[juliusmarminge]

Problem

/side <question> ignored the model picked in the composer and started the side chat on the thread's saved model. Repro on main (6f54322e4a), web dev server, fresh database: send one message on GPT-6-Astra, switch the composer's picker to GPT-6-Luna (or Claude Haiku 5.5) without sending, then send /side Reply with the single word: ping. The side chat's first run used codex/gpt-6-astra in both cases, while the composer still showed the picked model.

Change

The composer's model selection is only a client draft until a message is sent, and the /side path never forwarded it: the side chat's first startTurn had no modelSelection, so the server used the model the fork copied from the parent.

• Web: useComposerSend passes sendCtx.selectedModelSelection (the same selection a normal send uses) to onSideChat; ChatView hands it to useSideChat.open, which adds it to the side chat's first startTurn. When the composer has no usable provider the selection is the no-provider placeholder, so nothing is passed and the side chat keeps the model it forked with. The prompt-less "open side chat" entry points send no turn and are unchanged.
• Mobile: useThreadComposerState passes draft.modelSelection ?? thread.modelSelection (the normal send's value) to useStartSideChat, which adds it to startTurn.

No server or contract change: modelSelection on a turn start already exists, and the server already handles a provider switch on a side chat's first turn (portable context handoff).

Scope and approval

Fixes #31831

Verification

• vp test run src/components/ChatView.browser.test.tsx (apps/web): new test /side starts the side chat on the model picked in the composer fails on main with expected undefined to deeply equal { instanceId: 'codex', model: 'gpt-5.5' } and passes with the fix (2/2 in the file). The shared web test fake (apps/web/test/fakeEnvironment.ts) gains an opt-in side-chat capability, a second Codex model, command answers and command recording for it.
• vp test run src/state/thread-task-permissions.test.ts (apps/mobile): the existing /side test now picks a model different from the thread's and asserts it reaches startSideChat (11/11).
• vp run typecheck in apps/web and apps/mobile; vp run check:changed.
• Real app, web dev server, fresh database each side, 1280×800, light, same steps on both sides (before 6f54322e4a, after the branch head). Persisted model of each side chat's first run (orchestration_v2_projection_runs):

Row	Before (6f54322e4a)	After (branch head)
F1: thread on GPT-6-Astra, composer GPT-6-Luna, /side …	codex/gpt-6-astra	codex/gpt-6-luna
F2: thread on GPT-6-Astra, composer Claude Haiku 5.5, /side …	codex/gpt-6-astra	claudeAgent/claude-haiku-5-5 (context handoff 6 Astra → Haiku 5.5 shown in the side chat)

The side chat (right panel) composer chip shows the model its run used; the main composer shows the pick.

Row	Before (6f54322e4a)	After (branch head)
F1
F2

Capture environment: T3 preview browser (Chromium), no device emulation, DPR 1, 1280×800, light theme; each side's own dev server on a fresh database (T3CODE_DEV_SEED=0), driven through the UI with the steps above. No diff images: the before/after differ by design in the run model shown, and the frames come from separate live runs.

Not checked: the mobile client on a device (covered by the mobile unit test and typecheck only), desktop (wraps the web code path).

AI-assisted; verified with the tests and tools listed above.

[macroscopeapp]

Approvability

Verdict: Approved at 93d3340

Macroscope's review found this PR approvable — The /side command now uses the composer's selected model on web and mobile, while retaining existing fallback and failure handling. The change is limited to explicitly initiated side-chat sends and leaves product defaults unchanged.

^{You can add or adjust custom eligibility rules. Learn more.}

[coderabbitai]

No actionable comments were generated in the recent review.

Walkthrough

The /side flow now passes the composer’s model selection to the side chat’s first turn on mobile and web. Web browser coverage verifies that the selected model is included in the dispatched turn.

Changes: Side-chat model selection

Layer / File(s)	Summary
Web composer-to-side-chat model flow apps/web/src/components/chat/useComposerSend.ts, apps/web/src/components/ChatView.tsx, apps/web/src/components/chat/useSideChat.ts	The web composer passes an available model selection to the side-chat turn. If no selection is provided, the turn input omits it.
Mobile composer-to-side-chat model flow apps/mobile/src/state/use-thread-composer-state.ts, apps/mobile/src/features/threads/useStartSideChat.ts, apps/mobile/src/state/thread-task-permissions.test.ts	The mobile /side command passes the draft model selection, falling back to the thread’s selection. The test checks that startSideChat receives the selection.
Web browser coverage and fake side-chat environment apps/web/test/fakeEnvironment.ts, apps/web/src/components/ChatView.browser.test.tsx	The fake environment supports configurable side chats and records dispatched commands. The browser test checks that the fork’s first turn uses the selected model.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant Composer
  participant useComposerSend
  participant ChatView
  participant useSideChat
  Composer->>useComposerSend: Submit /side with available model selection
  useComposerSend->>ChatView: Pass prompt and model selection
  ChatView->>useSideChat: Open side chat with prompt and model selection
  useSideChat->>useSideChat: Include model selection in the turn input when provided

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Suggested reviewers: t3dotgg

Merge Risk: Minimal · up to 93d33

The change is mergeable after normal checks. A narrow mobile submission path may briefly create a side chat for an unavailable model, but its turn is rejected and cleanup is requested.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly describes the model-selection fix for web and mobile side chats.
Description check		The description is mostly complete. It explains the problem, change, scope, and verification. It links an issue but does not establish triage or approval, or explain why the fix qualifies without prio…
Linked Issues check		[#31831] Web and mobile pass the composer's model selection to the side chat's first turn. Web omits the placeholder when no provider is usable. The web browser test checks the dispatched turn; the mo…
Out of Scope Changes check		[#31831] The web and mobile changes implement the requested model selection. The tests and fake-environment changes support verification of that behavior.

Full details: Description check

Explanation

The description is mostly complete. It explains the problem, change, scope, and verification. It links an issue but does not establish triage or approval, or explain why the fix qualifies without prior approval. It also does not name the AI model and harness.

Full details: Linked Issues check

Explanation

[#31831] Web and mobile pass the composer's model selection to the side chat's first turn. Web omits the placeholder when no provider is usable. The web browser test checks the dispatched turn; the mobile test checks the selection passed to the side-chat hook.

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Recent review info

Run configuration

• Source mode: Auto
• Repository: pingdotgg/t3code
• Branch: fix/issue-31831
• File: .coderabbit.config.ts
• Commit: 93d3340
• Layers: Repository TS config
• Review profile: CHILL
• Plan: Team
• Run ID: 0abe7e5f-8cfa-4b91-8596-0f53003b3a6a

Commits

Reviewing files that changed from the base of the PR and between 5c004fd and 93d3340.

Files selected for processing: 8

• apps/mobile/src/features/threads/useStartSideChat.ts
• apps/mobile/src/state/thread-task-permissions.test.ts
• apps/mobile/src/state/use-thread-composer-state.ts
• apps/web/src/components/ChatView.browser.test.tsx
• apps/web/src/components/ChatView.tsx
• apps/web/src/components/chat/useComposerSend.ts
• apps/web/src/components/chat/useSideChat.ts
• apps/web/test/fakeEnvironment.ts

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 8 remain after this review.

[juliusmarminge]

Thanks @juliusmarminge for the fix and thorough verification! #32315 by @Iweisc landed as e455200077 and already carries the composer model selection into side chat turns on web and mobile, so I’m closing this as covered.