Claude Sonnet 5.5 Is Live: Building Guide, Migration, and Claude Code Defaults

Update — September 27 staging: Registry leak — Claude Sonnet 5.5 Droid greyscale evidence.

September 28, 2026 — Anthropic’s second Claude 5.5 model is live. @claudeai introduced Claude Sonnet 5.5 (5.6M+ views) as a clear upgrade over Sonnet 5: more than 30% faster, up to about 30% lower cost per task at the same per-token prices, and the everyday counterpart to Opus 5.5. @ClaudeDevs pointed builders to Addy Osmani’s official playbook: Building with Claude Sonnet 5.5.

Update — October 1, 2026: The Osmani playbook now sits on Anthropic's named developer hub — what claude.dev is (and is not).

Official launch videos (@claudeai)

The family intro clip is the 5.6M-view post. The two follow-ups Anthropic pinned in the same thread are the ones builders actually quote in Slack: same list price, fewer tokens, faster output, then design/slides.

Cost and speed (same $2/$10, up to 30% less per task, more than 30% faster):

Design and slides (UI polish, template-following decks):

Developer guide on X (@ClaudeDevs)

Official evals (Anthropic launch charts)

The digest headline “Sonnet 5.5 beats flagship Opus 5.5 on Terminal-Bench 4.0 at half the price” is half true. List price is $2/$10 vs $4/$20. On Terminal-Bench 4.0, Anthropic’s table really does put Sonnet ahead. On almost every other row, Opus still wins. Read the footnotes before you rewrite a default-model policy.

Figures below are Anthropic’s own launch graphics (Sonnet 5.5 System Card methodology). Attribution: Anthropic, September 28, 2026.

Claude Sonnet 5.5 vs Sonnet 5, Opus 5.5, and GPT-6 Sol across Terminal-Bench 4.0, FrontierCode 1.1, CursorBench 4.0, GDPval-AA, AA-Briefcase, Humanity’s Last Exam, OSWorld 2.1, and Chartography

Four footnotes that change how you use the table:

1. Opus Terminal-Bench is reported at Xhigh, not a mystery “default.”
2. FrontierCode Max vs Xhigh: Sonnet drops at Max (46.2% vs 52.1%). FrontierCode grades whether a change could merge without human edits and penalizes out-of-scope diffs even when they are useful. Anthropic says Max more often fires Claude Code’s code-review skill, which fans the review across subagents; Cognition found two cases that timed out or added extra out-of-scope edits. If your harness already runs a review swarm, do not assume Max is better.
3. Artificial Analysis ran GDPval-AA and AA-Briefcase on a pre-release Claude Platform deployment that had a structured-output bug. Anthropic says the bug is fixed and any remaining effect understates Sonnet 5.5.
4. GPT-6 Sol image-understanding scores may still reflect a Surge AI Chartography / AA snapshot before OpenAI’s later image bugfix. Anthropic does not expect large AA-Briefcase / GDPval moves from that fix; internal Chartography retests looked unimpacted.

Cost vs quality at each effort level

These four plots are the actual @claudeai claim in chart form: Low/Medium Sonnet 5.5 often matches Sonnet 5’s best score at a fraction of the dollars. X-axis is USD per task (log) except Terminal-Bench, which is USD per attempt.

AA-Briefcase v1.1 knowledge-work Elo versus cost per task for Sonnet 5.5, Opus 5.5, Sonnet 5, and GPT-6 Sol at Low through Max effort

On AA-Briefcase, Sonnet 5.5’s Med → High → Xhigh → Max curve sits on top of Opus until the far-right Max point, where Opus still edges Elo. Sonnet 5 is a lower parallel line. GPT-6 Sol is competitive at cheap Low/Med and then stops on this plot before Claude’s Max.

CursorBench 4.0 agentic coding score versus cost per task for Sonnet 5.5, Opus 5.5, Sonnet 5, and GPT-5.6 Sol

CursorBench 4.0 does not publish GPT-6 Sol, so Anthropic plots GPT-5.6 Sol. Sonnet 5.5 Low already sits near 36%; High/Xhigh track Opus; Max is still a hair under Opus. Sonnet 5 never leaves the 25–35% band at much higher spend.

FrontierCode 1.1 Main agentic coding score versus cost per task, showing Sonnet 5.5 dropping at Max effort

FrontierCode is the trap chart. Sonnet 5.5 peaks around Xhigh (~52%) then falls at Max toward ~46%. Opus stays in the mid-50s. If you “just turn Max on” because it sounds like more thinking, this eval punishes you. Pair this with the Claude Code build-eval / hillclimb loop: held-out tasks, one change per round, revert if only the train split moves.

Terminal-Bench 4.0 score versus cost per attempt; Sonnet 5.5 Max crosses above Opus 5.5

Terminal-Bench 4.0 is where Sonnet Max crosses above Opus. GPT-5.6 Sol is the OpenAI stand-in (no public GPT-6 Sol row). Sonnet 5 is almost flat in the single digits until late spend. This is the chart behind “half the price, higher terminal score” — it is real, and it is one eval.

How to route from these plots: default Sonnet 5.5 medium in Claude Code for scoped bugs and docs; API high for knowledge-work Elo; Xhigh not Max if FrontierCode-style merge-cleanliness is the grade; Opus 5.5 when CursorBench / HLE / OSWorld / Chartography are the job. Re-run your own eval — Anthropic’s harness is not yours.

