Cursor ACP in Zed: Composer 2.5 Fast Mode Issue

[niallobrien]

Reproduction steps

Cursor ACP in Zed: Composer 2.5 Fast Mode Issue

Date: 2026-05-23

Summary

When using Cursor as an external ACP agent inside Zed, selecting Composer 2.5 still produced Cursor dashboard usage entries marked as fast. The local Cursor config also repeatedly reverted to:

{
  "selectedModel": {
    "modelId": "composer-2.5",
    "parameters": [{ "id": "fast", "value": "true" }]
  }
}

The working fix was to run Cursor through a local wrapper that patches the ACP initialize request so Cursor exposes parameterized model options to Zed. With that enabled, Zed can set:

{
  "model": "composer-2.5",
  "fast": "false"
}

Symptoms

• Zed launched Cursor ACP successfully.
• ~/.cursor/cli-config.json kept being rewritten to Composer 2.5 Fast.
• Cursor dashboard usage at https://cursor.com/dashboard/usage reported fast after new Zed agent requests.
• Passing --model composer-2.5[fast=false] acp to cursor-agent was not enough once Zed connected to the ACP server.

Attempts That Did Not Work

Launch arg only

Zed settings:

{
  "agent_servers": {
    "cursor": {
      "type": "custom",
      "command": "/path/to/cursor-agent",
      "args": ["--model", "composer-2.5[fast=false]", "acp"]
    }
  }
}

Result: the process argv was correct, but after the ACP session was created Cursor rewrote its selected model to fast=true.

Zed default model as base model

"default_config_options": {
  "model": "composer-2.5",
  "fast": "false"
}

Zed log:

WARN [agent_servers::acp] `composer-2.5` is not a valid value for config option `model` in cursor

Zed default model as variant string

"default_config_options": {
  "model": "composer-2.5[fast=false]"
}

Zed log:

WARN [agent_servers::acp] `composer-2.5[fast=false]` is not a valid value for config option `model` in cursor

Root Cause

Cursor ACP has two model-picker modes.

Without parameterized model picker

When the ACP client does not advertise:

{
  "_meta": {
    "parameterizedModelPicker": true
  }
}

Cursor returns a single model config option whose valid values are exploded variant strings. In this mode, the Composer 2.5 entry advertised by Cursor was:

{
  "value": "composer-2.5[fast=true]",
  "name": "composer-2.5"
}

There was no advertised composer-2.5[fast=false] entry for Composer 2.5 in this list. That explains both Zed validation failures:

• composer-2.5 is invalid in variant-list mode.
• composer-2.5[fast=false] is also invalid because Cursor does not advertise it in that mode.

With parameterized model picker

When the client advertises:

{
  "_meta": {
    "parameterizedModelPicker": true
  }
}

Cursor returns:

{
  "id": "model",
  "currentValue": "composer-2.5",
  "options": [{ "value": "composer-2.5", "name": "Composer 2.5" }]
}

and separately:

{
  "id": "fast",
  "currentValue": "false",
  "options": [
    { "value": "false", "name": "Off" },
    { "value": "true", "name": "Fast" }
  ]
}

In this mode, model=composer-2.5 plus fast=false is valid and works.

Local Workaround

Created this wrapper:

~/.local/bin/cursor-agent-zed-nonfast

The wrapper:

1. Starts the real Cursor agent with --model composer-2.5[fast=false] acp.
2. Proxies newline-delimited ACP JSON between Zed and Cursor.
3. Patches the client-to-agent initialize request to add:

{
  "clientCapabilities": {
    "_meta": {
      "parameterizedModelPicker": true
    }
  }
}

4. Forces ~/.cursor/cli-config.json back to composer-2.5 with fast=false before start and after exit.

Zed settings now point at the wrapper:

{
  "agent_servers": {
    "cursor": {
      "type": "custom",
      "command": "/Users/johndoe/.local/bin/cursor-agent-zed-nonfast",
      "args": [],
      "default_config_options": {
        "model": "composer-2.5",
        "fast": "false"
      }
    }
  }
}

Verification

Direct ACP probe against the wrapper returned:

