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Third-party API users can now declare model capabilities via environment variables: ANTHROPIC_DEFAULT_OPUS_MODEL_SUPPORTED_CAPABILITIES, ANTHROPIC_DEFAULT_SONNET_MODEL_SUPPORTED_CAPABILITIES, and ANTHROPIC_DEFAULT_HAIKU_MODEL_SUPPORTED_CAPABILITIES. These are comma-separated capability strings that Claude Code checks when deciding which features to enable for custom models.
Evidence
Capability env vars (search for "ANTHROPIC_DEFAULT_OPUS_MODEL_SUPPORTED_CAPABILITIES")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
Related
Other releases about the same thing. Found by shared names or similar wording; neither means one caused the other.