What Changed
Anthropic released Claude Opus 5.5 on September 22, 2026, and replaced Claude Opus 5 as the default recommendation in its own model comparison table: "start with Claude Opus 5.5 for most workloads." Opus 5 is still available but now sits in Anthropic's legacy model list rather than the current lineup.
| Claude Opus 5.5 | Claude Opus 5 (legacy) | Change | |
|---|---|---|---|
| Input per MTok | $4 | $5 | -20% |
| Output per MTok | $20 | $25 | -20% |
| Cache reads per MTok | $0.20 (5% of input) | $0.50 (10% of input) | -60% |
| Cache writes per MTok | $5 | $6.25 | -20% |
| Default effort | Medium | High | Changed |
| Generation speed | Baseline | 30%+ slower | Opus 5.5 faster |
| Retirement (not sooner than) | Sep 22, 2027 | Jul 24, 2027 | n/a |
Anthropic's own framing of the headline change: Opus 5.5 "performs at the level of Claude Fable 5.1 on most work and costs 40% less to run than Opus 5." That's a capability jump and a price cut landing on the same release, not a tradeoff between them.
The Benchmark Numbers
Anthropic's own published benchmark deltas, Opus 5.5 versus Opus 5:
| Benchmark | Opus 5 | Opus 5.5 | Delta |
|---|---|---|---|
| Terminal-Bench 4.0 (agentic coding) | 52.3% | 66.4% | +14.1 pts |
| GDPval-AA v2.1 (knowledge work, Elo) | 1708 | 1846 | +138 Elo |
| OSWorld 2.0 (computer use) | 74.0% | 81.8% | +7.8 pts |
The Terminal-Bench 4.0 jump is worth putting next to this site's earlier Fable 5.1 vs Opus 5 comparison: Fable 5.1 scored 55.8% on that same benchmark against Fable 5's 42.0%. Opus 5.5's 66.4% now beats both Fable 5.1 and Fable 5 on agentic coding, at a fraction of Fable 5.1's $10/$50 pricing.
This Changes the Recommendation From the Last Piece
The earlier piece's framework was: start with Opus 5 at effort high, move to Fable 5.1 only after Opus 5 falls short on your own evals, and reserve Fable 5.1 for genuinely long-horizon agentic work where its benchmark gains actually concentrated.
That framework still holds structurally, but the numbers underneath it moved. Opus 5.5 now performs at Fable 5.1's level "on most work" per Anthropic's own claim, while costing 60% less than Fable 5.1's $10/$50 pricing. The practical effect: the set of workloads that genuinely need to reach for Fable 5.1 over Opus 5.5 is smaller than the set that needed to reach for it over the older Opus 5, since the baseline they're being compared against got both better and cheaper at the same time.
| Claude Fable 5.1 | Claude Opus 5.5 | Claude Sonnet 5 | Claude Haiku 4.5 | |
|---|---|---|---|---|
| Input / output per MTok | $10 / $50 | $4 / $20 | $2 / $10 | $1 / $5 |
| Context window | 1M tokens | 1M tokens | 1M tokens | 200K tokens |
| Default effort | High | Medium | High | Not supported |
| Thinking | Adaptive, always on | Adaptive, always on | Adaptive | Extended (manual) |
The updated default, in order: start with Opus 5.5, confirm on your own evals before reaching for Fable 5.1, and treat Fable 5.1 as the exception for long-horizon agentic work rather than the aspirational tier Opus models are compared against.
The Operational Gotcha: Default Effort Changed
Every other current-generation model (Fable 5.1, Sonnet 5) defaults to effort high when the parameter is omitted. Opus 5.5 defaults to medium.
Code migrated from claude-opus-5 to claude-opus-5-5 by changing only the model string, with no explicit output_config.effort set, will silently run at a lower effort level than it did before. That's not necessarily worse, medium effort on Opus 5.5 may well outperform high effort on the old Opus 5 given the benchmark deltas above, but it's a behavior change that won't show up as an error, just as a quality or verbosity difference that's easy to misattribute to something else during a migration.
// Migrated model string, but the effort default is now different.const response = await client.messages.create({ model: "claude-opus-5-5", max_tokens: 16000, // No output_config.effort set: this now runs at "medium", not "high". messages: [{ role: "user", content: "..." }],});// Explicit, if you want to match Opus 5's old default behavior:const response2 = await client.messages.create({ model: "claude-opus-5-5", max_tokens: 16000, output_config: { effort: "high" }, messages: [{ role: "user", content: "..." }],});Anyone migrating existing Opus 5 code should set effort explicitly rather than relying on the default carrying over unchanged.
What About the "Good at Explainer Videos" Buzz?
A secondary story circulated alongside the Opus 5.5 launch claiming particular strength at generating explainer videos. Checked directly against Anthropic's own Opus 5.5 announcement: there's no mention of video or explainer-content generation as a new capability. The announcement's stated improvements are agentic coding, knowledge work, and communication clarity, not multimedia generation. That claim traces to a third-party site, not to Anthropic's own materials, and this piece isn't repeating it as a confirmed capability.