writeonce/docs/plan/15-mcp-streamable-http.md

15 — MCP over Streamable HTTP: every writeonce app is an MCP server

Kanban: ⬜ not started (Track 4 — Language & API) — board: 00-kanban.md

Context sources: MCP specification 2025-06-18 — Transports (the normative Streamable HTTP contract this plan implements, verified 2026-07-12), reference/mcp-python-sdk/ (symlink to the official MCP Python SDK — grep src/mcp/server/streamable_http.py + streamable_http_manager.py for the reference server behaviour, src/mcp/client/streamable_http.py for what a conforming client expects; behaviour is ported, code is not), ../runtime/database/04-client-api.md (the wire-protocol design; its "REST + SSE gateway" row is what this plan makes concrete for agents), ./13-class-model-live-pricing.md (13b methods become MCP tools; 13c's subscription registry carries 15e), ./09-concurrency-scaleout.md (thread-per-core + shard bus the endpoint rides; 09d fan-out gates 15e), crates/rt/src/server.rs + crates/rt/src/http/ (the keep-alive HTTP layer and router this lands in).

Context

MCP (Model Context Protocol) is the open JSON-RPC 2.0 protocol LLM agents use to discover and call external capabilities: tools (typed functions), resources (readable content addressed by URI), and change notifications. Streamable HTTP is its HTTP transport (spec 2025-06-18, replacing the 2024-11-05 HTTP+SSE pair): one endpoint path serving POST and GET, where every client→server JSON-RPC message is a POST, the server answers each request with either a single application/json body or a text/event-stream (SSE) that may carry server messages before the final response, a GET opens a server→client SSE stream for unsolicited notifications, and an Mcp-Session-Id header carries optional stateful sessions.

The fit with writeonce is unusually direct. The runtime already is the database, the schema authority, and the HTTP server in one binary; REST routes are generated from type declarations and their service … expose lists. MCP is the same generation problem with a different wire shape: the catalog becomes tools/list and resources/templates/list, the engine's CRUD paths become tools/call, and — once Stage 3/13c lands — committed deltas become notifications/resources/updated. A .wo app then serves browsers (REST/htmlx), programs (REST), and agents (MCP) from one catalog, one engine, one port.

What exists today that this plan builds on: the hand-rolled epoll HTTP layer with keep-alive and pipelining (plan 03 + the 09 follow-up), thread-local routers per SO_REUSEPORT worker, sharded engine with owner-hop/fan-out on the shard bus (09b), and durable ack gating on the io_uring group commit (Response.gate — a mutation's response is parked until its fsync CQE). What does not exist yet: any non-buffered response (every Response is a full Vec<u8> with Content-Length), sessions, and the subscription registry (13c).

Transport contract (normative summary)

The rules the sub-phases implement, condensed from the spec — each MUST below is the spec's, not ours:

#	Rule
T1	One endpoint path (/mcp) MUST support POST and GET. Body of a POST is a single JSON-RPC message (batching is gone in 2025-06-18).
T2	POSTed request → server returns Content-Type: application/json (one object) or text/event-stream (SSE stream that eventually carries the response, then SHOULD close). The server chooses; clients MUST support both.
T3	POSTed notification/response → 202 Accepted, no body (or 4xx if rejected).
T4	GET → SSE stream for server-initiated messages, or 405 Method Not Allowed. No JSON-RPC responses on a GET stream except when resuming.
T5	Sessions: server MAY return Mcp-Session-Id on the InitializeResult response; clients MUST echo it on all subsequent requests; missing → 400; terminated/unknown → 404 (client then re-initializes); client DELETE terminates a session (server MAY answer 405).
T6	MCP-Protocol-Version header required on post-initialize requests; absent → assume 2025-03-26; invalid/unsupported → 400.
T7	Resumability: SSE events MAY carry id: (unique per stream, acting as a per-stream cursor); client reconnects with Last-Event-ID; server MAY replay messages from that stream only.
T8	Security: server MUST validate Origin (DNS-rebinding defence), SHOULD bind localhost when local, SHOULD authenticate.

Goal

cargo run --bin wo -- run docs/examples/blog serves POST /mcp alongside /api/*: an MCP client (MCP Inspector, Claude Code, or a curl script) performs initialize → tools/list → tools/call article_create → tools/call article_list and sees its write — with the ack held for the fsync CQE exactly as REST does. After 15e (with 13c + 09d): resources/subscribe on wo://product/1, a set_price commit in another terminal, and notifications/resources/updated arrives on the open SSE stream — the agent-shaped twin of the 13d browser demo.

