whats-new-fable-5-1 changed

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

---
title: What's new in Claude Fable 5.1
url: https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1
description: Overview of new features, breaking changes, and capability improvements in Claude Fable 5.1 and Claude Mythos 5.1.
---

Claude Fable 5.1 extends Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost, and brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work. For most workloads, start with Claude Opus 5 (see [Choosing a model](https://platform.claude.com/docs/en/about-claude/models/choosing-a-model)). Use Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Claude Opus 5 at higher effort still fall short. Claude Mythos 5.1 offers the same capabilities to [Project Glasswing](https://anthropic.com/glasswing) participants only.

If you already call Claude Fable 5, three changes are breaking: [forced tool use returns an error](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#forced-tool-use-is-not-supported), [earlier models can't read its thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#thinking-blocks-are-tied-to-the-model-that-produced-them), and [editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#editing-earlier-turns-invalidates-thinking-blocks). Five are additive: [per-message effort](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#change-effort-mid-conversation-beta) (beta), [turn-scoped system messages](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta) (beta), [readable progress updates between tool calls](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#progress-updates-between-tool-calls-beta) (`display: "updates"`, beta), a [lower cache read price](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing), and [content provenance](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#content-provenance).

## Models

| Model             | Claude API ID     | Description                                                                                | Availability                                                           |
| ----------------- | ----------------- | ------------------------------------------------------------------------------------------ | ---------------------------------------------------------------------- |
| Claude Fable 5.1  | claude-fable-5-1  | Successor to Claude Fable 5, for long-running agentic coding, knowledge work, and research | All customers, on the Claude API and partner platforms                 |
| Claude Mythos 5.1 | claude-mythos-5-1 | Same capabilities as Claude Fable 5.1. Successor to Claude Mythos 5.                       | [Project Glasswing](https://anthropic.com/glasswing) participants only |

Claude Fable 5.1 and Claude Mythos 5.1 share specs and pricing:

* **Context window and output:** a [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows) (default and maximum) at standard per-token pricing across the whole window, and 128k max output tokens.
* **Thinking:** [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) is always on. Use the [effort parameter](https://platform.claude.com/docs/en/build-with-claude/effort) to control thinking depth.
* **Pricing:** the same as Claude Fable 5, except for a [lower cache read price](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing).
* **Tokenizer:** the same as Claude Fable 5 (introduced with Claude Opus 4.7). Compared with models older than Claude Opus 4.7, the same text produces roughly 30% more tokens. See [Token counting](https://platform.claude.com/docs/en/build-with-claude/token-counting).

For all current models, see the [models overview](https://platform.claude.com/docs/en/models/overview).

## Breaking changes

### Forced tool use is not supported

Claude Fable 5.1 and Claude Mythos 5.1 don't support forced tool use. `tool_choice` set to `{"type": "any"}` or `{"type": "tool", "name": "..."}` returns a 400 `invalid_request_error`:

```text wrap
tool_choice: type "tool" and "any" are not supported for this model.
```

`tool_choice: {"type": "auto"}` (the default) and `{"type": "none"}` are unchanged. The same validation applies to the [token counting](https://platform.claude.com/docs/en/build-with-claude/token-counting) endpoint.

Thinking is always on for these models, and a forced tool call would skip it. The model would write its working-out into the tool arguments instead, which lowers argument quality. For schema-valid JSON, keep `tool_choice: {"type": "auto"}` and set `strict: true` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use), or move the schema to [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs). To make the model call a tool rather than reply in text, state in the prompt when the tool applies (for example, "Use the `get_weather` tool to answer"). Claude Fable 5.1 follows explicit tool instructions reliably.

### Earlier models can't read Claude Fable 5.1 thinking blocks

Every thinking block records which model produced it, and it's preserved in one direction only: Claude Fable 5.1 reads earlier models' thinking blocks, and no earlier model reads Claude Fable 5.1's. A conversation that moves onto Claude Fable 5.1 (from Claude Opus 5, Claude Fable 5, or any earlier Claude model) keeps its reasoning. A conversation that moves from Claude Fable 5.1 to any of those models loses it for the turns that run there.

