feat(client): Add capability_extensions parameter to ClientSession #1901
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Add a new `capability_extensions` parameter to `ClientSession.__init__()` that
allows clients to include additional capability fields in the initialize request.
This enables clients to advertise protocol extensions (like `io.modelcontextprotocol/ui`)
without having to override the `initialize()` method.
Example usage:
```python
session = ClientSession(
read_stream,
write_stream,
capability_extensions={
"extensions": {
"io.modelcontextprotocol/ui": {
"mimeTypes": ["text/html;profile=mcp-app"]
}
}
}
)
```
The extensions are merged into `ClientCapabilities` using Pydantic's extra fields
feature (`model_config = {'extra': 'allow'}`).
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Kludex
reviewed
Jan 17, 2026
| experimental=None, | ||
| roots=roots, | ||
| tasks=self._task_handlers.build_capability(), | ||
| **self._capability_extensions, |
Member
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I don't think it should allow any key here, this will be an issue if in the future ClientCapabilities has a new key but it's used by someone.
Suggested change
| **self._capability_extensions, | |
| extensions=self._capability_extensions, |
If the spec doesn't have this explicit, I think the python SDK should.
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Summary
Add a new
capability_extensionsparameter toClientSession.__init__()that allows clients to include additional capability fields in the initialize request.Motivation
Currently, if a client wants to advertise protocol extensions (like
io.modelcontextprotocol/uifor UI rendering) in the initialize request, they must override the entireinitialize()method and duplicate its internal logic. This is error-prone and creates maintenance burden when the SDK'sinitialize()implementation changes.This PR provides a clean escape hatch for clients to extend capabilities without subclassing.
Example Usage
Implementation
The extensions are merged into
ClientCapabilitiesusing Pydantic's extra fields feature (model_config = {'extra': 'allow'}onMCPModel).Testing
Added a test that verifies capability extensions are properly included in the initialize request.