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Slice 4d.4: namespaced MCP tool forwarder (DoD #3 dispatch half) - #294
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Pal Lakatos-Toth (pallakatos) merged 1 commit intoMay 13, 2026
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Closes Slice 4 DoD #3 dispatch half: /mcp now forwards tool calls to upstream McpServer.spec.url instead of serving only the in-tree EchoDispatcher. Producer (controller): - McpServerMeta gains url + allowed_tools fields, written into meta.json alongside the OAuth metadata from Slice 4d.3. Consumer (router): - New mcp::forwarder module with RouterToolDispatcher implementing AsyncToolDispatcher. - Startup discovery POSTs tools/list to each upstream, filters through allowed_tools ("*" exposes all; explicit list selects named subset; empty fails closed), prefixes each tool with snake_case(server_name). - Servers with empty URL, empty allow-list, outbound OAuth requirement (deferred to 4d.5), unreachable upstream, or HTTP errors are recorded in skipped[] with operator-actionable reason and excluded from the catalog. Router still starts. - Dispatch splits name on first '.', looks up server, forwards tools/call to upstream URL. JSON-RPC errors collapse to is_error:true; 5xx surfaces as DispatchError::ExecutionFailed; bad names surface as DispatchError::UnknownTool. - McpRouteState gains with_tools() builder; build_mcp_router (now async) swaps the dispatcher when registry is non-empty. Scope: - Unauthenticated upstreams only. Outbound OAuth is Slice 4d.5 (requires per-server credential source). Servers requiring bearer credentials are deliberately skipped per §5 (no scaffolding — only ship the consumer we can drive end-to-end). - Single tools/list page (multi-page support deferred until a real upstream hits the cap). Test coverage: 15 new unit tests covering every skip reason, both allow-list semantics, namespaced dispatch (single + multi server), upstream JSON-RPC errors, HTTP 5xx, and unknown-tool branches. Backward compatibility: registries with no meta.json or no url fall through to the existing EchoDispatcher unchanged. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Summary
Closes the dispatch half of Slice 4 DoD #3 ("per-server JWKS + namespaced tools"). The OAuth half landed in PR #293 (Slice 4d.3); this PR wires the actual upstream forwarder so
/mcpno longer ships only the in-treeEchoDispatcher.When the registry advertises at least one
McpServerwith a usable upstream URL,RouterToolDispatcherdiscovers each upstream'stools/listat startup, filters throughspec.allowedTools, and exposes the surviving tools under a{server_snake_case}.{tool}namespace. Tool calls are then forwarded to the matching upstream URL.Wire contract — what the controller emits
meta.json(per-server ConfigMap) gains two fields alongside Slice 4d.3's OAuth metadata:urlMcpServer.spec.urlallowedToolsMcpServer.spec.allowedTools[\"*\"]exposes all)Wire contract — what the router consumes
`mcp::registry::DiscoveredMcpServerMeta` matches the producer shape, and the new `mcp::forwarder::RouterToolDispatcher`:
Out-of-scope (Slice 4d.5)
Test coverage (15 new unit tests)
Every skip reason is asserted with the expected substring (operator-actionable). The happy path runs against a real `axum::Router` mock upstream — full JSON-RPC `tools/list` → `tools/call` round-trip through the dispatcher.
Negative branches:
Multi-server dispatch is asserted with two distinct mock upstreams — each invocation lands at the correct URL.
Backward compatibility
Registries without `meta.json` files or with empty `url` fall through to the existing `EchoDispatcher`. The single-issuer `MCP_JWKS_PATH` legacy path remains as the third fallback.
Verification
Slice 4 DoD scoreboard after this PR
Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com