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1111* Unsure which effort level to run, or latency and cost are higher than the task warrants: [Consider all effort levels](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#consider-all-effort-levels)
1212* Little or no text between tool calls: [Ask for user-facing progress updates](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#ask-for-user-facing-progress-updates)
13* One tool call per turn in agent loops: [Batch independent tool calls in agent loops](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#batch-independent-tool-calls-in-agent-loops)
13* One tool call for each turn in agent loops: [Batch independent tool calls in agent loops](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#batch-independent-tool-calls-in-agent-loops)
1414* Requests fail with `bound to a different conversation`, or your harness edits earlier turns between requests: [Keep the conversation history append-only](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#keep-the-conversation-history-append-only)
1515* Prose runs long and dense: [Writing density](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#writing-density)
1616* Chat replies carry less structure than the content needs: [Formatting in chat](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#formatting-in-chat)
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2828<Note>
2929 Claude Fable 5.1 runs safety classifiers and can return `stop_reason: "refusal"`. See [Refusals, fallback, and billing](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#refusals-fallback-and-billing) and [Reduce safeguard false positives](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#reduce-safeguard-false-positives).
3030
31 Prompts, skills, and tool descriptions that ask the model to write out its thinking or reasoning may be declined with the `reasoning_extraction` category. Ask for a short explanation or a summary of the actions taken instead, or read the reasoning from [summarized thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#summarized-thinking) blocks (`display: "summarized"`). See [Keep reasoning in thinking blocks](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#keep-reasoning-in-thinking-blocks).
31 Prompts, skills, and tool descriptions that ask the model to write its thinking or reasoning might be declined with the `reasoning_extraction` category. Ask for a short explanation or a summary of the actions taken instead, or read the reasoning from [summarized thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#summarized-thinking) blocks (`display: "summarized"`). See [Keep reasoning in thinking blocks](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#keep-reasoning-in-thinking-blocks).
3232</Note>
3333
3434## Consider all effort levels
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4343
4444Claude Fable 5.1's default behavior is to write fewer user-facing updates during long tool-calling turns than Claude Fable 5 does. This becomes more pronounced at higher effort and in longer tool chains. Users see the agent go quiet for minutes at a time, or a final message that covers only the last step rather than the whole task.
4545
46First, check that your client receives progress updates at all. The model's short notes between tool calls, what it just found and what it's doing next, come back as [progress-update `thinking` blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates), and those blocks are empty under the default `thinking.display` of `"omitted"`. Set `display: "updates"` (beta, `thinking-display-updates-2026-08-18` header) and render each non-empty `thinking` block as a status line, or set `"summarized"` to receive them along with summarized reasoning. If you aren't requesting them, the model's updates may simply not be reaching your users.
46First, check that your client receives progress updates at all. The model's short notes between tool calls, what it just found and what it's doing next, come back as [progress-update `thinking` blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates), and those blocks are empty under the default `thinking.display` of `"omitted"`. Set `display: "updates"` (beta, `thinking-display-updates-2026-08-18` header) and render each non-empty `thinking` block as a status line, or set `"summarized"` to receive them along with summarized reasoning. If you aren't requesting them, the model's updates might simply not be reaching your users.
4747
48Second, audit your prompt for instructions that suppress narration. Some earlier models were eager to give updates while working, which led to system prompt lines such as "hold all findings for the final response." Remove lines like that before adding anything.
48Second, audit your prompt for instructions that suppress narration. Some earlier models gave frequent updates while working, which led to system prompt lines such as "hold all findings for the final response." Remove lines like that before adding anything.
4949
5050If you still want more updates, for example when pair programming or in other human-in-the-loop work, add a short system prompt line that says when you want user-facing text from the model and what each update should contain:
5151
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5353Before you start, say in a line what you're about to do; brief updates while you work help the user follow along. Close with a short recap that stands on its own — what you found, what you did, and what's next — so a reader who only sees the last message has the full picture.
