[Bug] Opus model regression: Degraded performance on complex Rust tasks

Bug Description
Opus 4.6 keeps failing complex tasks in Rust. Never survives a code review by Codex. While asking Codex to fix something that Claude kept failing after several attempts, they fixed it in one shot. I don't know what was changed but, a few weeks ago, Claude felt unstoppable, now it is really sub-par. Hopefully, you guys can fix this because I love working with Claude.

Environment Info

• Platform: darwin
• Terminal: tmux
• Version: 2.1.97
• Feedback ID: 3d9414ca-12df-45bc-8742-8d0708ab279c

Errors

[{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server github invalid: Missing environment variables: GITHUB_PERSONAL_ACCESS_TOKEN\n    at XwH (/$bunfs/root/src/entrypoints/cli.js:1465:34314)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-13T19:53:03.141Z"},{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server github invalid: Missing environment variables: GITHUB_PERSONAL_ACCESS_TOKEN\n    at XwH (/$bunfs/root/src/entrypoints/cli.js:1465:34314)\n    at async W (/$bunfs/root/src/entrypoints/cli.js:5857:10283)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-13T19:53:07.100Z"},{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server github invalid: Missing environment variables: GITHUB_PERSONAL_ACCESS_TOKEN\n    at XwH (/$bunfs/root/src/entrypoints/cli.js:1465:34314)\n    at async Z (/$bunfs/root/src/entrypoints/cli.js:5857:11053)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-13T19:53:07.101Z"},{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server github invalid: Missing environment variables: GITHUB_PERSONAL_ACCESS_TOKEN\n    at XwH (/$bunfs/root/src/entrypoints/cli.js:1465:34314)\n    at async W (/$bunfs/root/src/entrypoints/cli.js:5857:10283)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-13T20:41:48.298Z"},{"error":"Error: Plugin MCP server error - mcp-config-invalid: MCP server github invalid: Missing environment variables: GITHUB_PERSONAL_ACCESS_TOKEN\n    at XwH (/$bunfs/root/src/entrypoints/cli.js:1465:34314)\n    at async Z (/$bunfs/root/src/entrypoints/cli.js:5857:11053)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-04-13T20:41:48.300Z"}]