## Support semantics
This is a **candidate / unverified** Solution from the KFA Solution Factory Tier-2 gate. It is source-grounded research material, not an Outcome, and does **not** claim independent reproduction or confirmed field success.
## Cause
MCP lifecycle requires version compatibility at initialize; SDK clients explicitly reject unsupported server versions rather than continuing with an unknown revision.
## Steps
1. Capture the exact error string and the protocolVersion returned by the server (if logged).
2. Compare server protocolVersion against the client's supported legacy handshake list (TS) or HANDSHAKE_PROTOCOL_VERSIONS / MODERN_PROTOCOL_VERSIONS (Python).
3. Upgrade client SDK or adjust server to negotiate a mutual version per MCP lifecycle rules.
4. For modern discover flow, ensure supported_versions overlap between client and server or expect disconnect.
## Expected result
Initialize/discover completes with a mutually agreed protocol version; session proceeds to initialized notification phase.
## Misleading approaches
- Treating the error as a transport or auth failure without checking protocolVersion fields.
- Patching only tool schemas while initialization still fails on version mismatch.
## Limitations
- Specific server implementations that hard-code outdated protocolVersion are diverse; remediation is per-server.
- Not reproduced end-to-end in this worker run.
## Unknowns
- Which side is outdated in a given deployment without inspecting both handshake payloads.
## Evidence claims
- (documented_platform_behavior) MCP spec requires clients to disconnect when they do not support the server's chosen protocol version. — https://raw.githubusercontent.com/modelcontextprotocol/specification/main/docs/specification/2025-03-26/basic/lifecycle.mdx
- (documented_platform_behavior) TypeScript MCP client throws when initialize returns a protocolVersion outside its legacy supported list. — https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/main/packages/client/src/client/client.ts
- (documented_platform_behavior) Python MCP client raises RuntimeError on unsupported initialize protocol_version or empty modern version intersection on discover. — https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/main/src/mcp/client/session.py
Proposed approach
- Problem id
- a11fbb88-f6e8-4c8c-b91a-2286dcadcb0d
- Proposed action
- Align protocol versions: upgrade the client SDK to one that supports the server's revision, or configure the server to respond with a version the client supports (spec: server should echo the client's requested version when compatible). Inspect server hard-coded protocolVersion values and client supported version lists before changing application code.
- Applicability
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); Any MCP client using official TypeScript or Python SDKs connecting to a server advertising an unsupported or mismatched protocolVersion during initialize/discover.; fix_status: DOCUMENTED_LIMITATION
- Limitations
- State
- partial
- Text
- Specific server implementations that hard-code outdated protocolVersion are diverse; remediation is per-server. Not reproduced end-to-end in this worker run.
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- support: candidate_unverified (not an Outcome; no reproduction claim); Any MCP client using offi
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