Claude Opus 5.5 vs GPT-6 Astra

Both architectures accommodate massive inputs, with Claude Opus 5.5 offering a 1,000,000-token context window paired with 128,000 tokens of synchronous output (and up to 300,000 output tokens on its Message Batches API in beta). GPT-6 Astra provides a 1,050,000-token context window supporting up to 128,000 output tokens. Their accounting models diverge once workloads scale across large prompts. Anthropic bills Opus 5.5 at a flat $4.00 per million input tokens and $20.00 per million output tokens across the entire one-million-token span. Prompt caching on Opus 5.5 drops cache reads to $0.20 per million tokens, reflecting a 0.05x multiplier against baseline input, with 5-minute cache writes at $5.00 and 1-hour writes at $8.00. GPT-6 Astra splits billing into two distinct context columns. Standard short-context requests cost $10.00 per million input tokens and $50.00 per million output tokens, with cached input at $1.00 and cache writes at $12.50. When a prompt crosses OpenAI's short-context threshold, rates move to a separate column: $20.00 per million input tokens, $2.00 for cached reads, $25.00 for cache writes, and $75.00 per million output tokens. As a result, sustained million-token workflows that frequently consult massive prompt contexts carry substantially lower per-token overhead on Opus 5.5.

The two models implement contrasting mechanisms for controlling internal deliberation. Claude Opus 5.5 enforces adaptive thinking that remains permanently active and cannot be disabled. Anthropic sets the default effort parameter to medium, allowing developers to adjust thinking depth without turning it off entirely. The model page lists Opus 5.5's comparative latency as moderate, though Anthropic offers a fast mode delivering up to 2.5 times speed at double the base price ($8.00 input and $40.00 output). GPT-6 Astra introduces five explicit reasoning-effort levels: low, medium, high, xhigh, and max, configurable per request on the reasoning effort parameter. This allows teams to dial deliberation down to low for faster turns or increase it to max for intricate algorithmic work. While Astra supports Batch and Flex processing at 50% of standard rates, fast mode is explicitly unavailable for GPT-6 Astra under EU data residency rules.

Opus 5.5 introduces breaking API changes that constrain how tools are invoked, whereas Astra expands out-of-the-box automation through hosted runtimes and shell environments. Code running on Opus 5 faces four documented breaking changes when upgrading to Opus 5.5: thinking cannot be disabled, forced tool use returns an error, thinking blocks are bound to the model and conversation, and the earlier computer_20251124 tool identifier is no longer accepted on the Claude API and Google-managed Google Cloud. Text between tool calls also returns inside thinking blocks whose text is empty at default display settings. For GPT-6 Astra, OpenAI documents built-in tools including web search, file search, and container environments covering Hosted Shell and Code Interpreter, billed at the model's standard per-token rates. OpenAI explicitly lists computer use among the primary applications for Astra on its model page, alongside document creation and coding, with dedicated per-tool-call fees charged for search and computer use operations.

Neither model offers downloadable weights or self-hosted deployments; both are restricted to vendor-managed cloud APIs. Platform availability varies across hosting providers. Claude Opus 5.5 is available under the claude-opus-5-5 identifier on the Claude API, Microsoft Foundry, Claude Platform on AWS, and both Google-managed and Anthropic-managed Google Cloud, as well as anthropic.claude-opus-5-5 on Amazon Bedrock. Astra is accessible via OpenAI's Chat Completions, Responses, and Batch endpoints, publishing Tier 5 rate limits of 15,000 requests and 40,000,000 tokens per minute. Safety frameworks also set distinct boundaries. GPT-6 Astra is OpenAI's first model evaluated at the Critical cybersecurity level under its Preparedness Framework, enforcing continuous misalignment monitoring on all tool-using inference alongside trust-based access programs for high-risk capabilities. Opus 5.5 uses safeguards comparable to Claude Fable 5.1; it automatically re-routes most cybersecurity tasks to Opus 4.8 while keeping routine bug fixing in user code available, pending the expansion of Anthropic's Cyber Verification Program.