Sonnet 5.5 vs Opus 5.5 — when to use which

Anthropic’s table (via claude.dev) is the routing doc teams should paste into runbooks:

Epic COO Daniel Vogel quoted in the guide: Sonnet 5.5 cleared a system design audit and data-flow review on large gameplay codebases with less prescriptive prompting — early enterprise signal, not a universal benchmark substitute.

For Opus-specific spend math, keep what a Claude Code task costs on Opus 5.5 beside this post; Sonnet 5.5 deserves the same dollars-per-shipped-feature treatment once your cache hit rate is known.

Migration breaking changes (API)

Osmani’s guide lists five breaking changes plus response-shape updates. The ones that break production silently:

1. thinking: disabled → between_tools

# Sonnet 5.5 — tool-heavy agent
client.messages.create(
    model="claude-sonnet-5-5",
    max_tokens=16000,
    thinking={"type": "between_tools"},
    output_config={"effort": "high"},
    messages=[{"role": "user", "content": "..."}],
)

between_tools only works at low/medium/high effort — not xhigh/max.

2. Forced tool_choice → auto + strict: true

tool_choice types any and tool return 400, including on token counting.

3. Read blocks by type

Default thinking means content[0].text is wrong — loop block.type for thinking vs text.

4. Computer use toolset

Claude API / Google Cloud: use computer_toolset_20260801; legacy computer_20251124 returns 400 (Bedrock still accepts the older declaration per Anthropic docs).

5. Claude Code migration skill

Run /claude-api migrate this project to claude-sonnet-5-5 in Claude Code to apply ID swaps and parameter fixes repo-wide.

Tuning and refusals builders should know

• Re-run effort sweeps — levels are recalibrated vs Sonnet 5; start high on API, medium for agentic coding unless evals say otherwise.
• Drop Sonnet 5 prompt hacks — remove “do not be lazy” shims and re-eval before adding new ones.
• Cyber refusals — Sonnet 5.5 adds cyber safeguards similar to flagship models; declines return stop_reason: "refusal" with categories like cyber and reasoning_extraction.
• Images — high-res tier up to 2576px long edge costs ~2.5× image tokens vs Sonnet 4.x — downscale when detail is unnecessary.

X controversy: reference photo credit

Launch creative included a code-to-painting demo comparing Sonnet 5, Sonnet 5.5, and Opus 5.5 on a window-seat photograph. @IceSolst noted the image matched their September 26 post; @fire argued uncredited commercial use raises liability questions for Anthropic. The guide credits @jkeatn for demo ideas and @IceSolst for the reference — the dispute is about permissions, not model capability. Treat it as a launch comms lesson if you ship model demos with user-generated references.

Claude Code defaults

From v2.1.284:

• /model sonnet → Sonnet 5.5, medium effort, 1M context.
• Default model remains Opus 5.5.
• No fast mode on Sonnet 5.5; thinking cannot be fully disabled in Claude Code — effort controls depth.

Pair with Claude Opus 5.5 prompting guide for effort and cache patterns on the same 5.5 family.

What @claudeai said besides the launch clip

The family intro, the cost/speed clip, and the design clip are one thread. The numbers teams actually ship against:

• Role in the family: faster, lower-cost complement to Opus 5.5 — strongest at well-scoped everyday tasks, bug fixes, and polished documents, slides, and spreadsheets.
• Same list price as Sonnet 5, fewer tokens per task — Anthropic’s testing: up to 30% less cost per task, more than 30% faster output. Fastest Sonnet to date.
• Effort economics: on several benchmarks, Sonnet 5.5 at Low or Medium effort beats Sonnet 5’s best score at about a tenth of the cost.
• Design: polish on UIs; follows templates for slides that need minimal editing.
• Writing: like Opus 5.5, clearer than the prior generation; speed favors fast iteration.
• Safety: automated behavioral audit improves or matches Sonnet 5 on most alignment/honesty measures. First Sonnet with cyber safeguards and fallbacks like flagship models. Routine software development unaffected.
• Availability: everywhere today; Haiku 5.5 still “coming weeks.”

That last point is why the Droid leak aged in 24 hours: the slug was real, the marketing switch flipped September 28.

X replies included the expected pacing-the-frontier joke — ship Opus 5.5, then Sonnet 5.5, while asking the industry to slow down. That is a politics thread, not a migration bug. For enterprise routing, ignore the joke and use the workload table above.

Availability IDs (copy into config)

US-only inference (inference_geo: "us") is 1.1× standard price. Cache write $2.50 / $4 (5 min / 1 hour); cache read $0.10 (cut from $0.20 on October 7, 2026; see the cache-read cost math) — half of Opus 5.5 on writes.

Related reading

• Opus 5.5 vs Sonnet 5.5 — AA 58 vs 56, when max Sonnet costs more per task
• Sonnet 5.5 vs GPT-6 Astra
• Claude Haiku 5.5 launch (Oct 7) — the small model arrives, and Sonnet 5.5 cache reads drop to $0.10
• Claude Sonnet 5.5 registry leak (Sep 27)
• Claude Opus 5.5 launch benchmarks
• Opus 5.5 task cost in Claude Code
• Claude Code build-eval and hillclimb — same-day claude-api skill for evals
• Official: Building with Claude Sonnet 5.5 · @claudeai intro · cost/speed · design · @ClaudeDevs guide

Model IDs, breaking changes, and pricing match Anthropic’s September 28, 2026 claude.dev guide — verify current docs before production cutover.