GPT-5.5 Hits an Invisible Wall: 44% of Complex Tasks Land at Exactly 516 Reasoning Tokens

A statistical analysis of 390,000 Codex responses reveals GPT-5.5 chain-of-thought tokens cluster at a fixed boundary — and the pattern has grown 300-fold since February.

A statistical investigation into OpenAI's Codex platform has uncovered a striking anomaly in GPT-5.5's behavior: the model's reasoning tokens disproportionately land at exactly 516, a pattern that has intensified dramatically since February and correlates with degraded performance on complex tasks.

The analysis, filed on the openai/codex issue tracker, examined 390,195 response-level token records across 865 sessions from February through June 2026. The findings are stark:

• GPT-5.5 accounts for only 19.3% of all Codex responses, but 82% of all responses that land exactly at 516 reasoning tokens
• 44% of GPT-5.5 responses that use 516 or more reasoning tokens stop at precisely 516 — compared to just 1.3% for all other models
• The clustering has grown from 0.11% of responses in February to 35.84% in June — a 300x increase

When GPT-5.5 responses hit this 516-token boundary, performance appears to degrade. A related earlier issue documented a case where a GPT-5.5 run ending at exactly 516 reasoning tokens returned the wrong answer. The new analysis adds aggregate evidence: as the exact-516 clustering intensified month-over-month, overall reasoning-token intensity decreased across the board.

The pattern is specific to GPT-5.5. GPT-5.3-codex and GPT-5.3-codex-spark show zero exact-516 events in the dataset. GPT-5.2 shows 0.34%. GPT-5.4 shows 19.8% — elevated but nowhere near GPT-5.5's 44%.

The investigator is careful not to claim this proves hidden chain-of-thought truncation. The narrower claim is that Codex telemetry shows a GPT-5.5-specific fixed-token clustering anomaly consistent with thresholded reasoning-budget behavior — and that users are noticing the quality impact.

The issue has drawn 190 reactions and 63 comments on GitHub, with the HN discussion reaching 273 points and 105 comments.

Sources: GitHub Issue #30364, Hacker News Discussion