Register the Copilot event handler via SessionConfig.OnEvent so session-lifecycle events are not dropped

[PratikDhanave]

What

The Copilot provider opened the session (openSession → client.CreateSession/ResumeSession) and only afterward called copilotSession.On(eventHandler). sessionConfig() and resumeSessionConfig() never set OnEvent.

This change threads the eventHandler created in run() into openSession and sets cfg.OnEvent = eventHandler inside both sessionConfig() and resumeSessionConfig(), then removes the now-redundant post-return .On(eventHandler).

Why

The Copilot SDK registers the session (and any config.OnEvent handler) synchronously before it issues the session.create / session.resume RPC, precisely so that lifecycle events the CLI emits during session creation (e.g. session.start) are not missed. The SDK read loop runs on a separate goroutine, so with the old post-return On there is a window between the RPC returning and the .On call during which such events are dispatched to a session with zero handlers and silently dropped (the event channel's consumer snapshots the handler list at dequeue time, so an event dequeued before .On runs is lost).

Registering through OnEvent closes that window and matches the cross-SDK behaviour: the .NET and Python Copilot integrations attach the provider's event handler at session-construction time (as part of the create/resume options) rather than after the create call returns, so no early events are missed.

This is not a duplicate of #710 (SessionConfig field carry-over) or #595 (Tools clone).

Testing

Added TestRun_SurfacesLifecycleEventEmittedDuringSessionResume in copilot_test.go. The fake runtime now emits a session.start event during session.resume handling, before the RPC response, and the test asserts the provider surfaces it. The test flakily fails against the old code (the event is dropped in the race window — reproduced deterministically with -count) and passes reliably with the fix, including under -race and -count=300.

• go build ./...
• go vet ./provider/copilotprovider/...
• go test ./provider/copilotprovider/... -race

[Copilot]

Pull request overview

This PR updates the Copilot provider’s session creation/resume path so the run-scoped Copilot event handler is registered via SessionConfig.OnEvent at session construction time, preventing session-lifecycle events emitted during session.create/session.resume from being dropped.

Changes:

• Thread the run’s eventHandler into openSession and set it on SessionConfig.OnEvent / ResumeSessionConfig.OnEvent before the create/resume RPC.
• Remove the post-return session.On(eventHandler) subscription that introduced an event-loss window.
• Add a regression test that emits a lifecycle event during session.resume (before the RPC response) and asserts it is surfaced.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
provider/copilotprovider/copilot.go	Registers the run event handler through config at session construction time to avoid dropping early lifecycle events.
provider/copilotprovider/copilot_test.go	Adds a fake-runtime mechanism to emit pre-response resume events and a test ensuring the provider surfaces them.

Comments suppressed due to low confidence (1)

provider/copilotprovider/copilot.go:252

• resumeSessionConfig unconditionally sets cfg.OnEvent = eventHandler, which prevents any handler configured on p.cfg.SessionConfig.OnEvent from ever running on resumed sessions (and also makes create/resume behavior inconsistent if callers rely on SessionConfig.OnEvent). Consider seeding cfg.OnEvent from p.cfg.SessionConfig.OnEvent and chaining it with the per-run handler.

func (p *provider) resumeSessionConfig(streaming bool, eventHandler copilot.SessionEventHandler, options []agent.Option) copilot.ResumeSessionConfig {
	cfg := copyResumeSessionConfig(p.cfg.SessionConfig)
	cfg.Streaming = copilot.Bool(streaming)
	cfg.OnEvent = eventHandler
	cfg.SystemMessage = systemMessageWithInstructions(cfg.SystemMessage, slices.Collect(agent.AllOptions(options, agent.WithInstructions)))
	cfg.Tools = append(cfg.Tools, copilotTools(options)...)

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[github-actions]

Parity Review ✅

This PR changes only unexported/internal methods in provider/copilotprovider/copilot.go:

• openSession, sessionConfig, resumeSessionConfig (all unexported)
• New unexported helper chainSessionEventHandlers
• Bug fix: moves Copilot event-handler registration from post-RPC session.On() to pre-RPC SessionConfig.OnEvent to avoid dropped lifecycle events

No exported Go API surface was added or changed, so the public-api-change label is not warranted.

Cross-repo parity: This is Copilot-specific session plumbing with no direct equivalent in the upstream .NET or Python microsoft/agent-framework implementations. The change is an internal correctness fix (race-window between RPC and handler registration) that does not affect public contracts or user-visible behavior semantics.

No parity issues found. 🟢

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