Claude Opus 5.5 API for Coding and Long-Running Agents

Claude Opus 5.5 is Anthropic's model for long-running agentic coding and knowledge work, released on September 22, 2026. The API accepts text and images and returns text. Anthropic lists a 1,000,000-token context window, a standard maximum output of 128,000 tokens, and adaptive thinking that cannot be disabled. The default effort setting is medium; an application can tune effort for the depth of work it needs. The official Claude API model ID is claude-opus-5-5. Confirm the exact GPTProto model string in this page's API Usage tab before sending production traffic.

The practical fit is work that needs repeated inspection and revision: debugging across files, migrations, code review, multi-tool agents, and document-heavy analysis. For routine extraction or brief responses, compare a less expensive model on the same tasks. A large context window makes it possible to send more material; it does not replace retrieval, selective context, or checks on the final answer.

Claude Opus 5.5 vs Opus 5 and Fable 5.1

The rates above are Anthropic's standard list prices, not GPTProto prices. Opus 5.5 has 20% lower official input and output token rates than Opus 5, but cost per finished task also depends on tool calls, cache hits, retries, and output length. Anthropic says Opus 5.5 reaches Fable 5.1's level on most work; that is a vendor claim, not a guarantee for your workload. Test each model with the same prompts, tools, and acceptance criteria. For current platform charges, use the fixed pricing panel above.

Migrating from Claude Opus 5: Four Checks That Matter

Replacing claude-opus-5 with claude-opus-5-5 is the first step. If your agent sends Anthropic-specific fields through GPTProto, check whether the selected endpoint forwards them and run a small canary before switching traffic:

1. Remove manual thinking settings. Opus 5.5 rejects both thinking.type: "disabled" and the older thinking.type: "enabled" with a token budget. On the native Claude Messages API, tune depth with output_config.effort instead. Confirm how GPTProto exposes this control.

2. Stop forcing a particular tool. Anthropic documents errors for tool_choice values any and tool on Opus 5.5. Use auto and validate your tool schema and output handling in the API format you actually call.

3. Preserve thinking blocks across turns. Thinking blocks are associated with the model and conversation. A fallback model may ignore prior Opus 5.5 thinking blocks; editing earlier turns or tool definitions can also break a conversation. Test a full multi-turn tool loop, not only a single request.

4. Recheck computer use integrations. On Anthropic's Claude API and Google Cloud, the older computer_20251124 tool is not supported by Opus 5.5. If you use computer control, confirm both Anthropic's current toolset and GPTProto's exposure before migrating.

These checks address request compatibility. Also measure completion rate, latency, total tokens, and cost per accepted result on a representative set of coding or agent tasks. Anthropic's migration guide documents the native API changes; GPTProto support for individual fields must be verified separately.