MCP x-mcp-header validation is easy to miss because the annotation looks like ordinary JSON Schema metadata. On the 2026-07-28 Streamable HTTP transport, it is a wire contract: the client copies selected tool arguments into Mcp-Param-* headers, intermediaries can act on those headers, and the se...
Source: [Dev.to](https://dev.to/ssukhpinder/mcp-x-mcp-header-validation-keep-bad-tool-schemas-out-of-toolslist-3j3d)