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

models/fable-5/migration-guide New page · 696 lines, new page

## Migrating to Claude Mythos 5 and Claude Fable 5 from Claude Mythos Preview ### Update your model name ### Features not available on Claude Mythos 5 and Claude Fable 5 ### Token counting and billing ### Migration checklist ## Migrating to Claude Mythos 5 and Claude Fable 5 from Claude Opus 5 ### Update your model name ### What changed ### Migration checklist ## Migrating to Claude Mythos 5 and Claude Fable 5 from Claude Opus 4.8 ### Update your model name ### What changed ### Migration checklist

A whole new page. There's nothing to diff it against, so here is what it says.

---
title: Migrating to Claude Mythos 5 and Claude Fable 5
url: https://platform.claude.com/docs/en/models/fable-5/migration-guide
description: "Migrate to Claude Mythos 5 and Claude Fable 5 from Claude Mythos Preview, Claude Opus 5, or Claude Opus 4.8: model IDs, API changes, and migration checklists."
---

<Note>
  This guide covers migrating [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) code. If you use [Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview), no changes beyond updating the model name are required.
</Note>

<Tip>
  **Automate your migration with the Claude API skill.** In Claude Code, run `/claude-api migrate` to invoke the bundled [Claude API skill](https://platform.claude.com/docs/en/agents-and-tools/agent-skills/claude-api-skill#migrating-to-a-newer-claude-model). It works for any current Claude model as the target:

  ```text wrap
  /claude-api migrate this project to claude-opus-5
  ```

  The skill applies the model ID swap and, as needed, breaking parameter changes, prefill replacement, and effort calibration for your target model across your code base, then produces a checklist of items to verify manually. It asks you to confirm the migration scope (entire working directory, a subdirectory, or a specific file list) before editing any files. The skill also detects Amazon Bedrock and Claude Platform on AWS clients and adjusts model ID formats and feature changes for those platforms.
</Tip>

[Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/introducing-claude-fable-5-and-claude-mythos-5) is Anthropic's most capable widely released model, available on the Claude API, [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Google Cloud](https://platform.claude.com/docs/en/build-with-claude/claude-on-vertex-ai), and [Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry). [Claude Mythos 5](https://anthropic.com/glasswing) shares the same capabilities and is offered only to approved customers in Project Glasswing.

The baseline settings shared by `claude-fable-5` and `claude-mythos-5`:

* **Thinking:** [Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is always on. The model determines when and how much to think on each request, and no `thinking` configuration is required. Both `thinking: {type: "disabled"}` and manual extended thinking (`thinking: {type: "enabled", budget_tokens: N}`) return a 400 error.
* **Prefill:** Prefilling the assistant message returns a 400 error. Use system prompt instructions instead.
* **Context window and output:** A [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) by default, and up to 128k output tokens per request.
* **Pricing:** $10 USD per million input tokens and $50 USD per million output tokens. See [Claude pricing](https://platform.claude.com/docs/en/about-claude/pricing).
* **Data retention:** Both models require 30-day data retention and are not available under zero data retention (ZDR) arrangements; both are designated Covered Models. On the Claude API, a request to Claude Fable 5 from an organization whose data retention configuration does not meet this requirement returns a 400 `invalid_request_error`. Organizations with a ZDR arrangement should contact their Anthropic account team to discuss data retention configuration. Alternatively, you can configure data retention per workspace. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements) for per-platform details.

Where the two models diverge:

* **Availability:** Claude Fable 5 does not require access approval. Claude Mythos 5 is available only to approved customers in [Project Glasswing](https://anthropic.com/glasswing).
* **Safety classifiers:** Claude Fable 5 runs safety classifiers that can decline requests with `stop_reason: "refusal"`. Claude Mythos 5 does not include these classifiers. See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
* **Priority Tier:** [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models) is supported on Claude Fable 5 but not on Claude Mythos 5.

