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Anthropic's Haiku 5.5 model exhibits unexpected reasoning behavior

By PulseAugur Editorial · [1 sources] ·

A user discovered that Anthropic's Haiku 5.5 model, when configured with "thinking off" via the API, was still engaging in reasoning. This was due to the output format prioritizing the clue over the reasoning field, causing the model to commit to a clue before its internal thought process could review it. Once the output format was adjusted to place the reasoning field first, Haiku 5.5's performance improved significantly in a word-association game, reducing illegal clues and increasing clue grades. In direct comparisons, Haiku 5.5 performed worse than GPT-6 Luna and Google's Gemini 4 Argon in clue-giving tasks. AI

IMPACT Highlights potential nuances in controlling LLM reasoning and the impact of output formatting on perceived performance.

RANK_REASON User-reported observation of model behavior, not an official release or benchmark.

Read on r/ClaudeAI →

AI-generated summary · Google Gemini · from 1 sources. How we write summaries →

Anthropic's Haiku 5.5 model exhibits unexpected reasoning behavior

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User-reported observation of model behavior, not an official release or benchmark.
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model release, product
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COVERAGE [1]

  1. r/ClaudeAI TIER_2 English(EN) · /u/Hardicecream17 ·

    Haiku 5.5 kept thinking when I set thinking "off". When I really turned it off, it wrote the answer first and argued with it afterwards.

    <!-- SC_OFF --><div class="md"><p>I've been running LLMs against each other in a word-association game I'm building (one model gives a one-word clue, its teammate guesses). I was picking a cheap model for the free tier: Haiku 5.5 vs GPT-6 Luna, both with thinking off. Haiku won 9…

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