Follow Discord
Sweep 10 Nov 2026 · 02:18Z Build v2.1.322 539 read Stable v2.1.311 Latest v2.1.322 Next v2.1.322 Feeds RSS JSON llms.txt llms-full.txt Unofficial
HomeDocsClaude Developer Platform 9 Sep 2026
One change · api

whats-new-fable-5-1 changedmodels/fable-5-1/whats-new-fable-5-1

Nearest release: v2.1.267, published under an hour after this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.

Recorded here
Lines+2added
Lines−2removed
From line 65 where the diff opens
First seen 1 Sep 2026 this site's first read of the page
Recorded edits27to this page, all time

The whole hunk

from line 65, old and new numbered
/
lines
from line 65
6565 
6666### Change effort mid-conversation (beta)
6767 
68On Claude Fable 5.1 you can change the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level mid-conversation without invalidating the prompt cache. Raise it for a hard step and lower it for routine ones. Per-message effort is in beta: include the `mid-conversation-output-config-2026-07-01` beta header. Claude Fable 5.1, Claude Mythos 5.1, and Claude Opus 5 support it on the Claude API.
68On Claude Fable 5.1 you can change the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level mid-conversation without invalidating the prompt cache. Raise it for a hard step and lower it for routine ones. Per-message effort is in beta: include the `mid-conversation-output-config-2026-07-01` beta header. Claude Fable 5.1, Claude Mythos 5.1, and Claude Opus 5 support it on the Claude API and Google Cloud.
6969 
7070<CodeGroup>
7171 ```bash cURL
from line 477
4774772. Pass thinking blocks back unchanged and keep the history append-only. If your code builds the `messages` array itself, run the [history-editing check](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-preserved-thinking): move per-turn reminders you currently inject and delete to [turn-scoped system messages](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta), move `system` and `tools` changes to mid-conversation system messages, trim context server-side or strip thinking blocks from turns you carry across a client-side summary, then pick a production [`prefix_mismatch_behavior`](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-thinking-controls) and monitor `input_transformations`.
4784783. Re-tune effort from the default (`high`), and consider [changing it mid-conversation](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#change-effort-mid-conversation-beta) instead of holding one level for the whole session.
4794794. In agent loops, watch for one tool call per turn where Claude Fable 5 batched several, and add the per-turn note from [Prompting Claude Fable 5.1](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1).
4805. Re-run your evals. Refusal handling, fallback, fallback credit, and token counts carry over unchanged. Cache reads cost less (see [Pricing](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing)), and default behavior differs in the ways listed under [Changed from Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#changed-from-claude-fable-5).
4805. Re-run your evals. Refusal handling, fallback, fallback credit, and the tokenizer carry over unchanged. Cache reads cost less (see [Pricing](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing)), and default behavior differs in the ways listed under [Changed from Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#changed-from-claude-fable-5).
481481 
482482See the [migration guide](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide) for step-by-step instructions, including from Claude Opus 5 and earlier models.
483483 
Feedback