[juliusmarminge]

Problem

A detached thread could not switch providers when the outgoing provider had never returned a native thread reference, even though the server could hand off its saved history.

Change

Allow the existing portable handoff once there is no live session or active run. Keep negotiated session capabilities and the active-run guard authoritative. Web, desktop and mobile consume the shared helper; no server or adapter behavior changes.

Scope and approval

This is a focused fix to the existing provider-switch workflow. Retains the work by @FabioLukas123 in #19687, with the pinned main revision merged and original commits preserved. It adds no providers, model IDs or wire fields.

Verification

• The regression is one changed assertion in an existing test. It fails without the fix; ten redundant PR-added cases were removed without deleting main tests.
• 22 focused shared/mobile tests pass. Existing tests still reject active runs and sessions that cannot hand off; a guard-removal probe makes them fail.
• 11 server planner tests and five existing portable-text-handoff integration scenarios pass. An independent review also passed 170 targeted shared/mobile/server/web tests. Those existing integration scenarios cover the transfer path; the live fixture below covers the missing-native-reference case.
• Targeted lint, formatting and client-runtime typecheck pass. Independent Opus and Sol full-diff reviews found no blockers.
• Root compared 952921f39a with 798ab31708 in the actual web app using a synthetic detached thread with no native reference. The alternate provider is disabled before and enabled after. A session denying handoff and an active run each keep it disabled.

Before: alternate provider disabled	After: alternate provider enabled

Control: live session denies handoff	Control: active run blocks switching

Root selected Fixture Handoff Target and sent through the real app pipeline to a copied repository Codex mock peer. The transport record contains thread creation, the exact prior-history marker T3_19687_CONTEXT_MARKER in thread/inject_items, and a completed subsequent turn. This proves history reached the target transport; it makes no claim about external model comprehension.

No external model or network call was made by the mock. Native desktop/mobile shells were not independently exercised. An identical temporary startup-recovery bypass preserved synthetic control rows on both variants; it was removed, the isolated database/settings restored, and the test server stopped.

Current-head CI runs after this update; earlier checks cover the original head only.

Implementation: Claude Fable 5.1. Test assessment and independent review: Claude Opus 5.5. Independent review and coordination: GPT-6.1 Sol, through Claude Code and Codex in T3 Code.

Takeover

Credit to @FabioLukas123 for identifying the detached-session regression in #19687.
Rebuilt on the pinned main revision: remove only the native-reference prerequisite; keep the source provider-thread association so saved history is handed off rather than silently omitted.
The original description above is preserved as historical context; verification below describes this replacement at 271e7b6f11.

Evidence

Before 3eb8366 (merge-base), after 271e7b6 (head). Real web app per checkout, fresh isolated state (T3CODE_DEV_SEED=0), synthetic data; source completed with sentinel T3_24641_HISTORY, detached through RPC, then nativeThreadRef nulled with the server stopped. Source is mock Grok, target mock Codex. Chromium 156/Linux, no device emulation, DPR 2, frozen clock, settled frames (two identical frames 500 ms apart). Full sheet with every row, diff image and script: Evidence.md, fixture matrix matrix.md.

F01 detached, null native ref	Before 3eb8366	After 271e7b6	Diff0	Diff16
Open picker, 1440×1000 light
Composer, 1440×1000 light
Open picker, 390×844 dark
Composer, 390×844 dark

Recordings (open picker → try target → search gpt-5.6-sol → select if allowed → send on head): 1440 before / after, 390 before / after. Before stays on the source; after selects the target, shows "Context handoff pending", sends, and the target transport receives the sentinel in thread/inject_items and completes (transport record). F02 control (native ref present, head): recording.

Guard controls on this same head (session denying handoff, active run, missing source association) are from the earlier round: controls, audited SUFFICIENT.

What else changed: enabled Codex/Claude icons move ahead of the selected Grok icon; the composer label and runtime-control position follow the shorter target name; the existing handoff-pending line appears. Not from this PR: a left-edge "=" glyph in the 1440 after-send still, mock-version notices in the F02 clip, a host-overload toast at the start of the raw 390 before clip (excluded from the stills).

Scripts and fixtures (synthetic, not committed): seed.mjs, client.mjs, null-ref-all.sql, browser-capture-driver.js, diffs.py, all scripts, fixtures, reproduce.md.

