opus 4.8 xhigh acting very dumb on toy reinforcement learning

Bug Description
I have lots of experience with Opus 4.8 and it is normally very capable.
I am retired, working on a retirement project to learn AI and coding a Reinforcement Learning
solution to learn using Opus 4.8 xHigh Thinking. Opus is making all kinds of mistakes, poor decisions. It feels like i am using a model of a year ago.

Environment Info

• Platform: linux
• Terminal: vscode
• Version: 2.1.172
• Feedback ID: 0bc4b1c7-0523-45ac-bf70-68d252936bf0

Errors

[{"error":"Error: 404 {\"type\":\"error\",\"error\":{\"type\":\"not_found_error\",\"message\":\"Claude Fable 5 is not available. Please use Opus 4.8. Learn more: https://www.anthropic.com/news/fable-mythos-access\"},\"request_id\":\"req_011Cc1fSidcP7vJBzX7MsFVB\"}\n    at generate (/$bunfs/root/src/entrypoints/cli.js:12:66109)\n    at makeRequest (/$bunfs/root/src/entrypoints/cli.js:52:7694)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-06-13T17:36:10.743Z"},{"error":"Error: 404 {\"type\":\"error\",\"error\":{\"type\":\"not_found_error\",\"message\":\"Claude Fable 5 is not available. Please use Opus 4.8. Learn more: https://www.anthropic.com/news/fable-mythos-access\"},\"request_id\":\"req_011Cc1fiJU1TG4Kjr2hekAC6\"}\n    at generate (/$bunfs/root/src/entrypoints/cli.js:12:66109)\n    at makeRequest (/$bunfs/root/src/entrypoints/cli.js:52:7694)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-06-13T17:39:42.134Z"},{"error":"Error: 404 {\"type\":\"error\",\"error\":{\"type\":\"not_found_error\",\"message\":\"Claude Fable 5 is not available. Please use Opus 4.8. Learn more: https://www.anthropic.com/news/fable-mythos-access\"},\"request_id\":\"req_011Cc1g2Ktq4xRZbrE85pmn2\"}\n    at generate (/$bunfs/root/src/entrypoints/cli.js:12:66109)\n    at makeRequest (/$bunfs/root/src/entrypoints/cli.js:52:7694)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-06-13T17:43:46.691Z"},{"error":"Error: 401 The socket connection was closed unexpectedly. For more information, pass `verbose: true` in the second argument to fetch()\n    at generate (/$bunfs/root/src/entrypoints/cli.js:12:66035)\n    at makeRequest (/$bunfs/root/src/entrypoints/cli.js:52:7694)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-06-20T15:12:42.457Z"}]