Handle session.canvas.closed by removing from open_canvases snapshot

[jmoseley]

What

Completes the symmetric pair with session.canvas.opened: the SDKs now remove a canvas from their in-memory open_canvases snapshot when the runtime emits session.canvas.closed.

Why

The runtime now emits a symmetric session.canvas.closed ephemeral event (payload { instanceId, extensionId, canvasId }, instanceId required + non-empty) when a canvas is genuinely removed — mirroring the existing session.canvas.opened (copilot-agent-runtime #9489, shipped in CLI 1.0.60).

The generated event types already landed via codegen, but the hand-written handlers only ever upserted on open, so the snapshot was append-only and never shrank on close. This left consumers' open-canvas state stale.

The consumer driving this is github-app's "sticky terminal canvas" fix: a right-panel canvas the user closes resurrects on warm session-reselect. That fix needs the SDK snapshot to shrink on close so reselect stops resurrecting closed canvases.

How

Adds a symmetric removal handler in every SDK that maintains the open-canvases snapshot — Rust, Node, Python, Go, .NET. Each:

• removes the matching instanceId on session.canvas.closed, acquiring the same lock the upsert path uses;
• warns and no-ops on an empty/missing instanceId;
• is idempotent (closing an absent instanceId is a no-op);
• leaves the resume path untouched and does not treat opened/stale events as removals (only the explicit closed event removes — provider unregister still marks instances stale via a re-emitted opened, which continues to upsert).

Generated files were not hand-edited; the change is purely in the hand-written session handlers + unit tests.

Java — intentionally excluded

Java has no open-canvases snapshot feature in java/src/main at all (no accessor, no opened-upsert handler, no resume capture) — only the generated event types exist. There is nothing symmetric to add; covering it would mean building the entire feature net-new, which is out of scope for this focused bug fix.

Tests

New unit tests in each language open two canvases, close one (assert the closed instance is gone and the other remains), and cover idempotency (closing an absent id) and the empty-instanceId no-op.

• Rust: cargo test --features test-support --test session_test — pass
• Node: vitest run test/client.test.ts — 95 pass
• Python: pytest test_canvas.py — 12 pass
• Go: go test . — pass
• .NET: dotnet test --filter CanvasTests — 10 pass

[Copilot]

Pull request overview

This PR completes the open-canvases snapshot lifecycle across the SDKs by removing entries from the in-memory open_canvases/openCanvases snapshot when a session.canvas.closed event is received, keeping consumer state aligned with the runtime’s authoritative “open canvas instances” view.

Changes:

• Add session.canvas.closed handling to remove the matching instanceId from the open-canvases snapshot in Rust, Node, Python, Go, and .NET.
• Update snapshot documentation/comments to reflect that both opened and closed events affect the snapshot.
• Add symmetric unit tests in each language to cover removal, idempotency, and empty/missing instanceId no-op behavior.

Show a summary per file

File	Description
rust/src/session.rs	Handles session.canvas.closed by removing the matching instance from the open_canvases snapshot.
rust/tests/session_test.rs	Adds coverage for close-driven removal, idempotency, and empty-instance no-op behavior.
python/copilot/session.py	Removes closed canvas instances from _open_canvases snapshot and updates docstring accordingly.
python/test_canvas.py	Adds unit test validating removal, idempotency, and warning/no-op behavior on empty instance id.
nodejs/src/session.ts	Dispatches session.canvas.closed to a new removal path that updates the openCanvasInstances snapshot.
nodejs/test/client.test.ts	Adds a unit test for close-driven snapshot removal and invalid payload warning behavior.
go/session.go	Extends event-driven snapshot updates to handle close events via removal under the same mutex.
go/session_test.go	Adds a unit test verifying close removes one instance, idempotency, and invalid close no-op behavior.
dotnet/src/Session.cs	Updates event processing to remove closed instances from OpenCanvases snapshot and updates remarks.
dotnet/test/Unit/CanvasTests.cs	Adds unit test verifying close removal, idempotency, and empty instance id no-op behavior.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 0

[github-actions]

Cross-SDK Consistency Review ✅

This PR adds the symmetric session.canvas.closed removal handler to all five SDKs that maintain the open-canvases snapshot: Node.js, Python, Go, .NET, and Rust.

Consistency across modified SDKs

All five implementations are symmetric with their existing session.canvas.opened upsert path:

Concern	Node.js	Python	Go	.NET	Rust
Removal logic	Array.filter	list comp	iterate + rebuild	LINQ Where	Vec::retain
Empty instanceId → warn + no-op	✅	✅	✅	✅	✅
Idempotent (absent id → no-op)	✅	✅	✅	✅	✅
Same lock as upsert path	N/A (single-threaded)	_open_canvases_lock	openCanvasesMu.Lock()	(no lock, consistent with upsert)	open_canvases.write()
Warning message	console.warn	logger.warning	fmt.Printf	_logger.LogWarning	warn!
Tests	✅	✅	✅	✅	✅

Warning messages all use the same text: "failed to deserialize session.canvas.closed payload", consistent with the existing opened-path wording.

Java exclusion

Confirmed: java/src/main has no open-canvases snapshot feature — no openCanvases accessor, no session.canvas.opened upsert handler, and no resume capture. Only the generated event types (SessionCanvasClosedEvent, OpenCanvasInstance, etc.) exist. The exclusion is correctly documented in the PR description and is the right call for a focused bug fix.

No cross-SDK consistency issues found.

Generated by SDK Consistency Review Agent for issue #1604 · sonnet46 1.1M · ◷

[stephentoub]

Did we not add these APIs to Java?

[jmoseley]

Did we not add these APIs to Java?

When they were originally added, the Java implementation was missing a bunch of stuff. I can kick off a session to build this into the Java client as well.