Quick answer
For a production API comparison, the specific models here are OpenAI GPT-6.1 Sol (gpt-6.1-sol) and Anthropic Claude Opus 5.5 (claude-opus-5-5). “Codex” is not the API model name: OpenAI’s model documentation identifies Sol as an API model and says its default context window in Codex and ChatGPT Work is 500,000 tokens. Codex access depends on plan, workspace settings, and rollout eligibility; API use is a separate billing path. (OpenAI API model documentation, OpenAI Help Center)
Both models document tool calling and structured outputs. Sol’s tool calling requires OpenAI’s Responses API; Opus 5.5 supports tools through Anthropic’s Messages API, but does not support forcing a tool call with tool_choice set to any or to a specific tool. Compare them on your own tasks rather than assuming either is the universal winner. (OpenAI API model documentation, Anthropic tool-use documentation, Opus 5.5 feature notes)
Comparison checked against provider documentation on November 12, 2026. Availability, limits, and prices can change.
What “Codex vs Claude Opus” means
This article compares API models, not the Codex app against a Claude model. OpenAI identifies gpt-6.1-sol as an API model and separately documents its use in Codex. For Codex access, the provider says eligibility depends on plan, workspace settings, and rollout access. Anthropic identifies claude-opus-5-5 as the Claude API model ID. (OpenAI API model documentation, OpenAI Help Center, Claude Opus 5.5 model documentation)
If you are choosing a coding environment rather than an API model, compare the complete products and workflows separately. An app or coding agent is not interchangeable with the model that an API request names.
Production API comparison
| Factor | GPT-6.1 Sol (gpt-6.1-sol) |
Claude Opus 5.5 (claude-opus-5-5) |
|---|---|---|
| Provider-documented positioning | Complex coding, computer use, and professional work. | Long-running agentic coding and knowledge work. |
| Tool/function calling | Supported. OpenAI says tool calling requires the Responses API. | Supported through the Messages API; Anthropic’s example uses Opus 5.5. Forced tool use is not supported with tool_choice: any or a named-tool choice. |
| Structured outputs | Supported in the model documentation. | Supported for this model through the Claude API; Anthropic lists Opus 5.5 among supported models. |
| Standard API token rates | $2 per million input tokens; $10 per million output tokens. Cached input: $0.10 per million. | $4 per million input tokens; $20 per million output tokens. Cache reads: $0.20 per million. |
| Context listed by provider | 1,050,000 tokens for the API model; the page separately gives a 500,000-token default in Codex and ChatGPT Work. | 1 million tokens. |
Sources: OpenAI GPT-6.1 Sol model page, Anthropic Opus 5.5 model page, Anthropic tool-use guide, Anthropic structured-outputs guide, and Opus 5.5 feature notes.
OpenAI’s model page says API prompts with more than 272,000 input tokens are charged at 2× input and cache rates and 1.5× output rates for the full request. Do not apply the standard-rate cost comparison below to a request that crosses that threshold. The advertised context window is not a promise that every request uses the same price or limits. (OpenAI GPT-6.1 Sol model page)
The feature lists do not make the providers’ tool runtimes identical. With OpenAI, the model can call developer-provided functions and, through the Responses API, supported built-in tools. With Anthropic, the Messages API supports tool use, where your application runs client tools and Anthropic runs supported server tools. If your integration requires the model to call a tool on every turn, Opus 5.5’s documented restriction on forced tool use may matter. Check the current API documentation for the exact tools, endpoint, and controls your application needs.
Which model should you choose?
- Shortlist GPT-6.1 Sol if its documented Responses API features fit your integration and its standard rates suit your budget. OpenAI’s provider positioning includes complex coding and computer use; that positioning is not a guarantee of performance on your workload.
- Shortlist Claude Opus 5.5 if its model or platform fits your application and you need its documented API capabilities. Account for the forced-tool-use restriction if your workflow requires a tool call every turn.
- Evaluate both when quality, reliability, or cost per successful task matters more than listed token rates. Use the same representative prompts, tools, output schema, and success criteria. Measure correctness, completion rate, latency, token use, retries, and human correction time. This is a suggested evaluation—not a test conducted for this article.
Both providers document tool calling and structured outputs, so those two features alone do not establish a winner. Check specific tool support and endpoint behavior for your design, then make the decision using your own evaluation.
API cost example
Assume 100 separate requests, each with 10,000 uncached input tokens and 1,000 output tokens. The aggregate is 1 million input tokens and 100,000 output tokens. Each individual request stays below OpenAI’s 272,000-input-token surcharge threshold. Assume no tools, retries, or other charges.
- GPT-6.1 Sol: 100 × [(10,000 ÷ 1,000,000 × $2) + (1,000 ÷ 1,000,000 × $10)] = $3.00.
- Claude Opus 5.5: 100 × [(10,000 ÷ 1,000,000 × $4) + (1,000 ÷ 1,000,000 × $20)] = $6.00.
These are calculations from listed standard token rates, not a measured workload result or a complete cost estimate. They do not account for cached tokens, tools, retries, or other charges. In particular, the $3/$6 ratio does not apply to OpenAI requests with more than 272,000 input tokens, where the full request receives the documented surcharge. Keep API charges separate from ChatGPT or Claude subscriptions. (OpenAI GPT-6.1 Sol model page, Claude Opus 5.5 model page)
Bottom line
For this API-focused interpretation of “Codex vs Claude Opus,” compare GPT-6.1 Sol with Claude Opus 5.5. Both document tool calling and structured outputs, with different API requirements and tool-use constraints. Sol has lower listed standard token rates, but the example above is not a prediction of cost per successful task. Choose based on tested workload results, applicable API limits, tool behavior, and total cost—not the Codex product name or a single price figure.
Sources
GPT-6.1 Sol Model | OpenAI API
ChatGPT Work and Codex | OpenAI Help Center
Claude Opus 5.5 | Claude Platform Docs
Tool use with Claude | Claude Platform Docs