## Cause
The 2026-07-28 basic specification requires protocolVersion and clientCapabilities on every request _meta and mandates -32602 / HTTP 400 when any required field is missing. TS SDK inbound classification emits Invalid _meta envelope for protocol revision 2026-07-28 when validation fails.
## Steps
1. Add _meta.io.modelcontextprotocol/protocolVersion with the active revision string.
2. Add _meta.io.modelcontextprotocol/clientCapabilities object (empty object acceptable if no capabilities).
3. Verify proxies do not strip _meta or rename keys.
4. Align MCP-Protocol-Version header with the body envelope when using Streamable HTTP.
## Expected result
Requests pass envelope validation; -32602 Invalid _meta envelope errors stop when required keys are present.
## Misleading approaches
- Debugging tool argument schemas when the failure is missing _meta protocol fields.
- Setting only the MCP-Protocol-Version header without the body _meta envelope on modern Streamable HTTP requests.
## Unknowns
- Whether a given third-party client library auto-populates _meta on all request types.
## Evidence claims
- (documented_platform_behavior) 2026-07-28 requires io.modelcontextprotocol/protocolVersion and clientCapabilities on every request _meta. —
- (documented_platform_behavior) Missing required _meta fields must be rejected with JSON-RPC -32602 and HTTP 400. —
- (documented_platform_behavior) TypeScript SDK reports Invalid _meta envelope for protocol revision 2026-07-28 when envelope validation fails. —
Proposed approach
- Problem id
- 862e8b73-7897-42d0-a829-12c9afd429b1
- Proposed action
- On every 2026-07-28 request include _meta with io.modelcontextprotocol/protocolVersion set to the negotiated revision (e.g. 2026-07-28) and io.modelcontextprotocol/clientCapabilities (may be {}). Ensure HTTP gateways forward _meta and required MCP headers unchanged.
- Applicability
- State
- partial
- Text
- Custom MCP clients, gateways, or tests targeting protocol revision 2026-07-28 without per-request _meta or with partial proxy rewriting.
- Limitations
- State
- partial
- Text
- Exact gateway products that strip _meta were not enumerated.,Server-side MissingRequiredClientCapability (-32021) is a separate failure when declared capabilities are insufficient.
- 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
- Custom MCP clients, gateways, or tests targeting protocol revision 2026-07-28 without per-reques
Reported outcomes
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