---
id: adapter-mcp
name: from_mcp + to_mcp (mcp feature)
status: completed
depends_on: [client-http-host, gateway-routes]
scope: broad
risk: medium
impact: component
level: implementation
tags: [adapters, mcp, phase-3]
---

## Description

Port both MCP adapters from
`/workspace/@alkdev/alknet/crates/alknet-http/src/adapters/{from_mcp/`,
`to_mcp.rs}` behind the `mcp` feature (rmcp, streamable HTTP only —
ADR-037). from_mcp: tools/list discovery, per-tool HandlerRegistration
(Mutation, HandlerKind::Once, FromMCP leaf), structuredContent-preferred
output handling, ContentBlock-union fallback, isError→CallError.
to_mcp: 4 fixed gateway tools (search/schema/call/batch — Sub AND Pub
excluded), StreamableHttpService into axum under /mcp with bearer
middleware. Shared dispatch spine with to_openapi via GatewayDispatch.

## Acceptance Criteria

- [x] Both adapters behind `mcp`; default features compile without rmcp
- [x] from_mcp round-trip against a mock MCP server (tools/list, tools/call, structured + content-block outputs)
- [x] to_mcp: 4 tools listed; call dispatch filtered by ACL; Pub/Sub ops excluded
- [x] stdio transport not present anywhere (dependency audit)
- [x] `cargo test --all-features` passes

## References

- docs/architecture/http-mcp.md
- docs/architecture/decisions/037-mcp-stdio-transport-exclusion.md, 041-mcp-tool-gateway-pattern.md
- Old source: `/workspace/@alkdev/alknet/crates/alknet-http/src/adapters/from_mcp/`, `to_mcp.rs`, `tests/from_mcp_integration.rs`

## Notes

Port split internally as from_mcp/ (dir: mod.rs + tests.rs) + to_mcp.rs,
mirroring the old crate layout. Streamable HTTP only (rmcp 1.8,
default-features off; client + server + streamable-http transport
features only — no stdio anywhere in the feature set).

## Summary

Ported both MCP adapters behind the `mcp` feature (rmcp 1.8):

- `src/adapters/from_mcp/` (mod.rs + tests.rs): tools/list discovery
  over StreamableHttpClientTransport; per-tool HandlerRegistration
  (Mutation, HandlerKind::Once, FromMCP leaf, Internal, ADR-015/022);
  structuredContent-preferred output, ContentBlock-union fallback
  schema (5 variants), isError -> MCP_TOOL_ERROR CallError with
  content blocks as details; bearer token via capabilities key "mcp"
  (ADR-014). 19 unit tests + 5 integration tests vs a real rmcp
  streamable-HTTP server (tests/from_mcp_integration.rs, gated on the
  mcp feature): tools round-trip incl. structured + legacy
  content-block outputs, no-env-vars, unreachable-server error
  classification.
- `src/adapters/to_mcp.rs`: ToMcpGateway with exactly 4 gateway tools
  (search/schema/call/batch — ADR-041), Sub ops excluded from search
  listings (op_type wire strings "sub"/"subscription"/"Sub"), call
  dispatch through the shared GatewayDispatch spine with
  ACL-filtered FORBIDDEN/NOT_FOUND mapping, batch returns per-call
  result array. Identity survives rmcp framing: bearer_auth_middleware
  stashes Option<Identity> in http::request::Parts extensions;
  extract_identity reads it back from RequestContext extensions.
  StreamableHttpService nested at /mcp in build_router (feature-gated)
  with bearer middleware around it. 19 tests.
- `GET|POST /mcp` reserved path unchanged; default build compiles
  without rmcp (cargo tree -e normal: 0 rmcp occurrences).

218 lib tests (default 182 / all-features 218) + 5 MCP integration +
10 WS integration green. clippy -D warnings both feature sets, fmt.