_try_activate_fallback() walked the chain by index without comparing
the candidate entry against the currently-failing backend. So a
misconfigured chain that listed the same provider+model as the primary,
or two custom_providers entries pointing at the same shim URL, would
loop the same failure 3x for the same backend.
After the fix, advance() skips:
- entries where (provider, model) match the current agent's
- entries with a base_url + model matching the current backend
(catches two custom_providers names pointing at the same shim)
Recursing through self._try_activate_fallback() continues to the next
chain entry; if everything matches, returns False and the caller
moves on without retrying the same broken path.
3 regression tests covering same-provider-same-model skip, same-base_url-
same-model skip, and the all-self-matching-returns-False exhaustion path.
Closes#22548 (the Hermes-side portion). The 120s timeout itself in
the downstream claude-cli shim is a deployment concern documented in
that issue's wherewolf87 comment.
Teknium
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