5454```
5555
56If your product collapses or hides tool output, tell the model. Otherwise it may run commands to "show" the user output that your UI never displays. Deliver the note in a [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages) (`clear_at: "next_user_message"`, beta):
56If your product collapses or hides tool output, tell the model. Otherwise it may run commands to "show" the user output that your UI never displays. Each time you send back tool output that your UI hides, follow it with the note in a [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages) (`clear_at: "next_user_message"`, beta):
5757
5858```text wrap
5959Only you see that command's output — the user's terminal shows at most a few lines of it. If the user needs to read any of it, put it in your reply.
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6161
6262## Batch independent tool calls in agent loops
6363
64Claude Fable 5.1 usually issues parallel tool calls as expected: when a request names several things to fetch, it issues those calls in parallel. The exception is coding and computer-use loops where the next independent calls are implied by the task rather than explicitly requested (custom coding agents, bash-and-editor harnesses, computer use): there it may issue them one per turn instead. This doesn't affect answer quality, but each extra turn costs tokens, a round trip, and wall-clock time. A one-sentence nudge at the end of the current request addresses it:
64Claude Fable 5.1 usually issues parallel tool calls as expected: when a request names several things to fetch, it issues those calls in parallel. The exception is coding and computer-use loops where the next independent calls are implied by the task rather than explicitly requested (custom coding agents, bash-and-editor harnesses, computer use): there it might issue them one in each turn instead. This doesn't affect answer quality, but each extra turn costs tokens, a round trip, and wall-clock time. A one-sentence nudge at the end of the current request addresses it:
6565
6666```text wrap
6767First privately list what you need next; then request every item that doesn't depend on another's result in this one response.
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7070Each time you send tool results back, append it after that user message as a [turn-scoped system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages): a `role: "system"` entry in `messages` with `clear_at: "next_user_message"`. Once a later user message exists, the API clears the earlier copies, so the model reads only the newest one. Turn-scoped system messages are in beta and require the [beta header](https://platform.claude.com/docs/en/api/beta-headers) `mid-conversation-system-clear-at-2026-08-21`. Without the beta, place the sentence in a text block after the `tool_result` blocks in the same user message instead.
7171
72Append a fresh copy each turn and leave the earlier copies where they are, byte-for-byte. They stay in the array, but once cleared the model doesn't see them and they cost no input tokens. Deleting or rewriting them is an edit to earlier turns: it restarts the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) from that point and invalidates the thinking blocks that came after them (see [Keep the conversation history append-only](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#keep-the-conversation-history-append-only)).
72Append a fresh copy each turn and leave the earlier copies where they are, byte-for-byte. They stay in the array, but once cleared the model doesn't see them and they cost no input tokens. Deleting or rewriting them is an edit to earlier turns: it invalidates the thinking blocks that came after them (see [Keep the conversation history append-only](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#keep-the-conversation-history-append-only)).
7373
7474The following loop shows this placement. Each assistant turn goes back exactly as returned, each user turn carries only the tool results, and a fresh turn-scoped copy of the nudge follows it.
7575
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711711 response = nil
712712 loop do
713713 response = client.beta.messages.create(
714 model: "claude-fable-5-1",
714 model: Anthropic::Model::CLAUDE_FABLE_5_1,
715715 max_tokens: 16000,
716716 betas: ["mid-conversation-system-clear-at-2026-08-21"],
717717 tools: tools,
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751751
752752Append each assistant turn to the history exactly as the API returned it, thinking blocks included, and don't edit earlier turns between requests. For new accounts created on or after August 31, 2026, Claude Fable 5.1's thinking blocks are valid [only in the exact conversation that produced them](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation): a request that replays a thinking block after its prefix (the system prompt, the tool list, or any earlier message) has changed returns a 400, or drops the affected blocks if you set `thinking.block_binding.prefix_mismatch_behavior: "drop_block"` (beta, `thinking-binding-controls-2026-08-01` header). Adopt the pattern even if the check isn't enforced for your account, so the same code works on every account.
