Claude Fable 5.1 Price & Mythos 5.1: What Anthropic's Sept 1 Release Means for Agency Budgets

Published September 1, 2026Updated September 4, 2026 (Claude FLT-in-Lean proof)By ABD Legacy LLC
Model pricingClaudeAgent tooling

Anthropic shipped a new flagship on September 1, 2026: Claude Fable 5.1 is generally available, and Claude Mythos 5.1 is the same model with different safeguards, restricted to trusted-access programs for cybersecurity and life sciences work. The Claude Fable 5.1 price is $10 per million input tokens and $50 per million output tokens — unchanged from Fable 5 — but lower cache-read pricing makes typical workloads about 25% cheaper and highly agentic workloads up to ~45% cheaper.

For agencies, this is the first Claude release that pairs a real per-token price cut with a serious capability jump. Here is the pricing you can quote, how Fable 5.1 compares to Fable 5, Opus 5, and GPT-5.6 Sol, and where the Mythos access question changes what you can promise clients.

Claude Fable 5.1 price: $10/$50 per million tokens, cheaper cache reads

Anthropic's published API pricing for Claude Fable 5.1 (API ID claude-fable-5-1):

SpecClaude Fable 5.1
Input (per 1M tokens)$10
Output (per 1M tokens)$50
Cache read (per 1M tokens)$0.25 (quarter of Fable 5's rate)
Context window1M tokens (default and max)
Max output128K tokens
ThinkingAdaptive, always on; default effort high
AvailabilityGA — Claude API, Bedrock, Vertex, Foundry

The cache-read cut is the headline. Fable 5.1 reads cached input at $0.25/1M versus $1.00 for Fable 5 — a 75% reduction. Anthropic estimates ~25% lower cost for typical token-billed workloads and up to ~45% lower for context-heavy agentic work, where cache reads dominate. Batch API keeps the standard 50% input/output discount.

Claude API pricing 2026: Fable 5.1 vs Opus 5 vs GPT-5.6 Sol

Here is how Fable 5.1 stacks up against the models agencies actually route between (verified against Anthropic's Sept 1, 2026 release notes and OpenAI's current GPT-5.6 Sol promo rates):

ModelInput / 1MOutput / 1MContextNotes
Claude Fable 5.1$10$501MNew GA flagship; $0.25 cache reads
Claude Fable 5$10$501MPrior flagship; $1.00 cache reads
Claude Opus 5$5$251MValue tier for most workloads
GPT-5.6 Sol$4$20~1MPromo rate through Nov 21, 2026; then expected $5/$30
Gemini 3.8 Flash (Sept 2, 2026)$0.75*$3.75*—Google's best reasoning & coding model yet; recommended for SWE and autonomous agents; Flash-first cadence

*Introductory rate through December 31, 2026; $1.50/$7.50 thereafter. Gemini 3.8 Flash context window not stated in launch coverage as of Sept 2, 2026.

Google's countermove landed the next day. On September 2, 2026 — the day after Fable 5.1 — Google released Gemini 3.8 Flash, its third Flash in six weeks, at $0.75/$3.75 per 1M through December 31 (introductory). Google calls it its best reasoning and coding model yet, says it tops the DeepSWE v1.1 leaderboard at a fraction of the cost of larger frontier models, and recommends it for software engineering and autonomous agent workloads — while Pro releases remain paused since early 2026. That makes the "cheapest frontier coding model 2026" conversation a four-way race (Fable 5.1, Sol, Grok 4.6, Gemini 3.8 Flash), and it is exactly why workload routing, not brand loyalty, is the agency skill that matters.

On Anthropic's published benchmarks, Fable 5.1 leads the coding pack: Terminal-Bench 4.0 at 55.8% (Fable 5: 42.0%, Opus 5: 52.3%, GPT-5.6 Sol: 37.3%), CursorBench 3.2.0 at 73.4% (Opus 5: 70.0%, Sol: 67.2%), and Terminal-Bench-Science 0.1 at 52.6% (Opus 5: 29.0%, Sol: 22.4%). Fable 5.1 is the strongest argument yet for the "best AI model for coding agents 2026" label — but the right pick still depends on workload (see our AI coding agent pricing guide and OpenAI vs Anthropic for business, plus the GPT-6 Astra computer-use comparison, for the full routing picture). Model your own per-task costs in the AI agent API cost calculator.

