Impossible tasks (Anthropic)
Claude Haiku 4.5 30% leads since October 2025
Only in system cards Agents work on coding problems designed to be impossible. Score is the rate at which they hack instead of giving up.
Run with no prompt and with an anti-hack prompt. The no-prompt rate is the default setting.
- Made by
- Anthropic
- Released
- May 2025
- Kind
- Deception and alignment
- Level
- Expert
- Scored
- Graded by a model
- Answers
- Agent task
- Tasks
- Kept private
- Strongest model tested
- Claude Opus 5.5 #4 on the capability index
- Last in a system card
- September 2026
- Last model added
- September 2026
11
Models scored in lab cards
11
System cards report it, from 1 lab
Claude Haiku 4.5 leads with 30% among models released by September 2026
Best score per model as labs report them in their own cards. Labs pick their own settings, so these compare less cleanly than independent runs.
The newest model with a score here came out in September 2026, and nobody has reported a newer one. Among the 20 highest-ranked models on the capability index, these have no score yet:
- Claude Fable 5.2
-
Gemini 4 Carbon - GPT-6.1 Sol
- GPT-6.1 Terra
- and 15 more
- 1 Claude Haiku 4.5 30%
- 2 Claude Opus 5.5 34.7%
- 3 Claude Mythos Preview 37.5%
- 4 Claude Sonnet 4.6 40%
- 5 Claude Sonnet 4 45%
- 6 Claude Opus 4.7 45%
- 7 Claude Opus 4 47%
- 8 Claude Opus 4.6 50%
- 9 Claude Opus 4.1 52%
- 10 Claude Sonnet 4.5 53%
- 11 Claude Opus 4.5 55%
Source: Labs’ model cards
| # | Model | Score | Setting | Date | Source |
|---|---|---|---|---|---|
| 1 | Claude Haiku 4.5 | 30% | Oct 2025 | Lab model card | |
| 2 | Claude Opus 5.5 | 34.7% | Sep 2026 | Lab model card | |
| 3 | Claude Mythos Preview | 37.5% | Apr 2026 | Lab model card | |
| 4 | Claude Sonnet 4.6 | 40% | Feb 2026 | Lab model card | |
| 5 | Claude Sonnet 4 | 45% | May 2025 | Lab model card | |
| 6 | Claude Opus 4.7 | 45% | Apr 2026 | Lab model card | |
| 7 | Claude Opus 4 | 47% | May 2025 | Lab model card | |
| 8 | Claude Opus 4.6 | 50% | Feb 2026 | Lab model card | |
| 9 | Claude Opus 4.1 | 52% | Aug 2025 | Lab model card | |
| 10 | Claude Sonnet 4.5 | 53% | Sep 2025 | Lab model card | |
| 11 | Claude Opus 4.5 | 55% | Nov 2025 | Lab model card |
2 models have held number one in labs’ own numbers since its release in May 2025
Lab claims count from the day the card came out, independent runs from the model’s release.
Anthropic
Source: Labs’ model cards
Scores over time
All the scores we have, one dot each, so a model tested at several settings appears more than once.
Reported by the lab in its model card
Best so far
Source: Labs’ model cards
Labs report it only in system cards
We read the headline results of 522 launch posts from 52 labs, and none of them list this benchmark. Some system cards do, below.
11 system cards report it, from 1 lab
Besides launch posts, we read the system cards, model cards and technical reports labs publish with a release. They report far more, including safety and frontier-risk tests.
| Date | Document | Score | As the document writes it | Section |
|---|---|---|---|---|
| Sep 22, 2026 | Claude Opus 5.5 system card | 34.7 | Impossible tasks case study, RL training episodes missing a needed file, attempted reward hack rate (successful 3.1%. Intact tasks from the same environments: 6.0% attempted, 0.4% successful), lower is better (6.2.2, Figure 6.2.2.A) | Alignment |
| Apr 16, 2026 | Claude Opus 4.7 system card | 45 | Impossible coding problems, hack rate with no prompt, printed on chart. 12.5 with anti-hack prompt (6.2.2.2, Figure 6.2.2.2.A) | Alignment |
| Apr 7, 2026 | Claude Mythos Preview system card | 37.5 | Impossible coding problems, hack rate with no prompt, lower is better, value printed on chart; 20.0 with anti-hack prompt (4.2.2.2, Figure 4.2.2.2.A) | Alignment |
| Feb 17, 2026 | Claude Sonnet 4.6 system card | 40 | Impossible tasks, classifier hack rate with no prompt; 28 with anti-hack prompt (Table 4.3.2.A) | Alignment |
| Feb 5, 2026 | Claude Opus 4.6 system card | 50 | Impossible tasks, classifier hack rate with no prompt, lower is better; 23 with anti-hack prompt (Table 6.2.3.2.A) | Alignment |
| Nov 24, 2025 | Claude Opus 4.5 system card | 55 | Impossible Tasks, classifier hack rate with no prompt, lower is better (Table 6.10.1.A; 35% with anti-hack prompt) | Alignment |
| Oct 15, 2025 | Claude Haiku 4.5 system card | 30 | Impossible Tasks, classifier hack rate with no prompt; 23 with anti-hack prompt (Table 5.B) | Alignment |
| Sep 29, 2025 | Claude Sonnet 4.5 system card | 53 | Impossible Tasks, simpler scaffold outside Claude Code, classifier hack rate with no prompt 53%, lower is better. With anti-hack prompt 20% (Table 6.1.A) | Alignment |
| Aug 5, 2025 | Claude Opus 4.1 system card addendum | 52 | Claude Code Impossible Tasks, classifier hack rate with no prompt 52%. With anti-hack prompt 18% (Table 5.B) | Alignment |
| May 22, 2025 | Claude Sonnet 4 system card | 45 | Claude Code Impossible Tasks, classifier hack rate with no prompt 45%. With anti-hack prompt 10% (the Opus 4.1 addendum later corrects this to 7%) (Table 6.2.A) | Alignment |
| May 22, 2025 | Claude Opus 4 system card | 47 | Claude Code Impossible Tasks, classifier hack rate with no prompt 47%. With anti-hack prompt 5% (the Opus 4.1 addendum later corrects this to 19%) (Table 6.2.A) | Alignment |
More in Safety and alignment
- FORTRESS In use Paired adversarial and benign national-security misuse prompts. Rubric-graded risk score on adversarial and over-refusal score on benign prompts. Claude Opus 5.5 5.78%
- StrongREJECT In use Forbidden-prompt jailbreak benchmark with a rubric grader that scores how useful a non-refusal actually is.
Inkling 98.6% - Gray Swan IPI Arena benchmark In use Indirect prompt-injection attacks from public red-teaming competitions replayed on tool, coding and browser agents. Attack success rate at k attempts. Gemini 4 Carbon 0%
- DistressBench In use Models reply as a chatbot to 718 simulated users in psychological crisis, in single-turn and multi-turn chats. Three judge models score replies against clinician-written rubrics.
Muse Spark 1.2 88.4% - Lakera B3 (Backbone Breaker Benchmark) In use Crowdsourced prompt-injection attacks hit backbone LLMs inside agent threat snapshots at three defense levels. The score is attack resistance, 100 minus the board's vulnerability percentage. Claude Opus 4.5 89%
- IHEval In use Models face conflicts between system, user and tool instructions. Reports a composite score for following the higher-priority instruction. GPT-4o 70%