Unverified50% confidenceFactExact time
The preserved-thinking check introduced by Fable 5.1 now applies to Opus 5.5 and Sonnet 5.5: thinking blocks are bound to the model and the conversation prefix, and sending thinking blocks back after editing system, tools or earlier turns returns a 400 on accounts created on or after August 31, 2026, while other accounts have them silently dropped.
1
Sources
50%
Confidence
Medium-term (~90 days)
Relevance
10/15/2026
First Seen
Valid until: 1/13/2027
Sources
Related Entities
Related Claims
UnverifiedIn Opus 5.5 and Sonnet 5.5, text between tool calls is now returned as thinking blocks, which are empty under the default display, so UIs that stream progress annotations silently stop working, and display: updates must be used (beta header thinking-display-updates-2026-08-18).67% similarUnverifiedSwitching effort levels mid-conversation on Opus 5.5 no longer clears the cache, though this feature is still in beta.62% similarUnverifiedIn the coffee shop quarterly report question, Gemini 4 Argon missed a space between two words in the code so they stuck together into a single identifier, causing the data on page two and page five to stop updating, and the profit page stayed stuck at 289080.56% similarUnverifiedJudging from the version number, Mythos 5.1 should be an iterative update built on 5.0, which typically implies performance optimization rather than a disruptive architectural change.55% similar
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