Claude Opus 4.8 launched with 1M token context window and mid-conversation system messages

Claude Opus 4.8 is now available as the most capable generally available model. Key features and updates:

• 1M token context window by default on Claude API, Amazon Bedrock, and Vertex AI (200k on Microsoft Foundry)
• 128k max output tokens
• Mid-conversation system messages: Send role: "system" messages after user turns to preserve prompt cache hits when instructions change
• Enhanced stop_details field on refusal responses with category and explanation
• Effort parameter defaults to high across all surfaces
• Minimum cacheable prompt length for prompt caching reduced to 1,024 tokens
• Adaptive thinking triggers reasoning only when needed, reducing wasted tokens
• High-resolution image input support (up to 2576 pixels on long edge)
• Task budgets, advisor tool, and computer use all supported
• Fast mode available as research preview on Claude API only
• Sampling parameters (temperature, top_p, top_k) set to non-default values return 400 error
• Claude Code: Auto mode expanded for long-running tasks; Max plan users default to fast mode; Workflows available as research preview
• Fast mode for Claude Opus 4.6 deprecated (removal ~30 days after launch)