optimizing-for-cost-and-intelligence changedabout-claude/models/optimizing-for-cost-and-intelligence
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199The 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.
199At list prices, the 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, except on Claude Mythos 5.1[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.
200200
201201**What to do.**
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1033103318. **Cache-break, compaction timing, and status-line measurements:**
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10351035 * **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.
1036 * **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.
1036 * **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 tokens, and on all but Claude Fable 5.1 with conversations of 100,000 tokens, all with 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.
10371037 * **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.
10381038 * **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).
10391039 * **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.
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