`.
+
+    ## Authentication
+
+    All endpoints except `GET /health` and `GET /metrics` require a bearer
+    token when the orchestrator is started with `--api-token` or
+    `NULLBOILER_API_TOKEN`. When no token is configured, the API is open.
+
+    ## Idempotency
+
+    `POST /runs` accepts an idempotency key either as the
+    `Idempotency-Key` request header (preferred) or as `idempotency_key`
+    in the body. Replays of the same payload return `200` with
+    `idempotent_replay: true`. Replays with a different payload return
+    `409 conflict`.
+
+    ## Drain mode
+
+    `POST /admin/drain` puts the orchestrator into drain mode. While
+    draining, `POST /runs` returns `503` and existing runs continue to
+    completion. Drain mode is process-local and resets on restart.
+  version: "2026.3.2"
+  license:
+    name: MIT
+    identifier: MIT
+  contact:
+    name: NullBoiler maintainers
+    url: https://github.com/nullclaw/nullboiler
+
+servers:
+  - url: http://localhost:8080
+    description: Local development (default port from docker-compose)
+  - url: http://localhost:3000
+    description: Local development (default Dockerfile CMD port)
+  - url: https://{host}
+    description: Custom deployment
+    variables:
+      host:
+        default: nullboiler.example.com
+
+tags:
+  - name: Health
+    description: Liveness, version, and aggregate health probes.
+  - name: Metrics
+    description: Prometheus-format scrape endpoint for `/metrics`.
+  - name: Runs
+    description: Workflow run lifecycle — create, inspect, cancel, retry, replay, fork.
+  - name: Steps
+    description: Individual step inspection within a run.
+  - name: Events
+    description: Run-scoped event feed and stream snapshots.
+  - name: Checkpoints
+    description: Run state snapshots used for resume, fork, and replay.
+  - name: Workers
+    description: Worker registry — register, list, delete worker runtimes.
+  - name: Workflows
+    description: Reusable workflow definitions (CRUD + validate + mermaid + run).
+  - name: Tracker
+    description: Bridge to NullTickets when running in tracker-driven mode.
+  - name: Admin
+    description: Operational controls — drain mode, rate limit inspection.
+  - name: Internal
+    description: Endpoints used by worker runtimes — not for external consumers.
+
+security:
+  - bearerAuth: []
+
+paths:
+  /health:
+    get:
+      tags: [Health]
+      summary: Health probe
+      description: Public health probe. Always returns 200 with current version and counters.
+      operationId: getHealth
+      security: []
+      responses:
+        '200':
+          description: Health snapshot
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/HealthResponse'
+              examples:
+                ok:
+                  value:
+                    status: ok
+                    version: "2026.3.2"
+                    active_runs: 3
+                    total_workers: 2
+
+  /metrics:
+    get:
+      tags: [Metrics]
+      summary: Prometheus metrics
+      description: |
+        Prometheus exposition format. Public (no auth) so that scrapers can
+        reach it without provisioning a token.
+      operationId: getMetrics
+      security: []
+      responses:
+        '200':
+          description: Metrics in text exposition format
+          content:
+            text/plain:
+              schema:
+                type: string
+              example: |
+                # TYPE nullboiler_http_requests_total counter
+                nullboiler_http_requests_total 1234
+                # TYPE nullboiler_runs_created_total counter
+                nullboiler_runs_created_total 42
+                # ... 9 more counters
+
+  /admin/drain:
+    post:
+      tags: [Admin]
+      summary: Enter drain mode
+      description: |
+        Refuses new runs. Existing runs continue. Drain mode is process-local
+        and resets on restart. There is no inverse endpoint — restart the
+        orchestrator to leave drain mode.
+      operationId: enableDrain
+      responses:
+        '200':
+          description: Drain mode enabled
+          content:
+            application/json:
+              schema:
+                type: object
+                required: [status]
+                properties:
+                  status:
+                    type: string
+                    enum: [draining]
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /rate-limits:
+    get:
+      tags: [Admin]
+      summary: Inspect per-worker rate limit state
+      description: |
+        Returns rate limit state per worker_id. Empty array when no rate
+        limits are configured or none have been observed yet. Useful for
+        investigating dispatch backpressure.
+      operationId: listRateLimits
+      responses:
+        '200':
+          description: Array of per-worker rate limit snapshots
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/RateLimitEntry'
+
+  /workers:
+    get:
+      tags: [Workers]
+      summary: List registered workers
+      operationId: listWorkers
+      responses:
+        '200':
+          description: Array of workers
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Worker'
+        '500':
+          $ref: '#/components/responses/InternalError'
+    post:
+      tags: [Workers]
+      summary: Register a worker
+      description: |
+        Registers a worker with the orchestrator. Workers can also be
+        configured statically in `config.json` — those have `source: config`.
+        Registered workers have `source: registered`.
+      operationId: registerWorker
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/WorkerRegistration'
+      responses:
+        '201':
+          description: Worker registered
+          content:
+            application/json:
+              schema:
+                type: object
+                required: [id, status, protocol]
+                properties:
+                  id:
+                    type: string
+                  status:
+                    type: string
+                    enum: [active]
+                  protocol:
+                    $ref: '#/components/schemas/WorkerProtocol'
+        '400':
+          $ref: '#/components/responses/BadRequest'
+        '409':
+          description: Worker id already exists
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /workers/{id}:
+    delete:
+      tags: [Workers]
+      summary: Remove a worker
+      operationId: deleteWorker
+      parameters:
+        - $ref: '#/components/parameters/WorkerId'
+      responses:
+        '200':
+          description: Worker removed
+          content:
+            application/json:
+              schema:
+                type: object
+                required: [ok]
+                properties:
+                  ok:
+                    type: boolean
+                    enum: [true]
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /runs:
+    post:
+      tags: [Runs]
+      summary: Create a workflow run
+      description: |
+        Creates a new run from an inline workflow definition. Use
+        `/workflows/{id}/run` to launch a stored workflow instead.
