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Ongoing case: AI-written mathematics and how it is checked 4 stories
Anthropic says 1,000 Fable 5.2 agents formalized Fermat's Last Theorem in Lean in 30 hours; the post names no checker.
The first formalization, by an unreleased research version of Claude, took 11 days; Anthropic's post does not say who checked the new result.
Check our sources · 7 facts from 1 sourceAnthropic says a team of 1,000 Claude Fable 5.2 agents fully formalized Fermat's Last Theorem in the Lean programming language in 30 hours, with minimal human intervention. The claim is in the launch post for Fable 5.2 and Mythos 5.2, dated 9 November 2026, and is Anthropic's own account.
The post says the first formalization, by an unreleased research version of Claude, took 11 days and relied on a purpose-built harness and guidance from a human researcher. It says the new run needed one-third as many action tokens and produced a proof seven times shorter.
In the same setup, Opus 5.5 completed only 26% of the formalization in 18 hours and barely progressed after 30. The post describes a single run per model, and does not say whether Lean's kernel or human reviewers checked the result.
Underlined sentences link to the facts we checked. Dotted words open a short explainer.
Fact 1 of 7 · from Anthropic, 9 Nov 2026
a team of 1,000 Fable 5.2 agents fully formalized the theorem in just 30 hours, with minimal human intervention.
Fact 3 of 7 · from Anthropic, 9 Nov 2026
took 11 days ... relied on a purpose-built harness along with guidance from a human researcher.
Fact 4 of 7 · from Anthropic, 9 Nov 2026
one-third as many action tokens as our first formalization ... a proof seven times shorter
Fact 7 of 7 · from Anthropic, 9 Nov 2026
(absence) the post does not say who verified the proof or whether the Lean kernel checked it
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Check our sources
Every sentence above is checked against this source.
1 Claude Fable 5.2 and Claude Mythos 5.2
Open the source archived copy-
Anthropic's launch post says a team of 1,000 Fable 5.2 agents fully formalized Fermat's Last Theorem in just 30 hours, with minimal human intervention. the company says
citea team of 1,000 Fable 5.2 agents fully formalized the theorem in just 30 hours, with minimal human intervention.
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The post says the formalization was in the mathematical programming language Lean. the company says
citeLean
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The post says the first formalization, performed by an unreleased research version of Claude, took 11 days and relied on a purpose-built harness along with guidance from a human researcher. the company says
citetook 11 days ... relied on a purpose-built harness along with guidance from a human researcher.
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The post says the new run needed one-third as many action tokens as the first formalization and produced a proof seven times shorter. the company says
citeone-third as many action tokens as our first formalization ... a proof seven times shorter
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The post says Opus 5.5, with the same 1,000-agent setup, completed only 26% of the formalization in 18 hours and barely made any more progress after 30. the company says
citebarely made any more progress after 30
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The post's caption describes a single run per model.
citea single run per model
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The post does not say whether Lean's kernel checked the result or whether humans reviewed it.
cite(absence) the post does not say who verified the proof or whether the Lean kernel checked it
Every sentence above is checked against this source. Each fact has its own address you can cite.
Source 1 · Anthropic · 9 Nov 2026
Claude Fable 5.2 and Claude Mythos 5.2
Anthropic's launch post says a team of 1,000 Fable 5.2 agents fully formalized Fermat's Last Theorem in just 30 hours, with minimal human intervention. the company says
a team of 1,000 Fable 5.2 agents fully formalized the theorem in just 30 hours, with minimal human intervention.
The post says the formalization was in the mathematical programming language Lean. the company says
Lean
The post says the first formalization, performed by an unreleased research version of Claude, took 11 days and relied on a purpose-built harness along with guidance from a human researcher. the company says
took 11 days ... relied on a purpose-built harness along with guidance from a human researcher.
Show all 7 factsShow fewer facts
The post says the new run needed one-third as many action tokens as the first formalization and produced a proof seven times shorter. the company says
one-third as many action tokens as our first formalization ... a proof seven times shorter
The post says Opus 5.5, with the same 1,000-agent setup, completed only 26% of the formalization in 18 hours and barely made any more progress after 30. the company says
barely made any more progress after 30
The post does not say whether Lean's kernel checked the result or whether humans reviewed it.
(absence) the post does not say who verified the proof or whether the Lean kernel checked it
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