# Claude Fable 5.1 vs Fable 5 vs Opus 5.5: which Claude model to use By SeedRouter · Published 2026-09-27 · Updated 2026-09-27 Start with Claude Opus 5.5. It is the cheapest of the three per token, Anthropic suggests it for most workloads, and on Anthropic's own benchmark table it scores at or above Claude Fable 5.1. Move a task to Claude Fable 5.1 when it needs deeper reasoning or when Opus 5.5 at higher effort still falls short. Keep Claude Fable 5 only for code that forces a tool call, which the two newer models reject. All three are sold on SeedRouter under one key, and switching is a change to `model`. ## How do Claude Fable 5.1, Fable 5 and Opus 5.5 compare on specs? | | Claude Fable 5.1 | Claude Fable 5 | Claude Opus 5.5 | | --------------------------- | ------------------- | ------------------- | ------------------- | | Model ID | `claude-fable-5-1` | `claude-fable-5` | `claude-opus-5-5` | | Released | September 1, 2026 | June 9, 2026 | September 22, 2026 | | Status | Active (latest) | Active (legacy) | Active (latest) | | Context window | 1M tokens | 1M tokens | 1M tokens | | Max output | 128K tokens | 128K tokens | 128K tokens | | Thinking | Adaptive, always on | Adaptive, always on | Adaptive, always on | | Default effort | `high` | `high` | `medium` | | Reliable knowledge cutoff | June 2026 | January 2026 | June 2026 | | Forced `tool_choice` | Rejected | Accepted | Rejected | | Latency (Anthropic's label) | Slower | — | Moderate | The figures come from Anthropic's model pages for [Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview) and [Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview). All three read text and images and write text only. ## Which one costs less? ### Claude Fable 5.1 Current rates are temporarily unavailable. No price estimate is provided; unavailable rates must not be interpreted as free usage. ### Claude Fable 5 Current rates are temporarily unavailable. No price estimate is provided; unavailable rates must not be interpreted as free usage. ### Claude Opus 5.5 Current rates are temporarily unavailable. No price estimate is provided; unavailable rates must not be interpreted as free usage. Claude Opus 5.5 is the lowest on every line. Claude Fable 5.1 and Claude Fable 5 share input and output rates, but Fable 5.1 reads the prompt cache at a quarter of Fable 5's rate. Anthropic estimates that change alone makes Fable 5.1 about 25% cheaper than Fable 5 on typical workloads, and up to about 45% on highly agentic work. The [Claude API pricing guide](https://seedrouter.ai/blog/claude-api-pricing) explains how each line is billed. ## How do they score on Anthropic's benchmarks? Anthropic published two benchmark tables, one per launch. They use different benchmark versions in places, so compare within a table, not across them. From the [Claude Opus 5.5 announcement](https://www.anthropic.com/claude-opus-5-5) (September 22, 2026): | Benchmark | Opus 5.5 | Fable 5.1 | Opus 5 | | --------------------------------------- | -------- | --------- | ------ | | Terminal-Bench 4.0 (agentic coding) | 66.4% | 55.8% | 52.3% | | FrontierCode v1.1 Main (agentic coding) | 54.4% | 50.3% | 48.0% | | CursorBench 4.0 (agentic coding) | 57.8% | 51.8% | 46.6% | | GDPval-AA v2.1 (knowledge work) | 1846 | 1735 | 1708 | | Humanity's Last Exam, with tools | 67.7% | 65.6% | 63.6% | | OSWorld 2.0, partial (computer use) | 81.8% | 80.7% | 74.0% | From the [Claude Fable 5.1 announcement](https://www.anthropic.com/claude-fable-and-mythos-5-1) (September 1, 2026): | Benchmark | Fable 5.1 | Fable 5 | Opus 5 | | ------------------------------------- | --------- | ------- | ------ | | Terminal-Bench-Science 0.1 (research) | 52.6% | 24.7% | 29.0% | | Terminal-Bench 4.0 (agentic coding) | 55.8% | 42.0% | 52.3% | | GDPval-AA v2 (knowledge work) | 1853 | 1723 | 1824 | | Humanity's Last Exam, no tools | 60.9% | 57.8% | 56.6% | | AutomationBench (business workflows) | 31.4% | 17.1% | 26.9% | | CursorBench 3.2.0 (agentic coding) | 73.4% | 70.5% | 70.0% | Anthropic adds a caveat worth keeping in mind: "In our own use, the gap between Opus 5.5 and Claude Fable 5.1 is narrower than these scores suggest." Benchmarks rank the models; your own tasks decide between them. ## What changed from Claude Fable 5 to Claude Fable 5.1? Claude Fable 5.1 keeps the input and output prices, cuts cache reads to a quarter, and is stronger at long-running coding, multistep research, and document, spreadsheet and slide work. Anthropic also reports that at low or medium effort it matches or beats Fable 5 at much lower cost. Three changes can break existing code: * Forcing a tool call (`tool_choice` of `any` or `tool`) returns an error. Use `auto` with strict tool definitions instead. * Earlier models cannot read Claude Fable 5.1's thinking blocks, so a conversation moved to another model loses them. * Editing earlier turns invalidates its thinking blocks. ## What changed from Claude Opus 5 to Claude Opus 5.5? Anthropic prices Claude Opus 5.5's input and output tokens 20% below Claude Opus 5, and says it costs about 40% less on typical workloads because it also uses fewer tokens per task. It generates output more than 30% faster than Opus 5. For code moving from Opus 5, the [migration guide](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide) lists what now returns a 400 error: disabling thinking, forcing a tool call, and a prefilled assistant turn. The default effort also drops from `high` to `medium`, so a request that omits effort thinks less than it did on Opus 5. SeedRouter sells Claude Opus 5.5, not Claude Opus 5. ## Which Claude model should I use? | If you need | Use | Why | | ---------------------------------------- | ---------------------------- | ----------------------------------------------------------- | | A default for coding and knowledge work | Claude Opus 5.5 | Lowest price, top scores on most of Anthropic's table | | The deepest reasoning on long agent runs | Claude Fable 5.1 | Anthropic's pick when Opus 5.5 at higher effort falls short | | Forced tool calls in existing code | Claude Fable 5 | The only one of the three that accepts them | | Knowledge past January 2026 | Claude Fable 5.1 or Opus 5.5 | Both have a June 2026 cutoff | ## Frequently asked questions ### Is Claude Fable 5.1 better than Claude Fable 5? Yes, on every benchmark in Anthropic's launch table, with the largest gains in research and business workflows. It also costs less in practice because cache reads are cheaper. Stay on Claude Fable 5 only if your code forces a tool call. ### Is Claude Opus 5.5 better than Claude Fable 5.1? On Anthropic's Opus 5.5 table it scores higher on all six rows shown above, and it costs less per token. Anthropic still positions Claude Fable 5.1 for the most demanding reasoning and says the real-world gap is narrower than the scores suggest. ### Is Claude Opus 5.5 better than Claude Opus 5? Anthropic calls it "a major step up from Opus 5": higher scores across its table, lower prices, fewer tokens per task and faster output. The trade is a stricter request format, listed above. ### Can I switch between these models in one conversation? You can send the next request with a different `model`, but thinking blocks do not always carry over. Claude Opus 5.5 does not read thinking from Claude Fable models, and earlier models do not read Claude Fable 5.1's. The conversation continues without them. ## Try all three on one key Run the same prompt on [Claude Opus 5.5](https://seedrouter.ai/models/claude-opus-5-5), [Claude Fable 5.1](https://seedrouter.ai/models/claude-fable-5-1) and [Claude Fable 5](https://seedrouter.ai/models/claude-fable-5) in the browser, compare the answers and the token counts, then call the winner through the API with the model ID from the table above.