+
+        Idempotency: pass `Idempotency-Key` header (preferred) or
+        `idempotency_key` body field. Replays of the exact same payload
+        return `200` with `idempotent_replay: true`. Different payloads
+        with the same key return `409 conflict`.
+      operationId: createRun
+      parameters:
+        - name: Idempotency-Key
+          in: header
+          required: false
+          schema:
+            type: string
+            minLength: 1
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/RunCreate'
+      responses:
+        '200':
+          description: Idempotent replay of an existing run
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RunCreated'
+              examples:
+                replay:
+                  value:
+                    id: r_abc123
+                    status: completed
+                    idempotent_replay: true
+        '201':
+          description: Run created
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RunCreated'
+              examples:
+                created:
+                  value:
+                    id: r_abc123
+                    status: running
+                    idempotent_replay: false
+        '400':
+          $ref: '#/components/responses/BadRequest'
+        '409':
+          description: Idempotency conflict — same key, different payload
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+        '503':
+          description: Orchestrator is in drain mode
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ErrorResponse'
+              examples:
+                draining:
+                  value:
+                    error:
+                      code: draining
+                      message: orchestrator is draining and does not accept new runs
+        '500':
+          $ref: '#/components/responses/InternalError'
+    get:
+      tags: [Runs]
+      summary: List runs
+      description: |
+        Returns a paginated wrapper. Items are the condensed run shape
+        (id, status, optional idempotency_key, optional workflow_id,
+        timestamps) — use `GET /runs/{id}` for the full detail with
+        embedded steps, workflow snapshot, input, callbacks, and state.
+      operationId: listRuns
+      parameters:
+        - name: status
+          in: query
+          required: false
+          schema:
+            $ref: '#/components/schemas/RunStatus'
+          description: Filter by run status.
+        - name: workflow_id
+          in: query
+          required: false
+          schema:
+            type: string
+          description: Filter to runs created from a specific stored workflow.
+        - name: limit
+          in: query
+          required: false
+          schema:
+            type: integer
+            minimum: 1
+            maximum: 1000
+            default: 100
+        - name: offset
+          in: query
+          required: false
+          schema:
+            type: integer
+            minimum: 0
+            maximum: 1000000000
+            default: 0
+      responses:
+        '200':
+          description: Paginated list of runs (newest first)
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RunListResponse'
+              examples:
+                page:
+                  value:
+                    items:
+                      - id: ea25c0ac-9c45-441c-a1ff-d2f4602a3b2e
+                        status: failed
+                        created_at_ms: 1778183970318
+                        updated_at_ms: 1778183975351
+                    limit: 100
+                    offset: 0
+                    next_offset: 1
+                    has_more: false
+
+  /runs/{id}:
+    get:
+      tags: [Runs]
+      summary: Get run detail
+      operationId: getRun
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+      responses:
+        '200':
+          description: Run with embedded steps
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RunDetail'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /runs/{id}/cancel:
+    post:
+      tags: [Runs]
+      summary: Cancel a run
+      operationId: cancelRun
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+      responses:
+        '200':
+          description: Cancellation accepted
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RunStatusResponse'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '409':
+          $ref: '#/components/responses/Conflict'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /runs/{id}/retry:
+    post:
+      tags: [Runs]
+      summary: Retry a failed run
+      description: Resets failed steps to ready and moves the run back to `running`.
+      operationId: retryRun
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+      responses:
+        '200':
+          description: Run moved back to running
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RunStatusResponse'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '409':
+          $ref: '#/components/responses/Conflict'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /runs/{id}/steps:
+    get:
+      tags: [Steps]
+      summary: List steps in a run
+      operationId: listRunSteps
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+      responses:
+        '200':
+          description: Array of steps
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Step'
+        '404':
+          $ref: '#/components/responses/NotFound'
+
+  /runs/{id}/steps/{step_id}:
+    get:
+      tags: [Steps]
+      summary: Get step detail
+      operationId: getStep
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+        - $ref: '#/components/parameters/StepId'
+      responses:
+        '200':
+          description: Step detail
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Step'
+        '404':
+          $ref: '#/components/responses/NotFound'
+
+  /runs/{id}/events:
+    get:
+      tags: [Events]
+      summary: List run events
+      description: |
+        Polling-mode event feed backed by persisted run events.
+      operationId: listRunEvents
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+      responses:
+        '200':
+          description: Array of events ordered by id ascending
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Event'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /runs/{id}/stream:
+    get:
+      tags: [Events]
+      summary: Snapshot run stream events
+      description: |
+        Returns the current run status/state, persisted events, and buffered
+        in-memory stream events as a regular JSON response. Use `after_seq`
+        to page through buffered stream events without sharing cursor state
+        between consumers.
+      operationId: streamRunEvents
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+        - name: mode
+          in: query
+          required: false
+          schema:
+            type: string
+            example: values,debug
+          description: Comma-separated stream modes to include (`values`, `updates`, `tasks`, `debug`, `custom`). Defaults to all modes.
+        - name: after_seq
+          in: query
+          required: false
+          schema:
+            type: integer
+            format: int64
+            minimum: 0
+            default: 0
+          description: Return buffered stream events after this sequence number.