Design decisions (locked)

1. One endpoint, same workers. /mcp is a route in the existing thread-local Router — no second listener, no port, no dedicated thread. It scales the way /api/* does: SO_REUSEPORT spreads connections, shard bus routes data ownership.
2. Catalog-driven and class-blind. Tools and resources are generated from the catalog + expose lists, never hand-registered — the 13a doctrine (storage/REST class-blind) extends to MCP. Until 15d, the existing service rest … expose list governs what MCP exposes; 15d adds service mcp for independent control.
3. JSON first, streaming second. 15a–15b answer every POSTed request in application/json mode — spec-legal per T2 — so the MCP surface is useful before any streaming machinery exists. SSE (T2's other arm, T4, T7) is additive in 15c/15e.
4. The durable-ack rule is transport-independent. A tools/call that mutates parks its JSON-RPC response on the group-commit gate exactly like a REST POST (Response.gate / Parked machinery from 09c). An MCP client never observes a result for a non-durable write.
5. Sessions are worker-owned. The worker that serves initialize mints Mcp-Session-Id = w<t>-<128-bit hex>; the embedded worker index lets any other worker forward session-scoped work over the shard bus (run_on, the existing point-op machinery). Stateless until 15c — no session header is issued, which the spec permits.
6. Protocol version 2025-06-18. Negotiated at initialize; absent header → assume 2025-03-26 (T6 — identical for the surface served here); anything else → 400.
7. serde_json for now. Same dependency posture as the rest of rt; migrates when phase 05 lands. No MCP SDK crates — the protocol layer is hand-rolled like the HTTP layer, per the zero-deps north star.
8. Origin validated on every /mcp request (T8): allow absent-Origin (non-browser clients) and a WO_MCP_ORIGINS allowlist defaulting to localhost origins; anything else → 403. The localhost-bind guidance is already satisfied — WO_LISTEN defaults to 127.0.0.1:8080. Authentication is deferred (non-scope; ties to the policy phase).

Dependency graph

15a JSON-RPC core + tools ──→ 15b resources ──→ 15c SSE + sessions ──→ 15e LIVE subscriptions
        │                                        (first streaming         (needs 13c + 09d)
        │                                         response in rt)
        └──→ 15d `service mcp` surface  (parser-only; any time after 15a)
15a needs nothing that isn't shipped: router, sharded engine, group commit.

Sub-phase sequence

15a-jsonrpc-core-and-tools.md — the endpoint speaks MCP, tools work

• Endpoint + envelope: POST /mcp in server.rs; parse a single JSON-RPC 2.0 message (T1); protocol errors as JSON-RPC errors (-32700 parse, -32600 invalid request, -32601 method not found, -32602 invalid params). Notifications/responses → 202 empty (T3). GET /mcp and DELETE /mcp → 405 (T4/T5 — legal until 15c).
• Header plumbing: surface Accept, Origin, MCP-Protocol-Version (and later Mcp-Session-Id, Last-Event-ID) on http::Request; enforce decisions 6 and 8.
• Lifecycle: initialize (version negotiation; capabilities {tools: {listChanged: false}}; serverInfo from the app directory name + crate version), notifications/initialized, ping.
• Tool generation: per exposed type×op → <type>_list, <type>_get, <type>_create, <type>_update, <type>_delete, with inputSchema (JSON Schema) derived from catalog field types (unions → enum, embedded structs → nested object) — same source of truth as describe_routes.
• tools/call dispatch through the same handler paths REST uses: creates local, point ops run_on(owner_of(id)), lists fan out — no second data path. Engine/validation failures return isError: true inside the tool result (the MCP rule: execution errors are results, protocol errors are JSON-RPC errors). Mutations park on the WAL gate (decision 4).

Exit: scripted flow (checked in beside reference/rest/) against the blog sample passes: initialize → 202 for initialized → tools/list enumerates exactly the exposed ops → article_create → article_list shows the row; runs green with WO_GROUP_COMMIT on and off; GET→405, DELETE→405, bad version→400, disallowed Origin→403; unit tests in the server.rs style cover envelope errors and gate parking.

15b-resources.md — the schema and rows become addressable

• URI scheme: wo://schema/<type> (field/shape listing as JSON) and wo://<type>/<id> (one row). resources/list returns the schema resources (bounded); resources/templates/list returns wo://<type>/{id} per exposed type; resources/read resolves both forms (row reads owner-hop like REST GET). Opaque id-based nextCursor pagination on list endpoints.
• Capabilities gain resources: {subscribe: false, listChanged: false} (flips in 15e).

Exit: resources/read wo://articles/1 body-equals GET /api/articles/1; templates enumerate every exposed type; unknown URI → resource-not-found error (-32002); cursor walks a 3-page listing without duplication or loss.

