Anthropic's 2026-09 frontier model — successor to Claude Fable 5, with gains in agentic coding, long-running agentic workflows, and knowledge work; 1M context, 128K output.
Claude Fable 5.1 is Anthropic's 2026-09 successor to Claude Fable 5, targeting gains in agentic coding, long-running agentic workflows, and knowledge work. In TheRouter it is cataloged as anthropic/claude-fable-5-1, with text and image input, text output, a 1M-token context window, and up to 128K output tokens flowing from the live model catalog.
Unlike Fable 5, which was blocked on Anthropic's first-party API for a period after launch, Fable 5.1 is directly reachable via Anthropic's first-party Claude API at TheRouter's launch of this model, with OpenRouter configured as a fallback route. Both routes decline the tool_choice: any and tool_choice: tool request modes with a 400; requests should use tool_choice: auto/none or the strict/output_schema migration path Anthropic documents for forcing structured tool use.
tool_choice: any or tool_choice: tool — both configured routes reject these with a 400; use auto/none or strict/output_schema insteadAnthropic operates Fable 5.1 natively through Claude API semantics, including its own tool_choice, effort, and prompt-caching controls.
TheRouter serves it as the catalog model id anthropic/claude-fable-5-1 on the OpenAI-compatible chat completions path, with an additional ingress-level rejection of tool_choice: any/tool (both entry points) that Anthropic itself would otherwise reject upstream with a 400 — TheRouter rejects it before dialing out, avoiding a wasted upstream round-trip.
| Type | Rate |
|---|---|
| Input | $10.80 / 1M tokens |
| Output | $54.00 / 1M tokens |
| Release date | 2026-09 | to verify |
| Anthropic model id | claude-fable-5-1 | verified |
| Context window | 1,000,000 tokens | verified |
| Maximum output | 128,000 tokens | verified |
| Training cutoff | Not publicly disclosed | unknown |
| License | Anthropic Usage Policy (proprietary, API-only)www.anthropic.com ↗ | verified |
| Pricing (input / output) | $10 / $50 per MTok (list price; TheRouter's PRICING_MARGIN applies once, at runtime)platform.claude.com ↗ | verified |
| Benchmark | Distribution | Score | Source |
|---|---|---|---|
Official benchmark table No benchmark table is published in the cited model documentation at curation time. | — | Not publicly disclosed | — |
Use the global api.therouter.ai endpoint shown below for new integrations; the legacy China accelerated endpoint is retired.
curl https://api.therouter.ai/v1/chat/completions -H "Content-Type: application/json" -H "Authorization: Bearer $THE_ROUTER_API_KEY" -d '{
"model": "anthropic/claude-fable-5-1",
"messages": [
{"role": "user", "content": "Summarize the key points from this input."}
]
}'Use the OpenAI-compatible chat completions endpoint when you want TheRouter to route Claude Fable 5.1 through its configured provider chain.
curl https://api.therouter.ai/v1/chat/completions \
-H "Authorization: Bearer $THEROUTER_API_KEY" \
-H "Content-Type: application/json" \
-d '{"model":"anthropic/claude-fable-5-1","messages":[{"role":"user","content":"Hello"}]}'TheRouter adds anthropic/claude-fable-5-1 with a first-party Anthropic route and an OpenRouter fallback; both routes reject tool_choice: any/tool with a 400 at the ingress layer.
No. Anthropic rejects these modes for this model with a 400, and TheRouter enforces the same rejection at both the /v1/messages and /v1/chat/completions entry points before any upstream call. Use tool_choice: auto/none, or the strict/output_schema migration path, instead.
| source | URL | retrieved | |
|---|---|---|---|
| Release date | — | — | to verify |
| Anthropic model id | — | — | verified |
| Context window | — | — | verified |
| Maximum output | — | — | verified |
| Training cutoff | — | — | unknown |
| License | www.anthropic.com ↗ | 2026-09-17 | verified |
| Pricing (input / output) | platform.claude.com ↗ | 2026-09-17 | verified |
| Official benchmark table | platform.claude.com ↗ | 2026-09-17 | unknown |
| Claude Fable 5.1 launches as successor to Fable 5 | platform.claude.com ↗ | 2026-09-17 | verified |