Junie ACP Rejects Custom Model Prefixes

**Bug Report: Junie ACP Rejects Custom Model Prefixes That CLI Accepts**

Environment:

• Junie CLI version: Build 1755.37
• Zed extension: junie-acp-zed/1.3.7+stable.289.f1567cfdac6a41a750de42dd70e90a7b83af54e8
• Operating System: macOS 26.3.1 aarch64

**Summary:**
The Junie ACP protocol's session/set_model command rejects custom model identifiers with the custom: prefix e.g., custom:deepseek-v4-flash, even though the Junie CLI accepts the same format without issue.

**Steps to Reproduce:**

1. Configure a custom model profile in Junie e.g., ~/.junie/models/deepseek-v4-flash.json
2. Connect to Junie via Zed IDE using the Junie ACP extension
3. Select the custom model from the model dropdown in Zed
4. Observe that the model selection is rejected

**Expected Behavior:**
Selecting a custom model in Zed should work the same as using the CLI:

`junie --model="custom:deepseek-v4-flash" ping # Works`

**Actual Behavior:**
The Junie ACP logs show:

`WARN SessionConfigController - ACP[session/set_model]: Unsupported Junie model 'custom:deepseek-v4-flash'`

**Root Cause:**
The CLI path --model argument bypasses model validation entirely, passing the model name directly. However, the ACP session/set_model command goes through SessionConfigController, which validates against a hardcoded list of known Junie models and rejects any custom: prefixed identifiers.

**Additional Context:**

• Logs show RPC models endpoint not available, falling back to legacy authentication
• The custom model profiles are correctly configured in ~/.junie/models/
• The Zed extension is correctly sending the model name as selected by the user—the rejection occurs entirely on the Junie ACP side

**Severity:**
Medium — Prevents users from selecting custom models via the Zed extension, forcing them to use the CLI directly for model selection.