optimizing-for-cost-and-intelligence changedabout-claude/models/optimizing-for-cost-and-intelligence
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104104| Pauses run for minutes | Claude Fable 5.1 | 5-minute cache kept warm, instead of the 1-hour duration |
105105| Pauses run toward an hour | Claude Fable 5.1 | 1-hour duration |
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107On Claude Haiku 5.5, use the "Any" rows. Keep-alive requests were not measured on it. By list prices, not by measurement, each one costs twice as much relative to the input price as on Claude Opus 5.5 and Claude Sonnet 5.5: Claude Haiku 5.5 reads the cache at the standard 0.1x, those two at 0.05x.
107On Claude Haiku 5.5, use the "Any" rows: it reads the cache at the standard 0.1x the input price, twice the 0.05x of Claude Opus 5.5 and Claude Sonnet 5.5, so by list prices each keep-alive request costs twice as much relative to the input price. A `max_tokens: 0` request refreshed its cache in a separate test (reference 16).
108108
109109**Why, row by row.**
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141141* The 1-in-20 rule leaves a margin over the break-even of about 1 turn in 30. At 1 turn in 20 the 5-minute and the 1-hour setting cost about the same on Claude Sonnet 5.5, within run-to-run variation; at 1 turn in 10 the 5-minute setting cost 54% more (reference 16).
142142* On models other than Claude Sonnet 5.5 and Claude Opus 5.5, prices put the break-even in the same range: every current model uses the same cache-write multipliers, and every model but Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, and Claude Sonnet 5.5 the same read price.
143* The Claude Opus 5.5 and Claude Sonnet 5.5 keep-alive figures come from requests sent with `max_tokens: 1`. Whether a `max_tokens: 0` request refreshes an existing cache entry was not measured on Claude Opus 5.5 (reference 16).
143* The Claude Opus 5.5 and Claude Sonnet 5.5 keep-alive figures come from requests sent with `max_tokens: 1`. A `max_tokens: 0` request also refreshed an existing cache entry on Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5 (reference 16).
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145145To keep the cache warm:
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187187Keep per-request text in the newest user turn, after the last cache breakpoint, and make every cache-invalidating change at a natural break, such as the first request after a compaction. The cache is a byte-exact prefix match over the request in order (tools, then system prompt, then messages), so a change anywhere invalidates everything after it.
188188
189The stakes are highest on Claude Fable 5.1 and Claude Mythos 5.1, where a break re-writes the prefix at 1.25x the input price instead of reading it at 0.025x. The following table gives what one broken turn costs on a 100,000-token prefix, computed from list prices with the 5-minute cache, not measured:
189When something must change mid-conversation, send it in the form that keeps the cache, where the model supports it. In Anthropic's test on Claude Fable 5.1, Claude Opus 5.5, and Claude Sonnet 5.5, on a cached conversation of about 100,000 tokens, editing the system prompt cost about 25 to 50 times what the same text cost as a mid-conversation system message. Changing effort at the top level cost 20 to 40 times what it cost per message[18](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#refs):
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191| Model | One broken turn | The read instead | Compared with the read |
192| -------------------------------------- | --------------- | ---------------- | ---------------------- |
193| Claude Fable 5.1 and Claude Mythos 5.1 | $1.25 USD | $0.025 USD | 50 times |
194| Claude Opus 5.5 | $0.50 USD | $0.02 USD | 25 times |
195| Claude Sonnet 5.5 | $0.25 USD | $0.01 USD | 25 times |
191<Frame>
192 
193</Frame>
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197On the other current models it costs 12.5 times the read.
195The stakes are highest on Claude Fable 5.1 and Claude Mythos 5.1, where a break re-writes the prefix at 1.25x the input price instead of reading it at 0.025x. On Claude Haiku 5.5 a system prompt edit costs 12.5 times an unchanged request. The edit re-wrote the whole conversation. By default, newer accounts get a 400 error instead[18](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#refs). The top-level effort change, in a conversation that held the model's first reply with its thinking, re-wrote all but the system prompt. These figures are from the Messages API.
