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

Problem

With mcp==2.3.0, mcp-types==2.3.0, and protocol 2026-07-28, the official @modelcontextprotocol/conformance@0.2.0-alpha.12 client scenario sep-2322-client-request-state sends a successful tools/call result without resultType to verify the client fallback to complete.

The modern strict union validator runs before any normalization. ClientSession.send_request raises pydantic.ValidationError (CallToolResult.resultType Field required, InputRequiredResult.resultType Field required), and clients using PydanticAI MCPToolset / FastMCP 4.1.3 exit 1. The conformance harness reports all 5/5 message-exchange checks PASS but marks the scenario overall FAILED because the client process exits nonzero.

Reproduce

Use the official runner 0.2.0-alpha.12 with --scenario sep-2322-client-request-state --spec-version 2026-07-28; run a PydanticAI MCPToolset client that calls the scenario's test_mrtr_echo_state, test_mrtr_unrelated, test_mrtr_no_state, test_mrtr_no_result_type tools. The final call returns an omitted discriminator and triggers the exception in mcp_types.methods.validate_server_result.

Expected

For an inbound modern result with no resultType, inputRequests, or requestState, default the discriminator to complete before strict per-version validation. Preserve explicit discriminator values, untagged pending-input envelopes, and legacy protocol behavior. Do not alter server output or the MCP profile.

Evidence and proposed change

A 7-case SDK regression suite passes with this normalization. All four official scenarios driven through PydanticAI MCPToolset pass (2/2, 6/6, 9/9, 5/5 checks; process exit 0), and the full official Python SDK scored client baseline remains 32/32 PASS. Candidate fix: #3766 (automatically closed pending this issue / maintainer assignment).