[BUG] Download failed. Check your connection when starting new session — 404 on model config

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Problem

Cannot start new sessions in Claude Code desktop app (macOS).
Error shown: "Download failed. Check your connection and try again."
Existing sessions continue to work normally.

Logs (~/Library/Logs/Claude/claude.ai-web.log)

Error 1 — model config 404:
endpoint: /api/organizations/.../model_configs/claude-opus-4-6[1m]
statusCode: 404
message: "Not found"

Error 2 — repeated fetch failures:
TypeError: Failed to fetch (assets-proxy.anthropic.com)

Steps to reproduce

1. Open Claude Code desktop app
2. Click "New session"
3. Error appears immediately

Environment

• macOS Darwin 25.2.0
• Tried: restart app, restart Mac, cleared cache — no effect
• Network: api.anthropic.com responds (curl returns HTTP/2 404 as expected)

What Should Happen?

Starting a new session should work normally — the app should connect to the Anthropic API, load the selected model config, and open an empty chat ready for input.

Instead, clicking "New session" shows "Download failed. Check your connection and try again." immediately. Existing sessions continue to work fine, which suggests the issue is specifically in the new-session initialization flow (model config fetch + asset proxy).

Logs show two related errors:

1. 404 Not Found on /api/organizations/{org_id}/model_configs/claude-opus-4-6[1m]
2. TypeError: Failed to fetch against assets-proxy.anthropic.com

App version: 1.1062.0 (macOS Darwin 25.2.0)
Restarting app, restarting Mac, and clearing cache did not help.
Direct curl to api.anthropic.com works fine.

Error Messages/Logs

Steps to Reproduce

1. Open Claude Desktop app (version 1.1062.0 on macOS)
2. Inside an existing session, run: /model claude-opus-4-6[1m]
3. Close the app completely (Cmd+Q)
4. Reopen the app
5. Click "New session" (or try to start any new conversation)

Expected: New session opens normally
Actual: Error "Download failed. Check your connection and try again."

The app appears to persist the selected model name including the [1m] suffix, then tries to fetch its config from:
/api/organizations/{org_id}/model_configs/claude-opus-4-6[1m]

This returns 404 (likely because brackets are not URL-encoded, or because this model variant is not available for my org). The error blocks new session creation entirely, even though existing sessions still work.

Attempted workarounds that did NOT help:

• Restart the app (Cmd+Q + reopen)
• Restart the Mac
• Clear ~/.claude/cache

Environment:

• App version: 1.1062.0
• OS: macOS Darwin 25.2.0
• Network: api.anthropic.com reachable (curl -I returns HTTP/2 as expected)

Log excerpt from ~/Library/Logs/Claude/claude.ai-web.log:
[REACT_QUERY_CLIENT] QueryClient error: {"type":"not_found_error","statusCode":404,
"endpoint":"/api/organizations/.../model_configs/claude-opus-4-6[1m]",
"method":"GET","message":"Not found"}

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude Desktop версия: 1.1062.0

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_