+      responses:
+        '200':
+          description: Stream snapshot
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/StreamSnapshot'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /runs/{id}/checkpoints:
+    get:
+      tags: [Checkpoints]
+      summary: List checkpoints for a run
+      operationId: listCheckpoints
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+      responses:
+        '200':
+          description: Array of checkpoints
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Checkpoint'
+        '404':
+          $ref: '#/components/responses/NotFound'
+
+  /runs/{id}/checkpoints/{cp_id}:
+    get:
+      tags: [Checkpoints]
+      summary: Get checkpoint detail
+      operationId: getCheckpoint
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+        - $ref: '#/components/parameters/CheckpointId'
+      responses:
+        '200':
+          description: Checkpoint with full state
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Checkpoint'
+        '404':
+          $ref: '#/components/responses/NotFound'
+
+  /runs/{id}/resume:
+    post:
+      tags: [Runs]
+      summary: Resume an interrupted run
+      description: |
+        Resumes the latest checkpoint for a run currently in `interrupted`
+        status. Optional `state_updates` are applied before the run is moved
+        back to `running`.
+      operationId: resumeRun
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+      requestBody:
+        required: false
+        content:
+          application/json:
+            schema:
+              type: object
+              properties:
+                state_updates:
+                  type: object
+                  additionalProperties: true
+                  description: Optional state values to merge before resuming.
+      responses:
+        '200':
+          description: Run resumed
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RunStatusResponse'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '409':
+          $ref: '#/components/responses/Conflict'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /runs/fork:
+    post:
+      tags: [Runs]
+      summary: Fork a run from a checkpoint into a new run
+      description: |
+        Creates a new run that branches from the parent's checkpoint.
+        Useful for what-if exploration without disturbing the parent run.
+      operationId: forkRun
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              required: [checkpoint_id]
+              properties:
+                checkpoint_id:
+                  type: string
+                  description: Existing checkpoint to fork from; the parent run is inferred from this checkpoint.
+                state_overrides:
+                  type: object
+                  additionalProperties: true
+      responses:
+        '201':
+          description: Forked run created
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ForkRunResponse'
+        '400':
+          $ref: '#/components/responses/BadRequest'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /runs/{id}/state:
+    post:
+      tags: [Runs]
+      summary: Inject state into a running run
+      description: |
+        Schedules an injection of state updates into the run. Optionally
+        delays application until after a specified step completes.
+      operationId: injectRunState
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              required: [updates]
+              properties:
+                updates:
+                  type: object
+                  additionalProperties: true
+                  description: State key/value updates to merge.
+                apply_after_step:
+                  type: string
+                  description: Optional def_step_id — defer until that step finishes.
+      responses:
+        '200':
+          description: State update applied immediately or queued for a later step
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/StateInjectionResponse'
+        '400':
+          $ref: '#/components/responses/BadRequest'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /runs/{id}/replay:
+    post:
+      tags: [Runs]
+      summary: Replay a run from a checkpoint
+      description: |
+        Resets the existing run back to a checkpoint, deletes later steps and
+        checkpoints, and moves the same run back to `running`.
+      operationId: replayRun
+      parameters:
+        - $ref: '#/components/parameters/RunId'
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              anyOf:
+                - required: [from_checkpoint_id]
+                - required: [checkpoint_id]
+              properties:
+                from_checkpoint_id:
+                  type: string
+                  description: Canonical checkpoint id to replay from.
+                checkpoint_id:
+                  type: string
+                  description: Backward-compatible alias for `from_checkpoint_id`.
+      responses:
+        '200':
+          description: Run reset to checkpoint and moved to running
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ReplayRunResponse'
+        '400':
+          $ref: '#/components/responses/BadRequest'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /workflows:
+    post:
+      tags: [Workflows]
+      summary: Store a reusable workflow definition
+      operationId: createWorkflow
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/WorkflowCreate'
+      responses:
+        '201':
+          description: Workflow stored
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/WorkflowCreated'
+        '400':
+          $ref: '#/components/responses/BadRequest'
+        '500':
+          $ref: '#/components/responses/InternalError'
+    get:
+      tags: [Workflows]
+      summary: List stored workflows
+      operationId: listWorkflows
+      responses:
+        '200':
+          description: Array of workflows including definitions
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Workflow'
+
+  /workflows/{id}:
+    get:
+      tags: [Workflows]
+      summary: Get workflow with full definition
+      operationId: getWorkflow
+      parameters:
+        - $ref: '#/components/parameters/WorkflowId'
+      responses:
+        '200':
+          description: Workflow with definition
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Workflow'
+        '404':
+          $ref: '#/components/responses/NotFound'
+    put:
+      tags: [Workflows]
+      summary: Update workflow definition
+      description: |
+        Replaces the workflow definition. If `version` is omitted, the stored
+        version is preserved. Existing runs already created from older versions
+        are unaffected because they hold a frozen snapshot of the workflow.
+      operationId: updateWorkflow
+      parameters:
+        - $ref: '#/components/parameters/WorkflowId'
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/WorkflowUpdate'
+      responses:
+        '200':
+          description: Updated workflow
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/OkResponse'
+        '400':
+          $ref: '#/components/responses/BadRequest'
+        '404':
+          $ref: '#/components/responses/NotFound'
+    delete:
+      tags: [Workflows]
+      summary: Delete a workflow
+      operationId: deleteWorkflow
+      parameters:
+        - $ref: '#/components/parameters/WorkflowId'
+      responses:
+        '200':
+          description: Deleted
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/OkResponse'
+        '404':
+          $ref: '#/components/responses/NotFound'
+
+  /workflows/{id}/validate:
+    post:
+      tags: [Workflows]
+      summary: Validate a stored workflow definition
+      description: |
+        Runs graph-based workflow validation without creating a run. Returns
+        a list of validation errors (empty array on success) and, when
+        generation succeeds, a Mermaid diagram.
+      operationId: validateWorkflow
+      parameters:
+        - $ref: '#/components/parameters/WorkflowId'
+      responses:
+        '200':
+          description: Validation result
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ValidationResult'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /workflows/{id}/mermaid:
+    get:
+      tags: [Workflows]
+      summary: Render workflow as Mermaid diagram
+      description: Returns a Mermaid `flowchart` string suitable for embedding in docs.