When a request carries a block the target model can't read (a router or fallback that switches models mid-conversation, for example), the API drops the block before the model sees it. Dropped blocks don't count toward `input_tokens` and aren't billed. With the `thinking-binding-controls-2026-08-01` beta header, the drop is reported in a top-level `input_transformations` array. Without it, the drop is silent. See [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-for-model).

### Editing earlier turns invalidates thinking blocks

Modifying anything before a Claude Fable 5.1 thinking block (the `system` prompt, the `tools`, or an earlier message) results in an error on the next request, or in the block being dropped if you opt into that. Claude Mythos 5.1 doesn't run this check. Claude Code, claude.ai, [Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview), and the [Claude Agent SDK](https://code.claude.com/docs/en/agent-sdk/overview) keep that prefix intact for you. If your code builds the `messages` array itself, check it before you migrate: [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) walks through the check and each fix. The check is enforced for new accounts created on or after August 31, 2026. For accounts created earlier, the API records the mismatch but acts on it only when the request sets `thinking.block_binding.prefix_mismatch_behavior`.

These patterns invalidate every later thinking block:

* Editing, reordering, or removing an earlier turn while keeping later ones.
* Injecting per-request text into an earlier turn (a reminder or status line) that you remove on the next request.
* Rebuilding the top-level `system` prompt or `tools` array between requests in the same conversation.
* An image or document URL that serves different bytes on a later request (the check covers the bytes, not the URL, so a rotating signed URL for the same file is fine).

These keep later blocks valid: removing a leading run of thinking blocks (oldest first), letting server-side compaction or context editing trim the history, moving `cache_control` markers, and changing `effort` between requests. Removing a thinking block from anywhere other than the start of the run invalidates every thinking block after it.

Where the check is enforced, a request that replays an invalidated block is rejected with a 400 whose message says `The block is bound to a different conversation`. To drop the block and continue instead, send the `thinking-binding-controls-2026-08-01` beta header with `thinking.block_binding.prefix_mismatch_behavior: "drop_block"`. The drop is reported in `input_transformations` with `reason: "prefix_binding_mismatch"`.

To keep thinking valid across a long session, treat the conversation as append-only. Add instructions with a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) ([turn-scoped](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta) if it should apply to one turn only) and change tools with [mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes) rather than editing `system` or `tools`. Trim context with server-side [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing) or [compaction](https://platform.claude.com/docs/en/build-with-claude/compaction), which don't count as edits. These patterns also keep the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) warm. To find out whether your integration edits history, run a session with `prefix_mismatch_behavior: "drop_block"` and log `input_transformations`: the [migration guide](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-preserved-thinking) has the three-step check. See [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation) for the full rules.

## New features

### Change effort mid-conversation (beta)

On Claude Fable 5.1 you can change the [effort](https://platform.claude.com/docs/en/build-with-claude/effort) level mid-conversation without invalidating the prompt cache. Raise it for a hard step and lower it for routine ones. Per-message effort is in beta: include the `mid-conversation-output-config-2026-07-01` beta header. Claude Fable 5.1, Claude Mythos 5.1, and Claude Opus 5 support it on the Claude API.

