feat(sep-2243): add x-mcp-header static-reachability checks#352
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Upstream spec PR modelcontextprotocol/modelcontextprotocol#2891 tightens the x-mcp-header placement rule: the annotation MUST only appear on properties statically reachable from the inputSchema root via a chain of `properties` keys, never through `items`, `oneOf`/`anyOf`/`allOf`/`not`, `if`/`then`/`else`, or `$ref`. It also clarifies that header values are extracted at the annotated property's exact properties-path. - http-invalid-tool-headers: serve seven new invalid tools (one per disallowed placement keyword) under new check ID `sep-2243-x-mcp-header-statically-reachable`; clients MUST reject each. - http-custom-headers: add `test_custom_headers_nested` with x-mcp-header on `properties.outer.properties.inner` and assert under new check ID `sep-2243-client-extract-nested-path` that the client sends Mcp-Param-Nested when outer.inner is present and omits it when absent. - sep-2243.yaml: add traceability rows for both new check IDs with verbatim spec text from #2891. - tests: extend the conforming-client path to call the nested tool and add negative cases for nested-header-missing and invalid_under_items-called. Claude-Session: https://claude.ai/code/session_01Gbqx5LodtAghT5fRYFxeja
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Summary
Upstream spec PR modelcontextprotocol/modelcontextprotocol#2891 tightens the
x-mcp-headerplacement rule for SEP-2243: the annotation MUST only appear on properties that are statically reachable from theinputSchemaroot via a chain ofpropertieskeys. The chain MUST NOT pass throughitems,oneOf/anyOf/allOf/not,if/then/else, or$ref. It also clarifies that header values are read at the annotated property's exact properties-path, and omitted when no value is present there.This PR adds conformance coverage for both halves.
New check IDs
sep-2243-x-mcp-header-statically-reachable—http-invalid-tool-headersnow serves seven additional invalid tool definitions, each placingx-mcp-headerunder one of the disallowed keywords (items,oneOf,anyOf,allOf,not,then,$ref). Clients MUST exclude every one of them.sep-2243-client-extract-nested-path—http-custom-headersnow servestest_custom_headers_nestedwithx-mcp-headeronproperties.outer.properties.inner. The scenario asserts the client sendsMcp-Param-Nestedwhenouter.inneris present and omits it when absent.What changed
src/scenarios/client/http-custom-headers.ts— new tool fixtures, declared-ID lists,INVALID_TOOL_CONSTRAINT_IDSentries, and the nested-tool call handler.src/scenarios/client/http-custom-headers.test.ts— extends the conforming-client path to call the nested tool (present + absent), and adds negative cases for nested-header-missing andinvalid_under_itemscalled.src/seps/sep-2243.yaml— traceability rows for both new check IDs with verbatim spec text from #2891.Verification
npx tsc --noEmit— cleannpm run lint— cleannpm test— 295 passedhttps://claude.ai/code/session_01Gbqx5LodtAghT5fRYFxeja