Client 2026-07-28: missing resultType causes ValidationError despite required complete fallback #3767

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1. claude

   added

   bugSomething isn't workingSomething isn't working

   spec-2026-07-28Concerns the SDK's implementation of the 2026-07-28 MCP spec revisionConcerns the SDK's implementation of the 2026-07-28 MCP spec revision

   v2Affects the v2 line (2.x on main)Affects the v2 line (2.x on main)

   on Oct 26, 2026
2. Ben-Home commented on Oct 28, 2026

   

   Ben-Home

   on Oct 28, 2026 · Hidden as spam

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3. Gowthaman90 commented on Nov 4, 2026

   

   Gowthaman90

   on Nov 4, 2026

   More actions

   I hit this from a different direction, and I think it needs a decision on intended behaviour
   before a fix.

   I was scoring a stdio security proxy with my benchmark
   (mcp-defense-bench-py). When the proxy
   blocks a call, it writes its own {"content": [...], "isError": true} with no resultType, on
   a session that negotiated 2026-07-28 (the proxy forwards server/discover to a v2 server).
   FastMCP 4.1 on mcp 2.3.0 raises the ValidationError described here. I reported the missing
   tag to the proxy (vdalal/agentx-security-sdk#1).

   Two things point to rejection being the intended behaviour:

   • The 2026-07-28 schema says servers implementing that version MUST include resultType, and
     scopes the absent-means-complete fallback to servers on earlier versions.
     sep-2322 absent-resultType probe is an invalid stimulus on the 2026-07-28 wire it negotiates conformance#475 argues that the conformance probe this issue relies on
     is invalid on the 2026-07-28 wire it negotiates.
   • The TypeScript client (@modelcontextprotocol/client 2.4.0, versionNegotiation: { mode: "auto" })
     rejects the same reply: SdkError: Invalid result for tools/call: missing required resultType — servers implementing protocol revision 2026-07-28 MUST include it (the absent-means-complete bridge applies only to earlier-revision servers).

   If rejection stays, the Python error could be as direct as the TypeScript one. Today it reads
   2 validation errors for union[CallToolResult,InputRequiredResult], and its second line,
   InputRequiredResult.resultType Field required, sent me looking at input-required handling.
   The actual cause was a missing tag from a non-conformant intermediary.

   Minimal repro (plain-Python stdio server, no SDK, so the reply bytes are exact):

   repro.py

   """A 2026-07-28 tools/call result without `resultType`: what the Python client raises.
   
   The "server" is a few lines of plain Python speaking JSON-RPC over stdio, so the reply
   bytes are exact. It answers server/discover (2026-07-28) and returns an isError tool
   result with no resultType, the shape a stdio security proxy writes when it blocks a call.
   """
   import sys
   
   import anyio
   from mcp import Client, StdioServerParameters
   
   SERVER = r'''
   import json, sys
   for line in sys.stdin:
       msg = json.loads(line)
       if "id" not in msg:
           continue
       reply = {"jsonrpc": "2.0", "id": msg["id"]}
       if msg["method"] == "server/discover":
           reply["result"] = {"resultType": "complete", "supportedVersions": ["2026-07-28"],
                              "capabilities": {"tools": {}}, "ttlMs": 0, "cacheScope": "private"}
       elif msg["method"] == "tools/call":
           reply["result"] = {"content": [{"type": "text", "text": "blocked by policy"}], "isError": True}
       else:
           reply["error"] = {"code": -32601, "message": "Method not found"}
       print(json.dumps(reply), flush=True)
   '''
   
   
   async def main() -> None:
       async with Client(StdioServerParameters(command=sys.executable, args=["-c", SERVER])) as client:
           print("negotiated:", client.protocol_version)
           await client.call_tool("echo", {"text": "hi"})
   
   
   anyio.run(main)

   On a 2025-11-25 session the same untagged reply is accepted as complete, as the spec says.

   mcp 2.3.0, mcp-types 2.3.0, pydantic 2.14.2, Python 3.12.13, macOS arm64.

   AI disclosure: I used Claude Code to search the SDK source, write the repro and run it on my machine; I reviewed the reproduction and output and can answer questions on it.