Fix include_tags/exclude_tags ignored without tools in MCPConfig

[jlowin]

Closes #3177.

When a server config in mcpServers used include_tags or exclude_tags without also specifying tools, the tags were silently ignored. The config parsed as a plain RemoteMCPServer/StdioMCPServer (which absorb unknown fields via extra="allow") instead of the Transforming* variant that actually applies tag filtering.

The fix makes tools optional in _TransformingMCPServerMixin (defaulting to {}) and adds a model validator that requires at least one of tools, include_tags, or exclude_tags to be present — so plain configs still parse as base types, but any config with transform-related fields gets the right type:

config = {
    "mcpServers": {
        "my_server": {
            "url": "http://localhost:8003/mcp",
            "include_tags": ["production"],  # now works without "tools"
        }
    }
}

async with Client(config) as client:
    tools = await client.list_tools()  # only tools tagged "production"

[coderabbitai]

Walkthrough

The pull request includes documentation link updates and introduces validation logic for MCP server transformation configurations. In src/fastmcp/mcp_config.py, the _TransformingMCPServerMixin class is updated with a new pre-validation method that enforces at least one of the transform fields (tools, include_tags, or exclude_tags) to be provided when using this mixin. The tools field signature is also updated to use Field(default_factory=dict) for initialization. These changes ensure that transformation configurations cannot be created with empty or unspecified transform parameters.

🚥 Pre-merge checks | ✅ 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: fixing the bug where include_tags/exclude_tags were ignored when tools were not specified in MCPConfig.
Description check	✅ Passed	The description adequately covers the issue (references #3177), explains the root cause, describes the fix, and provides a concrete example demonstrating the corrected behavior.
Linked Issues check	✅ Passed	The code changes directly address the requirements in #3177 by making tools optional with a default empty dict and adding a validator to ensure transform fields are properly recognized, enabling tag filtering without requiring explicit tools.
Out of Scope Changes check	✅ Passed	All changes are within scope: the mcp_config.py modifications implement the fix for the linked issue, and the documentation link updates are standard maintenance for consistency.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/config-include-tags-single-server

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