fix(subtask): derive subtask state from messages, remove render-time mutation (#3147)

[fancyboi999]

Closes #3147. Split from #3138.

Why

MessageList reconstructed the subtask card map by calling updateSubtask() during render from its per-message render loop. That helper actually mutated the SubtaskContext tasks object in place — only the SSE-driven latestMessage path called setState. Three consequences:

1. React 19 Strict Mode's double render surfaces the pattern as "Cannot update a component while rendering a different component".
2. Lifecycle regressions like [Stability][v2.0-m1-rc1] Release-blocking issues found in Ultra/user workflow testing #3107 BUG-007 (cards stuck on "running") were masked: when the backend's final ToolMessage arrived, the render-time mutation silently updated the shared object but did not trigger a re-render unless a follow-up latestMessage event happened to land.
3. SubtaskCard had to useSubtask(id)! non-null-assert the value, which only worked because the same render had just mutated it in.

Design

Two write paths split by data origin.

Base fields (status / result / error / description / ...) are derived from the message list by a pure buildSubtaskMapFromMessages(messages). MessageList passes each entry directly to SubtaskCard as a task prop. No round trip through context, so the first render is already correct — no effect lag, no ! non-null assertion.

Latest streaming AIMessage still comes from the task_running custom SSE event handled in useThreadStream. That fires outside React render, so writing it into context is safe. The provider in tasks/context.tsx now owns only this map; SubtaskCard reads it via useLatestSubtaskMessage(taskId) and merges into the prop inside a useMemo.

The old API surface (useSubtask / useUpdateSubtask) is gone — replaced by useLatestSubtaskMessage / useUpdateLatestMessage.

Changes

• core/tasks/derive.ts — new pure buildSubtaskMapFromMessages(messages). Handles multi-tool-call AI messages, missing matching tool messages (stays in_progress), and non-task tool_calls (ignored). Forward-compatible with Follow-up: use structured subtask status instead of parsing backend text #3146 / PR fix(subagent): structured subagent_status field over text parsing (#3146) #3154 — when parseSubtaskResult grows the second additional_kwargs argument the call site will start preferring the structured field automatically.
• core/tasks/context.tsx — provider trimmed to the latest-message map, exposes useLatestSubtaskMessage(id) and useUpdateLatestMessage().
• components/.../message-list.tsx — useMemo the derived map, removes the render-time updateSubtask loop, passes each task straight into SubtaskCard. Drops unused extractTextFromMessage import.
• components/.../subtask-card.tsx — takes task: Subtask as a prop, merges latestMessage from context via useLatestSubtaskMessage.
• core/threads/hooks.ts — onCustomEvent handler renames the import to useUpdateLatestMessage and calls it with (taskId, message).
• vitest.config.ts — include *.test.tsx, set environment: \"jsdom\", globals: true so the new React hook / render tests can run.
• package.json — jsdom + @testing-library/react + @testing-library/dom dev deps for the StrictMode regression test.

Tests

• tests/unit/core/tasks/derive-subtask-map.test.ts — 7 cases covering empty, in_progress seed, completed / failed flips, multi-tool-call, and non-task-tool-ignored.
• tests/unit/core/tasks/context.test.tsx — 4 cases including a <StrictMode> render that spies on console.error and asserts the "Cannot update a component while rendering" warning is not logged. That assertion is the regression test for the root cause of Follow-up: avoid mutating subtask state during render #3147.

Totals: backend unchanged, frontend 94 passed (was 76, +18 new).

Live verification

Started make dev, sent "use the task tool with general-purpose subagent to run echo derive_v2", watched the subtask card go from the shimmering in_progress state to the completed-with-output state, then saw the AI's final answer rendered cleanly.

A previous attempt at this PR (which kept base state in a context + effect, instead of passing the task in as a prop) reproduced Cannot read properties of undefined (reading 'status') in SubtaskCard on the very first render — the effect-based publish lagged behind the first paint. The prop-driven version this PR ships does not have that problem.

Out of scope (tracked by #3138 follow-ups)

• Follow-up: replace string-based export marker stripping with structured internal context filtering #3145 structured internal context filtering

[Copilot]

Pull request overview

This PR fixes React 19 Strict Mode warnings and subtask-card lifecycle glitches by removing render-time mutations of subtask state and switching to a pure “derive from messages” data flow, while keeping only the streaming task_running message in context.

Changes:

• Introduces a pure buildSubtaskMapFromMessages(messages) to derive base subtask fields from the thread message list.
• Refactors subtask context to store only latest streaming AI messages (task_running) and updates consumers accordingly.
• Adds/updates unit tests and Vitest configuration to run React hook/render tests under JSDOM.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
frontend/vitest.config.ts	Expands test glob to include TSX; sets JSDOM + globals for React tests.
frontend/tests/unit/core/tasks/derive-subtask-map.test.ts	Adds coverage for deriving subtask map from messages (tool-call + tool-result cases).
frontend/tests/unit/core/tasks/context.test.tsx	Adds provider/hook tests, including StrictMode regression coverage.
frontend/src/core/threads/hooks.ts	Updates SSE handler to write latest streaming message via new context API.
frontend/src/core/tasks/derive.ts	Adds pure derivation helper to compute subtask base state from messages.
frontend/src/core/tasks/context.tsx	Replaces old mutable subtask map API with latest-message-only context.
frontend/src/components/workspace/messages/subtask-card.tsx	Switches to prop-driven base task + merges latest streaming message from context.
frontend/src/components/workspace/messages/message-list.tsx	Removes render-time subtask mutations; derives map via useMemo and passes tasks into cards.
frontend/pnpm-lock.yaml	Locks new testing dependencies (jsdom, testing-library).
frontend/package.json	Adds jsdom and Testing Library dev dependencies for new React tests.

Files not reviewed (1)

• frontend/pnpm-lock.yaml: Language not supported