+      operationId: getWorkflowMermaid
+      parameters:
+        - $ref: '#/components/parameters/WorkflowId'
+      responses:
+        '200':
+          description: Mermaid source
+          content:
+            text/plain:
+              schema:
+                type: string
+              example: |
+                flowchart TD
+                  start --> plan
+                  plan --> build
+                  build --> review
+        '404':
+          $ref: '#/components/responses/NotFound'
+
+  /workflows/{id}/run:
+    post:
+      tags: [Workflows]
+      summary: Launch a run from a stored workflow
+      description: |
+        Convenience over `POST /runs` when the graph workflow definition is
+        already stored. The current implementation accepts an optional
+        `input` object and does not implement idempotency for this endpoint.
+      operationId: runWorkflow
+      parameters:
+        - $ref: '#/components/parameters/WorkflowId'
+      requestBody:
+        required: false
+        content:
+          application/json:
+            schema:
+              type: object
+              properties:
+                input:
+                  type: object
+                  additionalProperties: true
+      responses:
+        '201':
+          description: Run created
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/RunStatusResponse'
+        '400':
+          $ref: '#/components/responses/BadRequest'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /tracker/status:
+    get:
+      tags: [Tracker]
+      summary: NullTickets bridge status
+      description: |
+        Reports whether NullBoiler is connected to a NullTickets tracker
+        and basic queue health. Returns `404 tracker_disabled` when no
+        tracker is configured.
+      operationId: getTrackerStatus
+      responses:
+        '200':
+          description: Tracker bridge status
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/TrackerStatus'
+        '404':
+          $ref: '#/components/responses/TrackerDisabled'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /tracker/tasks:
+    get:
+      tags: [Tracker]
+      summary: List active tracker tasks
+      operationId: listTrackerTasks
+      responses:
+        '200':
+          description: Array of tracker tasks
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/TrackerTask'
+        '404':
+          $ref: '#/components/responses/TrackerDisabled'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /tracker/tasks/{task_id}:
+    get:
+      tags: [Tracker]
+      summary: Tracker task detail
+      operationId: getTrackerTask
+      parameters:
+        - name: task_id
+          in: path
+          required: true
+          schema:
+            type: string
+      responses:
+        '200':
+          description: Tracker task detail
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/TrackerTask'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /tracker/stats:
+    get:
+      tags: [Tracker]
+      summary: Aggregate tracker statistics
+      operationId: getTrackerStats
+      responses:
+        '200':
+          description: Aggregate counts by state
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/TrackerStats'
+        '404':
+          $ref: '#/components/responses/TrackerDisabled'
+        '500':
+          $ref: '#/components/responses/InternalError'
+
+  /tracker/refresh:
+    post:
+      tags: [Tracker]
+      summary: Force a tracker poll cycle
+      description: |
+        Acknowledges an operator refresh request. The current handler returns
+        success immediately.
+      operationId: refreshTracker
+      responses:
+        '200':
+          description: Refresh request acknowledged
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/TrackerRefreshResponse'
+
+  /internal/agent-events/{run_id}/{step_id}:
+    post:
+      tags: [Internal]
+      summary: Worker callback — agent event ingest
+      description: |
+        Workers post agent execution events here (tool calls, model
+        results, status updates). Not for external consumers.
+      operationId: postAgentEvent
+      parameters:
+        - name: run_id
+          in: path
+          required: true
+          schema:
+            type: string
+        - name: step_id
+          in: path
+          required: true
+          schema:
+            type: string
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/AgentEventPayload'
+      responses:
+        '200':
+          description: Event accepted
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  ok: { type: boolean, enum: [true] }
+        '400':
+          $ref: '#/components/responses/BadRequest'
+        '404':
+          $ref: '#/components/responses/NotFound'
+
+components:
+  securitySchemes:
+    bearerAuth:
+      type: http
+      scheme: bearer
+      bearerFormat: token
+      description: |
+        Configured via `--api-token` CLI flag or `NULLBOILER_API_TOKEN`
+        environment variable. Health and metrics endpoints are unauthenticated.
+
+  parameters:
+    RunId:
+      name: id
+      in: path
+      required: true
+      schema:
+        type: string
+      description: Run identifier returned from `POST /runs`.