## Migrating to Claude Mythos 5 and Claude Fable 5 from Claude Mythos Preview

[Claude Mythos 5](https://anthropic.com/glasswing) is the access-gated successor to [Claude Mythos Preview](https://anthropic.com/glasswing), the invitation-only research preview. [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/introducing-claude-fable-5-and-claude-mythos-5) offers the same capabilities and does not require access approval. The changes in this section apply equally to both targets.

Migration is mostly drop-in. Claude Mythos 5 and Claude Fable 5 use the same [Messages API](https://platform.claude.com/docs/en/build-with-claude/working-with-messages) and the same [tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview) patterns as Claude Mythos Preview, and token counts are roughly unchanged because all three models use the same tokenizer. The key changes to check are the features that are no longer available (listed in the next section) and thinking output. If you migrate to Claude Fable 5, also plan for safety classifier refusals, which Claude Mythos Preview and Claude Mythos 5 do not have; see [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).

For the Claude Mythos Preview retirement timeline, see [Model deprecations](https://platform.claude.com/docs/en/about-claude/model-deprecations).

### Update your model name

```python
model = "claude-mythos-preview"  # Before
model = "claude-mythos-5"  # After

# Or, for the model with the same capabilities and no access approval requirement:
model = "claude-fable-5"  # After
```

### Features not available on Claude Mythos 5 and Claude Fable 5

1. **Extended thinking and thinking token budgets:** Manual extended thinking (`thinking: {type: "enabled", budget_tokens: N}`) is not supported on `claude-mythos-5` or `claude-fable-5` and returns a 400 error. [Adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is always on: the model determines when and how much to think on each request, and no `thinking` configuration is required. `thinking: {type: "disabled"}` returns an error. `budget_tokens` has no direct replacement: thinking is adaptive, and the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) is a separate output-level control, not a thinking budget.

   Before (Claude Mythos Preview):

   <CodeGroup>
     ```bash cURL
     curl https://api.anthropic.com/v1/messages \
       -H "x-api-key: $ANTHROPIC_API_KEY" \
       -H "anthropic-version: 2023-06-01" \
       -H "content-type: application/json" \
       -d '{
         "model": "claude-mythos-preview",
         "max_tokens": 16000,
         "thinking": {
           "type": "enabled",
           "budget_tokens": 10000
         },
         "messages": [
           {
             "role": "user",
             "content": "..."
           }
         ]
       }'
     ```

     ```bash CLI
     ant messages create <<'YAML'
     model: claude-mythos-preview
     max_tokens: 16000
     thinking:
       type: enabled
       budget_tokens: 10000
     messages:
       - role: user
         content: "..."
     YAML
     ```

     ```python Python
     client.messages.create(
         model="claude-mythos-preview",
         max_tokens=16000,
         thinking={"type": "enabled", "budget_tokens": 10000},
         messages=[{"role": "user", "content": "..."}],
     )
     ```

     ```typescript TypeScript
     await client.messages.create({
       model: "claude-mythos-preview",
       max_tokens: 16000,
       thinking: { type: "enabled", budget_tokens: 10000 },
       messages: [{ role: "user", content: "..." }]
     });
     ```

     ```csharp C#
     using Anthropic;
     using Anthropic.Models.Messages;

     AnthropicClient client = new();

     var parameters = new MessageCreateParams
     {
         Model = "claude-mythos-preview",
         MaxTokens = 16000,
         Thinking = new ThinkingConfigEnabled(budgetTokens: 10000),
         Messages = [new() { Role = Role.User, Content = "..." }]
     };

     var response = await client.Messages.Create(parameters);
     Console.WriteLine(response);
     ```

     ```go Go
     client := anthropic.NewClient()

     response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
     	Model:     "claude-mythos-preview",
     	MaxTokens: 16000,
     	Thinking:  anthropic.ThinkingConfigParamOfEnabled(10000),
     	Messages: []anthropic.MessageParam{
     		anthropic.NewUserMessage(anthropic.NewTextBlock("...")),
     	},
     })
     if err != nil {
     	log.Fatal(err)
     }
     fmt.Println(response)
     ```

