Claude Fable 5.1 output carries text watermarks and C2PA Content Credentials

UpdateSep 1, 2026VerifiedAdded Sep 22, 2026

Text from Claude Fable 5.1 and Mythos 5.1 carries Anthropic's text watermark, and image, video and audio files produced via code execution carry C2PA Content Credentials when retrieved through the Files API. No request changes are needed.

LLMs Developer tools

Read Anthropic's release notes

https://platform.claude.com/docs/en/release-notes/overview#september-1-2026

Summaries of vendors' own notes. Product names and logos belong to their owners; logos via logo.dev.

More Claude Platform releases

Claude PlatformSep 1, 2026

Claude Fable 5.1 launches alongside Claude Mythos 5.1 with 1M token context

Claude Fable 5.1 succeeds Claude Fable 5 for long-running agentic coding, knowledge work and research, with Claude Mythos 5.1 for Project Glasswing participants. Both have a 1M context, 128k output and adaptive thinking at $10 / $50 per MTok, with cache reads cut to $0.25 per MTok.

GAAI agentsLLMsDeveloper toolsSource

Claude PlatformSep 1, 2026

Prompt cache reads on Claude Fable 5.1 and Mythos 5.1 drop to $0.25 per MTok

Cache reads on Claude Fable 5.1 and Claude Mythos 5.1 cost 0.025x the base input price instead of the usual 0.1x, which lowers the cost of cache-heavy agent workloads. Cache write pricing is unchanged.

PricingLLMsPricingSource

Claude PlatformSep 1, 2026

Claude Fable 5.1 and Mythos 5.1 reject tool_choice any and tool

On Claude Fable 5.1 and Claude Mythos 5.1, tool_choice types any and tool return a 400 error while auto and none are unchanged. Use strict tool use or structured outputs to guarantee schema-conformant tool inputs.

UpdateLLMsSource

Claude PlatformSep 1, 2026

Claude Fable 5.1 thinking blocks only replay to the same or newer models

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UpdateLLMsDeveloper toolsSource

Claude PlatformSep 1, 2026

Per-message effort changes enter beta on the Claude API

A role system message carrying output_config.effort inside messages changes effort for later turns while preserving the prompt cache, on Claude Fable 5.1, Mythos 5.1 and Opus 5. Requests need the mid-conversation-output-config-2026-07-01 beta header.

BetaLLMsDeveloper toolsSource

Claude PlatformSep 1, 2026

Claude API adds turn-scoped system messages in beta

Setting clear_at next_user_message on a mid-conversation system message renders it for the current turn only, then keeps it in history at no token cost. Per-turn reminders no longer accumulate or invalidate the prompt cache and later thinking blocks.

BetaPricingSource

Claude PlatformSep 1, 2026

Claude API thinking.display adds an updates mode in beta

The new updates value, behind the thinking-display-updates-2026-08-18 header, returns reasoning empty like omitted but surfaces the short progress updates Fable and Mythos models write between tool calls as text.

BetaLLMsSource

Claude PlatformSep 1, 2026

Claude Fable 5.1 and Mythos 5.1 require 30-day data retention

Like Claude Fable 5, both new models require 30-day data retention and are not available under zero data retention unless Anthropic expressly authorizes it, which affects teams with ZDR agreements.

UpdateLLMsSource

Also shipped on Sep 1, 2026

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BetaSource

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