PR #1762 fixed the rsplit grammar collision for plain @openrouter:model:free
qualifiers, but skipped the fallback whenever the provider hint started with
'custom:' on the assumption that custom providers route directly. That left
'@custom:my-key:some-model:free' broken: rsplit yields
provider='custom:my-key:some-model', bare='free' → custom guard skips the
split-fallback → returns provider='custom:my-key:some-model', model='free'.
Detect the over-split structurally instead of using a known-suffix allowlist:
custom hints carry exactly one segment after 'custom:' (constructed at
api/config.py:1363 as 'custom:' + entry_name). So any rsplit result of
'custom:<a>:<b>' with bare model '<c>' has eaten one model segment — peel
it back with a second rsplit and prepend it to the bare model.
This is robust for :free / :beta / :thinking / :preview / any future
OpenRouter suffix without an allowlist to maintain.
Adds 5 regression tests covering the matrix (free/beta/thinking/preview/
slashed-model). All 7 existing #1744 tests still pass; #1228 tests
unaffected.
Co-authored-by: Cake <51058514+Sanjays2402@users.noreply.github.com>
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