+    StepId:
+      name: step_id
+      in: path
+      required: true
+      schema:
+        type: string
+    CheckpointId:
+      name: cp_id
+      in: path
+      required: true
+      schema:
+        type: string
+    WorkerId:
+      name: id
+      in: path
+      required: true
+      schema:
+        type: string
+    WorkflowId:
+      name: id
+      in: path
+      required: true
+      schema:
+        type: string
+
+  responses:
+    BadRequest:
+      description: Validation error
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/ErrorResponse'
+          examples:
+            invalid_json:
+              value:
+                error:
+                  code: bad_request
+                  message: invalid JSON body
+            missing_field:
+              value:
+                error:
+                  code: bad_request
+                  message: 'missing required field: steps'
+    NotFound:
+      description: Resource not found
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/ErrorResponse'
+          examples:
+            not_found:
+              value:
+                error:
+                  code: not_found
+                  message: run not found
+    Conflict:
+      description: Request conflicts with current resource state
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/ErrorResponse'
+          examples:
+            conflict:
+              value:
+                error:
+                  code: conflict
+                  message: run is not failed
+    TrackerDisabled:
+      description: Pull-mode tracker is not configured
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/ErrorResponse'
+          examples:
+            disabled:
+              value:
+                error:
+                  code: tracker_disabled
+                  message: pull-mode tracker is not configured
+    InternalError:
+      description: Internal server error
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/ErrorResponse'
+          examples:
+            oom:
+              value:
+                error:
+                  code: internal
+                  message: out of memory
+
+  schemas:
+    HealthResponse:
+      type: object
+      required: [status, version, active_runs, total_workers]
+      properties:
+        status:
+          type: string
+          enum: [ok]
+        version:
+          type: string
+          example: "2026.3.2"
+        active_runs:
+          type: integer
+          format: int64
+          minimum: 0
+        total_workers:
+          type: integer
+          format: int64
+          minimum: 0
+
+    ErrorResponse:
+      type: object
+      required: [error]
+      properties:
+        error:
+          $ref: '#/components/schemas/ErrorDetail'
+
+    ErrorDetail:
+      type: object
+      required: [code, message]
+      properties:
+        code:
+          type: string
+          enum:
+            - bad_request
+            - conflict
+            - not_found
+            - internal
+            - draining
+            - unauthorized
+            - tracker_disabled
+        message:
+          type: string
+
+    RunStatus:
+      type: string
+      enum: [pending, running, interrupted, completed, failed, cancelled, forked]
+
+    StepStatus:
+      type: string
+      enum: [pending, ready, running, completed, failed, skipped, interrupted]
+
+    StepType:
+      type: string
+      enum: [task, route, interrupt, agent, send, transform, subgraph]
+
+    WorkerStatus:
+      type: string
+      enum: [active, draining, dead]
+
+    WorkerSource:
+      type: string
+      enum: [config, registered]
+
+    WorkerProtocol:
+      type: string
+      enum: [webhook, api_chat, openai_chat, mqtt, redis_stream, a2a]
+      description: |
+        - `webhook` — requires URL with explicit path (e.g. `/webhook`)
+        - `api_chat` / `openai_chat` — OpenAI-compatible chat completion
+        - `mqtt` — pub/sub MQTT broker
+        - `redis_stream` — Redis Streams
+        - `a2a` — Google Agent-to-Agent protocol
+
+    TrackerTaskState:
+      type: string
+      enum:
+        - claiming
+        - workspace_setup
+        - spawning
+        - running
+        - completing
+        - completed
+        - failed
+        - stalled
+        - cooldown
+        - removing
+
+    ReducerType:
+      type: string
+      enum: [last_value, append, merge, add, min, max, add_messages]
+
+    Worker:
+      type: object
+      required: [id, url, protocol, model, tags, max_concurrent, source, status, consecutive_failures, created_at_ms]
+      properties:
+        id: { type: string }
+        url: { type: string, format: uri }
+        protocol: { $ref: '#/components/schemas/WorkerProtocol' }
+        model:
+          type: string
+          nullable: true
+        tags:
+          type: array
+          items:
+            type: string
+        max_concurrent:
+          type: integer
+          format: int64
+          minimum: 1
+        source: { $ref: '#/components/schemas/WorkerSource' }
+        status: { $ref: '#/components/schemas/WorkerStatus' }
+        consecutive_failures:
+          type: integer
+          format: int64
+          minimum: 0
+        circuit_open_until_ms:
+          type: integer
+          format: int64
+          nullable: true
+        last_error_text:
+          type: string
+          nullable: true
+        created_at_ms:
+          type: integer
+          format: int64
+
+    WorkerRegistration:
+      type: object
+      required: [id, url]
+      properties:
+        id:
+          type: string
+          minLength: 1
+          example: claw-1
+        url:
+          type: string
+          format: uri
+          description: Must include explicit path for `webhook` protocol (e.g. `http://nullclaw:3000/webhook`).
+          example: http://nullclaw:3000/webhook
+        token:
+          type: string
+          default: ""
+        protocol:
+          allOf:
+            - $ref: '#/components/schemas/WorkerProtocol'
+          default: webhook
+        model:
+          type: string
+          description: Required when `protocol` is `openai_chat`.
+        tags:
+          type: array
+          items:
+            type: string
+          default: []
+        max_concurrent:
+          type: integer
+          minimum: 1
+          default: 1
+
+    RunSummary:
+      type: object
+      description: |
+        Condensed run shape returned in list responses. Optional
+        `idempotency_key` and `workflow_id` fields are present only when
+        non-null on the row.
+      required: [id, status, created_at_ms, updated_at_ms]
+      properties:
+        id: { type: string }
+        status: { $ref: '#/components/schemas/RunStatus' }
+        idempotency_key:
+          type: string
+          description: Present only when set on the run.
+        workflow_id:
+          type: string
+          description: Present only when the run was launched from a stored workflow.
+        created_at_ms: { type: integer, format: int64 }
+        updated_at_ms: { type: integer, format: int64 }
+
+    RunListResponse:
+      type: object
+      required: [items, limit, offset, next_offset, has_more]
+      properties:
+        items:
+          type: array
+          items:
+            $ref: '#/components/schemas/RunSummary'
+        limit:
+          type: integer
+          format: int64
+          minimum: 1
+          maximum: 1000
+          description: Page size echoed back.
+        offset:
+          type: integer
+          format: int64
+          minimum: 0
+          description: Page offset echoed back.
+        next_offset:
+          type: integer
+          format: int64
+          description: |
+            Pass as `offset` to the next request. Equals
+            `offset + items.length` on the last page.
+        has_more:
+          type: boolean
+          description: |
+            True when at least one more page exists. Computed by
+            fetching `limit + 1` rows and checking the overflow.
+
+    RunDetail:
+      type: object
+      required:
+        - id
+        - status
+        - created_at_ms
+        - updated_at_ms
+        - checkpoint_count
+        - total_input_tokens
+        - total_output_tokens
+        - total_tokens
+        - steps
+      properties:
+        id: { type: string }
+        status: { $ref: '#/components/schemas/RunStatus' }
+        idempotency_key:
+          type: string
+          description: Present only when the run was created with an idempotency key.
+        workflow_id:
+          type: string
+          description: Present only when the run was launched from a stored workflow.
+        error_text:
+          type: string
+          description: Present only when the run has an error.
