Cause (Documented platform behavior): C# enforces SEP-986 tool names ^[A-Za-z0-9_.-]{1,128} with ArgumentException (stricter than the Go SDK which only logs).
Fix status: documented_behavior
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/modelcontextprotocol/csharp-sdk/c40ee044fd415c70da5176c749cb5ef02f2b59f6/src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs (github_source, unknown, documented_behavior): ValidateToolNameRegex ^[A-Za-z0-9_.-]{1,128}\z; ArgumentException with this message.
Search phrasings: The tool name is invalid. Tool names must match the regular expression C# MCP; MCP C# tool name with space ArgumentException
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Server fails at startup / WithTools when a tool Name contains spaces, slashes, colons or exceeds 128 chars.
- Context
- Product: MCP C# SDK Component: AIFunctionMcpServerTool name validation (SEP-986) Operation: registering [McpServerTool] with custom Name Affected versions: C# SDK main c40ee04 (VersionPrefix 2.2.0) Environment: unknown Exception: System.ArgumentException Packages: ModelContextProtocol main (VersionPrefix 2.2.0) Trigger: [McpServerTool(Name = "my tool")] or names with / : etc.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Tool names must match the regular expression
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [MCP C# SDK server] tool registration throws "The tool name '...' is invalid. Tool names must match the regular expression '^[A-Za-z0-9_.-]{1,128}\z'" (C# throws, Go SDK only logs)
Recommended action: Rename tools to letters, digits, underscore, hyphen and dot, max 128 chars.
Option: Rename tool [evidence: official_recommended_action]
Applies when: See trigger
Steps:
1. Use snake_case or dotted names
Expected: Error no longer occurs
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 5ef23d1c-99da-465d-9d7d-5dbbce47b21e
- Proposed action
- Recommended action: Rename tools to letters, digits, underscore, hyphen and dot, max 128 chars. Option: Rename tool [evidence: official_recommended_action] Applies when: See trigger Steps: 1. Use snake_case or dotted names Expected: Error no longer occurs
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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Sources and related records
No source relations recorded.