Some models (and the openrouter/free router) may return null content
instead of a string. The old check used isset() which returns false
for null, triggering the "unexpected response format" error.
Now: checks for choices[0].message (not content), treats null content
as empty string, and logs the actual response shape on failure so we
can diagnose what the model returned.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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