Verification log

• No code changes this round; head unchanged at 271e7b6. Tests as described above (regression assertion fails without the fix).
• Earlier rounds: two review rounds, final round both MERGE; proof audit pass 3 SUFFICIENT for web at this head.
• This round: proof audit pass 1 derived a 15-row matrix; rows exercising picker layout only (long lists, unavailable source, keyboard routes, awaiting projection) are not affected by this diff. Captured F01 at 1440 light and 390 dark with history delivery, and F02. Proof audit pass 2 not run this round (time budget).
• Unverified: native mobile (Android Pixel 9, iOS iPhone 17 Pro). The Android dev client build did not finish in budget and the iOS simulator build failed on a missing GhosttyKit (error). Mobile consumes the same shared helper via apps/mobile/src/state/thread-provider-switching.ts, covered by its unit tests.
• CI: all 39 checks green on this head.
• Models/harness: Claude Opus 5.5 lead; GPT-6.1 Sol proof audit and capture, through T3 Code Claude and Codex harnesses.

[macroscopeapp]

Approvability

Verdict: Approved at 4cef96c

Macroscope's review found this PR approvable — Allows users to switch providers after detaching a session even when its native thread reference is absent. Existing session-capability, active-run, and source-association safeguards remain, and no provider defaults change.

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

[coderabbitai]

No actionable comments were generated in the recent review.

Walkthrough

The provider handoff eligibility check no longer requires a native thread reference on the active provider thread. The test now expects eligibility when that reference is absent and verifies that an empty provider thread list does not qualify.

Changes: Provider handoff eligibility

Layer / File(s)	Summary
Update handoff eligibility and test packages/client-runtime/src/state/threadWorkflows.ts, packages/client-runtime/src/state/threadWorkflows.test.ts	A matching active provider thread can qualify for handoff without a native thread reference. The test covers this case and verifies that an empty provider thread list does not qualify.

Estimated code review effort: 2 (Simple) | ~8 minutes

Change: Bug fix

Suggested reviewers: t3dotgg, yash-singh1

Merge Risk: Minimal · up to 4cef9

Detached threads can now hand off their saved history to a different provider even when the previous provider has no native thread reference. Session capability and active-run guards are unchanged, and no merge-blocking risk was found.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title names the main change: detached threads can now hand off portable provider history after detach. It uses a conventional commit format. The changeset removes the native-thread-reference prere…
Description check		The description follows the repository template. It has Problem, Change, Scope, and Verification sections with substantive content. Scope links issue #19687 and explains the takeover. Verification giv…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title names the main change: detached threads can now hand off portable provider history after detach. It uses a conventional commit format. The changeset removes the native-thread-reference prerequisite in threadSupportsProviderHandoff and updates the matching test assertion, which fits the title.

Full details: Description check

Explanation

The description follows the repository template. It has Problem, Change, Scope, and Verification sections with substantive content. Scope links issue #19687 and explains the takeover. Verification gives specific test counts, a before/after regression result, and live evidence. Minor gaps: the template asks for focused tests and states what could not be checked, and the author did state unverified native mobile shells. The description is also long and includes material outside the template's intent, such as evidence hosted on external links and a long verification log. These do not make it incomplete.

Finishing Touches

Generate unit tests (beta)

• Commit to this branch
• Create a new PR

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

Recent review info

Run configuration

• Source mode: Auto
• Repository: pingdotgg/t3code
• Branch: takeover/pr-19687
• File: .coderabbit.config.ts
• Commit: 4cef96c
• Layers: Repository TS config
• Review profile: CHILL
• Plan: Team
• Run ID: 122e51c7-f8df-49dd-9f33-7905db1dba05

Commits

Reviewing files that changed from the base of the PR and between 271e7b6 and 4cef96c.

Files selected for processing: 2

• packages/client-runtime/src/state/threadWorkflows.test.ts
• packages/client-runtime/src/state/threadWorkflows.ts

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 3 remain after this review.

[juliusmarminge]

Taking over this one

Dismissing prior approval to re-evaluate 4cef96c

[juliusmarminge]

Evidence

Complete a Replay todo-list turn, settle the thread, stop the server, clear only its source provider-thread nativeThreadRef, restart, and open a freshly paired incognito tab. Open the model picker and click Codex. Base blocks switching; head permits selecting GPT-6-Luna. Sending a request for the previous assistant sentence completes with the correct historical text and a visible context handoff.

Environment: real web client, Chromium incognito, 1280×800 CSS pixels, light theme, separate fresh databases. Both selected stills passed pixel-identical stability checks at least 500 ms apart.

Before	After
25c5032b3b	4cef96c259
Picker interaction recording	Enabled picker recording, send and response recording

Offline fixture script · Seed commands and browser procedure · Synthetic persisted state evidence

Not shown:

• A natural user flow producing the missing native reference was not established; the reference was cleared offline by hand, matching triage.
• Mobile and Electron were not exercised; this proof covers the real web client.
• Sending after switching on base could not run because the picker blocks switching; no bypass was used.