MCP-SCH-005
x-mcp-header property is statically reachable via properties only
“The chain MUST NOT pass through items (or any other array keyword), composition keywords (oneOf, anyOf, allOf, not), conditional keywords (if/then/else), or $ref.” — spec anchor
How to fix this
Consequence, not a separate rule: a violation makes the WHOLE tool definition invalid, and conforming clients MUST exclude it from tools/list — the tool silently disappears from every compliant client.
How the validator checks this
Probe P2.1, against a streamable-http server on the modern protocol. What it reports:
- pass
- Every x-mcp-header property is statically reachable via properties
- fail
- x-mcp-header must not be reached through
Quoted from the probe that runs this check, so it cannot drift from what the validator actually reports.
This rule is checked deterministically: a fail here is a certain violation, not an inference.
Checked in the same request as MCP-HDR-008, MCP-HDR-009, MCP-RES-003, MCP-RES-006, MCP-RES-007, MCP-ERR-003, MCP-ERR-004, MCP-ERR-005, MCP-ERR-008, MCP-CAC-001, MCP-CAC-002, MCP-CAC-003, MCP-CAC-004, MCP-CAC-005, MCP-SCH-001, MCP-SCH-002, MCP-SCH-003, MCP-SCH-004, MCP-SCH-006, MCP-SCH-007, MCP-SEC-004, MCP-SEC-005, MCP-SEC-006, MCP-DEP-005.
Check your own server against this rule
The validator makes real protocol requests and reports this rule as pass, warn or fail alongside the other 78. Validate a server or read how the check works.
Other Tool/prompt schema and x-mcp-header rules
- MCP-SCH-001x-mcp-header is non-empty and matches RFC 9110 token syntax
- MCP-SCH-002x-mcp-header contains no control characters, CR, or LF
- MCP-SCH-003x-mcp-header values are case-insensitively unique per inputSchema
- MCP-SCH-004x-mcp-header only on string/integer/boolean — never number
- MCP-SCH-006inputSchema/outputSchema validate against the declared or default 2020-12 dialect
- MCP-SCH-007Tool, prompt, and resource names are header-safe ASCII