753753
754The history edits that trip the check are the same ones that restart the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching): injecting and removing per-turn reminders, summarizing older turns in place, or changing the system prompt mid-session. Send per-turn reminders as [turn-scoped system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages), change instructions or tools with a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) instead of rewriting `system` or `tools`, and let server-side [compaction](https://platform.claude.com/docs/en/build-with-claude/compaction) or [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing) do any trimming. If you compact on the client, the simplest shape is to replace the whole history with one summary message plus the new user turn and replay nothing else: no thinking blocks carry over, so nothing fails, and the model thinks afresh on the compacted conversation (see [Custom compaction on the client](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#custom-compaction-on-the-client)). Because cache reads are now cheaper (see [Pricing](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing)), compacting early to save cost may no longer be the right cost-intelligence tradeoff on Claude Fable 5.1, so experiment with later compaction points.
754Most history edits that trip the check also restart the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching): injecting and removing per-turn reminders, summarizing older turns in place, or changing the system prompt mid-session. Send per-turn reminders as [turn-scoped system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages), change instructions or tools with a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) instead of rewriting `system` or `tools`, and let server-side [compaction](https://platform.claude.com/docs/en/build-with-claude/compaction) or [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing) do any trimming. If you compact on the client, the simplest shape is to replace the whole history with one summary message plus the new user turn and replay nothing else: no thinking blocks carry over, so nothing fails, and the model thinks afresh on the compacted conversation (see [Custom compaction on the client](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#custom-compaction-on-the-client)). Because cache reads are now cheaper (see [Pricing](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing)), compacting early to save cost may no longer be the right cost-intelligence tradeoff on Claude Fable 5.1, so experiment with later compaction points.
755755
756756To find edits your harness already makes, run a session with `prefix_mismatch_behavior: "drop_block"` and log `input_transformations`, as described in [How to tell whether your integration is impacted](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#how-to-tell-whether-your-integration-is-impacted), or capture the exact requests it sends over a few normal turns and confirm that consecutive requests are byte-identical up to the appended turns.
757757
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771771
772772## Formatting in chat
773773
774Earlier models overused bullets and bold in chat, and many prompts carry anti-formatting rules written to hold that down. Claude Fable 5.1 leans the other way: it uses bold less and is less likely to reach for headers, lists, or quotation marks. If your prompt contains anti-formatting language, remove it or replace it with a rule that says when specific formatting is appropriate, such as the following:
774Earlier models overused bullets and bold in chat, and many prompts carry anti-formatting rules written to reduce that. Claude Fable 5.1 tends toward the opposite: it uses bold less and is less likely to reach for headers, lists, or quotation marks. If your prompt contains anti-formatting language, remove it or replace it with a rule that says when specific formatting is appropriate, such as the following:
775775
776776```text wrap
777777Use lists and bullet points when asked to, or when the content is multifaceted enough that they help with clarity. If the person explicitly requests minimal formatting, always format your responses without bullet points, headers, lists, or bold emphasis, as requested. In conversational, personal, or emotional exchanges, keep to plain prose.
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797797
798798## Finish the whole task
799799
800Claude Fable 5.1 can execute very long tasks without much guidance on methodology, especially when the goal is clear. On complex asynchronous workloads, though, nudge it not to end its turn before the work is done. Without the nudge, the model sometimes describes what it would do next instead of doing it ("Next, I'll …") or stops to ask permission for a step the original request already covered ("Shall I apply this?"). Users have to reply "continue" or "go ahead," which suits pair programming and other human-in-the-loop work but doesn't use the model's full long-horizon capability.
800Claude Fable 5.1 can run very long tasks without much guidance on methodology, especially when the goal is clear. On complex asynchronous workloads, though, nudge it not to end its turn before the work is done. Without the nudge, the model sometimes describes what it would do next instead of doing it ("Next, I'll …") or stops to ask permission for a step the original request already covered ("Shall I apply this?"). Users have to reply "continue" or "go ahead," which suits pair programming and other human-in-the-loop work but doesn't use the model's full long-horizon capability.
