Or, for the model with the same capabilities and no access approval requirement:

 
 1. **Thinking can no longer be disabled:** On Claude Opus 5, thinking is on by default and can be turned off with `thinking: {type: "disabled"}` at an [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level of `high` or below. On `claude-fable-5` and `claude-mythos-5`, [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is always on, and `thinking: {type: "disabled"}` returns a 400 error at any effort level. Remove the `thinking: {type: "disabled"}` configuration and use lower effort levels to control token spend instead.
 
+   If your Claude Opus 5 requests disabled thinking, the response shape changes: a response can begin with one or more `thinking` blocks before the first `text` block, returned with an empty `thinking` field at the default `display: "omitted"` (the same default as Claude Opus 5). Code that reads the reply by position, such as `content[0].text` or a stream handler that treats the first content block as text, must select content blocks by their `type` field instead, and tool-use loops must pass `thinking` blocks back complete and unmodified with their tool results. The API rejects edited, reordered, or partially dropped thinking blocks with a 400 error (see [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks)). Thinking tokens are billed as output tokens even when the thinking text is not returned.
+
 2. **Pricing:** Claude Fable 5 and Claude Mythos 5 are priced at $10 USD per million input tokens and $50 USD per million output tokens, compared with $5 USD and $25 USD for Claude Opus 5. See [Claude pricing](https://platform.claude.com/docs/en/about-claude/pricing).
 
 3. **Priority Tier:** [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models) is not supported on Claude Opus 5, so no existing traffic is affected. If your organization has a Priority Tier commitment, Claude Fable 5 supports it; Claude Mythos 5 does not.

 
 * Update the model name from `claude-opus-5` to `claude-fable-5` (or `claude-mythos-5`).
 * Remove any `thinking: {type: "disabled"}` configuration; it returns a 400 error on `claude-fable-5` and `claude-mythos-5`. Use lower [effort](https://platform.claude.com/docs/en/build-with-claude/effort) levels to control token spend instead, and revisit `max_tokens` for workloads that ran with thinking disabled on Claude Opus 5.
+* If those workloads read content by position, such as `content[0].text`, update them to select content blocks by `type`: `thinking` blocks now arrive before `text` blocks. Pass `thinking` blocks back complete and unmodified in tool-use loops; modified blocks return a 400 error.
 * If your organization has a zero data retention (ZDR) arrangement, confirm eligibility before migrating. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
-* Re-baseline cost on your own workloads. Token counts are roughly unchanged; per-token pricing differs.
+* Re-baseline cost on your own workloads. Token counts are roughly unchanged; per-token pricing differs, and workloads that ran with thinking disabled now produce thinking tokens, which are billed as output tokens.
 
 ## Migrating to Claude Mythos 5 and Claude Fable 5 from Claude Opus 4.8
 

 
 1. **Adaptive thinking is always on:** [Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is the only thinking mode on `claude-fable-5` and `claude-mythos-5`. The model determines when and how much to think on each request, and no `thinking` configuration is required. `thinking: {type: "disabled"}` returns an error. Use the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) to control thinking depth.
 
-   The behavior change to check: on Claude Opus 4.8, requests without a `thinking` field run without thinking; on `claude-fable-5` and `claude-mythos-5`, those same requests run with adaptive thinking. `max_tokens` remains a hard limit on total output, thinking plus response text, so revisit it for workloads that ran without thinking on Claude Opus 4.8. See [Cost control](https://platform.claude.com/docs/en/build-with-claude/thinking-steering-and-cost#cost-control).
+   The behavior change to check: on Claude Opus 4.8, requests without a `thinking` field run without thinking; on `claude-fable-5` and `claude-mythos-5`, those same requests run with adaptive thinking. `max_tokens` remains a hard limit on total output, thinking plus response text, so revisit it for workloads that ran without thinking on Claude Opus 4.8. See [Cost control](https://platform.claude.com/docs/en/build-with-claude/thinking-steering-and-cost#cost-control). Responses can also begin with one or more `thinking` blocks before the first `text` block, so code that reads the reply by position (for example, `content[0].text`, or a stream handler that treats the first content block as text) must select content blocks by their `type` field instead. Thinking tokens are billed as output tokens even when the thinking text is not returned to you, so a workload that ran without thinking on Claude Opus 4.8 produces more output tokens per request, in addition to the per-token price difference.
 
+   If you run a tool-use loop, pass the `thinking` blocks from each assistant response back to the API complete and unmodified when you return tool results, including blocks whose `thinking` field is empty. Echo the assistant message as received rather than filtering its content blocks by type or rebuilding it: the API rejects edited, reordered, or partially dropped thinking blocks with a 400 error. See [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks).
+
    Before (Claude Opus 4.8):
 
    <CodeGroup>

 * If your organization has a zero data retention (ZDR) arrangement, confirm eligibility before migrating. `claude-fable-5` and `claude-mythos-5` require 30-day data retention; on the Claude API, requests to `claude-fable-5` that do not meet this requirement return a 400 `invalid_request_error`. Claude Opus 4.8 remains available under ZDR. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
 * Update the model name from `claude-opus-4-8` to `claude-fable-5` (or `claude-mythos-5`).
 * Remove any `thinking: {type: "disabled"}` configuration. Disabling thinking returns an error on `claude-fable-5` and `claude-mythos-5`, and requests without a `thinking` field run with adaptive thinking.
+* Update response parsing that reads content by position, such as `content[0].text`: with adaptive thinking always on, `thinking` blocks arrive before `text` blocks. Select content blocks by `type` instead, and pass `thinking` blocks back complete and unmodified in tool-use loops; modified blocks return a 400 error. See [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks).
 * If you removed manual extended thinking and assistant prefills during earlier migrations, no action is needed: both remain unsupported on `claude-fable-5` and `claude-mythos-5`.
 * Verify any code that parses the `thinking` field treats it as display text only and passes thinking blocks back unchanged when continuing on the same model. `thinking.display` defaults to `"omitted"` on `claude-fable-5` and `claude-mythos-5`, the same as on Claude Opus 4.8; set `display: "summarized"` to receive readable summaries. See [Thinking output on Claude Fable 5 and Claude Mythos 5](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-output-on-claude-fable-5-and-claude-mythos-5).
 * If you replay conversation history on another model, strip `thinking` and `redacted_thinking` blocks from prior assistant turns first. Thinking blocks from `claude-fable-5` and `claude-mythos-5` are tied to the model that produced them, and models other than Claude Fable 5 and Claude Mythos 5 silently ignore them. Stripping keeps cross-model requests minimal and uniform. The exception is redeeming a [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit), which requires the request body echoed under that feature's exact rules.
 * If you migrate to Claude Fable 5, handle `stop_reason: "refusal"` and read the `stop_details.category` field. To re-run refused requests on another model automatically, consider the opt-in `fallbacks` parameter (beta). See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
 * Re-evaluate your `effort` setting. Start at `high` for most tasks, including workloads that ran at `xhigh` on Claude Opus 4.8.
-* Re-baseline cost and latency on your own workloads. Token counts are roughly unchanged when migrating from `claude-opus-4-8`; per-token pricing differs.
+* Re-baseline cost and latency on your own workloads. Token counts are roughly unchanged when migrating from `claude-opus-4-8`; per-token pricing differs, and thinking tokens are billed as output tokens, so workloads that ran without thinking produce more output tokens per request.