Document client meta parameter

[jlowin]

Adds documentation for the meta parameter on call_tool() and call_tool_mcp() (added in PR #2206).

The meta parameter allows clients to send ancillary information alongside requests - useful for observability, debugging, client identification, or any context the server may need beyond tool arguments.

Client usage:

result = await client.call_tool(
    name="send_email",
    arguments={"to": "[email protected]", "subject": "Hello"},
    meta={"trace_id": "abc-123", "request_source": "mobile_app"}
)

Server access:

@mcp.tool
def send_email(to: str, subject: str, ctx: Context) -> str:
    meta = ctx.request_context.meta
    if meta:
        trace_id = meta.trace_id if hasattr(meta, 'trace_id') else None
        # Use metadata for logging, observability, etc.
    return f"Email sent to {to}"

Documentation is intentionally neutral about use cases - the structure and usage of meta is application-dependent.

[coderabbitai]

Walkthrough

This pull request adds documentation and docstring updates for a new meta parameter in tool calls. The meta parameter enables clients to send metadata with tool calls that becomes accessible server-side via ctx.request_context.meta. Changes include a new "Sending Metadata" section in the client tools documentation, a "Client Metadata" section in the server context documentation (both marked with VersionBadge 2.13.1), and updated parameter docstrings in the client implementation explaining the meta parameter's purpose and server-side accessibility.

Possibly related issues

• Allow clients to send _meta to server #1315: Directly addresses the same meta parameter exposure for tool calls on both client and server sides with documentation and docstring updates.

Pre-merge checks and finishing touches

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The pull request title accurately summarizes the main change: adding documentation for the client meta parameter across documentation files and code docstrings.
Description check	✅ Passed	The description explains the purpose of the meta parameter, provides clear client and server usage examples, and notes the intentionally neutral documentation approach. However, it does not include the contributors checklist or review checklist sections from the template.

✨ 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 docs/client-meta-parameter

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📥 Commits

Reviewing files that changed from the base of the PR and between 5747cb6 and 40421a8.

📒 Files selected for processing (3)

• docs/clients/tools.mdx (1 hunks)
• docs/servers/context.mdx (1 hunks)
• src/fastmcp/client/client.py (2 hunks)

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docs/**/*.mdx

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docs/**/*.mdx: Use clear, direct language appropriate for technical audiences
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Files:

• docs/clients/tools.mdx
• docs/servers/context.mdx

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• GitHub Check: Run tests: Python 3.10 on windows-latest

🔇 Additional comments (4)

docs/servers/context.mdx (1)

316-343: Excellent documentation of the client metadata feature.

The new "Client Metadata" section is well-written, clearly explains the purpose and accessibility of the meta parameter, and provides a realistic, safe example using hasattr() checks. The documentation correctly emphasizes that metadata structure is application-determined, positioning it as a flexible, neutral mechanism. Consistent version badge (2.13.1) with client-side documentation.

src/fastmcp/client/client.py (2)

895-898: Clear meta parameter documentation in call_tool_mcp.

The docstring correctly describes meta as additional metadata for contextual information, distinguishes it from tool arguments, and documents server-side accessibility via context.request_context.meta. Consistent with call_tool documentation below.

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941-944: Clear meta parameter documentation in call_tool.

The docstring mirrors call_tool_mcp and clearly explains the meta parameter's purpose, proper usage, and server-side accessibility. Documentation is consistent and aligns with the feature described in the server-side context documentation.

docs/clients/tools.mdx (1)

104-128: Clear, user-focused documentation of the meta parameter.

The new "Sending Metadata" section fits well after "Advanced Execution Options," explains the purpose clearly with concrete examples (trace_id, request_source), and maintains consistency with server-side documentation. The cross-reference to Client Metadata provides good navigation. VersionBadge 2.13.1 aligns with the feature version across all documentation.

Please verify that the cross-reference anchor #client-metadata correctly maps to the h4 heading "#### Client Metadata" in docs/servers/context.mdx (line 316). Mintlify typically auto-slugifies headings to lowercase with dashes, so this should work, but confirm the anchor resolves correctly.

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