198196
199197**What to do.**
200198
201* Put nothing that changes per request, such as a timestamp or a queue position, ahead of the stable prefix: it turns every request into a full cache write. In a measurement on an earlier model, a 25-token status line at the front of the system prompt made a run of a small agent that triages bug reports ([Trim input and context tokens](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#trim-input-and-context-tokens)) cost more than running with caching off[18](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#refs).
202* The [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching#what-invalidates-the-cache) page lists the changes that invalidate the cache, such as a change to [`effort`](https://platform.claude.com/docs/en/build-with-claude/effort), the thinking configuration, the system prompt, or a tool definition. Changing the output format invalidates the cache for that conversation (see [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs#prompt-modification-and-token-costs)).
203* On the most recent models, change instructions with a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages), a `{"role": "system"}` message appended to `messages`, instead of editing the top-level `system` field: the cached prefix stays intact. Check that page for which models support it. A per-message effort change leaves the cached prefix intact too, but on fewer models: [Change effort mid-conversation](https://platform.claude.com/docs/en/build-with-claude/effort#changing-effort-mid-conversation) lists them.
199* Put nothing that changes per request, such as a timestamp or a queue position, ahead of the stable prefix: it turns every request into a full cache write. In Anthropic's test, a 25-token status line at the front of the system prompt made each request write the whole conversation; moved to the end of the newest user turn, it left the cache read intact[18](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#refs).
200* The [prompt caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching#what-invalidates-the-cache) page lists the changes that invalidate the cache, such as a change to [`effort`](https://platform.claude.com/docs/en/build-with-claude/effort), the thinking configuration, the [`speed`](https://platform.claude.com/docs/en/build-with-claude/fast-mode) setting, the system prompt, or a tool definition. Changing the output format invalidates the cache for that conversation (see [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs#prompt-modification-and-token-costs)).
201* Check which models support each form: [mid-conversation system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages), and [per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort#changing-effort-mid-conversation), which is in beta.
204202* After a cache-invalidating change, confirm cache reads have not dropped; if they have, [cache diagnostics](https://platform.claude.com/docs/en/build-with-claude/cache-diagnostics) shows where the prefix diverged.
205203
206Anthropic measured the timing of a change on an earlier model, on the triage agent's long sessions[18](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#refs). In that measurement, sessions cost more when an effort change and an added tool were made mid-session, or on the request that triggered the compaction, than when the same two changes were made on the first request after compaction. Accuracy was within run-to-run noise in every case.
204On an earlier model, Anthropic measured when to make such a change, on the triage agent's long sessions[18](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#refs). An effort change and an added tool cost more mid-session, or on the request that triggered the compaction, than on the first request after compaction. Accuracy was within run-to-run noise. The sessions with no changes cost more than those changed after compaction for another reason: they ran at the default effort throughout, and two of them compacted a second time, while the others started at low effort:
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208The sessions with no changes cost more than the sessions changed after compaction, but not because of caching. They ran at the default effort throughout and two compacted a second time, while the others started at low effort (reference 18):
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10121008 * **5-minute and 1-hour settings, Claude Sonnet 5 and Claude Opus 5.5:** In the 5% cell the 5-minute and 1-hour settings tied on Claude Sonnet 5, because that draw's pauses fell on small prefixes; on Claude Opus 5.5 they nearly tied. On Claude Sonnet 5, at its cache-read price of $0.20 USD per million tokens, a session cost $0.60 USD, $0.69 USD, and $0.99 USD on the 5-minute setting with no pauses, 5%, and 10% of turns paused, and $0.70 USD, $0.70 USD, and $0.71 USD on the 1-hour setting.
10131009 * **Keep-alive requests, Claude Sonnet 5 and Claude Opus 5:** Anthropic measured keep-alive requests that refresh the 5-minute cache on Claude Sonnet 5 and Claude Opus 5 on August 23, 2026, and on Claude Opus 5.5 in the runs above, always sent with `max_tokens: 1`. On Claude Sonnet 5 they cost 7.7% less than the 1-hour setting with 5% of turns paused and about the same with 10%; on Claude Opus 5 no difference was measurable at either share; on both they cost more with a pause of 6 minutes or more before every turn. So on those two models the setting to use is the 1-hour duration, not keep-alive requests.