+        started_at_ms: { type: integer, format: int64 }
+        ended_at_ms: { type: integer, format: int64 }
+        state_json:
+          type: object
+          additionalProperties: true
+          description: Present only when persisted state exists for the run.
+        checkpoint_count: { type: integer, format: int64, minimum: 0 }
+        total_input_tokens: { type: integer, format: int64, minimum: 0 }
+        total_output_tokens: { type: integer, format: int64, minimum: 0 }
+        total_tokens: { type: integer, format: int64, minimum: 0 }
+        created_at_ms: { type: integer, format: int64 }
+        updated_at_ms: { type: integer, format: int64 }
+        steps:
+          type: array
+          items:
+            $ref: '#/components/schemas/Step'
+
+    RunCreate:
+      type: object
+      required: [steps]
+      properties:
+        steps:
+          type: array
+          minItems: 1
+          items:
+            $ref: '#/components/schemas/StepDefinition'
+        strategy:
+          type: string
+          description: Name of a registered strategy (e.g. `sequential`, `parallel`).
+        idempotency_key:
+          type: string
+          minLength: 1
+        input:
+          type: object
+          additionalProperties: true
+        callbacks:
+          type: object
+          additionalProperties: true
+        config:
+          type: object
+          additionalProperties: true
+          description: Run-scoped config (`tracker_url`, `tracker_api_token`, etc.).
+        reduce:
+          type: object
+          description: Optional reducer step appended after parallel strategy.
+          properties:
+            id:
+              type: string
+              default: __reduce
+            prompt_template:
+              type: string
+
+    RunCreated:
+      type: object
+      required: [id, status, idempotent_replay]
+      properties:
+        id: { type: string }
+        status: { $ref: '#/components/schemas/RunStatus' }
+        idempotent_replay:
+          type: boolean
+
+    RunStatusResponse:
+      type: object
+      required: [id, status]
+      properties:
+        id: { type: string }
+        status: { $ref: '#/components/schemas/RunStatus' }
+
+    ForkRunResponse:
+      type: object
+      required: [id, status, forked_from_checkpoint]
+      properties:
+        id: { type: string }
+        status: { $ref: '#/components/schemas/RunStatus' }
+        forked_from_checkpoint: { type: string }
+
+    ReplayRunResponse:
+      type: object
+      required: [id, status, replayed_from_checkpoint]
+      properties:
+        id: { type: string }
+        status: { $ref: '#/components/schemas/RunStatus' }
+        replayed_from_checkpoint: { type: string }
+
+    StateInjectionResponse:
+      type: object
+      required: [applied]
+      properties:
+        applied:
+          type: boolean
+          description: True when updates were applied immediately.
+        pending:
+          type: boolean
+          description: Present and true when updates were queued until `apply_after_step`.
+
+    StepDefinition:
+      type: object
+      required: [id]
+      properties:
+        id:
+          type: string
+          description: Definition step id (unique within the workflow).
+        type:
+          allOf:
+            - $ref: '#/components/schemas/StepType'
+          default: task
+        depends_on:
+          type: array
+          items:
+            type: string
+          description: Definition step ids this step depends on.
+        retry:
+          type: object
+          properties:
+            max_attempts:
+              type: integer
+              minimum: 1
+              default: 1
+        timeout_ms:
+          type: integer
+          format: int64
+          minimum: 1
+        prompt_template:
+          type: string
+        worker:
+          type: string
+          description: Constrain dispatch to a specific worker id.
+        tags:
+          type: array
+          items: { type: string }
+          description: Worker-tag selector.
+        output_key:
+          type: string
+        output_mapping:
+          type: object
+          additionalProperties: true
+        items_key:
+          type: string
+          description: For `send` (fan-out) steps — state key holding the items array.
+        steps:
+          type: array
+          items:
+            $ref: '#/components/schemas/StepDefinition'
+          description: For `subgraph` or strategy-nested steps.
+      additionalProperties: true
+
+    Step:
+      type: object
+      required:
+        - id
+        - run_id
+        - def_step_id
+        - type
+        - status
+        - attempt
+        - max_attempts
+        - created_at_ms
+        - updated_at_ms
+        - iteration_index
+      properties:
+        id: { type: string }
+        run_id: { type: string }
+        def_step_id: { type: string }
+        type: { $ref: '#/components/schemas/StepType' }
+        status: { $ref: '#/components/schemas/StepStatus' }
+        worker_id:
+          type: string
+          nullable: true
+        output_json:
+          type: object
+          additionalProperties: true
+          nullable: true
+          description: Present only after a step has output.
+        error_text:
+          type: string
+          nullable: true
+        attempt: { type: integer, format: int64, minimum: 1 }
+        max_attempts: { type: integer, format: int64, minimum: 1 }
+        timeout_ms:
+          type: integer
+          format: int64
+          nullable: true
+        next_attempt_at_ms:
+          type: integer
+          format: int64
+          nullable: true
+        parent_step_id:
+          type: string
+          nullable: true
+        item_index:
+          type: integer
+          format: int64
+          nullable: true
+        created_at_ms: { type: integer, format: int64 }
+        updated_at_ms: { type: integer, format: int64 }
+        started_at_ms:
+          type: integer
+          format: int64
+          nullable: true
+        ended_at_ms:
+          type: integer
+          format: int64
+          nullable: true
+        child_run_id:
+          type: string
+          nullable: true
+        iteration_index: { type: integer, format: int64 }
+
+    Event:
+      type: object
+      required: [id, run_id, kind, ts_ms]
+      properties:
+        id: { type: integer, format: int64 }
+        run_id: { type: string }
+        step_id:
+          type: string
+          nullable: true
+        kind:
+          type: string
+          description: Event kind (e.g. `step.started`, `step.completed`, `run.completed`).
