What is Claude Opus 5.5 Agent? Complete 2026 Guide: Pricing, Benchmarks & Migration

Claude Opus 5.5 is Anthropic's flagship large language model, released on September 22, 2026 as the first member of the new Claude 5.5 family (Sonnet 5.5 followed on September 28). In Anthropic's lineup, the Opus series has always been the intelligence ceiling — above Sonnet, just below the pricier Fable tier. Anthropic states that Opus 5.5 performs at the level of Claude Fable 5.1 on most work while costing roughly 40% as much to run. As an agent engine, its core strength is long-running, multi-step autonomous work. In a real case disclosed by Anthropic, an early tester used it to audit and fix a 200,000-line codebase in under three hours — the same job took Opus 5 more than 20 hours and 2.5x the tokens. For coding agents that run for hours, codebase-wide migrations, and system-level audits, that is exactly what a flagship model is for.

1. 20% price cut: input/output pricing dropped from $5/$25 to $4/$20 per million tokens, and cache reads fell 60% from $0.50 to $0.20. The Batch API still carries a 50% discount, and a Fast mode at $8/$40 is available. Anthropic says typical workloads cost about 40% less to run than on Opus 5. 2. More than 30% faster output: Anthropic's official figures show Opus 5.5 generating output over 30% faster than Opus 5, noticeably shortening wait times on long tasks. 3. Adaptive thinking, always on: thinking is now adaptive and cannot be disabled — intensity is steered by the new effort parameter (low, medium, high, xhigh, max), defaulting to medium on the API (Opus 5 defaulted to high). Testing shows medium reaches the same FrontierCode scores as max effort at far lower cost. 4. Leading benchmarks: 66.4% on Terminal-Bench 4.0 (vs 55.8% for Fable 5.1), the top score on the Artificial Analysis Intelligence Index at 58, and 1846 Elo on GDPval-AA v2.1. Against GPT-6 Astra it leads on four of six shared benchmarks at 40% of the price. 5. Bigger context and fresher knowledge: a 1M-token context window, up to 128K output tokens per request (300K on the Batch API with a beta header), knowledge cutoff updated to June 2026, and no surcharge for long prompts.

Step 1: Get access and set the model ID. Obtain API credentials from the Anthropic Console, Amazon Bedrock, Google Cloud, or Microsoft Foundry, and pass claude-opus-5-5 as the model ID (anthropic.claude-opus-5-5 on Bedrock). It is also live in the Claude apps and Claude Code. Step 2: Pick the effort tier for your workload. This is the most important new parameter in the 5.5 family: stick with the medium default for everyday tasks, use high for terminal-based coding agents, and reserve xhigh or max for knowledge-work deliverables like long reports and complex analyses. Higher tiers cost more — get things working on medium first, then scale up only where evals justify it. Step 3: Define tool schemas and build the execution loop. Declare tools with strict JSON Schema and assemble a ReAct or Plan-and-Solve loop: the model emits tool_use requests, your server executes functions and feeds tool_result payloads back until the task completes. Note that thinking blocks in 5.5 are tied to the generating model and conversation, which matters when designing multi-turn state management. Step 4: Use caching aggressively. At just $0.20 per million cache-read tokens, the large system prompts and repo context resent every turn in agent loops are ideal cache candidates and can substantially cut the cost of long tasks.

Anthropic's documentation lists four changes that will return errors on code running cleanly against Opus 5 — handle all of them before migrating: 1. Thinking cannot be disabled: adaptive thinking in 5.5 is always on; attempts to turn it off fail. Control intensity with the effort parameter instead. 2. Forced tool use returns 400: the legacy pattern for forcing tool calls now errors out and must be replaced with the recommended tool-choice parameters. 3. Thinking blocks are bound to the model and conversation: thinking blocks are no longer portable text — reusing them across models or conversations fails. 4. The old computer tool is rejected: tool definitions based on computer_20251124 are rejected on the Claude API and Google Cloud and must be upgraded. Additionally, plain text between tool calls now lands inside thinking blocks, so adjust response parsing accordingly. Recommended migration path: get the core loop passing in staging at medium effort, fix the four breakages above, run A/B comparisons to confirm quality, then roll out gradually while keeping Opus 5 pinned as a fallback.

1. Treat effort as a cost dial, not a quality dial. Cover 80% of tasks on medium and only move to high or above where evals prove it helps. 2. Enforce hard iteration limits: cap agent runs at 10 to 15 steps per session to guard against runaway loops from flaky upstream APIs. 3. Keep humans in the loop for irreversible actions: database writes, production deploys, and financial operations must require synchronous human approval. 4. Instrument everything: log the effort tier, token consumption, tool latency, and thinking length per turn. Opus 5.5's cost is highly sensitive to the effort setting — flying without telemetry means flying blind. 5. Plan around the retirement window: Anthropic commits to serving Opus 5.5 until no sooner than September 22, 2027 — a full year of stability, making it a safe long-term foundation.