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Claude Opus 5 generates lengthy content from specific markdown trigger

By PulseAugur Editorial · [2 sources] ·

A peculiar behavior in Anthropic's Claude Opus 5 model has been observed where sending a specific markdown sequence, '---', can cause it to generate lengthy, unsolicited content. This issue, initially flagged on X, appears to be triggered by the model misinterpreting the markdown as an unfinished user turn, leading it to complete a document or essay. Experiments showed a wide variance in output length, from 17 to over 4,800 tokens, even with identical inputs. Further investigation revealed that the trigger is tied to markdown grammar rather than a general 'empty message' problem, as other delimiters did not produce the same effect. AI

IMPACT This behavior highlights potential vulnerabilities in LLM context window management and prompt interpretation, which could impact reliability in production systems.

RANK_REASON The cluster details a specific failure mode and behavior of an AI model, including experimental analysis and hypotheses, which falls under research into AI capabilities and limitations.

Read on HN — claude cli stories →

AI-generated summary · Google Gemini · from 2 sources. How we write summaries →

Claude Opus 5 generates lengthy content from specific markdown trigger

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The cluster details a specific failure mode and behavior of an AI model, including experimental analysis and hypotheses, which falls under research into AI capabilities and limitations.
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2 independent sources
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model release, product
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High
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77 days old
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COVERAGE [2]

  1. HN — claude cli stories TIER_1 English(EN) · auspiv ·

    I sent Claude Opus 5 '–-' and it wrote me 5k tokens about a cartographer

  2. Mastodon — mastodon.social TIER_1 English(EN) · [email protected] ·

    I sent Claude Opus 5 '--' and it wrote me 5k tokens about a cartographer https://austinsnerdythings.com/2026/07/28/claude-opus-5-dangling-document-effect/ # Hac

    I sent Claude Opus 5 '--' and it wrote me 5k tokens about a cartographer https://austinsnerdythings.com/2026/07/28/claude-opus-5-dangling-document-effect/ # HackerNews # Tech # AI

    LINKS austinsnerdythings.com/…/claude-opus-5-da…

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