When was Claude Mythos 5.1 released?

Anthropic released Claude Mythos 5.1 on 1 September 2026.

How big is Claude Mythos 5.1's context window?

Claude Mythos 5.1 takes 1M tokens of context, and returns up to 128K tokens.

How much does Claude Mythos 5.1 cost?

Anthropic's list price is $10.00 per million input tokens and $50.00 per million output tokens. Caching and batch discounts are not included.

What do people say about Claude Mythos 5.1?

We have logged 3 Reddit threads that name Claude Mythos 5.1. The page links each thread; it does not reproduce the comments.

Model Watch · Anthropic

Claude Mythos 5.1

Same price as Fable 5.1 and same shape, but you can't just sign up for it. Anthropic gates Mythos behind an invite.

Status

released

Released

1 September 2026

Context

1M tokens

Price / 1M

$10.00 in, $50.00 out

Mentions

Threads

What is Claude Mythos 5.1?

Claude Mythos 5.1 is a model from Anthropic, built in the US. It's out, and you can call it today.

When did Claude Mythos 5.1 come out?

Anthropic released it on 1 September 2026. That date comes from the lab's own docs.

The tracker first logged the name on 3 September 2026 and last saw it on 3 September 2026.

What does it cost, and how much context does it hold?

Lab	Anthropic
Built in	US
Status	released
API model id	`claude-mythos-5-1`
Released	1 September 2026
Context window	1M tokens
Max output	128K tokens
Input, per 1M tokens	$10.00
Output, per 1M tokens	$50.00

List prices per million tokens, read off the lab's own docs. Caching, batching and volume deals all cut the real number, so treat this as the ceiling. Check the source before you budget against it.

How does it compare to Anthropic's other models?

Model	Status	Date	Context	In / 1M	Out / 1M
Claude Mythos 5.1	released	1 September 2026	1M tokens	$10.00	$50.00
Claude Fable 5.1	released	1 September 2026	1M tokens	$10.00	$50.00
Claude Opus 5	released	24 July 2026	1M tokens	$5.00	$25.00
Claude Sonnet 5	released	30 June 2026	1M tokens	$2.00	$10.00
Claude Fable 5	released	9 June 2026	1M tokens	$10.00	$50.00
Claude Opus 4.5	released	24 November 2025	200K tokens	$5.00	$25.00
Claude Haiku 4.5	released	15 October 2025	200K tokens	$1.00	$5.00
Claude Sonnet 4.5	released	29 September 2025	200K tokens	$3.00	$15.00
Claude Opus 5.1	rumoured	—	—	—	—
Claude Opus 4.1	deprecated	—	—	$15.00	$75.00

A dash means the lab hasn't published that number for that model.

What are people discussing about Claude Mythos 5.1?

We tracked 3 public discussions naming Claude Mythos 5.1 across r/ClaudeAI and r/singularity on 1 September 2026.

None of them settled on a single subject clearly enough to group, so there is nothing more specific to report yet.

Every mention we've logged

Every tracked post, release, changelog entry or incident whose title or summary names Claude Mythos 5.1. Newest first, each one linking to where the lab published it.

Thu 03 Sep 2026

Anthropicincident claude-fable-5-1 claude-mythos-5-1 claude-opus-516:23 UTCanthropic-status

Elevated errors for multiple models

Sep 3 , 16:23 UTC Resolved - The issue affecting Claude Mythos 5.1, Claude Fable 5.1, and Claude Opus 5 has been resolved. Impact has ended as of 9:16 PT / 16:16 UTC. Sep 3 , 16:06 UTC Monitoring - A fix has been deployed for Claude Mythos 5.1, Claude Fable 5.1, and Claude Opus 5 and we are monitoring for recovery. Sep 3 , 15:25 UTC Update - The only affected models right now are Opus 4.8 and Opus 5. The rest of the models have recovered to baseline error rate. Sep 3 , 14:49 UTC Update - We are continuing to work on a fix for this issue. Sep 3 , 13:50 UTC Update - An exhaustive list of…

Keep reading

The release timeline is every model we know about, filterable by lab, country and date. The tracker is the live feed these mentions come from, and Model Watch is the whole catalogue on one page.