Fix prompt return type documentation

[jlowin]

The documentation example for prompts showed returning a single Message object with -> Message, but this is not a supported return type. The convert_result method only handles:

• str → wrapped as single user Message
• list[Message | str] → converted to list[Message]
• PromptResult → passed through

A single Message would raise TypeError: Prompt must return str, list[Message], or PromptResult.

Updated the example to return list[Message] which demonstrates the multi-message capability, and corrected the docstrings in Prompt.from_function and FunctionPrompt.from_function to match the actual supported types.

[coderabbitai]

Walkthrough

The pull request updates prompt-related documentation and docstrings to reflect changes in return types. An example function generate_code_request in the prompts documentation was modified to return list[Message] instead of a single Message. The function now returns a two-item messages list comprising the original code message and an assistant follow-up. Additionally, docstrings in src/fastmcp/prompts/prompt.py for Prompt.from_function and FunctionPrompt.from_function were updated to document the new accepted return possibilities, clarifying how different return types (str, list[Message | str], PromptResult) are handled. No functional code changes were made in the source file.

Possibly related PRs

• Introduce Message and PromptResult as canonical prompt types #2738: Updates prompt example and docstring to return list[Message], matching changes to documented prompt return shapes and Message/PromptResult usage patterns.
• Refactor prompt behavior and add meta support #2610: Prompt refactor that introduces PromptResult and updates examples to return list[Message], aligning with the documented return form changes.
• Refactor prompt behavior and add meta support #2600: Modifies the same prompt documentation and return-value descriptions in docstrings, sharing overlap in prompt return type clarifications.

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: fixing documentation for prompt return types to match the actual supported types.
Description check	✅ Passed	The description clearly explains the issue, the problem with the current documentation, and what was changed. However, the Contributors Checklist items are not checked off.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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• docs/servers/prompts.mdx
• src/fastmcp/prompts/prompt.py

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• src/fastmcp/prompts/prompt.py

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• docs/servers/prompts.mdx

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• docs/servers/prompts.mdx

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• docs/servers/prompts.mdx

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🔇 Additional comments (3)

src/fastmcp/prompts/prompt.py (2)

244-247: LGTM! Documentation now accurately reflects supported return types.

The docstring correction properly documents the three supported return types and their handling behavior, which matches the actual convert_result implementation (lines 273-307).

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386-389: LGTM! Consistent documentation update.

This docstring mirrors the correction in Prompt.from_function and accurately documents the supported return types for function-based prompts.

docs/servers/prompts.mdx (1)

41-48: LGTM! Example corrected to use supported return type.

The example now correctly demonstrates returning list[Message] instead of a single Message object, which aligns with the supported return types documented in the Python file. The multi-message example also better demonstrates the capability to create conversation-style prompts with different roles.

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