Claude Fable 5.1

Cookie settings We use cookies to deliver and improve our services, analyze site usage, and if you agree, to customize or personalize your experience and market our services to you. You can read our Cookie Policy here . Customize Customize Cookie Settings Reject Reject All Cookies Accept Accept All Cookies Loading Loading Loading Loading Loading Loading Loading Loading Loading Loading Loading Loading Ask Docs   Copy page  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 ). 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 participants only. If you already call Claude Fable 5, three changes are breaking: forced tool use returns an error , earlier models can't read its thinking blocks , and editing earlier turns invalidates thinking blocks . Five are additive: per-message effort (beta), turn-scoped system messages (beta), readable progress updates between tool calls ( display: "updates" , beta), a lower cache read price , and 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 participants only Claude Fable 5.1 and Claude Mythos 5.1 share specs and pricing: Context window and output: a 1M token context window (default and maximum) at standard per-token pricing across the whole window, and 128k max output tokens. Thinking: adaptive thinking is always on. Use the effort parameter to control thinking depth. Pricing: the same as Claude Fable 5, except for a lower cache read price . 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 . For all current models, see the 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 : 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 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 , or move the schema to 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 . 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 , and the Claude Agent SDK keep that prefix intact for you. If your code builds the messages array itself, check it before you migrate: 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 ( turn-scoped if it should apply to one turn only) and change tools with mid-conversation tool changes rather than editing system or tools . Trim context with server-side context editing or compaction , which don't count as edits. These patterns also keep the prompt cache 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 has the three-step check. See Preserved thinking for the full rules. New features  Change effort mid-conversation (beta)  On Claude Fable 5.1 you can change the 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. cURL CLI Python TypeScript C# Go Java PHP Ruby  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. { "ro