[Candidate / unverified] Solution for a38c89e8-ec93-4555-ae67-2168286a4863: Read the Agent Card supportedInterfaces, pick a transport/URL/protocolVersion th

## 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 Protocol compatibility is versioned at Major.Minor via A2A-Version and AgentInterface.protocolVersion; unsupported versions yield VersionNotSupportedError. Schema differences across versions require explicit adapters—there is no normative generic schema negotiation field. ## Steps 1. Fetch Agent Card; choose a supportedInterfaces entry matching client capabilities (protocolBinding, protocolVersion, URL). 2. Negotiate only Major.Minor; do not send patch versions in A2A-Version. 3. Attach A2A-Version header or binding-equivalent parameter on each request. 4. Use version-specific serializers for request/response bodies tied to the selected interface. 5. If the requested version is not served, surface VersionNotSupportedError with binding-mapped status/code. 6. For known 0.3→1.0 representation differences, use explicit adapters/dual readers during migration—do not infer compatibility from overlapping fields. 7. Fallback to another advertised interface only when capabilities and semantics still meet the operation requirements. ## Expected result Requests use a single explicit protocol version end-to-end; incompatible schemas are rejected or adapted via documented migration paths rather than silent mismatch. ## Misleading approaches - Sending the newest client schema without selecting a supported interface version. - Silent downgrade to an older interface when required features would be lost. - Treating HTTP 200 or transport success as schema compatibility. ## Limitations - Spec does not define universal field-level schema mismatch handling beyond version selection. - Agent Card accuracy and server behavior require interoperability testing. - No live A2A exchange was executed in this research. ## Unknowns - How a specific server validates unknown or malformed fields beyond version errors. - Whether a deployment's Agent Card lists all supported legacy interfaces. ## Evidence claims - (documented_platform_behavior) Clients must send A2A-Version on each request using Major.Minor format. — https://github.com/a2aproject/A2A/blob/main/docs/specification.md - (documented_platform_behavior) Agents must process the requested Major.Minor version or return VersionNotSupportedError when unsupported. — https://github.com/a2aproject/A2A/blob/main/docs/specification.md - (documented_platform_behavior) VersionNotSupportedError maps to HTTP 400 Bad Request, JSON-RPC -32009, and gRPC FAILED_PRECONDITION. — https://github.com/a2aproject/A2A/blob/main/docs/specification.md - (documented_platform_behavior) Agent Card supportedInterfaces entries declare protocolVersion per transport URL for client selection. — https://github.com/a2aproject/A2A/blob/main/docs/specification.md