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fix(ai-mcp): send Mcp-Param headers on spec 2026 tool calls - #1871
AlemTuzlak merged 1 commit into
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Spec 2026 tools/call went through a raw transport send, so the SDK never added the Mcp-Param-* headers from the x-mcp-header schema fields. Route those calls (and the input_required retry) through Client.callTool with the listed input schema, and delete the raw request path. Closes #1794
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Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation | Resolution |
|---|---|---|---|
| Docstring Coverage |  |
Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 4 files. (1 skipped: … | Write docstrings for the functions missing them to satisfy the coverage threshold. |
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Check name Status Explanation Title check The title clearly and concisely describes the main change: sending Mcp-Param headers for spec 2026 tool calls. Description check The description explains the root cause, implementation, testing, release impact, risk, rollback, and linked issue. It includes the required sections and a changeset. The local test checklist remains … Linked Issues check Issue [ #1794] requires schema-drivenMcp-Param-*headers for modern tool calls.callMcpToolnow usesClient.callTooland passestoolDefinition.inputSchema.tools()andMCPClient.callTool()…Out of Scope Changes check The reviewed changes stay within issue [ #1794]. They replace the raw modern request path, preserve input-required retry behavior, pass schemas through tool construction, and add regression coverage. S…Full details: Description check
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Explanation
The description explains the root cause, implementation, testing, release impact, risk, rollback, and linked issue. It includes the required sections and a changeset. The local test checklist remains unchecked, but the testing section clearly states which tests were not run and that CI runs them.
Full details: Linked Issues check
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Explanation
Issue [
#1794] requires schema-drivenMcp-Param-*headers for modern tool calls.callMcpToolnow usesClient.callTooland passestoolDefinition.inputSchema.tools()andMCPClient.callTool()pass the discovered schema, and explicit tool definitions pass their schema. The new strictStreamableHTTPClientTransporttests verifyMcp-Param-Region: eufor both paths. The input-required retry uses the same SDK call path.
Full details: Out of Scope Changes check
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Explanation
The reviewed changes stay within issue [
#1794]. They replace the raw modern request path, preserve input-required retry behavior, pass schemas through tool construction, and add regression coverage. SDK timeout, abort, and progress handling follow from usingClient.callTool. The changeset supports the published package fix. No unrelated source, test, or documentation changes are present.
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Full details: Docstring Coverage
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Explanation
Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 4 files. (1 skipped: 1 unsupported.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
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commit: |
Spec 2026-07-28 tool calls from
@tanstack/ai-mcpdid not send theMcp-Param-*headers that a tool'sx-mcp-headerschema fields ask for. Strict servers, such as GitHub's remote MCP server, reject those calls with HTTP 400 and error-32020. This PR sends spec 2026 tool calls through the SDKClient.callToolwith the listed input schema, so the SDK adds the headers.🎯 Changes
callMcpToolsends spec 2026 calls, and theinput_requiredretry, throughclient.callTool. It passestoolDefinition: { name, inputSchema }.tools(),tools([defs]), andMCPClient.callTool()pass the listed tool'sinputSchemato the call.transport.sendpath (rawRequest, the transport tap, and the_metaenvelope copy). The SDK now does timeout, abort, and progress for these calls.@tanstack/ai-mcp. No docs change: no page describes this behavior.✅ Checklist
pnpm run test:pr, or these tests do not apply to this pull request.docs/for this change, or this change is not user-facing.pnpm changeset), or this PR does not change a published package.🚀 Release Impact
Root cause
Issue. On a spec 2026-07-28 connection, a tool with an
x-mcp-headerargument fails on strict servers. TheMcp-Param-*header is missing, so the server returns 400 /-32020. Tools without those arguments work, so a basic smoke test does not show the bug.Cause.
callMcpToolinpackages/ai-mcp/src/tools.tssent spec 2026tools/callthroughrawRequest, which callstransport.send(message)directly. OnlyClient.callToolbuilds theMcp-Param-*headers from the tool schema. The raw path skipped it. The SDK does not export its header builder.Fix. Spec 2026 calls go through
Client.callToolwithtoolDefinition: { name, inputSchema }. The SDK readsx-mcp-headerfrom that schema and sends the headers. The definition has nooutputSchema, so the SDK skips its output check. That check would reject aninput_requiredresult.Possible alternatives
listTools()on spec 2026, and callcallToolwith notoolDefinition. This is the spec 2025 path. The SDK output check then runs and rejects aninput_requiredresult from a tool with anoutputSchema.rawRequestand pass headers totransport.send(message, { headers }). The SDK does not exportbuildMcpParamHeaders, so this copies the SDK header and encoding rules. More code, and it can drift from the SDK.Testing
Commands run
createMCPClient→tools()→execute({ region: 'eu' }), assert theMcp-Param-Regionheader):main(3e84606af): fails.tsc --noEmitonpackages/ai-mcp: no new errors in the changed files.pnpm test:prand the fullai-mcpunit suite. CI runs them.Manual test
main, connectcreateMCPClientto a spec 2026 server whose tool declaresregion: { type: 'string', 'x-mcp-header': 'Region' }. Call the tool. The request has noMcp-Param-Regionheader.Mcp-Param-Region: eu, and a strict server accepts it.How this PR makes testing easy
tests/client.test.ts→spec 2026 Mcp-Param headers: a strict fixture server rejects a missing or wrong header. It coverstools()andMCPClient.callTool().tests/tools.test.ts→callMcpTool — spec 2026 calls: checks thetoolDefinitionand the signal passed to the SDK, and theinputResponses/requestStateretry.Linked issues
Closes #1794
Risk / rollback
Low to medium. Spec 2026 tool calls now use the SDK request path, not a raw send. Timeout, abort, and progress now come from the SDK. In a browser, the SDK does not send
Mcp-Param-*headers (the SDK decides this). Rollback: revert this PR.Summary by CodeRabbit
Mcp-Param-*headers for arguments designated by the server, improving compatibility with the 2026-07-28 MCP specification.