feat: support SEP-2575 make MCP stateless by Yuan325 · Pull Request #3211 · googleapis/mcp-toolbox

Yuan325 · 2026-05-11T22:42:16Z

This PR also updates our custom auth error code since it contradicts with some new errors added in this SEP.:

// Custom auth error codes
UNAUTHORIZED = -401
FORBIDDEN    = -403

For stdio transport:

If _meta.io.modelcontextprotocol/protocolVersion was provided, it will use the protocol version specified (take precedence over existing version).
Else, it will use the version during initialization method.
If no initialization method was called, it will default to v2024-11-05 similar to Toolbox's HTTP transport. Toolbox had a custom transport on v2024-11-05 --> "regular" HTTP POST (without SSE), that does not enforce the initialize method

For HTTP transport:

Use the version that was specified in header as protocol version.
If it specified the DRAFT version, it will check against _meta.io.modelcontextprotocol/protocolVersion to validate.

gemini-code-assist

Code Review

This pull request introduces support for the MCP stateless protocol version (draft), adding the server/discover method and refactoring JSON-RPC structures to handle per-request metadata like protocol versions and client information. Review feedback highlights several critical issues: JSON field shadowing in request structs that may break unmarshaling, a missing case for the draft version in the method processing loop, and a duplicate error code. Furthermore, the metadata validation logic requires adjustments to maintain backward compatibility with clients not using the new metadata fields and to ensure negotiated protocol versions are not inadvertently overwritten.

Yuan325 · 2026-05-12T03:09:55Z

/gemini review

gemini-code-assist

Code Review

This pull request implements support for the MCP 2026 protocol draft, including metadata validation for protocol versions and client information. It refactors the JSON-RPC request structures to include a standardized _meta object and introduces the server/discover method. Feedback highlights a critical potential nil pointer dereference in the protocol validation logic and a conflict where a new error code duplicates an existing one.

Yuan325 · 2026-05-12T03:18:36Z

/gemini review

gemini-code-assist

Code Review

This pull request introduces support for the Model Context Protocol (MCP) v2026+ by implementing protocol version negotiation, client information validation, and a new server/discover method. It also refactors JSON-RPC message structures to support per-request metadata and adds a standard error for unsupported protocol versions. Feedback includes a critical fix for a colliding error code, the removal of a redundant string check, and suggestions to eliminate code duplication by utilizing shared types from the jsonrpc package.

Yuan325 · 2026-05-13T18:29:49Z

/gemini review

gemini-code-assist

Code Review

This pull request implements support for the MCP 2026 draft protocol, introducing the server/discover method and per-request metadata validation for protocol versions, client info, and capabilities. It refactors JSON-RPC structures to support the _meta field and updates error handling for protocol mismatches. Feedback identifies opportunities to improve HTTP status code mapping for JSON-RPC errors, remove a redundant protocol version check, and correct a header name in an error message.

Yuan325 · 2026-05-19T23:48:18Z

/gemini review

gemini-code-assist

Code Review

This pull request introduces support for the MCP draft protocol version, replacing the initialization and ping methods with a new server discovery mechanism. It refactors JSON-RPC request and error structures to support per-request metadata validation, including protocol version and client capabilities. Feedback suggests optimizing performance by avoiding redundant JSON unmarshaling in the draft method processor and improving error handling by replacing fragile string-based checks with structured error codes.

averikitsch · 2026-06-12T19:11:24Z

 	case TOOLS_LIST:
-		return toolsListHandler(id, resourceMgr, toolset, body)
+		return toolsListHandler(id, resourceMgr, toolset, body, stdio)
 	case TOOLS_CALL:


Does toolCallHandler need the stdio boolean too? Will this error if header is nil accessToken := tools.AccessToken(header.Get(authTokenHeadername))?

Yuan325 requested a review from a team as a code owner May 11, 2026 22:42

blunderbuss-gcf Bot assigned duwenxin99 May 11, 2026

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Yuan325 force-pushed the sep-2575 branch 2 times, most recently from cb4b8a9 to dcf9c44 Compare May 12, 2026 03:09

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Yuan325 force-pushed the sep-2575 branch from dcf9c44 to 22bafd3 Compare May 12, 2026 03:18

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Yuan325 force-pushed the sep-2575 branch from 22bafd3 to 3477693 Compare May 13, 2026 18:22

Yuan325 force-pushed the sep-2575 branch from 3477693 to a1fca7e Compare May 13, 2026 18:31

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Yuan325 mentioned this pull request May 15, 2026

feat: add server conformance tests for SEP-2575 modelcontextprotocol/conformance#271

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Yuan325 force-pushed the move-initialize branch from 5b354e6 to f3c296a Compare May 19, 2026 19:57

Yuan325 changed the base branch from move-initialize to mcp-v202606 May 19, 2026 20:03

Yuan325 force-pushed the mcp-v202606 branch from 8349937 to 661cd26 Compare May 19, 2026 22:16

Yuan325 force-pushed the sep-2575 branch from a1fca7e to 2052a9f Compare May 19, 2026 22:20

Yuan325 force-pushed the mcp-v202606 branch from 661cd26 to 9797f9d Compare May 19, 2026 22:30

Yuan325 force-pushed the sep-2575 branch 4 times, most recently from 18fb620 to a60ede4 Compare May 19, 2026 23:44

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anubhav756 mentioned this pull request May 29, 2026

fix: align stateless protocol version rejection error with SEP-2575 spec #3318

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Yuan325 force-pushed the sep-2575 branch 2 times, most recently from c004bf7 to 1bbcc67 Compare May 29, 2026 23:07

Yuan325 force-pushed the mcp-v202606 branch from 9797f9d to 0fe87b5 Compare June 9, 2026 18:40

feat: support SEP-2575 make MCP stateless

257275c

Yuan325 force-pushed the sep-2575 branch from 1bbcc67 to 257275c Compare June 9, 2026 18:40

averikitsch reviewed Jun 12, 2026

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averikitsch approved these changes Jun 12, 2026

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Yuan325 commented May 11, 2026 •

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