+        data:
+          type: object
+          additionalProperties: true
+        ts_ms: { type: integer, format: int64 }
+
+    StreamEvent:
+      type: object
+      required: [seq, event, mode, data]
+      properties:
+        seq: { type: integer, format: int64, minimum: 1 }
+        event: { type: string }
+        mode:
+          type: string
+          enum: [values, updates, tasks, debug, custom]
+        data:
+          type: object
+          additionalProperties: true
+
+    StreamPersistedEvent:
+      type: object
+      required: [kind, data, ts_ms]
+      properties:
+        kind: { type: string }
+        data:
+          type: object
+          additionalProperties: true
+        ts_ms: { type: integer, format: int64 }
+
+    StreamSnapshot:
+      type: object
+      required: [status, events, stream_events, next_stream_seq, stream_oldest_seq, stream_gap]
+      properties:
+        status: { $ref: '#/components/schemas/RunStatus' }
+        state:
+          type: object
+          additionalProperties: true
+          description: Present only when persisted state exists for the run.
+        events:
+          type: array
+          items:
+            $ref: '#/components/schemas/StreamPersistedEvent'
+        stream_events:
+          type: array
+          items:
+            $ref: '#/components/schemas/StreamEvent'
+        next_stream_seq: { type: integer, format: int64, minimum: 0 }
+        stream_oldest_seq: { type: integer, format: int64, minimum: 0 }
+        stream_gap:
+          type: boolean
+          description: True when requested events are older than the retained buffer.
+
+    Checkpoint:
+      type: object
+      required: [id, run_id, step_id, version, created_at_ms]
+      properties:
+        id: { type: string }
+        run_id: { type: string }
+        step_id: { type: string }
+        parent_id:
+          type: string
+          nullable: true
+        state:
+          type: object
+          additionalProperties: true
+        completed_nodes:
+          type: array
+          items:
+            type: string
+        version: { type: integer, format: int64 }
+        metadata:
+          type: object
+          additionalProperties: true
+          nullable: true
+        created_at_ms: { type: integer, format: int64 }
+
+    Workflow:
+      type: object
+      required: [id, name, definition, version, created_at_ms, updated_at_ms]
+      properties:
+        id: { type: string }
+        name: { type: string }
+        definition:
+          $ref: '#/components/schemas/WorkflowDefinition'
+        version: { type: integer, format: int64, minimum: 1 }
+        created_at_ms: { type: integer, format: int64 }
+        updated_at_ms: { type: integer, format: int64 }
+
+    WorkflowCreated:
+      type: object
+      required: [id, name, version]
+      properties:
+        id: { type: string }
+        name: { type: string }
+        version: { type: integer, format: int64 }
+
+    OkResponse:
+      type: object
+      required: [ok]
+      properties:
+        ok: { type: boolean, enum: [true] }
+
+    WorkflowDefinition:
+      type: object
+      description: Graph workflow definition consumed by the engine.
+      required: [nodes, edges]
+      properties:
+        nodes:
+          type: object
+          additionalProperties:
+            type: object
+            additionalProperties: true
+          description: Map of node name to node configuration.
+        edges:
+          type: array
+          items:
+            type: array
+            minItems: 2
+            maxItems: 2
+            items:
+              type: string
+          description: Directed edges such as `["__start__", "node"]` or `["router:yes", "next"]`.
+        state_schema:
+          type: object
+          additionalProperties:
+            type: object
+            additionalProperties: true
+        interrupt_before:
+          type: array
+          items: { type: string }
+        interrupt_after:
+          type: array
+          items: { type: string }
+      additionalProperties: true
+
+    WorkflowCreate:
+      type: object
+      properties:
+        id:
+          type: string
+          minLength: 1
+          description: Optional. Generated when omitted.
+        name:
+          type: string
+          default: untitled
+        version:
+          type: integer
+          format: int64
+          minimum: 1
+          default: 1
+        definition_json:
+          $ref: '#/components/schemas/WorkflowDefinition'
+      additionalProperties: true
+
+    WorkflowUpdate:
+      type: object
+      properties:
+        name:
+          type: string
+          default: untitled
+        version:
+          type: integer
+          format: int64
+          minimum: 1
+          description: Optional explicit version. When omitted, the existing version is preserved.
+        definition_json:
+          $ref: '#/components/schemas/WorkflowDefinition'
+      additionalProperties: true
+
+    ValidationResult:
+      type: object
+      required: [valid, errors]
+      properties:
+        valid:
+          type: boolean
+        errors:
+          type: array
+          items:
+            type: object
+            required: [type, message]
+            properties:
+              type:
+                type: string
+              node:
+                type: string
+              key:
+                type: string
+              message:
+                type: string
+        mermaid:
+          type: string
+          description: Present when Mermaid generation succeeds.
