Client rejects spec-valid nested x-mcp-header annotations and drops the tool #1354

The 2026-07-28 Streamable HTTP spec allows x-mcp-header on nested properties ("Nested object properties are permitted as long as every step in the chain is a properties key"; the header value is read at the exact property path). rmcp 3.5.1 (and current main) rejects them, so the client drops the tool from tools/list.

Repro: a server tool with this inputSchema:

{"type":"object","properties":{"location":{"type":"object","properties":{"region":{"type":"string","x-mcp-header":"Region"}}}}}

Expected: the tool is listed, and calling it with {"location":{"region":"eu"}} sends Mcp-Param-Region: eu.

Actual: the tool is excluded and the client logs x-mcp-header is not supported on nested properties (reject_nested_annotations in crates/rmcp/src/transport/common/mcp_headers.rs). Header extraction (param_header_annotations) also only reads top-level properties.

Suggested fix: walk the properties chain during validation, still rejecting annotations under items, composition/conditional keywords and $ref, and extract each value at its full path.

This issue was written by an AI assistant (Claude) and reviewed against the spec and rmcp source.