Surface AG-UI MESSAGES_SNAPSHOT events as metadata instead of dropping them

[PratikDhanave (PratikDhanave)]

What

onEvent in provider/aguiprovider/agui.go switched over StateSnapshotEvent and StateDeltaEvent and then hit default: return nil, nil. There was no case *aguiEvents.MessagesSnapshotEvent. The vendored AG-UI SDK decodes MESSAGES_SNAPSHOT into a distinct typed *MessagesSnapshotEvent{ *BaseEvent; Messages []Message } and feeds it to onEvent, so every MESSAGES_SNAPSHOT frame reached the default branch and was silently discarded.

This adds a case *aguiEvents.MessagesSnapshotEvent that emits a single assistant *agent.ResponseUpdate with CreatedAt: eventTime(evt) and the snapshot exposed under AdditionalProperties["agui_messages_snapshot"].

Why

MESSAGES_SNAPSHOT is a first-class AG-UI event that lets a server re-sync the full conversation history to the client. Dropping it means consumers lose that history-continuity signal entirely. The metadata-surfacing form is the lowest-risk mapping and matches how RunStarted metadata and the state snapshot/delta events are already surfaced instead of dropped, keeping consistent behavior across the provider's event handling. Surfacing the snapshot as provider-specific metadata (rather than fabricating message content) aligns with the .NET/Python AG-UI clients, which expose the snapshot to callers rather than discarding it.

Testing

Added TestAGUIAgentRun_SurfacesMessagesSnapshotEvent to agui_test.go (parallel to the existing state-snapshot test): it streams a NewMessagesSnapshotEvent with one message during a run and asserts a collected message carries AdditionalProperties["agui_messages_snapshot"] holding that message list. The test fails before the change (the frame yields no such update) and passes after. go build ./..., go vet ./provider/aguiprovider/..., and go test ./provider/aguiprovider/... all pass.

[Copilot]

Pull request overview

This PR updates the AG-UI provider’s event handling so MESSAGES_SNAPSHOT frames are no longer silently dropped, and instead are surfaced to consumers as provider-specific metadata on a streamed agent.ResponseUpdate.

Changes:

• Handle *aguiEvents.MessagesSnapshotEvent in toolCallAccumulator.onEvent and emit an assistant ResponseUpdate containing AdditionalProperties["agui_messages_snapshot"].
• Add a regression test that streams a NewMessagesSnapshotEvent and asserts the snapshot is present in collected message metadata.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
provider/aguiprovider/agui.go	Adds a MessagesSnapshotEvent case that returns a metadata-only ResponseUpdate instead of falling through to the default drop path.
provider/aguiprovider/agui_test.go	Adds a test ensuring agui_messages_snapshot metadata is preserved in the collected response when the event is streamed.

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[github-actions]

Parity Review: ✅ Approved

This PR fixes a silent event-discard bug in the AG-UI provider by handling MessagesSnapshotEvent that previously fell through to default: return nil, nil.

Scope check: Both changed files are in provider/aguiprovider/ — an internal implementation detail. No exported Go types, functions, methods, or option structs were added or changed. The fix surfaces data through the existing AdditionalProperties mechanism already used by RunStartedEvent, StateSnapshotEvent, and StateDeltaEvent handling in the same file.

Cross-repo parity: The upstream Python AG-UI package (python/packages/ag-ui/) is the producer of MESSAGES_SNAPSHOT events. The Go aguiprovider is a consumer. Surfacing the event as AdditionalProperties["agui_messages_snapshot"] rather than silently discarding it matches the intent: callers that care about history continuity can read it. This is consistent with the metadata-surfacing pattern the provider already uses for other AG-UI-specific events.

No public API change: The fix is confined to the unexported onEvent method on the unexported toolCallAccumulator type. No public-api-change label is warranted.

Verdict: The change preserves cross-repo semantic alignment and does not introduce divergence from .NET or Python behavior.

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