+
+    RateLimitEntry:
+      type: object
+      required: [worker_id, remaining, limit, reset_ms, updated_at_ms]
+      properties:
+        worker_id: { type: string }
+        remaining: { type: integer, format: int64, minimum: 0 }
+        limit: { type: integer, format: int64, minimum: 0 }
+        reset_ms: { type: integer, format: int64 }
+        updated_at_ms: { type: integer, format: int64 }
+
+    TrackerStatus:
+      type: object
+      required: [mode, running_count, max_concurrent, completed_count, failed_count, poll_interval_ms, running]
+      properties:
+        mode:
+          type: string
+          enum: [pull]
+        running_count: { type: integer, format: int64, minimum: 0 }
+        max_concurrent: { type: integer, format: int64, minimum: 1 }
+        completed_count: { type: integer, format: int64, minimum: 0 }
+        failed_count: { type: integer, format: int64, minimum: 0 }
+        poll_interval_ms: { type: integer, format: int64, minimum: 1 }
+        running:
+          type: array
+          items:
+            $ref: '#/components/schemas/TrackerTask'
+
+    TrackerTask:
+      type: object
+      required:
+        - task_id
+        - task_title
+        - pipeline_id
+        - agent_role
+        - execution
+        - current_turn
+        - max_turns
+        - started_at_ms
+        - last_activity_ms
+        - state
+      properties:
+        task_id: { type: string }
+        task_title: { type: string }
+        pipeline_id: { type: string }
+        agent_role: { type: string }
+        execution:
+          type: string
+          enum: [subprocess, dispatch]
+        current_turn: { type: integer, format: int64, minimum: 0 }
+        max_turns: { type: integer, format: int64, minimum: 1 }
+        started_at_ms: { type: integer, format: int64 }
+        last_activity_ms: { type: integer, format: int64 }
+        state: { $ref: '#/components/schemas/TrackerTaskState' }
+
+    TrackerStats:
+      type: object
+      required: [running, completed, failed, total, max_concurrent]
+      properties:
+        running: { type: integer, format: int64, minimum: 0 }
+        completed: { type: integer, format: int64, minimum: 0 }
+        failed: { type: integer, format: int64, minimum: 0 }
+        total: { type: integer, format: int64, minimum: 0 }
+        max_concurrent: { type: integer, format: int64, minimum: 1 }
+
+    TrackerRefreshResponse:
+      type: object
+      required: [status, message]
+      properties:
+        status:
+          type: string
+          enum: [ok]
+        message:
+          type: string
+
+    AgentEventPayload:
+      type: object
+      properties:
+        iteration:
+          type: integer
+          format: int64
+          minimum: 0
+        tool:
+          type: string
+          nullable: true
+        args:
+          type: object
+          additionalProperties: true
+          nullable: true
+        result:
+          type: string
+          nullable: true
+        status:
+          type: string
+          default: running
diff --git a/src/api.zig b/src/api.zig
index 3ddc132..b5ee09e 100644
--- a/src/api.zig
+++ b/src/api.zig
@@ -232,7 +232,7 @@ pub fn handleRequest(ctx: *Context, method: []const u8, target: []const u8, body
         return handleInjectState(ctx, seg1.?, body);
     }
 
-    // ── SSE stream endpoint ─────────────────────────────────────────
+    // ── Stream snapshot endpoint ────────────────────────────────────
 
     // GET /runs/{id}/stream
     if (is_get and eql(seg0, "runs") and seg1 != null and eql(seg2, "stream") and seg3 == null) {
@@ -1506,9 +1506,10 @@ fn handleForkRun(ctx: *Context, body: []const u8) HttpResponse {
     };
 
     const run_id_json = jsonQuoted(ctx.allocator, new_run_id) catch return jsonResponse(500, "{\"error\":{\"code\":\"internal\",\"message\":\"out of memory\"}}");
+    const checkpoint_id_json = jsonQuoted(ctx.allocator, checkpoint_id) catch return jsonResponse(500, "{\"error\":{\"code\":\"internal\",\"message\":\"out of memory\"}}");
     const resp = std.fmt.allocPrint(ctx.allocator,
         \\{{"id":{s},"status":"running","forked_from_checkpoint":{s}}}
-    , .{ run_id_json, checkpoint_id }) catch return jsonResponse(500, "{\"error\":{\"code\":\"internal\",\"message\":\"out of memory\"}}");
+    , .{ run_id_json, checkpoint_id_json }) catch return jsonResponse(500, "{\"error\":{\"code\":\"internal\",\"message\":\"out of memory\"}}");
     return jsonResponse(201, resp);
 }
 
@@ -1630,12 +1631,12 @@ fn handleInjectState(ctx: *Context, run_id: []const u8, body: []const u8) HttpRe
     }
 }
 
-// ── SSE Stream Handler ──────────────────────────────────────────────
+// ── Stream Snapshot Handler ─────────────────────────────────────────
 
 fn handleStream(ctx: *Context, run_id: []const u8, target: []const u8) HttpResponse {
     // For now, return the current state and events as a regular JSON response.
-    // Full SSE streaming with held-open connections will be implemented
-    // when the threading model is wired in main.zig (Task 12).
+    // Held-open SSE is handled internally by the hub; the HTTP endpoint
+    // returns snapshots so independent consumers can use after_seq cursors.
     //
     // Supports ?mode=values,tasks,debug,updates,custom query param to filter
     // which streaming modes the client wants. Default: all modes.
@@ -2667,6 +2668,30 @@ test "API: list runs supports workflow_id filter" {
     try std.testing.expect(std.mem.indexOf(u8, resp.body, "\"workflow_id\":\"wf_2\"") == null);
 }
 
+test "API: fork run returns quoted checkpoint id" {
+    const allocator = std.testing.allocator;
+    var store = try Store.init(allocator, ":memory:");
+    defer store.deinit();
+
+    var arena = std.heap.ArenaAllocator.init(allocator);
+    defer arena.deinit();
+
+    try store.createRunWithState("r1", null, "{\"nodes\":{}}", "{}", "{\"x\":1}");
+    try store.createCheckpoint("cp-one", "r1", "step_a", null, "{\"x\":1}", "[\"step_a\"]", 1, null);
+
+    var ctx = Context{
+        .store = &store,
+        .allocator = arena.allocator(),
+    };
+
+    const resp = handleRequest(&ctx, "POST", "/runs/fork", "{\"checkpoint_id\":\"cp-one\"}");
+    try std.testing.expectEqual(@as(u16, 201), resp.status_code);
+
+    const parsed = try std.json.parseFromSlice(std.json.Value, allocator, resp.body, .{});
+    defer parsed.deinit();
+    try std.testing.expectEqualStrings("cp-one", parsed.value.object.get("forked_from_checkpoint").?.string);
+}
+
 test "API: replay run from checkpoint" {
     const allocator = std.testing.allocator;
     var store = try Store.init(allocator, ":memory:");