fix(desktop): preserve active stream across hydration

[yinkev]

What does this PR do?

Prevents a lagging Desktop transcript hydration from splitting one live assistant turn into two bubbles.

The post-turn hydrator can finish after the user has already started the next turn. It replaced the renderer's message array with the older stored snapshot while leaving the active streamId unchanged. The next message.delta could no longer find that id and created a second assistant row, producing the intermittent byte-for-byte duplicate described in #38319 and #71857.

This PR closes the race at both seams:

1. Stored-session hydration now reconciles through the existing live-tail contract, preserving the newest optimistic user row and pending assistant before merging renderer-only assistant errors.
2. If another hydration/reconnect source still leaves an established streamId orphaned, the next delta rebinds the newest pending assistant to that id instead of appending a second bubble.

The fallback is deliberately narrow. It runs only when a prior stream id exists and a non-hidden pending assistant is present. A genuinely new stream still creates a new assistant message, and completed historical assistants are never reused by text equality.

Related Issue

Fixes #38319
Fixes #71857

Related but not duplicated by #68478: that broader session-reliability PR explicitly leaves the streaming-versus-hydration duplicate-response race out of scope.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• add reconcileStoredSessionMessages() as the shared stored-history reconciliation seam;
• preserve the current live turn in both post-turn hydration and active messaging transcript refresh;
• recover an orphaned streamId by rebinding the newest pending assistant;
• add an exact stream regression for the hydrated-pending-row → next-delta sequence;
• add a utility regression proving a lagging stored projection keeps the optimistic user and pending assistant tail.

How to Test

cd apps/desktop

../../node_modules/.bin/vitest run --project ui \
  src/app/session/hooks/use-message-stream/hydration-stream-race.test.tsx \
  src/app/session/hooks/use-session-actions/utils.test.ts

npm run typecheck

Then run ESLint on the five changed TypeScript files and git diff --check.

Expected focused result: 40 passed.

The stream regression was run against the pre-fix behavior first and failed with two assistant rows instead of one. The utility regression likewise failed because the shared reconciliation seam did not exist.

Checklist

Code

• I've read the Contributing Guide
• My commit message follows Conventional Commits
• I searched open and merged issues/PRs and compared Duplicate messages in desktop app — hydration race condition with active stream #38319, [Bug]: Desktop — identical assistant final answer delivered twice #71857, and fix(desktop): session reliability — duplicate sessions, blocked receipts, cross-window re-keying, following-prompt projection, external worktree project continuity #68478
• My PR contains only changes related to this fix
• I've run pytest tests/ -q and all tests pass — N/A for this Desktop-only TypeScript change; the focused UI tests and complete Desktop typecheck pass
• I've added tests for the bug
• I've tested on macOS Apple Silicon

Documentation & Housekeeping

• Relevant documentation — N/A; no user-facing configuration or workflow changed
• cli-config.yaml.example — N/A; no config keys changed
• CONTRIBUTING.md / AGENTS.md — N/A; no contributor workflow or architecture changed
• Cross-platform impact considered; the state reconciliation is renderer-platform-neutral
• Tool descriptions/schemas — N/A; no model-facing tool behavior changed

Verification

Focused Desktop tests: 40 passed
Desktop typecheck:      passed (renderer, Electron, E2E tsconfigs)
ESLint:                 passed
Diff hygiene:           passed

[teknium1]

Thanks for the focused race fix. The premise remains present on current main: apps/desktop/src/app/contrib/wiring.tsx:321-326 replaces the stored transcript through preserveLocalAssistantErrors() only, while apps/desktop/src/app/session/hooks/use-message-stream/index.ts:101-116 appends a new assistant row whenever the retained streamId no longer exists in the hydrated array.

The proposed reconciliation composes the existing live-tail contract in apps/desktop/src/app/session/hooks/use-session-actions/utils.ts:342-437, and the orphaned-stream fallback is narrow to an established stream plus a visible pending assistant. The two changed hydration callers cover the post-turn and active-messaging refresh paths. The PR tests exercise both the stored-tail reconciliation and the subsequent delta/rebind sequence.

The targeted hunks remain applicable against current main despite unrelated Desktop movement since the PR base.

Automated hermes-sweeper review.

[yinkev]

Author-status audit — corrected 2026-08-05: GitHub now reports this head as mergeable, current-head CI is green, and there are no unresolved inline review threads. The earlier conflict classification was a stale mergeability result, not a branch defect. Restoring the PR to ready for review. No branch rewrite is required.

[frizikk]

Independent current-main validation on Linux (CachyOS), 2026-08-08:

• merged exact PR head b63a1fcdd2d8fa249ba0dc2f2b022312a35821c3 without conflicts onto current main b9aa9289a8083f2e9d248ad6837b2938f5ee92d7 in an isolated worktree;
• focused regressions: 84/84 passed (hydration-stream-race.test.tsx + use-session-actions/utils.test.ts);
• Desktop TypeScript typecheck passed;
• ESLint passed on all five changed files;
• git diff --check passed.

The merged tree was discarded after verification and the worktree returned clean to b9aa9289. This confirms the targeted hydration fix remains applicable and green on today's upstream base. Maintainer review/merge would close the still-present transcript-clobber race.

— 🤖 Bot comment · LLM: gpt-5.6-sol · not written by a human