Generate dedicated Python session event types

[stephentoub]

Summary

Python session events still used quicktype and collapsed payload members into one merged Data model, unlike the newer Go implementation. That made the language bindings inconsistent and increased the chance of naming conflicts as the runtime schema grows.

This change switches Python session-event generation to dedicated per-event payload dataclasses, matching the newer Go-style approach. SessionEvent.from_dict() now dispatches to typed payload classes, unknown events still round-trip through RawSessionEventData, and the Python runtime/tests were updated to handle the typed payload union.

The one non-obvious part is compatibility: the generator also restores legacy top-level helper exports and keeps arbitrary nested mappings in the Data shim as plain dicts so older callers do not break while the generated model becomes more structured.

Validation

• python -m pytest test_event_forward_compatibility.py test_commands_and_elicitation.py
• python -m pytest --ignore=e2e

[Copilot]

Pull request overview

This PR updates the Python SDK’s session-events generation to emit dedicated per-event payload dataclasses (instead of a single merged Data payload), and updates the Python runtime/tests to consume these typed payloads while retaining compatibility shims.

Changes:

• Replaced Python session-events quicktype generation with a custom generator that emits per-event payload dataclasses and dispatches in SessionEvent.from_dict().
• Updated Python session handling and tests to use typed payload classes (e.g., CommandExecuteData, ElicitationRequestedData) and validate legacy top-level exports + Data shim behavior.
• Preserved forward compatibility for unknown event types via SessionEventType.UNKNOWN + RawSessionEventData, plus added several legacy helper aliases.

Show a summary per file

File	Description
scripts/codegen/python.ts	Implements custom Python session-events codegen emitting per-event payload dataclasses, compatibility shims, and typed dispatch.
python/test_event_forward_compatibility.py	Extends forward-compat tests to cover legacy helper exports and Data shim dict-preservation behavior.
python/test_commands_and_elicitation.py	Updates tests to construct typed event payloads instead of using the legacy Data container.
python/copilot/session.py	Updates broadcast event handling to cast event.data to the appropriate generated payload dataclass before accessing fields.

Copilot's findings

• Files reviewed: 4/5 changed files
• Comments generated: 2

[github-actions]

Generated by SDK Consistency Review Agent for issue #1063 · ● 488.3K

[SteveSandersonMS]

Generally looks good to me, would be interesting to run it by @brettcannon if you want more authoritative Python feedback.

[github-actions]

Cross-SDK Consistency Review ✅

This PR improves Python–Go consistency by switching Python session-event generation to the same dedicated-per-event-payload-dataclass approach that Go already uses. The structural alignment is good.

Positive consistency improvements

Pattern	Before (Python)	After (Python)	Go	.NET
Event dispatch	if event.type == SessionEventType.X	match event.data: case XData()	type switch on event.Data	pattern match on event class (is AssistantMessageEvent)
Unknown events	collapsed into flat Data	RawSessionEventData	RawSessionEventData	unknown discriminator silently deserializes to base SessionEvent
Per-event payloads	merged Data struct	XxxData per event	XxxData per event	XxxEvent.Data property typed as XxxData

Python and Go are now well-aligned.

Cross-SDK naming observation: PermissionRequestedDataPermissionRequest

The inner permission-request payload type is named:

• Python (after this PR): PermissionRequestedDataPermissionRequest
• Go: PermissionRequestedDataPermissionRequest
• .NET: PermissionRequest (base class) with PermissionRequestShell, PermissionRequestWrite, etc. as typed subclasses

Python and Go are now consistent with each other, but both use a verbose auto-generated name compared to .NET's ergonomic PermissionRequest. This is an existing codegen design difference, not introduced by this PR.

Backward-compatibility note

PermissionRequest (the old Python public name, documented in the README and used in permission handler signatures) is renamed to PermissionRequestedDataPermissionRequest without a legacy alias. This is a breaking change for existing users of the permission handler API.

Consider whether a compatibility alias in the generated file is warranted:

# Backward-compat alias
PermissionRequest = PermissionRequestedDataPermissionRequest

This would also bring the Python public name into closer alignment with .NET's PermissionRequest — a win for cross-SDK ergonomics. The codegen script could be updated to emit this alias in the session-events output alongside the other legacy helper exports.

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Overall, this PR is a solid step toward Python–Go consistency. The main cross-SDK gap worth considering is the PermissionRequest rename ergonomics relative to .NET's cleaner public type name.

Generated by SDK Consistency Review Agent for issue #1063 · ● 1.3M · ◷