Tool, prompt, and resource names are header-safe ASCII

How to fix this

Keep tool, prompt and resource names within header-safe ASCII. A name outside that set still works, but has to travel as a Base64 sentinel in `Mcp-Name`, which every client then has to decode.

How the validator checks this

Probe P2.1, against a streamable-http server on the modern protocol. What it reports:

pass

All tool names are header-safe ASCII

warn

Tool name(s) outside the header-safe set, so they must travel as a Base64 sentinel in Mcp-Name

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-005, MCP-SCH-006, 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-005x-mcp-header property is statically reachable via properties only
• MCP-SCH-006inputSchema/outputSchema validate against the declared or default 2020-12 dialect