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gsxdsm 4435ca2c54 FN-6158: handle Codex auth-tier model incompatibility

Gracefully recover from unsupported Codex model selections on ChatGPT-account auth tiers.

- detect Codex and general model-auth-tier incompatibility errors as retryable model-selection failures
- trigger configured fallback models during session creation and prompt-time retries, with actionable operator guidance when no fallback works
- document the expanded fallback behavior and add engine regression coverage plus a CLI changeset

Files changed:
 .changeset/fn-6158-model-compatibility-fallback.md |   5 +
 docs/settings-reference.md                         |   2 +-
 .../pi-prompt-session-and-check-recursion.test.ts  |  25 ++++
 packages/engine/src/__tests__/pi.test.ts           | 136 ++++++++++++++++++++-
 .../src/__tests__/transient-error-detector.test.ts |  24 ++++
 packages/engine/src/pi.ts                          |  21 +++-
 packages/engine/src/transient-error-detector.ts    |  24 ++++
 7 files changed, 234 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-6158
Fusion-Task-Lineage: 3e91f647-40f1-4296-afa4-40861310e1c1

2026-06-09 21:16:42 -07:00