801801
802802Two system prompt additions together mitigate this. Apply both. If you need to limit prompt length, use only the first, which keeps most of the effect. The first tells the model not to ask about work already requested and to carry out the next steps it has stated:
803803
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834834
835835## Keep changes and tests to what the task asks for
836836
837When asked to implement an open-ended feature, Claude Fable 5.1 delivers what's asked for and sometimes more: it may fix nearby code, extend behavior the task didn't mention, or commit more test files than the change warrants. It responds well to explicit instructions about what to leave out. With the following instruction, unrequested additions and committed test code drop substantially with no measurable change in task success:
837When asked to implement an open-ended feature, Claude Fable 5.1 delivers what's asked for and sometimes more: it might fix nearby code, extend behavior the task didn't mention, or commit more test files than the change warrants. It responds well to explicit instructions about what to leave out. With the following instruction, unrequested additions and committed test code drop substantially with no measurable change in task success:
838838
839839```text wrap
840840If, while working or testing, you find a pre-existing bug, a performance concern, or behavior the task doesn't mention, don't fix, optimize or extend it in this change unless the requested behavior cannot work without it; report it as a follow-up in your summary. Where the task is ambiguous, implement the reading its wording and the surrounding code most directly support, state that assumption in your summary, and don't build for the other readings as well. Verify your work however you like; scratch scripts and quick checks need not be kept. Commit tests only where the task asks for them or this repository already keeps tests for this kind of change, sized like the neighboring test files — roughly one focused test per stated behavior — and don't turn scratch checks into additional permanent test files. This is about extras only: implement every behavior the task asks for, completely.
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860860
861861## Prefer targeted edits over whole-file rewrites
862862
863If Claude Fable 5.1 rewrites whole files for small changes, append the following instruction to the system prompt or the first user message. Claude Fable 5.1 is more likely than Claude Fable 5 to rewrite an entire text file rather than make a targeted edit. The resulting file is usually the same, but unless the file is short or most of it is changing, a rewrite costs more output tokens and time. The instruction brings Claude Fable 5.1 back in line with Claude Fable 5 for small and medium changes.
863If Claude Fable 5.1 rewrites whole files for small changes, append the following instruction to the system prompt or the first user message. Claude Fable 5.1 is more likely than Claude Fable 5 to rewrite an entire text file rather than make a targeted edit. The resulting file is usually the same, but unless the file is short or most of it is changing, a rewrite costs more output tokens and time. With the instruction, Claude Fable 5.1 behaves like Claude Fable 5 on small and medium changes.
864864
865865```text wrap
866866The number of tokens used to edit files is best minimized, all else being equal. Therefore, when it will not affect the end result, try to surgically edit a file rather than rewrite the entire thing.
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868868
869869## Leave room for long outputs at xhigh and max effort
870870
871At `xhigh` and especially `max` effort, Claude Fable 5.1 can think for longer before it starts writing its reply. When a single request asks for a long deliverable, such as a full rewrite of a long document, it may draft much of that deliverable in its thinking and then write it out again as the reply, which means a longer wait and more output tokens. The simplest approach is to run requests like these at `high`, the recommended starting point, and move to `xhigh` or `max` only where you've measured a quality gain (see [Consider all effort levels](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#consider-all-effort-levels)). If you do run them at `xhigh` or `max`:
871At `xhigh` and especially `max` effort, Claude Fable 5.1 can think for longer before it starts writing its reply. When a single request asks for a long deliverable, such as a full rewrite of a long document, it might draft much of that deliverable in its thinking and then write it again as the reply, which means a longer wait and more output tokens. The simplest approach is to run requests like these at `high`, the recommended starting point, and move to `xhigh` or `max` only where you've measured a quality gain (see [Consider all effort levels](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#consider-all-effort-levels)). If you do run them at `xhigh` or `max`:
872872
873873* Set `max_tokens` to leave room for the thinking and the reply, not just the reply length you expect.
874874* Append the following note to the end of the user message. It makes the thinking much shorter on prose and code requests. Replace `[max_tokens]` with the request's actual `max_tokens` value, for example 64,000.
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885885
886886* Have the tool that starts a subagent return immediately.
887887* Pass each subagent's result back to the lead in a later `user` message once it's ready.
888* Give the lead a separate tool it can call when it wants to wait for a result.
888* Give the lead a separate tool it can call to wait for a result.
889889
890The model still often chooses to wait. The time savings come from the runs where it carries on with other work.
890The model still often waits. The time savings come from the runs where it carries on with other work.
891891
892892## Give vision work tools to crop and zoom
893893
No line in this hunk matches that.