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Fix media type inference for URLs with query parameters #3501

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Fix media type inference for URLs with query parameters #3501

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@fedexman fedexman commented Nov 21, 2025

Fix media type inference for URLs with query parameters

When using presigned URLs (eg AWS S3) with ImageUrl, AudioUrl, or VideoUrl, the media type inference fails

https://pics.s3.ap-northeast-1.amazonaws.com/test/Capture-2025-11-21-112402.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=fdafdas%2F20251121%2Fap-northeast-1%2Fs3%2Faws4_request&X-Amz-Date=20251121T023200Z&X-Amz-Expires=3600&X-Amz-SignedHeaders=host&X-Amz-Security-Token=IQoJb3JpZ2luX2VjEDoaD

to the agent we get this error,

Internal server error: Could not infer media type from image URL: 
...
Explicitly provide a `media_type` instead

the reason is that the _infer_media_type function only check the end of the url with url.endswith('.mkv') but do not parse the url.

I propose to parse the url with

from urllib.parse import urlparse
path = urlparse(self.url).path
if path.endswith('.mkv'):
            return 'video/x-matroska'

Viicos
Viicos reviewed Nov 21, 2025
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pydantic_ai_slim/pydantic_ai/messages.py
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@Viicos Viicos Nov 21, 2025

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Why aren't we using the mimetypes stdlib module? mimetypes.guess_type() already parses URLs and the current implementation doesn't take into account case insensitivity, etc.

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@DouweM DouweM Nov 21, 2025

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@Viicos Interestingly we already use that in DocumentUrl._infer_media_type, after checking a bunch of types ourselves :/

@fedexman Can you see if we can use mimetypes.guess_type() for all of these?

The method can be changed to just return str rather than XMediaType, as I don't think that type is used on any public fields.

DouweM
DouweM requested changes Nov 21, 2025
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pydantic_ai_slim/pydantic_ai/messages.py
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@DouweM DouweM Nov 21, 2025

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@Viicos Interestingly we already use that in DocumentUrl._infer_media_type, after checking a bunch of types ourselves :/

@fedexman Can you see if we can use mimetypes.guess_type() for all of these?

The method can be changed to just return str rather than XMediaType, as I don't think that type is used on any public fields.

@DouweM DouweM self-assigned this Nov 21, 2025
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@fedexman @DouweM @Viicos