<CodeGroup>
  ```bash cURL
  # Effort-only system message: the new level takes effect from the next user turn.
  curl https://api.anthropic.com/v1/messages \
    -H "x-api-key: $ANTHROPIC_API_KEY" \
    -H "anthropic-version: 2023-06-01" \
    -H "anthropic-beta: mid-conversation-output-config-2026-07-01" \
    -H "content-type: application/json" \
    -d '{
      "model": "claude-fable-5-1",
      "max_tokens": 4096,
      "output_config": {"effort": "high"},
      "messages": [
        {"role": "user", "content": "Plan a migration from SQLite to PostgreSQL in three short steps."},
        {"role": "assistant", "content": "1. Export the SQLite data. 2. Create the PostgreSQL schema. 3. Import the data and verify row counts."},
        {"role": "system", "content": [], "output_config": {"effort": "low"}},
        {"role": "user", "content": "Summarize the plan in one sentence."}
      ]
    }'
  ```

  ```bash CLI
  ant beta:messages create --beta mid-conversation-output-config-2026-07-01 \
    --transform 'content.#(type=="text").text' --raw-output <<'YAML'
  model: claude-fable-5-1
  max_tokens: 4096
  output_config:
    effort: high
  messages:
    - role: user
      content: Plan a migration from SQLite to PostgreSQL in three short steps.
    - role: assistant
      content: "1. Export the SQLite data. 2. Create the PostgreSQL schema. 3. Import the data and verify row counts."
    # Effort-only system message: the new level takes effect from the next user turn.
    - role: system
      content: []
      output_config:
        effort: low
    - role: user
      content: Summarize the plan in one sentence.
  YAML
  ```

  ```python Python
  client = anthropic.Anthropic()

  response = client.beta.messages.create(
      model="claude-fable-5-1",
      max_tokens=4096,
      output_config={"effort": "high"},
      messages=[
          {
              "role": "user",
              "content": "Plan a migration from SQLite to PostgreSQL in three short steps.",
          },
          {
              "role": "assistant",
              "content": "1. Export the SQLite data. 2. Create the PostgreSQL schema. 3. Import the data and verify row counts.",
          },
          # Effort-only system message: the new level takes effect from the next user turn.
          {"role": "system", "content": [], "output_config": {"effort": "low"}},
          {"role": "user", "content": "Summarize the plan in one sentence."},
      ],
      betas=["mid-conversation-output-config-2026-07-01"],
  )

  for block in response.content:
      if block.type == "text":
          print(block.text)
  ```

  ```typescript TypeScript
  const client = new Anthropic();

  const response = await client.beta.messages.create({
    model: "claude-fable-5-1",
    max_tokens: 4096,
    output_config: { effort: "high" },
    messages: [
      {
        role: "user",
        content: "Plan a migration from SQLite to PostgreSQL in three short steps."
      },
      {
        role: "assistant",
        content:
          "1. Export the SQLite data. 2. Create the PostgreSQL schema. 3. Import the data and verify row counts."
      },
      // Effort-only system message: the new level takes effect from the next user turn.
      { role: "system", content: [], output_config: { effort: "low" } },
      { role: "user", content: "Summarize the plan in one sentence." }
    ],
    betas: ["mid-conversation-output-config-2026-07-01"]
  });

  for (const block of response.content) {
    if (block.type === "text") {
      console.log(block.text);
    }
  }
  ```

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

  AnthropicClient client = new();

  var response = await client.Beta.Messages.Create(new MessageCreateParams
  {
      Model = "claude-fable-5-1",
      MaxTokens = 4096,
      OutputConfig = new() { Effort = Effort.High },
      Messages =
      [
          new() { Role = Role.User, Content = "Plan a migration from SQLite to PostgreSQL in three short steps." },
          new() { Role = Role.Assistant, Content = "1. Export the SQLite data. 2. Create the PostgreSQL schema. 3. Import the data and verify row counts." },
          // Effort-only system message: the new level takes effect from the next user turn.
          new()
          {
              Role = Role.System,
              Content = new([]),
              OutputConfig = new() { Effort = BetaSystemMessageOutputConfigEffort.Low },
          },
          new() { Role = Role.User, Content = "Summarize the plan in one sentence." },
      ],
      Betas = [AnthropicBeta.MidConversationOutputConfig2026_07_01],
  });

