fix(desktop): preserve pre-tool assistant text on turn complete

[erict16]

What does this PR do?

Desktop was dropping the first half of multi-step assistant turns once the turn finished.

You would see the model narrate, call tools, answer - then on message.complete the pre-tool narration vanished. It was still in state.db and came back after restart. Display-only bug.

mergeFinalAssistantText replaced every text part in the live bubble with the complete payload. That payload is usually only the last model reply. When earlier narration shared the bubble with tools and message.interim never sealed it first, the pre-tool half got wiped.

Related issue

Fixes the pre-tool narration wipe path described on #46606 (see mxcrowe's comment).

Does not claim to fix pure long-message head truncation / live render bounds. Separate problem.

Type of change

• Bug fix

Changes

• apps/desktop/src/lib/chat-messages.ts - segment-aware mergeFinalAssistantText: tool/image parts seal earlier text; only the open trailing segment is replaced by the final reply. Full-transcript dedupe only when the final is the sealed prefix itself or a real long-enough prefix continuation (no bare substring match, so short "OK." openers do not wipe the turn).
• Unit tests for multi-tool segments, empty final, full-transcript dedupe, short-opener false positive, mid-string mention false positive.
• Stream test: deltas + tools without message.interim, then complete - both halves stay.

Image-echo stripping stays on the caller (use-message-stream), same as main today.

Why this instead of #53350

Same bug class. Different shape:

	#53350	this PR
Hook	new reconcileCompletedTextParts + complete path	extends existing mergeFinalAssistantText
Fits current main	older use-message-stream.ts layout	matches today's index.ts choke point
No-interim path	yes	yes + stream test
Multi-tool sealed segments	partial	explicit test
Short-prefix false positive	n/a (always keeps head)	guarded (min length + real prefix)

Happy to close this if you prefer #53350's shape, or pull any of their cases over. Just pick one.

How to test

cd apps/desktop
npx vitest run src/lib/chat-messages.test.ts src/app/session/hooks/use-message-stream/interim-sealing.test.tsx

51 passed here.

Manual: model that narrates, tools, then answers. Pre-tool text should still be there after the turn settles. Plain text-only turns should still collapse to the final reply.

Checklist

• Contributing guide / conventional commits
• Searched existing PRs (related: fix(desktop): keep assistant text streamed before an inline tool call #53350, closed fix(desktop): honor display.interim_assistant_messages — stop collapsing mid-turn narration #61447 - different semantics, not a blind dupe)
• Scoped to this fix
• Tests added
• Desktop-only pure TS; macOS shares the same path

[alt-glitch]

This was generated by AI during triage.

Related to #53350 and #61447, which address the same Desktop completion-time narration loss. This patch uses segment sealing and a full-transcript dedupe fallback; the existing alternatives preserve earlier segments with different reconciliation semantics, so this needs a maintainer choice rather than a duplicate label.

[erict16]

Follow-up push on the dedupe path.

Earlier version used a bare substring check (final contains sealed text). That was too loose - a short sealed opener, or a phrase the final merely quotes, could force a full text replace and drop multi-step narration again.

Now full-replace only when:

• final equals the sealed prefix, or
• final starts with the sealed prefix, sealed is at least 20 chars, and the rest looks like a real continuation

Added tests for short "OK." openers and mid-string mentions.

Vitest: chat-messages.test.ts + interim-sealing.test.tsx - 51 passed.

Still sitting on needs-decision next to #53350. This PR patches the current mergeFinalAssistantText choke point on main; #53350 adds a parallel helper. Either is fine - I'd rather one lands cleanly than both sit.

[teknium1]

Thanks for the focused fix. The reported live-path loss is present on current main: mergeFinalAssistantText removes every text part at apps/desktop/src/lib/chat-messages.ts:230-254, while tool events deliberately flush prior deltas into the same bubble at apps/desktop/src/app/session/hooks/use-message-stream/index.ts:368-400; completion then invokes that helper at :495-512.

The proposed last-boundary segmentation addresses that exact state shape without introducing a new surface, and its unit and stream tests cover both the direct no-message.interim path and the dedupe false-positive cases described in the PR.

This is an automated hermes-sweeper review.