15c-sse-and-sessions.md — the "streamable" half

• First streaming response in the runtime: a streaming variant beside the buffered Response (ConnState::Streaming) that writes SSE frames (event: message\ndata: <json>\n\n) incrementally under epoll writability, honours backpressure (a slow reader parks on EPOLLOUT, never blocks the worker), and holds the connection out of keep-alive reuse until the stream closes. This is the piece 15e and Stage 3 inherit.
• POST answering mode: requests that will emit interim server messages answer in text/event-stream mode (response as the final SSE event, then close — T2); plain requests stay JSON. In 15c itself only long tools/calls use it; the machinery is the deliverable.
• Sessions (T5, decision 5): Mcp-Session-Id minted at initialize; missing on later requests → 400; unknown → 404; DELETE /mcp terminates → 200. Per-worker session table; cross-worker requests forward via the worker index in the id.
• GET /mcp opens the session's server→client SSE stream (heartbeat comments to keep intermediaries happy; never carries responses — T4).

Exit: the spec's own sequence diagram replayed end-to-end by script (init+session → 202 → JSON answer → GET stream stays open across ≥2 heartbeats); 400/404/DELETE conformance matrix green; a deliberately unread client stalls only its own connection (other connections' p99 unaffected, measured).

15d-service-mcp-surface.md — the language names the capability

• Parser: ServiceKind::Mcp + an ident arm for mcp in parse_service (ident, not keyword — the expose gotcha stands); service mcp "/mcp" expose list, get, set_price inside a type/class controls generation independently of REST. Precedence: service mcp present → it alone governs MCP exposure; absent → fall back to the service rest list (15a behaviour, now documented in the spec doc 02-wo-language.md).
• Methods become tools: a 13b class method in an expose list generates <type>_<method> with inputSchema from the method's parameter list — the agent-facing twin of POST /api/<t>/:id/<method>. (Parses and lists from this phase; round-trips once 13b ships.)

Exit: parser tests for the new arm and precedence; the pricing sample gains a service mcp block; tools/list reflects it (method tools listed; callable gated on 13b).

15e-live-subscriptions.md — commits push to agents

• Capabilities flip to resources: {subscribe: true}. resources/subscribe {uri: wo://<type>/<id>} registers a keyed (O(1)) subscription in the 13c registry, bound to the session's GET stream; commit → notifications/resources/updated {uri} pushed as an SSE event; cross-shard commits reach the session's worker via 09d fan-out; resources/unsubscribe and session teardown free registry slots (the EPOLLHUP → unsubscribe philosophy of the v1 datalayer).
• Resumability (T7): per-stream monotonic SSE id:s; a bounded per-session ring buffer of undelivered notifications; reconnect GET with Last-Event-ID replays from the cursor, stream continues.

Exit: two-terminal demo — subscribe to wo://products/1 over the GET stream, set_price via REST curl in the other terminal, the notification arrives without polling; kill the client mid-stream, reconnect with Last-Event-ID, the missed notification is replayed exactly once. Requires 13c + 09d.

Verification targets (after 15e)

Check	Target	How
Spec conformance	T1–T8 matrix green (status codes, headers, content types)	scripted curl flow checked in beside reference/rest/
Interop	MCP Inspector connects, lists tools/resources, calls a tool	manual check, noted per release
Parity	tools/call <type>_get ≡ GET /api/<type>/:id byte-for-byte on the row payload	unit test
Durability	mutation results never precede their fsync CQE (WO_GROUP_COMMIT on)	gate test in server.rs style
Latency	tools/call read p99 within 1 ms of the REST equivalent under the plan-09 bench load	bench harness rerun
Dep budget	no new crates; serde_json only, dropped with phase 05	Cargo.toml review

Non-scope

• No 2024-11-05 HTTP+SSE backwards compatibility. Only Streamable HTTP; old-transport clients are not served.
• No stdio transport. A wo mcp-stdio subcommand would be cheap later; out of scope here.
• No authorization. The MCP auth spec (OAuth 2.1) waits for the policy phase; until then the endpoint trusts what the Origin check and bind address admit.
• No prompts capability, no client-feature counterparts (sampling, elicitation, roots) — server capabilities only.
• No JSON-RPC batching — removed from the protocol in 2025-06-18; single message per POST, enforced.
• No WebSocket. MCP rides SSE only; the 13c browser WebSocket at /api/<type>/live is a separate surface sharing the same registry.

Cross-references

• ../runtime/database/04-client-api.md — the protocol-tier survey; this plan implements its "REST + SSE gateway" row for agents, on the same subscription registry it specifies.
• ./13-class-model-live-pricing.md — 13b gates method tools (15d); 13c gates 15e; 13d's demo has an agent-shaped twin in 15e's exit.
• ./09-concurrency-scaleout.md — 09d gates cross-shard notification fan-out (15e); the shard-bus ownership rules 15a/15c reuse.
• ./05-hand-rolled-json.md — removes this plan's serde_json use when it lands.
• ./07-inotify-content-watcher.md — a future notifications/tools/list_changed on hot reload would pair with it (not scheduled).
• MCP specification 2025-06-18 — the normative transport text summarized in T1–T8.