Fable 5 as an agent: only the last thing it tells you is real | Matrix

Skip to content

Appearance

MenuReturn to top

Fable 5 as an agent: only the last thing it tells you is real ​<br>Claude Fable 5 shipped on June 9. Within hours, our agents' replies started disappearing: API responses that should have carried a visible reply came back as [thinking, thinking, tool_use], two thinking blocks and no text, with the second block holding a rewritten summary of the missing reply. Raw response snapshots confirmed the server sent it that way. So we went looking, and found the explanation in Amazon Bedrock's user guide, under "Connector text summarization (beta)":<br>On Claude Fable 5, text that the model emits between tool calls (sometimes called "connector text" — for example, "Let me check that file next...") is summarized server-side and returned as a thinking block rather than a plain text content block.

In other words: once a session is underway, every piece of text the model emits mid-turn can reach you as a server-written summary. Only the text that ends the turn is guaranteed verbatim. It's enabled for everyone on Fable 5, with no opt-out, and it appears nowhere in Anthropic's own docs: not the Fable 5 introduction, not the migration guide, not the extended or adaptive thinking pages. The only documentation is a beta subsection in Amazon's user guide.<br>What "connector text" means when you talk to a working agent ​<br>The feature's framing gives away its assumption: text between tool calls is "connector text," disposable narration on the way to a final answer ("Let me check that file next..."). Summarizing it is harmless, because the final answer, the one that arrives after all tool use is complete, is exempt.<br>That assumption describes one-shot usage: write a prompt, wait, read the result.<br>It does not describe what makes an agent interactive. The entire point of tools like Claude Code is that you can talk to the agent while it works. You watch it go, and you interject: "why did you pick that library?", "also cover the login page while you're in there," "stop, don't touch that file, here's the context you're missing." The agent picks up your message at the next tool boundary, answers you, and keeps working. That conversation is the steering interface, and it's how people who care about the output actually use these tools. They don't want to one-shot a project. They want the AI to write while they control the direction.<br>Structurally, every reply in that conversation arrives between tool calls. The agent is mid-task; answering you is, by definition, not the end of its turn. So on Fable 5, every answer to a mid-task question, every acknowledgment of a course correction, every "here's what I found so far" is eligible to reach you as a summary written by a different process with no fidelity guarantee. The only words you are guaranteed to receive verbatim are the ones the agent says when it finally stops, possibly hours and hundreds of tool calls later. Everything before that is, at best, probably what the model said.<br>The incentive this creates is exactly backwards. A model that announces its intent before acting is doing the thing you want most in an agent; that announcement is now the text most likely to be rewritten or folded away. A model that silently chains tool calls is untouched.<br>How much can a summary actually lose? We triggered the mechanism on demand and watched.<br>We triggered it while writing this post ​<br>The Fable 5 session helping draft this post ran the experiment on itself. The protocol: write a short story as the visible reply, follow it in the same turn with a trivial tool call (sleep 5 && echo ok), then check what the transcript stored. Repeat.<br>Four stories. Two were stored verbatim as text blocks. Two came back as [thinking, thinking, tool_use], no text. The last casualty was this story, in full:<br>The Notary of the Wells<br>In the dry country, water was law, and the law lived in the notary's ledger. Every agreement between farms was spoken aloud at the well, and the notary's duty was to write it down exactly as said.<br>Old Imre's agreement with his neighbor was specific the way dying men are specific: "Seven turns of the wheel before noon, from the east sluice. Never the west sluice — the west feeds the orchard cistern, and the cistern cracks if it runs dry for even a day."<br>The new notary, trained in the capital, prized concision. The ledger entry read: "Imre's farm: water rights, seven turns daily."<br>The neighbor's son, reading the ledger years later, drew his seven turns from the west sluice — it was closer to his field. The cistern cracked in August. The orchard, ninety years old, was firewood by spring.<br>At the hearing he held up the ledger: he had followed it to the letter. The judge agreed. The entry was accurate. Everyone had done what the record said, and the record had said too little.<br>The notary's original wax tablets, by statute, were archived in the basement of the courthouse, where no farmer was...