Beyond coding: the Sept 4, 2026 reasoning milestone. On September 4, Anthropic announced that Claude produced the first computer-checked formalization of Fermat's Last Theorem in Lean — a 13M+ line proof completed in roughly 11 days, largely autonomously, which Anthropic calls "the largest Lean proof ever constructed." The run used an internal research model roughly comparable to Claude Fable 5.1 on the open Prove2Me platform, and the proof was machine-checked by Lean using only its three standard axioms. Kevin Buzzard's Xena blog — the formal-math community project that has been chasing FLT since 2024 — confirmed the result: "a gigantic proof (over 13.4 million lines of code)" that "took only 11 days." Coding benchmarks are not the whole Claude story: this is machine-verified reasoning on a centuries-old open problem, and it is the proof point to cite when clients ask whether Claude-class models can be trusted for verification-heavy knowledge work.

Claude Mythos 5.1 vs Fable 5.1: same model, different safeguards

The Claude Mythos 5.1 vs Fable 5.1 question is not about capability — they are the same model. The difference is the safety posture:

Anthropic also cut false positives: its newest cyber safeguards block 60% fewer benign requests than before, and Fable 5.1 can now help discover software vulnerabilities — though not develop exploits. That is a direct answer to the over-refusal complaints that dominated Claude discourse in late August.

When Mythos access matters for your clients

Mythos 5.1 matters when the client's work lives where general safeguards throttle it:

For everyone else, Fable 5.1 with production safeguards is the answer. Mythos is a procurement conversation for a narrow client set, not a general tooling decision — plan on trusted-access application cycles, not a public launch.

What this means for agency tooling and client AI budgets

Bottom line

Claude Fable 5.1 is the first genuinely cheaper flagship: same $10/$50 list price, but cache reads at a quarter of the cost make agentic workloads materially cheaper, with the best coding benchmark scores Anthropic has published. Mythos 5.1 is a trusted-access variant for cyber and life sciences clients — same model, different safeguards, no public date. Re-quote Fable pricing, route coding agents to 5.1, and verify the false-positive improvements before promising clients a friction-free Claude.

Frequently asked questions

What is the Claude Fable 5.1 price?

Claude Fable 5.1 costs $10 per million input tokens and $50 per million output tokens, with cache reads at $0.25 per million — about 25% less than Fable 5 for typical workloads and up to roughly 45% less for highly agentic, cache-heavy work. It has a 1M-token context window and 128K max output.

What is the difference between Claude Mythos 5.1 and Fable 5.1?

Claude Mythos 5.1 and Claude Fable 5.1 are the same underlying model with different safeguards. Fable 5.1 is generally available; Mythos 5.1 is available only through Anthropic's trusted access programs, with safeguards tuned for cybersecurity and life sciences work.

What is the best AI model for coding agents in 2026?

On Anthropic's September 2026 benchmarks, Claude Fable 5.1 leads agentic coding (Terminal-Bench 4.0: 55.8% vs Opus 5 at 52.3% and GPT-5.6 Sol at 37.3%) and costs $10/$50 per million tokens with cache reads at $0.25. Claude Opus 5 remains the value pick at $5/$25 for most workloads. The day after Fable 5.1 shipped, Google released Gemini 3.8 Flash at a $0.75/$3.75 introductory rate through Dec 31, 2026 — claiming the top of the DeepSWE v1.1 leaderboard at a fraction of the cost of larger frontier models — so the "best" label is now workload-dependent across Fable 5.1, GPT-5.6 Sol, Grok 4.6, and Gemini 3.8 Flash.

Which AI is best for math, research, and deep reasoning?

Claude is the strongest current answer for math, research, and deep-reasoning work. On September 4, 2026, Anthropic announced that Claude produced the first computer-checked formalization of Fermat's Last Theorem in Lean — a 13M+ line proof completed in roughly 11 days, largely autonomously, which Anthropic calls the largest Lean proof ever constructed. Claude Fable 5.1 also leads Anthropic's science-focused agentic benchmark, Terminal-Bench-Science 0.1 (52.6%, vs Opus 5 at 29.0% and GPT-5.6 Sol at 22.4%).

Can AI verify mathematical proofs?

Yes. In September 2026, Claude produced the first complete computer-checked formalization of Fermat's Last Theorem in the Lean proof assistant — a proof totaling over 13 million lines of code, machine-verified by Lean using its three standard axioms. Because the result is machine-checked rather than just human-reviewed, it is the strongest public demonstration to date that an AI can produce verifiable mathematics, not merely plausible output.

How long did Claude take to formalize Fermat's Last Theorem?

Roughly 11 days. Anthropic announced on September 4, 2026, that Claude formalized Fermat's Last Theorem in Lean — a 13M+ line proof produced largely autonomously in about 11 days on a problem the formal-math community had been pursuing since 2024 and expected to take years. Kevin Buzzard's Xena blog, the project that had been chasing FLT, confirmed the result and the timeline.

When is Claude Mythos 5.1 available?

Claude Mythos 5.1 is available only through Anthropic's trusted access programs (Project Glasswing participants) as of September 1, 2026. There is no public release date; access is restricted to vetted cybersecurity and life sciences organizations.

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