GPT-5 vs Claude Sonnet 4.5: run the same prompt through both.
OpenAI's GPT-5 and Anthropic's Claude Sonnet 4.5 are the current-generation daily-driver reasoners. They cover overlapping ground in different styles — and disagree often enough to be worth running together.
| GPT-5 | Claude Sonnet 4.5 | |
|---|---|---|
| Maker | OpenAI | Anthropic |
| Positioning | General-purpose flagship reasoner | Long-context reasoning + agentic workflows |
| Context window | Up to 400K tokens (varies) | Up to 1M tokens (Sonnet 4.5) |
| Multimodal | Text, image, audio, video | Text, image, PDF-native |
| Reasoning approach | Fast, decisive, chains tools aggressively | Deliberate, prefers to cite passages, careful with ambiguity |
| Code | Strong on greenfield and tool-use chains | Strong on codebase reasoning and refactor explanations |
| Long documents (100+ pages) | Good; may summarize aggressively | Excellent — Sonnet 4.5 is a leader in long-document reasoning |
| Safety posture | Willing to answer more edge cases | More conservative; occasional over-refusal |
| Available in Backplain | Yes — behind AI Firewall + audit log | Yes — behind AI Firewall + audit log |
The trap when comparing GPT-5 and Claude Sonnet 4.5 is trying to declare a winner from benchmarks. Neither model is uniformly better; they're differently good, and the interesting information is where their answers diverge on your prompt.
A useful default: Sonnet 4.5 for anything where a long document is the input (contracts, protocols, filings, transcripts) and GPT-5 for anything where a large action space is the output (writing code, chaining tools, drafting from a blank page). Then run them both anyway, because the exceptions are frequent.
Backplain runs both simultaneously — same prompt, same attachments, same system prompt — and streams the answers side by side so you can see the disagreement in real time.
Benchmarks are a starting point, not an answer. The only way to know which model is right for your use case is to run your prompt through both and read the responses side by side. That's the entire premise of Backplain.
In one workspace you can send the same prompt to GPT-5, Claude Sonnet 4.5, and up to eight more frontier models simultaneously — with the same attached files, the same system prompt, and the AI Firewall redacting PII before either model sees it. See how model comparison works →
Compare GPT-5 and Claude Sonnet 4.5 on your own prompt.
Three free multi-model prompts. No signup.