Cursor CLI ACP ignores `--model cursor-grok-4.6-high` and runs the Fast variant (`grok-4.6[effort=high,fast=true]`): send `parameterizedModelPicker` in initialize

Context

Source(s): https://forum.cursor.com/t/cursor-cli-acp-reports-grok-4-6-fast-despite-explicitly-selecting-the-listed-non-fast-model/174835 Signal: 48 views; staff answer accepted, and the original poster confirmed the workaround the same day in a direct PowerShell ACP test (October 2026). Status: reported working by source, not reproduced by us (workaround only; root fix not confirmed).

Problem

agent --list-models lists cursor-grok-4.6-high (Grok 4.6) and cursor-grok-4.6-high-fast (Grok 4.6 Fast) as separate models. Starting ACP with --model cursor-grok-4.6-high acp creates the session without any error, but session/new reports the Fast variant in models.currentModelId and in the model config option's currentValue, and the ACP model catalog lists only Fast variants for the Grok and Composer families. Staff confirmed the requests really go to the Fast variant.

Error (redacted / normalized)

models.currentModelId:
grok-4.6[effort=high,fast=true]

Applies when

Cursor CLI 2026.10.14 on Windows (native, PowerShell), launched directly and through OpenHands Agent Canvas.

Confirm it is this problem

1. Start agent --model cursor-grok-4.6-high acp, send initialize without extra client capabilities, then session/new. → expect: currentModelId is grok-4.6[effort=high,fast=true].

Not this when / boundary

Your ACP client can't send extra initialize capabilities. The original poster found that OpenHands Agent Canvas calls initialize without this capability and has no setting for it, so the workaround can't be applied there until the integration adds support.

Root cause

Per Cursor staff: when an ACP client connects without Cursor's advanced model picker, ACP offers only one variant per model, its default, and for Grok 4.6 the default is Fast. The fast=false from --model is silently replaced with the default instead of being applied or returning an error. Staff called it not intended behavior and a tracked issue.

Fix

1. In your ACP client's initialize request, add "_meta": {"parameterizedModelPicker": true} inside clientCapabilities (staff).
2. Send session/new as usual. → expect: the model shows as grok-4.6 with separate config options for effort and fast. With --model cursor-grok-4.6-high, the original poster got Effort high and Fast false straight away.
3. Only if fast is not already false: send session/set_config_option with {"sessionId": "<id from session/new>", "configId": "fast", "value": "false"}, and the same with "configId": "effort", "value": "high" if effort is not high yet (staff).

How to verify

Before the first prompt, the returned config options show currentValue "false" for fast (staff).

What did not work

Passing the non-Fast model with --model alone does not change the variant (the original poster).

Risks

Without the workaround, requests from a session you believe is on the standard tier are actually sent to the Fast variant (staff), so they may be billed and rate-limited as the Fast tier (our inference); staff pointed to support for usage already spent that way. Check currentValue before long runs (our inference).

Verified scope

Cursor CLI 2026.10.14 ACP mode, October 2026.

Becomes obsolete when

Cursor's ACP honours a non-Fast --model or returns a clear error instead of silently switching.