10141010 * **Keep-alive requests, Claude Opus 5.5:** On Claude Opus 5.5 keep-alive requests cost 8% to 18% less than the 1-hour setting with 5% and 10% of turns paused (8% to 13% less at its default effort, `medium`, and 10% to 18% less at `high`; about 10% to 15% once between-session noise is removed by re-billing each keep-alive session's own tokens at 1-hour cache prices), and more with a pause before every turn: 4% to 6% more at 6 minutes, 9% to 10% at 20 minutes, and 56% to 58% at 45 minutes.
1015 * **Requests with `max_tokens: 0`, Claude Opus 5.5:** Anthropic's pre-launch API tests on Claude Opus 5.5 show that a `max_tokens: 0` request writes the cache and that the next request reads it; whether such a request refreshes an existing entry was not measured on Opus 5.5.
1011 * **Requests with `max_tokens: 0`:** Measured October 20, 2026, on Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, three times per model, on a 10,000-token system prompt with the 5-minute cache. A `max_tokens: 0` request sent 4 minutes after the cache write returned no output, read the whole entry, and wrote nothing. A request 4 minutes later, 8 minutes after the write, still read the whole entry. Without the `max_tokens: 0` request, the request at 8 minutes read nothing and wrote the prefix again, all 12 times.
10161012 * **5-minute and 1-hour settings, Claude Sonnet 5.5:** With 5% of turns paused the 5-minute setting cost 5% more than the 1-hour setting on average (2% less to 12% more across the three runs), which is within run-to-run variation: that draw's pauses fell on small prefixes, as on Claude Sonnet 5, and re-billing each 5-minute session's own tokens at 1-hour cache prices gives about 1% more. With 10% paused it cost 54% more (36% to 73% more). With no pauses, the 5-minute setting cost 19% less than the 1-hour setting on average (3% to 32% less across the three runs; the 1-hour setting's higher write price predicts about 20%), and 21% to 24% less in every run with 2-minute pauses.
10171013 * **Keep-alive requests, Claude Sonnet 5.5:** Keep-alive requests cost 16% less than the 1-hour setting on average with 5% of turns paused (13% to 20% less across the three runs) and 8% less with 10% (3% to 10% less), and cost more with a pause of 6 minutes or more before every turn. With 5% and 10% of turns paused, the pauses lasted 6 to 32 minutes on Claude Sonnet 5.5 and on Claude Opus 5.5. Re-billing each keep-alive session's own tokens at 1-hour cache prices, as for Claude Opus 5.5 above, gives 15% to 16% less at 5% paused and about 10% less at 10%, and with a pause before every turn 7% more at 6 minutes, 9% to 10% more at 20 minutes, and 66% to 73% more at 45 minutes. The 6-minute figures leave out run 3, in which 2 of that session's 39 keep-alive requests timed out: the cache expired before those two turns, the session wrote 214,000 tokens to the cache against 80,000 to 85,000 in runs 1 and 2, and it cost 38% more than run 3's 1-hour session.
10181014 * **Keep-alive requests, Claude Sonnet 5.5, by run:** Comparing each run's keep-alive session with the same run's 1-hour session instead, keep-alive cost from about the same to 10% more at 6 minutes (runs 1 and 2; 4% more on their average), from about the same to 16% more at 20 minutes, and 64% to 80% more at 45 minutes.
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1026102217. **Cache-read share in production:** Aggregated first-party Claude API usage for the 14 days ending August 23, 2026, direct API product only, Anthropic-internal organizations excluded, no organization identified. An organization-day counts as an agent loop when its requests carry tool definitions and tool results, its prompts hold 9 or more prior tool calls on average, caching was used, and it made at least 10 such requests (the API has no conversation identifier, so this stands in for conversation length): 303,003 organization-days across 106,487 organizations, median cache-read share 84.2% of all input tokens, upper quartile 91.7%. Use-case labels (the organization's declared use case, or otherwise its classified one) cover 74% of those organization-days and 99% of their tokens; about 72% of coding organization-days read 80% or more. The top decile of organization-days reads 95.9% or more for coding and 94.2% to 94.8% for support, research, data, and other agents. The request-level split at 25 or more prior tool calls comes from a six-hour sample: coding 92% read, 7% write, under 1% uncached.
