Claude Mythos 5
23 values about Claude Mythos 5 from 5 documents, in 7 metric families. In each family, values from its own document come first.
- Developer
- Anthropic
- Release date
- 9 Jun 2026The date of its first card in the dataset, the Claude Fable 5 & Claude Mythos 5 System Card.
- Availability
- Public
- Its own document
- Claude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026
- Also reported in
- Claude Sonnet 5 System Card · 30 Jun 2026
- Claude Opus 5 System Card · 24 Jul 2026
- Claude Fable 5.1 & Claude Mythos 5.1 System Card · 1 Sep 2026
- Claude Opus 5.5 System Card · 22 Sep 2026
Data as of 26 Sep 2026 · Dataset v0.1 · Methodology v0.1 · Changelog
Values by metric family
Each value as the document printed it. Select a value for its source, its checks and its history. Values from other documents are listed after the model’s own, each marked as the first report of that measure or as a restatement.
KeyVerifiedUnverifiedDisputedCorrected read off a figure or stated in wordsA value opens its source and history.
M1 · Evaluation awareness 1 value
| Evaluation and metric | Value | Document |
|---|---|---|
| From its own documentClaude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 | ||
| Grader awarenessShare of episodes aware of graderCondition: grader at high risk of hacking | Claude Fable 5 & Claude Mythos 5 System Card9 Jun 2026 | |
M3 · Sabotage and sandbagging 1 value
| Evaluation and metric | Value | Document |
|---|---|---|
| From other documents | ||
| Safety research compromiseRate of compromising safety researchClaude Opus 5 System Card24 Jul 2026First reported | Claude Opus 5 System Card24 Jul 2026First reported | |
M4 · Misalignment audits 1 value
| Evaluation and metric | Value | Document |
|---|---|---|
| From its own documentClaude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 | ||
| Internal deployment monitoringMisaligned actions in monitored traffic | Claude Fable 5 & Claude Mythos 5 System Card9 Jun 2026 | |
M5 · Honesty and hallucination 1 value
| Evaluation and metric | Value | Document |
|---|---|---|
| From its own documentClaude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 | ||
| Input hallucinationRate of hallucinating missing reference | Claude Fable 5 & Claude Mythos 5 System Card9 Jun 2026 | |
M7 · Harmful compliance and over-refusal 3 values
| Evaluation and metric | Value | Document |
|---|---|---|
| From other documents | ||
| Malicious Claude Code useRefusal rate, malicious requestsClaude Sonnet 5 System Card30 Jun 2026First reported | Claude Sonnet 5 System Card30 Jun 2026First reported | |
| Malicious computer useRefusal rateClaude Sonnet 5 System Card30 Jun 2026First reported | Claude Sonnet 5 System Card30 Jun 2026First reported | |
| Single-turn violative requestsHarmless response rateCondition: no system prompt; APIClaude Sonnet 5 System Card30 Jun 2026First reported | Claude Sonnet 5 System Card30 Jun 2026First reported | |
M9 · Prompt injection 1 value
| Evaluation and metric | Value | Document |
|---|---|---|
| From its own documentClaude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 | ||
| External prompt injection benchmarkRank vs prior models | Claude Fable 5 & Claude Mythos 5 System Card9 Jun 2026 | |
M10 · Dangerous capabilities and risk determinations 15 values
Restated in later documents
A later document reported values about Claude Mythos 5 again. Each pair is shown side by side: both values are kept with their own documents, and the later one does not replace the earlier.
| Earlier value | Later value |
|---|---|
| M10 · Dangerous capabilities and risk determinations | |
| Anthropic ECI (AECI)Capability index | |
| Claude Fable 5 & Claude Mythos 5 System Card9 Jun 2026 · its own document | Claude Opus 5 System Card24 Jul 2026No reason stated |
| Anthropic ECI (AECI)Capability index | |
| Claude Fable 5 & Claude Mythos 5 System Card9 Jun 2026 · its own document | Claude Fable 5.1 & Claude Mythos 5.1 System Card1 Sep 2026No reason stated |
Risk determinations
The developer's formal decisions about Claude Mythos 5 under its framework, as printed. Levels from different frameworks do not map onto one another.
| Domain | Level as printed | Framework and document |
|---|---|---|
| Bio/chem | applicable code, not the printed wording Responsible Scaling Policy Claude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 | Responsible Scaling Policy Claude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 |
| Bio/chem | not crossed (elevated, less clear) Responsible Scaling Policy Claude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 | Responsible Scaling Policy Claude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 |
| AI R&D / autonomy | not applicable Responsible Scaling Policy Claude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 | Responsible Scaling Policy Claude Fable 5 & Claude Mythos 5 System Card · 9 Jun 2026 |
“Code, not the printed wording”: the version 0 file stored a code for this level rather than the words the document printed. The wording will be read again from the source.
Revisions
No recorded revision changes a value about Claude Mythos 5. The Claude Fable 5 & Claude Mythos 5 System Card has a recorded revision that changes no value about Claude Mythos 5; see the document's page.