[Bug]: Grok model switching is blocked after a thread starts even though ACP supports it #8385

Before submitting

• I searched existing issues and did not find an active duplicate.
• I included a deterministic reproduction and a verified provider probe.

Related but not duplicate: #6075 tracks the invalid synthetic grok-build model ID. This issue concerns switching between valid models advertised by Grok ACP.

Area

apps/server

Steps to reproduce

1. Enable Grok in T3 Code and start a thread with an advertised model such as grok-4.6.
2. Complete one turn.
3. Open the model picker and select another advertised model such as grok-4.5.

Expected behavior

T3 should allow the selection and apply it to the active Grok session before the next prompt, matching Codex model switching.

Actual behavior

The model is disabled after the thread starts. T3 shows:

Start a new chat to change models
This provider does not allow switching models after a conversation has started.

The server also rejects an equivalent turn request, so bypassing the web guard does not help.

Impact

Minor bug or occasional failure

The thread remains usable, but changing models requires abandoning its existing context and starting a new thread.

Version or commit

Current main after #8358 (a6797b3b97dca6b6941574ff062d069c45c89b9a).

Environment

• T3 Code: current main
• Grok CLI: 1.0.5 stable
• Client: web or desktop

Root cause

GrokProvider still publishes:

requiresNewThreadForModelChange: true

The web model picker and ProviderCommandReactor both consume this flag and reject the change before GrokAdapter.sendTurn runs.

This guard no longer matches the adapter or Grok ACP:

• GrokAdapter.sendTurn already compares the requested model with the active model.
• When they differ, it calls session/set_model before session/prompt.
• A mock adapter test produced this request order after one completed turn:
  1. session/prompt
  2. session/set_model with the alternate model
  3. session/prompt
• A real Grok 1.0.5 probe completed a prompt and then successfully switched the same ACP session to a different advertised model.

Proposed fix

Remove requiresNewThreadForModelChange: true from Grok's provider presentation. Omission matches providers such as Codex and allows the existing web and server behavior to pass the changed selection to GrokAdapter.

No new switching implementation is needed.

Add focused coverage for:

• Grok's provider snapshot no longer publishing the lock.
• A started Grok thread being allowed to select another model in the web guard.
• GrokAdapter sending session/set_model between two completed prompts.
• An opt-in real Grok probe switching to a different advertised model after a completed turn.

Prior work

• feat(server): Grok Build effort, auth, rewind, and usage [WIP - for content] #6383 and its superseding feat(grok): Grok Build workflows, usage, rewind — supersedes #6383 #7070 both contained this exact lock removal, but both PRs were closed.
• fix(grok): improve skills, plans, usage, and turn reliability #8358 merged the current Grok reliability and reasoning work while retaining the original lock.
• No active issue or pull request currently carries the model-switch fix.

Acceptance criteria

• A started Grok thread can select another valid advertised Grok model.
• The next turn sends session/set_model before session/prompt.
• Existing providers that genuinely require new threads remain locked.
• Invalid or synthetic model IDs remain handled separately under [Bug]: Grok model slug grok-build advertised but rejected by Grok CLI ACP (session/set_model → Invalid params / unknown model id) #6075.

Workaround

Start a new Grok thread with the desired model.