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102818. **Compaction timing and status-line measurements:**
102418. **Cache-break, compaction timing, and status-line measurements:**
10291025
1026 * **October 2026 test, one change to a long conversation:** Measured October 20, 2026, on Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, on the Messages API with no tools, adaptive thinking, each model's default effort, the 5-minute cache, a cache breakpoint on the system prompt, and [automatic caching](https://platform.claude.com/docs/en/build-with-claude/prompt-caching#automatic-caching). The conversation held about 101,000 tokens: a 20,000-token system prompt, 13 user turns of filler text, the model's own reply to the first turn with its thinking block, and one-word replies to the rest. Once one request had cached the conversation, a second added one short turn, and each measured request sent that longer conversation with one change. Two conversations per model gave the same result.
1027 * **October 2026 test, tokens read and written:** With no change, and with a per-message effort change, the request read all 101,000 tokens and wrote none. With a 25-token mid-conversation system message, or a 25-token status line at the end of the newest user turn, it read them all and wrote only the new text. A top-level effort change (to `low` on Claude Fable 5.1 and Claude Sonnet 5.5, to `high` on Claude Opus 5.5 and Claude Haiku 5.5) read the 20,000-token system prompt and wrote 81,000 tokens. The same 25 tokens at the front or the end of the system prompt read nothing and wrote all 101,000. The request after a mid-conversation system message, a top-level effort change, or 25 tokens added to the end of the system prompt read the new prefix; a new status line at the front wrote everything again. A further test on October 21, 2026, on the same four models with conversations of 5,000 or 100,000 tokens and one-word replies, sent a beta header for the first time mid-conversation, as a first per-message effort change may: that request read everything the request before it had cached, all 54 times. A system message sent with no beta header did the same, all 15 times.
1028 * **October 2026 test, how priced:** The chart gives the prompt side of one request, from its `usage` fields at list prices. The chart's values, in the order system prompt edit, system message, top-level effort change, and per-message effort change: Claude Fable 5.1 $1.26 USD, $0.026 USD, $1.02 USD, and $0.025 USD; Claude Opus 5.5 $0.50 USD, $0.020 USD, $0.41 USD, and $0.020 USD; Claude Sonnet 5.5 $0.25 USD, $0.010 USD, $0.20 USD, and $0.010 USD. The unchanged request repeated an earlier request byte for byte; a new turn of about 25 tokens adds under $0.001 USD. On Claude Haiku 5.5, whose requests were over 100,000 prompt tokens and so at its [long context prices](https://platform.claude.com/docs/en/about-claude/pricing#long-context-pricing): $0.063 USD for the system prompt edit, $0.052 USD for the effort change, and $0.005 USD for the read. The requests ran on an Anthropic-internal organization whose usage is metered the same way as a customer organization's, and on which the API does not enforce the [prefix check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) by default, so its system prompt edits were accepted. On accounts created on or after August 31, 2026, 00:00 UTC, a request to one of these four models that sends earlier thinking blocks back gets a 400 error by default if it changes `system`, `tools`, or a message before one of them.
10301029 * **What was measured:** The triage agent's long variant from [Trim input and context tokens](https://platform.claude.com/docs/en/about-claude/models/optimizing-for-cost-and-intelligence#trim-input-and-context-tokens), run August 24, 2026, on Claude Sonnet 5 with the 5-minute cache, cost from the usage fields at list prices, five sessions per arm: a no-change arm at the default effort throughout ($0.81 USD per session), and two arms that start at low effort and make the same two cache-breaking changes, a switch to the default effort and one added tool, either mid-session at requests 12 and 17 ($0.95 USD) or together on the first request after the first compaction ($0.75 USD).
10311030 * **Fourth arm:** A fourth arm of six sessions, run August 25, 2026, made the same two changes on the request that triggered the first compaction ($0.92 USD per session): that request's summarization pass wrote the 81,000-token context to the cache instead of reading it, so that pass cost $0.21 USD against $0.04 USD for the same pass in the boundary arm.
10321031 * **Compaction:** Sessions first compacted at request 21 to 25 (16 of the 21 sessions at request 22), once the prompt passed the 80,000-token compaction trigger, and two no-change sessions compacted a second time near the end. The boundary arm's lower total than the no-change arm reflects its low-effort requests before the change and those second compactions rather than caching: the two arms' re-write costs differ by under $0.01 USD.
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