MCP compatibility

GaugeMesh uses the official Rust MCP SDK rmcp 3.1.4. The dependency is exact-pinned in Cargo.toml. GaugeMesh is both a downstream-facing MCP server and an upstream-facing MCP client.

Revision	Lifecycle	stdio client/server	Streamable HTTP client/server	Tools	Resources/templates	Prompts	Tasks	Cache/result metadata	MRTR
2025-11-25	`initialize` session	tested	tested	list/call	list/templates/read	list/get	not advertised	TTL and private scope	SDK relay primitives
2026-07-28	`server/discover`, self-contained request metadata	tested	tested	list/call	list/templates/read	list/get	conditional durable route	TTL, cache scope, result `_meta`	SDK relay primitives

The 2026 path uses server/discover and supplies the protocol version and client capabilities on each request. The 2025 path uses the legacy initialization session. GaugeMesh does not infer a stateless request’s revision from an earlier request. Its adapter rejects header/body disagreement. The SDK validates current MCP-Protocol-Version, Mcp-Method, Mcp-Name, _meta, client/server identity, capabilities, and response metadata rules.

The Tasks extension is advertised only when SQLite task-route storage and at least one reviewed, capability-pinned 2026-07-28 upstream advertising Tasks are both present. An unpinned source retains ordinary-call compatibility but cannot authorize this durable task path. A downstream caller must also declare Tasks support. GaugeMesh gives the caller a public task ID, keeps the upstream ID private to the durable route, and forwards get and cancellation without automatic resubmission. Interactive tasks/update is rejected with GM_TASK_UPDATE_UNSUPPORTED_DURABLE because this route schema has no durable update intent and receipt. See DURABLE_TASKS.md for the protocol, recovery, and non-claim boundaries.

The transport tests exercise a real RMCP client against GaugeMesh over Tokio duplex, a real TCP Streamable HTTP listener, and an actual child process over stdio. They enumerate tools, resources, resource templates, and prompts, and then cancel and join their owned transport.

Limitations for 0.1:

The official conformance inventory is recorded in CONFORMANCE.md; an SDK claim is not a GaugeMesh conformance or certification claim.
Upstream change notifications are represented by immutable capability revision and schema snapshots in the federation. Live notification fan-out is not yet implemented.
Current MRTR wire handling comes from RMCP. Unsupported bridge requests return explicit errors in the model-broker layer rather than disappearing.
add mcp performs live bounded discovery, pins the complete capability-manifest digest, and writes configuration. serve loads configured sources transactionally into the bounded runtime; startup and restart reject identity drift.

Federation modes

Transparent mode uses stable readable aliases such as docs-a__search; policy and leases always authorize the full capability identity, never the alias. Compressed mode exposes five fixed meta-tools: search, describe, lease, invoke, and release. A sixth, gaugemesh_submit, is listed only for a 2026-07-28 caller that declares Tasks support while a durable task route is available. Discovery is deterministic lexical ranking and does not require an embedding model or LLM. Neither mode is claimed to be universally better; transparent mode remains the compatibility fallback for clients that do not reliably use meta-tools.

Resources use gaugemesh://resource/<source>/<opaque-id> and retain an internal mapping to the exact native URI. Composite cursors are HMAC-protected and bounded to 4 KiB. Prompts carry the same source, schema, revision, and configuration identity as tools and resources.