{
  "models": {
    "currentModelId": "composer-2.5"
  },
  "configOptions": [
    {
      "id": "model",
      "currentValue": "composer-2.5"
    },
    {
      "id": "fast",
      "currentValue": "false"
    }
  ]
}

After using Zed, the wrapper log showed Zed invoking the wrapper and the wrapper patching the initialize capabilities:

{
  "event": "initialize.patched",
  "payload": {
    "fs": { "readTextFile": true, "writeTextFile": true },
    "terminal": true,
    "auth": { "terminal": true },
    "_meta": {
      "terminal_output": true,
      "terminal-auth": true,
      "parameterizedModelPicker": true
    }
  }
}

Cursor dashboard usage then stopped reporting Composer 2.5 requests as fast.

Suggested Fix For Zed

For ACP agents, Zed should consider advertising parameterized model picker support in clientCapabilities._meta when initializing the agent:

{
  "clientCapabilities": {
    "_meta": {
      "parameterizedModelPicker": true
    }
  }
}

At least for Cursor ACP, this is required for model parameters such as Composer's fast flag to be represented as separate config options. Without it, Cursor falls back to an exploded variant list where composer-2.5[fast=false] is not advertised, so Zed cannot select the non-fast Composer 2.5 configuration.

Files Changed Locally

• /Users/johndoe/.config/zed/settings.json
• /Users/johndoe/.local/bin/cursor-agent-zed-nonfast
• ~/.cursor/cli-config.json
• ~/.cursor/acp-config.json

Current vs. Expected behavior

Set to off instead of fast

Zed version and system specs

Zed: v1.3.6
OS: Mac 26.5
Memory: 36GB

Attach Zed log file

Zed.log

Relevant Zed settings

settings.json

Relevant Keymap

keymap.json

(for AI issues) Model provider details

No response

If you are using WSL on Windows, what flavor of Linux are you using?

None

[yebei199]

Ran into the same root cause on Linux (NixOS, Zed 1.3.5), but the symptom is broader than fast mode: the cursor ACP server ignores all UI model selection — not just parameters within a model, but switching between models entirely.

Symptom

Any model chosen in the Zed cursor agent panel is ignored. Actual requests always go to whichever model is stored in ~/.cursor/cli-config.json → selectedModel (in our case gpt-5.5-medium). The UI selection is written to ~/.cursor/acp-config.json → selectedModelVariantId, but has no effect on the actual requests sent.

Confirmed root cause (matches this issue exactly)

1. cursor-agent acp (bundled at ~/.local/share/zed/external_agents/registry/cursor/v_2026.05.20_.../dist-package/) runs getModelPickerMode() at session init.
2. Zed 1.3.5 does not advertise _meta.parameterizedModelPicker: true in the ACP initialize handshake → cursor falls back to "variants" mode.
3. In variants mode, applyVariantModelConfigOption() matches by variantStringRepresentation — this match silently fails in our configuration, so modelManager.currentModel is never updated, and stays at the cli-config.json selectedModel value for the entire session.

Live experiment

• cli-config.selectedModel = "gpt-5.5", UI switched to gpt-5.4-mini → acp-config.selectedModelVariantId updated, but actual requests still went to gpt-5.5.
• After clearing hasChangedDefaultModel and setting cli-config.selectedModel = "gpt-5.4" → all requests now go to gpt-5.4 regardless of UI choice. Confirms the model is pinned to cli-config, not driven by UI.

Workaround (same approach as described here)

A JSON-RPC proxy wrapper that patches the ACP initialize request to inject clientCapabilities._meta.parameterizedModelPicker: true, then proxies all messages transparently. This causes cursor to enter parameterized mode where model switching works correctly.

Tracking our nixos_config side of this at yebei199/nixos_config#57.

Suggested fix (same as proposed above)

Zed should advertise parameterizedModelPicker: true in clientCapabilities._meta during ACP initialize. Without it, the cursor ACP server's variants-mode model picker is broken for model switching (not just parameter tuning), making the UI model selector effectively a no-op.

[nunopi]

is there a different workaround that doesn't require a proxy wrapper?

[niallobrien]

is there a different workaround that doesn't require a proxy wrapper?

I've no idea tbh. This doesn't even fix all of the issues with the Cursor agent in Zed either - I just needed a way to disable fast mode.

[morgankrey]

#59694