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Support ImageContent and AudioContent in Message class#3396

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jlowin merged 2 commits intoPrefectHQ:mainfrom
ericrobinson-indeed:fix-prompts-get-with-image-compatibility
Mar 6, 2026
Merged

Support ImageContent and AudioContent in Message class#3396
jlowin merged 2 commits intoPrefectHQ:mainfrom
ericrobinson-indeed:fix-prompts-get-with-image-compatibility

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@ericrobinson-indeed ericrobinson-indeed commented Mar 5, 2026

Description

Message.content now accepts ImageContent and AudioContent in addition to TextContent and EmbeddedResource, matching MCP's ContentBlock type. This fixes ProxyPrompt.render() silently JSON-serializing image/audio content instead of preserving it.

🤖 Generated with Claude Code

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@ericrobinson-indeed @claude
Support ImageContent and AudioContent in Message class
42ddd8f 
Message.content now accepts ImageContent and AudioContent in addition to
TextContent and EmbeddedResource, matching MCP's ContentBlock type. This
fixes ProxyPrompt.render() silently JSON-serializing image/audio content
instead of preserving it.

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 <[email protected]>
@marvin-context-protocol
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marvin-context-protocol Bot commented Mar 5, 2026 •
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Test Failure Analysis

✏️ Edited to reflect the latest analysis (previous failure was a GitHub infrastructure issue; this is a real code failure).

Summary: The ty type checker fails because CachableMessage.content in src/fastmcp/server/middleware/caching.py is too narrow — it only accepts TextContent | EmbeddedResource, but this PR expanded Message.content to also include ImageContent | AudioContent.

Root Cause: CachableMessage (used for caching prompt results) was not updated alongside the Message class. When CachablePromptResult.wrap() constructs CachableMessage instances from Message objects (line 121), ty correctly flags that ImageContent and AudioContent cannot be assigned to the narrower TextContent | EmbeddedResource union.

Suggested Solution: Update CachableMessage.content in src/fastmcp/server/middleware/caching.py (line 104) to match the expanded union:

# Before
content: mcp.types.TextContent | mcp.types.EmbeddedResource

# After
content: mcp.types.TextContent | mcp.types.ImageContent | mcp.types.AudioContent | mcp.types.EmbeddedResource

Also check whether the # type: ignore[arg-type] on line 133 (in unwrap()) can be removed after this fix.

Detailed Analysis

The ty check step failed with:

error[invalid-argument-type]: Argument is incorrect
   --> src/fastmcp/server/middleware/caching.py:121:46
    |
119 |         return cls(
120 |             messages=[
121 |                 CachableMessage(role=m.role, content=m.content) for m in value.messages
    |                                              ^^^^^^^^^^^^^^^^^ Expected `TextContent | EmbeddedResource`, found `TextContent | ImageContent | AudioContent | EmbeddedResource`
    |
info: Union elements `ImageContent` and `AudioContent` are not assignable to `TextContent | EmbeddedResource`

The root: CachableMessage at line 104 of caching.py still has the old narrow type, while Message.content was expanded by this PR. The CachableMessage class exists solely as a serializable wrapper around Message for caching, so its content field should mirror Message.content exactly.

Related Files
  • src/fastmcp/server/middleware/caching.py:104 — CachableMessage.content type needs to include ImageContent and AudioContent
  • src/fastmcp/prompts/prompt.py — Message.content was expanded by this PR (the intended change)

This was referenced Mar 6, 2026
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jlowin
jlowin approved these changes Mar 6, 2026
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Thank you!

@jlowin jlowin merged commit 72d04d7 into PrefectHQ:main Mar 6, 2026
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@jlowin jlowin added the enhancement Improvement to existing functionality. For issues and smaller PR improvements. label Mar 30, 2026
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Regression: Conformance test prompts-get-with-image fails in 3.X

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