Implement SEP-986: Tool name validation #1655
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Add validation for tool names according to SEP-986 specification. Tool names are now validated at registration time in both FastMCP and low-level server: - Names must be 1-128 characters - Only A-Za-z0-9._- characters allowed - Warnings issued for non-conforming names (doesn't block registration) - Leading/trailing dots or dashes trigger advisory warnings Validation is implemented in mcp.shared.tool_name_validation module with comprehensive test coverage. Github-Issue:#1537
pcarleton
reviewed
Nov 24, 2025
Co-authored-by: Paul Carleton <paulcarletonjr@gmail.com>
pcarleton
reviewed
Nov 24, 2025
| TOOL_NAME_REGEX = re.compile(r"^[A-Za-z0-9._-]{1,128}$") | ||
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| # SEP reference URL for warning messages | ||
| SEP_986_URL = "https://github.com/modelcontextprotocol/modelcontextprotocol/issues/986" |
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| SEP_986_URL = "https://github.com/modelcontextprotocol/modelcontextprotocol/issues/986" | |
| SEP_986_URL = "https://modelcontextprotocol.io/specification/2025-11-25/server/tools#tool-names" |
pcarleton
reviewed
Nov 24, 2025
| digits (0-9), underscore (_), dash (-), and dot (.). | ||
| Tool names SHOULD NOT contain spaces, commas, or other special characters. | ||
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| See: https://modelcontextprotocol.io/specification/draft/server/tools#tool-names |
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| See: https://modelcontextprotocol.io/specification/draft/server/tools#tool-names | |
| See: https://modelcontextprotocol.io/specification/2025-11-25/server/tools#tool-names |
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Motivation and Context
modelcontextprotocol/modelcontextprotocol#986
Implements tool name validation according to SEP-986 specification. Tool names are validated at registration time in both FastMCP and low-level server paths, issuing warnings for non-conforming names without blocking registration.
Taking similar approach to reference implementation in TS: modelcontextprotocol/typescript-sdk#900
How Has This Been Tested?
Unit tests - 29 new tests for the validation module covering:
All existing tests continue to pass.
Breaking Changes
None, but console warnings may appear for SDK users who have tool names that do not satisfy the SEP.
Types of changes
Checklist