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Claude Fable 5

Released 9 Jun 2026$10 input, $50 output per million tokens1.0M contextproprietaryAlso written as Fable 5, Mythos 5, Claude Mythos 5, Claude Fable 5 (Adaptive Reasoning, Max Effort, Opus 4.8 Fallback), Claude Mythos 5 (same underlying model)

7 results on 6 of 13 boards, latest measured Sep 2026. Compare with other models

Clinical Benchmarks Index61.1Rank 3 of 1495 of 10 boardsAverage × multiplier 66.9 × 0.913 = 61.1This site’s own calculation. Working How it is computed

Results

Each line sits on its own board's scale. Tall tick: the board leader.

Clinical reasoning and knowledge

  1. HealthBench Professional
    length-adjusted, Anthropic protocol: adaptive thinking at max effort, Claude Opus 4.8 grader, averaged over 5 trials, no tools or custom system prompt (raw 70.3%). Measured as Claude Mythos 5; Anthropic itself prints 66.0 in the Fable 5 column of the Opus 5 card with footnote Mythos 5.
    0.660
    Rank 3 of 35Leader GPT-6 Astra (Anthropic run) 0.703
    Vendor-reported
    Measured Jun 2026
  2. HealthBench Professional
    Measured as Claude Fable 5; Anthropic September card; length-adjusted; adaptive max effort; Opus 4.8 grader; five trials; no tools or custom system prompt; raw 68.9%
    0.633
    Rank 9 of 35Leader GPT-6 Astra (Anthropic run) 0.703
    Vendor-reported
    Measured Sep 2026
  3. MedXpertQA (MM)
    Qwen-run comparison in the Qwen3.8-Max launch post
    80.0
    Rank 4 of 22 here, 5 models on the boardLeader GPT-5.6 Sol 81.5
    Independent run

Documentation and coding

  1. MedScribe (Vals AI)
    88.52
    Rank 10 of 105Leader Claude Opus 5.5 91.43
    Official leaderboard
    Measured Sep 2026
  2. MedCode (Vals AI)
    56.07
    Rank 3 of 103Leader Claude Opus 5 63.57
    Official leaderboard
    Measured Sep 2026

EHR and workflow agents

  1. CHI-Bench
    Listed as claude-code + claude-fable-5
    24.0
    Rank 10 of 43 here, 45 models on the boardLeader erius + claude-opus-5 54.7
    Official leaderboard
    Measured Jul 2026

Safety

  1. First, Do NOHARM (v2)
    65.0
    Rank 11 of 17 here, 19 models on the boardLeader LiSA 2.5 86.2
    Official leaderboard
    Measured Aug 2026

Index working

The Clinical Benchmarks Index is this site's own calculation: a weighted average of the board scores times a multiplier for the boards behind it. How it is computed

  • HealthBench Professional0.66066.0×0.743length-adjusted, Anthropic protocol: adaptive thinking at max effort, Claude Opus 4.8 grader, averaged over 5 trials, no tools or custom system prompt (raw 70.3%). Measured as Claude Mythos 5; Anthropic itself prints 66.0 in the Fable 5 column of the Opus 5 card with footnote Mythos 5.
  • MedScribe (Vals AI)88.5288.5×0.25, near its ceilingNear its ceiling: leader 91.4, weight 0.25
  • MedCode (Vals AI)56.0756.1×0.911
  • MedXpertQA (MM)80.080.0×0.463, near its ceilingNear its ceiling: leader 81.5, weight 0.463Qwen-run comparison in the Qwen3.8-Max launch post
  • First, Do NOHARM (v2)65.065.0×0.507
Weighted average of 5 boards
66.9
Multiplier for 5 of 10 boards
× 0.913
Index
66.9 × 0.913 = 61.1

Score: the result on the board’s own 0 to 100 scale. Dot: this model; tick: the board’s leader. Weight: how much the board counts in the weighted average; bold where the board is near its ceiling and counts for less. Each result links to its quote and page; the line under it is the setting it was measured with.

