feat(sessions): refill composer when cancelling before the agent starts

[MattPua]

Problem

Cancelling a turn right after sending (before the agent has produced any output) loses the just-sent message. A quick "oops, wrong send" means retyping it.

Changes

CleanShot.2026-06-29.at.17.45.39.mp4

• Cancelling before the agent starts now refills the just-sent message back into the composer so it can be edited and re-sent.
• The message stays in conversation history — it's already committed to the agent's context and the persisted log, so removing it would desync the UI from what the model actually sees.
• Refill is skipped once the agent has started working, and when you're not focused on that chat.
• Attachments and mentions are restored as chips (via xmlToContent), not raw XML.
• Steered messages are excluded (a steer folds into the running turn rather than starting its own).

Implementation:

• New pure helper hasAgentStartedTurn({ events, hasOptimisticItems }) in core owns the decision: false while still optimistic (echo not landed, no output possible), otherwise checks for agent output after the latest session/prompt. Works for local and cloud (reads only persisted store state, avoids the local-only liveTurnContent).
• SessionService.hasAgentStartedCurrentTurn(taskId) is the thin session-aware wrapper.
• useSessionCallbacks captures the serialized text on send and applies the queued-wins / focus gating on cancel.

Danger level: 1/5 — additive, client-side only, no backend or protocol changes; gated behind narrow conditions and covered by tests.

How did you test this?

• pnpm --filter @posthog/core test hasAgentStartedTurn — 9 cases (optimistic guard, prior-turn isolation, each agent output kind), all passing.
• pnpm --filter @posthog/core typecheck and pnpm --filter @posthog/ui typecheck — both clean.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 4dc1401.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(sessions): refill composer when can..." | Re-trigger Greptile}

[stamphog]

The logic and architecture are correct, but this behavioral change to the session cancel flow (refilling the composer when the agent hasn't started) has no human team-member review — only bot comments and the author's own replies.