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The Day Opus 4.8 Went Haywire: tool call cannot be parsed

A one-day timeline from 2026-06-02: from hopping on X for moral support, to someone finally pinning down the root cause of tool call cannot be parsed.


Every time Claude breaks, I hop on X. Over there, I feel like I’m not alone (some kind of huddling-together-for-warmth mentality).

I feel like today is the perfect time to ship GPT-5.6. OpenAI was right to hold back last week.

Because Claude is going haywire across the board. This is the “kick a man while he’s down” moment!

New trick today: tool call cannot be parsed (retry also failed).

Someone found the root cause of Opus 4.8 thinking past the timeout and ending up returning tool call could not be parsed: the thinking block returns an abnormal empty output, and tool_use has no proper tool-call block. The diagnosis is that the extended thinking mechanism is misbehaving.

The fixes:

  1. Turn Opus 4.8 effort down below high, and there won’t be any thinking tokens — but the capability drops accordingly.

  2. Falling back to 4.6 + max works best.


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