Claude Mythos 5: Context Limits and Release Scope
Anthropic released Claude Mythos 5 on June 9, 2026. The model supports a 1M token context window, 128k max output, and adaptive thinking for Project Glasswing.
Release Announcement and Availability
The source event date is June 9, 2026. On this date, Anthropic launched Claude Mythos 5 alongside Claude Fable 5. The official changelog identifies Claude Mythos 5 as available specifically for Project Glasswing participants. In contrast, Claude Fable 5 is described as the most capable widely released model open to all customers. The provided facts do not specify access criteria for Project Glasswing or provide details on how to join the program.
Documented Technical Specifications
Both Claude Mythos 5 and Claude Fable 5 support a 1M token context window by default. The maximum output length is documented as 128k tokens. Both models feature always-on adaptive thinking. The input modalities listed for the model are text and image, while the output modality is text. The record indicates that open weights are not confirmed for this model. No pricing information is included in the source facts.
- Default context window: 1M tokens
- Maximum output tokens: 128k
- Thinking mode: Always-on adaptive thinking
- Input types: Text, Image
- Output type: Text
Evaluation Procedure and Evidence Limits
Engineers can evaluate the model by testing its performance within the specified 1M token context limit. A practical procedure involves measuring latency and accuracy when processing inputs near the 1M token threshold. Compare the output quality against previous baselines using standardized prompts. Validate that the adaptive thinking feature activates consistently across different query types. The source does not confirm pricing, specific API endpoint changes, or local installation requirements. It remains unknown whether the adaptive thinking mechanism adjusts computational depth based on query complexity, as no architectural details are provided.
Primary sources
Frequently asked questions
What is the exact date of the Claude Mythos 5 announcement?
The source indicates the event occurred on June 9, 2026. This historical date distinguishes the release from the current publication date of this article.
Can I install Claude Mythos 5 locally?
The source states that open weights are not confirmed. It does not establish that local installation is impossible, nor does it confirm that API access is mandatory. No deployment architecture or hosting arrangement is detailed in the provided facts.