[adampeterhiggins]

Switching models in a thread leaves a "Model changed" divider, but changing a parameter of the same model (effort, fast mode, context window, thinking, OpenCode agent/variant, and so on) left nothing in the transcript, and the divider never said which parameters either side ran with.

How

• One divider for every change. The model_change turn item is now recorded when the model or any of its options change, and it always renders as Model changed. Each side names the model and its parameters, labelled exactly as the composer's traits picker labels them, e.g. Claude Opus 5.5 High → Claude Opus 5.5 Max Fast, or Claude Opus 5.5 High → Claude Sonnet 5.5 High for a model switch.
• Provider-agnostic. Labels and defaults come from the option descriptors every provider already advertises, so Codex, Claude, Cursor, Grok, OpenCode, Antigravity, ACP and the rest work without per-adapter code.
• No phantom markers. With descriptors (clients), an omitted option resolves to the provider default, prompt-injected values such as Claude's ultrathink resolve to the default they dispatch as, and stale options the model doesn't offer are ignored. The orchestrator stays pure, so it compares raw values; if that records something the client can't resolve to a real difference, the row renders nothing.
• No contract change. The item already carried the full from/to ModelSelection. The steer-restart path already compared full selections.
• buildTraitsTriggerDisplay moved unchanged from the web TraitsPicker to @t3tools/shared/model so mobile's ThreadModelChangeRow renders the same text. Mobile's activity preview/inspector and the MCP thread text describe both selections with their options.
• Updated docs/fork-differences.md.

Screenshots

Same thread on Claude Opus 5.5 at High effort, composer changed before sending.

Before: effort → Max + Fast Mode on, no marker	After: same change	After: model switch

Verification

• vp test run for modelChangeMarker.test.ts (option-only change recorded; explicit default and ultrathink produce no pending divider; composer-style labels) and TraitsPicker.test.ts, plus server orchestration/MCP suites, mobile threadActivity* and web session-logic tests.
• tsc-rs --noEmit passes for shared, server and mobile. Web's only error is pre-existing, in terminal/ghostty/surface.test.ts.
• Checked the pending divider in the web client against a copy of real thread data (screenshots above). Mobile not checked in a simulator.

🤖 Generated with Claude Code