Groq Returned Empty Content. The Bug Was Hiding in Reasoning Tokens.

This article was originally published on Jo4 Blog . We use Groq's gpt-oss-safeguard model to classify pages behind freshly created short links. Most pages take a few hundred tokens to score. Some don't. And the ones that don't were silently failing — for weeks — until we noticed the symptom: a small but consistent stream of links stuck in "preview pending" forever. Here's what we found. The Problem The classifier wraps a single Groq chat completion. Send page text, get back a JSON verdict ( safe , unsafe , with category codes). For 95% of links, this works in well under a second. For the other 5%, we'd see this in logs: WARN Empty content in Groq response WARN Classification failed for shortUrl=xyz123 — preview stays enabled Empty content. Not a network error, not a rate limit, not malformed JSON. The API returned 200, the choices array had one entry, and choices[0].message.content was "" . What did those pages have in common? They weren't obvious spam. They weren't obvious safe. They were ambiguous — a wellness blog that mentioned medication dosages, a forum thread about firearms law, a satire site quoting violent rhetoric. The kind of content where a human reviewer would also pause. The Wrong First Guess Our first instinct: the model is rate-limited or degraded for hard inputs. We added retries. The empty-content rate didn't budge. Second guess: we're hitting max_tokens . We had set it to 200. Maybe ambiguous pages produce longer verdicts. We bumped it to 400. Empty content rate didn't budge. The clue we kept missing was sitting in the response body itself, in a field we weren't parsing. The Root Cause Groq's response includes a usage block, and usage.completion_tokens_details.reasoning_tokens was the smoking gun: { "choices" : [{ "message" : { "content" : "" }, "finish_reason" : "length" }], "usage" : { "completion_tokens" : 200 , "completion_tokens_details" : { "reasoning_tokens" : 200 } } } gpt-oss-safeguard is a reasoning model. Before emitting a single charac