Quick answer
Make Claude Opus 5.5 your default for long-running coding agents. It costs $4/$20 per million tokens, and GPT-6 Astra costs $10/$50, so Astra is 2.5x the price on every token. Move a task to Astra when it's the hardest thing on your board: computer and browser use, heavy science or math, or work where one failure costs more than the tokens.
Nobody has published a same-harness, head-to-head benchmark of these two yet. Run both on your own tasks before you commit.
OpenAI shipped GPT-6 Astra on September 3, 2026. Anthropic followed with Claude Opus 5.5 on September 22. Both have a context window of about a million tokens, both output up to 128K tokens, and both vendors call their model the best for long, agentic work. So the useful question isn't which one is smarter. It's which one you should reach for, and when. This guide covers official pricing, the documented tradeoffs, and a simple routing rule. For the cheaper tiers, see our Opus 5.5 vs GPT-6 Sol, Luna and Grok 4.7 comparison.
The specs side by side
| Claude Opus 5.5 | GPT-6 Astra | |
|---|---|---|
| Released | Sep 22, 2026 | Sep 3, 2026 |
| Input / output (per 1M tokens) | $4 / $20 | $10 / $50 |
| Cached input | $0.20 read, $5 write | $1.00 read, $12.50 write |
| Long-prompt surcharge | None listed | Over 272K input tokens: 2x input, 1.5x output |
| Discounts | Cache reads | Batch and Flex at 50% off |
| Context / max output | 1M / 128K | 1.05M / 128K |
| Knowledge cutoff | June 2026 | April 30, 2026 |
| Reasoning control | Adaptive thinking, always on (default effort: medium) | Effort from low to max |
Sources: Anthropic Opus 5.5 docs, OpenAI GPT-6 Astra model page. Prices are standard API rates as of September 27, 2026.
What the price gap looks like on a real task
Take a typical agent step: 200K tokens of codebase context in, 20K tokens out. Opus 5.5 costs about $1.20 for it ($0.80 input plus $0.40 output). Astra costs about $3.00 ($2.00 plus $1.00). Over a few thousand steps a month, that gap gets big.
Two things move the math. Astra's long-context surcharge starts at 272K input tokens, so an agent that loads a big repo into every call pays double on input with Astra, while Opus 5.5 doesn't list a surcharge. On the other side, Anthropic says Opus 5.5 finishes typical work with fewer tokens and puts the overall saving vs. Opus 5 at about 40% (VentureBeat). That claim isn't independent, so token counts from your own runs matter more than the list price. Our Claude pricing guide covers the full Anthropic rate card.
Where Opus 5.5 is the better pick
Long coding sessions in a real codebase. That's what Anthropic built Opus 5.5 for. Anthropic reports it beats the pricier Fable 5.1 on Terminal-Bench 4.0, FrontierCode v1.1 Main, and CursorBench 4.0 (per VentureBeat). Those are Anthropic's numbers, but the pitch is efficiency, and that's what matters when an agent runs for hours.
Volume. If you run many agent jobs in parallel, paying 2.5x per token for Astra across all of them is hard to justify. Save the flagship for the jobs that need it.
You already use Claude tooling. If your team works in Claude Code, Opus 5.5 slots in with no new harness.
Where GPT-6 Astra is worth paying for
Computer and browser use. This one's clear-cut. OpenAI says Astra leads on computer use and browsing. Anthropic's docs say the computer_20251124 tool is no longer accepted for Opus 5.5 on the Claude API and Google Cloud. If your agent needs to drive a desktop or browser through that tool, check Anthropic's docs before you plan around Opus 5.5.
The hardest reasoning. OpenAI reports 98% on FrontierMath Tier 4, 99.9% on ARC-AGI-3, and 64.6% on Terminal-Bench Science 0.1, compared with 52.6% for Claude Fable 5.1 (OpenAI launch post). Those are OpenAI's numbers and don't include Opus 5.5. Still, for research-grade analysis, simulation, or security work, Astra is the model OpenAI is pointing at.
When one failure costs more than the tokens. A migration that breaks production or an audit that misses something is worth far more than a $3 call. If a task has already failed on a cheaper model, move it up.
Breaking changes to know before you switch
Opus 5.5 isn't a drop-in swap for Opus 5. Per Anthropic: you can't turn thinking off, forced tool use can return errors, and on the Claude API the text between tool calls comes back inside thinking blocks with empty display by default. So streamed progress messages can go quiet unless you change the display setting.
Astra works on the Responses, Chat Completions, and Batch endpoints. It doesn't support fine-tuning (OpenAI docs). Its rollout was also bumpy. Third-party reporting says enterprise access is off by default until an admin turns it on (OrcaRouter). Confirm your org actually has access before you build a pipeline on it.
A routing rule you can use this week
- Default to Opus 5.5 for coding agents, refactors, and long knowledge-work jobs.
- Escalate to Astra when a task fails twice, needs computer or browser use, or is hard science, math, or security work.
- Drop down to GPT-6 Sol ($2/$10) or Luna for high-volume, simpler steps. Locking into one model leaves money on the table.
- Run your own bake-off. Pick 10 real tasks, run them on both models with the same harness, and record pass rate, tokens, and wall-clock time. That beats any vendor chart.
If you're working in the terminal, our Codex review covers how OpenAI's agent harness compares.
Bottom line
Opus 5.5 is the better value for most of what developers do all day. Astra is the specialist you call in for work that justifies 2.5x the price. The teams that spend least per shipped feature will be the ones routing between the two, not the ones who pick a single winner.
Frequently Asked Questions
Is GPT-6 Astra better than Claude Opus 5.5?
There's no same-harness head-to-head benchmark yet. OpenAI reports record scores for Astra on math, ARC-AGI-3, and science benchmarks. Anthropic reports strong agentic coding scores for Opus 5.5. Test both on your own tasks.
How much more does GPT-6 Astra cost than Opus 5.5?
Astra lists at $10 per million input tokens and $50 per million output tokens, and Opus 5.5 lists at $4 and $20. That makes Astra 2.5x more per token. It also charges 2x input and 1.5x output on prompts over 272K input tokens.
Which model is better for coding agents?
For most long-running coding work, Opus 5.5 is the better default because it's cheaper per token and Anthropic reports it beats Fable 5.1 on Terminal-Bench 4.0 and CursorBench 4.0. Move to Astra for the hardest tasks or ones that failed on a cheaper model.
Can Opus 5.5 do computer use?
Anthropic's docs say the computer_20251124 tool is no longer accepted for Opus 5.5 on the Claude API and Google Cloud. OpenAI says GPT-6 Astra leads on computer use.
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