     ```java Java
     AnthropicClient client = AnthropicOkHttpClient.fromEnv();

     MessageCreateParams params = MessageCreateParams.builder()
         .model("claude-mythos-preview")
         .maxTokens(16000L)
         .enabledThinking(10000L)
         .addUserMessage("...")
         .build();

     Message response = client.messages().create(params);
     IO.println(response);
     ```

     ```php PHP
     $client = new Client();

     $message = $client->messages->create(
         maxTokens: 16000,
         messages: [['role' => 'user', 'content' => '...']],
         model: 'claude-mythos-preview',
         thinking: ['type' => 'enabled', 'budget_tokens' => 10000],
     );
     ```

     ```ruby Ruby
     client = Anthropic::Client.new

     message = client.messages.create(
       model: "claude-mythos-preview",
       max_tokens: 16000,
       thinking: {
         type: "enabled",
         budget_tokens: 10000
       },
       messages: [
         { role: "user", content: "..." }
       ]
     )
     ```
   </CodeGroup>

   After (Claude Mythos 5):

   <CodeGroup>
     ```bash cURL
     curl https://api.anthropic.com/v1/messages \
       -H "x-api-key: $ANTHROPIC_API_KEY" \
       -H "anthropic-version: 2023-06-01" \
       -H "content-type: application/json" \
       -d '{
         "model": "claude-mythos-5",
         "max_tokens": 16000,
         "messages": [
           {
             "role": "user",
             "content": "..."
           }
         ]
       }'
     ```

     ```bash CLI
     ant messages create <<'YAML'
     model: claude-mythos-5
     max_tokens: 16000
     messages:
       - role: user
         content: "..."
     YAML
     ```

     ```python Python
     client.messages.create(
         model="claude-mythos-5",
         max_tokens=16000,
         messages=[{"role": "user", "content": "..."}],
     )
     ```

     ```typescript TypeScript
     await client.messages.create({
       model: "claude-mythos-5",
       max_tokens: 16000,
       messages: [{ role: "user", content: "..." }]
     });
     ```

     ```csharp C#
     using Anthropic;
     using Anthropic.Models.Messages;

     AnthropicClient client = new();

     var parameters = new MessageCreateParams
     {
         Model = "claude-mythos-5",
         MaxTokens = 16000,
         Messages = [new() { Role = Role.User, Content = "..." }]
     };

     var response = await client.Messages.Create(parameters);
     Console.WriteLine(response);
     ```

     ```go Go
     client := anthropic.NewClient()

     response, err := client.Messages.New(context.TODO(), anthropic.MessageNewParams{
     	Model:     "claude-mythos-5",
     	MaxTokens: 16000,
     	Messages: []anthropic.MessageParam{
     		anthropic.NewUserMessage(anthropic.NewTextBlock("...")),
     	},
     })
     if err != nil {
     	log.Fatal(err)
     }
     fmt.Println(response)
     ```

     ```java Java
     AnthropicClient client = AnthropicOkHttpClient.fromEnv();

     MessageCreateParams params = MessageCreateParams.builder()
         .model("claude-mythos-5")
         .maxTokens(16000L)
         .addUserMessage("...")
         .build();

     Message response = client.messages().create(params);
     IO.println(response);
     ```

     ```php PHP
     $client = new Client();

     $message = $client->messages->create(
         maxTokens: 16000,
         messages: [['role' => 'user', 'content' => '...']],
         model: 'claude-mythos-5',
     );
     ```

     ```ruby Ruby
     client = Anthropic::Client.new

     message = client.messages.create(
       model: "claude-mythos-5",
       max_tokens: 16000,
       messages: [
         { role: "user", content: "..." }

Cut at 300 lines. The page has the rest.