Sources

The quote, page and document for each of the 7 results
  1. 3HealthBench Professional length-adjusted 0.660
    Printed as 66.0Vendor-reported, measured Jun 2026Configuration: length-adjusted, Anthropic protocol: adaptive thinking at max effort, Claude Opus 4.8 grader, averaged over 5 trials, no tools or custom system prompt (raw 70.3%). Measured as Claude Mythos 5; Anthropic itself prints 66.0 in the Fable 5 column of the Opus 5 card with footnote Mythos 5.
    Claude Fable 5 and Claude Mythos 5 System Card system card, Anthropic, 9 Jun 2026. p. 252, Table 8.1.A, row HealthBench Professional, column Mythos 5 (Fable 5 column '-'); Figure 8.18.2.A p. 298 bar label 66.0% on Claude Mythos 5
    HealthBench Professional 66.0 - 64.7 56.9 51.8 -
    Every result from this document
  2. 9HealthBench Professional Anthropic September card 0.633
    Printed as 63.3%Vendor-reported, measured Sep 2026Configuration: Measured as Claude Fable 5; Anthropic September card; length-adjusted; adaptive max effort; Opus 4.8 grader; five trials; no tools or custom system prompt; raw 68.9%
    Claude Fable 5.1 and Claude Mythos 5.1 System Card system card, Anthropic, 1 Sep 2026. p. 199, Figure 8.17.2.A, Claude Fable 5 length-adjusted bar (visually read printed labels).
    HealthBench Professional | Length-adjusted score | Claude Fable 5 | 63.3%
    Every result from this document
  3. 10MedScribe (Vals AI) 88.52
    Printed as 88.52%Official leaderboard, measured Sep 2026
    Vals AI MedScribe leaderboard official leaderboard, Vals AI, 3 Sep 2026. Vals AI MedScribe leaderboard, View: All Models, Task: Overall, row 10 of 105 (Claude Fable 5), Accuracy column; Updated 9/29/2026. Rendered BenchmarkView table; configuration from embedded astro-island BenchmarkView props, benchmarkView.default.tasks.overall["anthropic/claude-fable-5"].
    10 | Claude Fable 5 | 88.52%±1.95 | $10/$50 | 119.47s
    Every result from this document
  4. 3MedCode (Vals AI) 56.07
    Printed as 56.07%Official leaderboard, measured Sep 2026
    Vals AI MedCode leaderboard official leaderboard, Vals AI, 29 Sep 2026. MedCode leaderboard, Overall task, All Models expanded, rank 3 of 103; Accuracy column; board Updated 9/29/2026. Rendered table row; configuration from embedded BenchmarkView props benchmarkView.default.tasks.overall["anthropic/claude-fable-5"].
    3 | Claude Fable 5 | 56.07%±2.20 | $10/$50 | 91.44s
    Every result from this document
  5. 4MedXpertQA (MM) Qwen-run comparison… 80.0
    Printed as 80.0Independent runConfiguration: Qwen-run comparison in the Qwen3.8-Max launch post
    Qwen3.8-Max: A New Bar for Coding and Cowork launch post, Alibaba, 2 Aug 2026. Full Benchmark Table, second (multimodal) table, row MedXpertQA-MM; columns Opus4.8 / Fable5 / Gemini3.1-Pro / GPT5.6-Sol / Qwen3.7-Plus / Qwen3.8-Max
    | MedXpertQA-MM | 71.7 | 80.0 | 80.7 | 81.5 | 71.0 | 80.4 |
    Every result from this document
  6. 11First, Do NOHARM (v2) 65.0
    Printed as 65.0%Official leaderboard, measured Aug 2026
    MAST technical leaderboard (First Do NOHARM v2 and per-benchmark results) official leaderboard, ARISE AI Research Network, 15 Aug 2026. arise-ai.org/mast/technical, 'First Do NOHARM v2 overall metric across 19 models' ranking (Latest Flagships view)
    10Claude Fable 5Anthropic 65.0%
    Every result from this document
  7. 10CHI-Bench 24.0
    Printed as 24.0%Official leaderboard, measured Jul 2026
    CHI-Bench leaderboard (actAVA) official leaderboard, actAVA, 12 Aug 2026. CHI-Bench v1.0.0, All Domains, rank 10, Agent/Model and Accuracy columns; PA/UM/CM follow; board last updated 2026-08-12; board Date column 2026-07-22; accessed 2026-09-30. Linked submission provenance: run 2026-07-22T07:16:27.207645Z to 2026-07-22T10:25:29.658582Z; submitted_at 2026-07-22T23:50:25Z.
    10 | claude-code | claude-fable-5 | Proprietary | 24.0% | 24.0% | 24.0% | 24.0% | 2026-07-22
    Every result from this document

Other Anthropic models: Claude 3.7 Sonnet, Claude Fable 5.1, Claude Haiku 4.5, Claude Haiku 5.5, Claude Opus 4.1, Claude Opus 4.5, Claude Opus 4.6, Claude Opus 4.7, Claude Opus 4.8, Claude Opus 5, Claude Opus 5.5, Claude Sonnet 4, Claude Sonnet 4.5, Claude Sonnet 4.6, Claude Sonnet 5, Claude Sonnet 5.5