{
  "id": 5015150,
  "title": "Claude Fable 5.1 Pensée Conservée : Résoudre l'erreur 'Le bloc est lié à une conversation différente'",
  "url": "https://urgent.news/2026/09/02/claude-fable-5-1-pensee-conservee-resoudre-lerreur-le-bloc-est-lie-a",
  "topic": "ai",
  "section": "AI",
  "published": "2026-09-02T04:50:22.000Z",
  "source": {
    "name": "Dev.to",
    "slug": "dev-to",
    "url": "https://dev.to/antoine_laurentt/claude-fable-51-pensee-conservee-resoudre-lerreur-le-bloc-est-lie-a-une-conversation-2moh"
  },
  "original_language": "fr",
  "account": "Claude Fable 5.1 now prevents blocks of reasoning from being linked to different conversations. If you migrate an agent harness to Claude Fable 5.1 and encounter a 400 error indicating that a reasoning block \"is linked to a different conversation,\" your code is modifying the history between two requests. Claude 5.1 is the first Claude model to stop this behavior. This guide explains how to verify, identify the affected cases, triggers, workarounds, and single-model fixes that resolve the error while preserving a hot prompt cache. The verification process is documented in the Preserved Thinking section and Claude Fable 5.1's release notes. This is the third major change to Fable 5.1, and the only one likely to silently degrade an harness. Review the migration guide for the other two. The message \"Invalid `signature` in `thinking` block. The block is bound to a different conversation\" signals the first reasoning block that no longer matches. The message may also name the first modified message - this is the most useful diagnostic indication. The token counter endpoint enforces the same check. A similar but different error may begin with the same clause. If it lacks the phrase \"linked to a different conversation,\" the signature is likely corrupted or unreadable. In that case, prefix_mismatch_behavior does not apply. How the verification works Each Claude 5.1 reasoning block contains a signature that records: the model that produced it; the exact prefix of the previous conversation; the top-level system prompt; the set of tools; all messages preceding the block. Blocks are chained together. When you return the transcript, the API checks that the prefix is identical byte-by-byte to the one that produced the block. Anthropic cites two reasons: anti-distillation - new API accounts can no longer manually modify the previous context of Claude while preserving previous reasoning blocks; cache consistency - the same changes that invalidate the verification also reset the prompt cache. Code that respects the verification also benefits from zero-delay token reads at each turn. Accounts created on or after August 31, 2026 are affected, including Claude API organizations; Amazon Bedrock; Google Cloud; Microsoft Foundry. Recorded but unapplied verification records discrepancies but does not block the request unless thinking.block_binding.prefix_mismatch_behavior is set, including to error. Anthropic says future models will apply this check to all accounts. Unaffected surfaces Claude Code, claude.ai, Claude Managed Agents, and the Claude Agent SDK are not affected. They maintain the prefix intact for you. Mythos 5.1 does not perform this verification, even if history modifications restart its cache. Affected code Any code that constructs the messages array itself is affected: custom agent loops; chat backends; frameworks that encapsulate the Messages API. If you distribute a tool used with your users' API keys, test with the verification enabled. Your account may be old while a user's account may be subject to the new check. Changes that invalidate subsequent blocks Modifying, rearranging, or deleting a previous turn; deleting old tool results; truncating the transcript mid-series of turns; using client-side compacting that retains recent turns in a summary; injecting temporary content like reminders, status lines, or token counters; reconstructing system or tools between requests; updating the current date in the system prompt; adding or removing a tool during a session; referencing an image or document URL that later returns different bytes; deleting a reasoning block other than at the beginning of execution. Initial blocks can only be removed from oldest to newest. A block in the middle of the transcript cannot be removed alone. Changes that preserve validity Adding reasoning blocks one at a time; adding role: system system prompts; leaving system prompts in place once they are erased; removing an initial series of reasoning blocks from oldest to newest; changing settings outside system, tools, and messages, such as max_tokens, output_config, effort, tool_choice, or metadata; adding, moving, or deleting cache_control markers; using server-side compacting and context editing. Server-side compacting occurs after the verification. The verified prefix starts at the compaction block. Workaround: drop_block To continue execution despite a divergence, send the beta header and explicitly set the behavior: response = client.",
  "summary": "Corriger l’erreur de liaison des blocs de réflexion avec Claude Fable 5.1 Si vous avez migré un harnais d’agent vers Claude Fable 5.1 et rencontrez une erreur 400 indiquant qu’un bloc de réflexion « est lié à une conversation différente », votre code modifie l’historique entre deux requêtes. Fable 5.1 est le premier modèle Claude à bloquer ce comportement. Ce guide explique la vérification, les…",
  "key_points": [],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 5,
    "also_reported_by": [
      {
        "outlet": "The New Stack",
        "title": "Claude Fable 5.1 watermark: It has a blind spot developers can’t ignore",
        "url": "https://urgent.news/2026/09/01/claude-fable-5-1-watermark-it-has-a-blind-spot-developers-cant-ignore",
        "published": "2026-09-01T20:59:09.000Z"
      },
      {
        "outlet": "Digital Trends",
        "title": "Claude Fable 5.1 and Mythos 5.1 arrive with better coding and cost cuts for developers",
        "url": "https://urgent.news/2026/09/02/claude-fable-5-1-and-mythos-5-1-arrive-with-better-coding-and-cost",
        "published": "2026-09-02T01:52:35.000Z"
      },
      {
        "outlet": "Dev.to",
        "title": "Claude Fable 5.1 Tư duy bảo toàn: Khắc phục lỗi The Block Is Bound to a Different Conversation",
        "url": "https://urgent.news/2026/09/02/claude-fable-5-1-tu-duy-bao-toan-khac-phuc-loi-the-block-is-bound-to",
        "published": "2026-09-02T04:46:43.000Z"
      },
      {
        "outlet": "Silicon Republic",
        "title": "Anthropic launches Claude Fable 5.1 and Mythos 5.1",
        "url": "https://urgent.news/2026/09/02/anthropic-launches-claude-fable-5-1-and-mythos-5-1",
        "published": "2026-09-02T09:44:12.000Z"
      }
    ]
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}