  foreach (var block in response.Content)
  {
      if (block.TryPickText(out var textBlock))
      {
          Console.WriteLine(textBlock.Text);
      }
  }
  ```

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

  response, err := client.Beta.Messages.New(context.Background(), anthropic.BetaMessageNewParams{
  	Model:     "claude-fable-5-1",
  	MaxTokens: 4096,
  	OutputConfig: anthropic.BetaOutputConfigParam{
  		Effort: anthropic.BetaOutputConfigEffortHigh,
  	},
  	Messages: []anthropic.BetaMessageParam{
  		anthropic.NewBetaUserMessage(anthropic.NewBetaTextBlock("Plan a migration from SQLite to PostgreSQL in three short steps.")),
  		{
  			Role:    anthropic.BetaMessageParamRoleAssistant,
  			Content: []anthropic.BetaContentBlockParamUnion{anthropic.NewBetaTextBlock("1. Export the SQLite data. 2. Create the PostgreSQL schema. 3. Import the data and verify row counts.")},
  		},
  		// Effort-only system message: the new level takes effect from the next user turn.
  		anthropic.NewBetaSystemMessage(anthropic.BetaSystemMessageOutputConfigParam{
  			Effort: anthropic.BetaSystemMessageOutputConfigEffortLow,
  		}),
  		anthropic.NewBetaUserMessage(anthropic.NewBetaTextBlock("Summarize the plan in one sentence.")),
  	},
  	Betas: []anthropic.AnthropicBeta{anthropic.AnthropicBetaMidConversationOutputConfig2026_07_01},
  })
  if err != nil {
  	log.Fatal(err)
  }

  for _, block := range response.Content {
  	if textBlock, ok := block.AsAny().(anthropic.BetaTextBlock); ok {
  		fmt.Println(textBlock.Text)
  	}
  }
  ```

  ```java Java
  import com.anthropic.models.beta.AnthropicBeta;
  import com.anthropic.models.beta.messages.BetaMessage;
  import com.anthropic.models.beta.messages.BetaMessageParam;
  import com.anthropic.models.beta.messages.BetaOutputConfig;
  import com.anthropic.models.beta.messages.BetaSystemMessageOutputConfig;
  import com.anthropic.models.beta.messages.MessageCreateParams;

  void main() {
      AnthropicClient client = AnthropicOkHttpClient.fromEnv();

      MessageCreateParams params = MessageCreateParams.builder()
          .model("claude-fable-5-1")
          .maxTokens(4096L)
          .addBeta(AnthropicBeta.MID_CONVERSATION_OUTPUT_CONFIG_2026_07_01)
          .outputConfig(BetaOutputConfig.builder()
              .effort(BetaOutputConfig.Effort.HIGH)
              .build())
          .addUserMessage("Plan a migration from SQLite to PostgreSQL in three short steps.")
          .addAssistantMessage("1. Export the SQLite data. 2. Create the PostgreSQL schema. 3. Import the data and verify row counts.")
          // Effort-only system message: the new level takes effect from the next user turn.
          .addMessage(BetaMessageParam.builder()
              .role(BetaMessageParam.Role.SYSTEM)
              .contentOfBetaContentBlockParams(List.of())
              .outputConfig(BetaSystemMessageOutputConfig.builder()
                  .effort(BetaSystemMessageOutputConfig.Effort.LOW)
                  .build())
              .build())
          .addUserMessage("Summarize the plan in one sentence.")
          .build();

      BetaMessage response = client.beta().messages().create(params);
      response.content().stream()
          .flatMap(block -> block.text().stream())
          .forEach(textBlock -> IO.println(textBlock.text()));
  }
  ```

  ```php PHP
  use Anthropic\Beta\AnthropicBeta;
  use Anthropic\Beta\Messages\BetaMessageParam;
  use Anthropic\Beta\Messages\BetaOutputConfig;
  use Anthropic\Beta\Messages\BetaSystemMessageOutputConfig;
  use Anthropic\Client;

  $client = new Client();

  $response = $client->beta->messages->create(
      model: 'claude-fable-5-1',
      maxTokens: 4096,
      outputConfig: BetaOutputConfig::with(effort: 'high'),
      messages: [
          BetaMessageParam::with(role: 'user', content: 'Plan a migration from SQLite to PostgreSQL in three short steps.'),
          BetaMessageParam::with(role: 'assistant', content: '1. Export the SQLite data. 2. Create the PostgreSQL schema. 3. Import the data and verify row counts.'),
          // Effort-only system message: the new level takes effect from the next user turn.
          BetaMessageParam::with(
              role: 'system',
              content: [],
              outputConfig: BetaSystemMessageOutputConfig::with(effort: 'low'),

Cut at 300 lines. The page has the rest.