model-migration-guide

name	model-migration-guide
description	Step-by-step instructions for migrating existing code to newer Claude models, covering breaking changes, deprecated parameters, per-SDK syntax, prompt-behavior shifts, and migration checklists
metadata	{"originalName":"Skill: Model migration guide","ccVersion":"2.1.246","sourceUrl":"https://github.com/Piebald-AI/claude-code-system-prompts/blob/main/system-prompts/skill-model-migration-guide.md","source":{"owner":"Piebald-AI","repo":"claude-code-system-prompts","ref":"main","path":"system-prompts/skill-model-migration-guide.md"}}

Model Migration Guide

If you arrived via /claude-api migrate: this is the right file. Execute the steps below in order - do not summarize them back to the user. Start with Step 0 (confirm scope) before touching any file.

How to move existing code to newer Claude models. Covers breaking changes, deprecated parameters, and drop-in replacements for retired models.

For the latest, authoritative version (with code samples in every supported language), WebFetch the Migration Guide URL from shared/live-sources.md. Use this file for the consolidated, skill-resident reference; fall back to the live docs whenever a model launch or breaking change may have shifted the picture.

This file is large. Use the section names below to jump (or Grep this file for the heading text). Read Step 0 and Step 1 first - they apply to every migration. Then read only the per-target section for the model you are migrating to.

Section	When you need it
Step 0: Confirm the migration scope	Always - before any edits
Step 1: Classify each file	Always - decides whether to swap, add-alongside, or skip
Per-SDK Syntax Reference	Translate the Python examples in this guide to TypeScript / Go / Ruby / Java / C# / PHP
Destination Models / Retired Model Replacements	Picking a target model
Breaking Changes by Source Model	Migrating to Opus 4.6 / Sonnet 4.6
Migrating to Opus 4.7	Migrating to Opus 4.7 (breaking changes, silent defaults, behavioral shifts)
Opus 4.7 Migration Checklist	The required vs optional items for 4.7, tagged [BLOCKS] / [TUNE]
Migrating to Opus 4.8	Migrating to Opus 4.8 (no new breaking changes; mid-session system prompts; behavioral re-tuning)
Opus 4.8 Migration Checklist	The required vs optional items for 4.8, tagged [BLOCKS] / [TUNE]
Migrating to {{OPUS_NAME}}	Migrating Opus 4.8 -> {{OPUS_NAME}} (thinking-disabled effort-gated; mid-conversation tool changes; per-turn effort and task budget; verbosity, over-verification, and scope re-tuning)
{{OPUS_NAME}} Migration Checklist	The required vs optional items for {{OPUS_NAME}}, tagged [BLOCKS] / [TUNE]
Migrating to {{SONNET_NEXT_NAME}}	Migrating Sonnet 4.6 -> {{SONNET_NEXT_NAME}} (adaptive thinking on by default; non-default sampling params 400; new tokenizer; xhigh effort for coding/agentic; high-res vision; behavioral re-tuning)
{{SONNET_NEXT_NAME}} Migration Checklist	The required vs optional items, tagged [BLOCKS] / [TUNE]
Migrating to {{FABLE_NAME}}	Migrating to {{FABLE_NAME}} or {{MYTHOS_NAME}} (always-on thinking, raw chain of thought never returned, refusal handling, data retention, behavioral shifts + prompting guidance)
{{FABLE_NAME}} Migration Checklist	The required vs optional items for {{FABLE_NAME}}, tagged [BLOCKS] / [TUNE]
Verify the Migration	After edits - runtime spot-check

TL;DR: Change the model ID string. If you were using budget_tokens, switch to thinking: {type: "adaptive"}. If you were using assistant prefills, they 400 on both Opus 4.6 and Sonnet 4.6 - switch to one of the prefill replacements (most often output_config.format; see the table in Breaking Changes by Source Model). If you're moving from Sonnet 4.5 to Sonnet 4.6, set effort explicitly - 4.6 defaults to high. Remove the effort-2025-11-24 and fine-grained-tool-streaming-2025-05-14 beta headers (GA on 4.6); remove interleaved-thinking-2025-05-14 once you're on adaptive thinking (keep it only while using the transitional budget_tokens escape hatch). Then drop back from client.beta.messages.create to client.messages.create. Dial back any aggressive "CRITICAL: YOU MUST" tool instructions; 4.6 follows the system prompt much more closely.

---

Step 0: Confirm the migration scope

Before any Write, Edit, or MultiEdit call, confirm the scope. If the user's request does not explicitly name a single file, a specific directory, or an explicit file list, ask first - do not start editing. This is non-negotiable: even imperative-sounding requests like "migrate my codebase", "move my project to X", "upgrade to Sonnet 4.6", or bare "migrate to Opus 4.7" leave the scope ambiguous and require a clarifying question. Phrases like "my project", "my code", "my codebase", "the whole thing", "everywhere", or "across the repo" are ambiguous, not directive - they tell you what to do but not where. Ask before doing.

Offer the common scopes explicitly and wait for the answer before touching any file:

1. The entire working directory
2. A specific subdirectory (e.g. src/, app/, services/billing/)
3. A specific file or a list of files

Surface this as a single clarifying question so the user can answer in one turn. Proceed without asking only when the scope is already unambiguous - the user named an exact file ("migrate extract.py to Sonnet 4.6"), pointed at a specific directory ("migrate everything under services/billing/ to Opus 4.6"), listed specific files ("update a.py and b.py"), or already answered the scope question in an earlier turn. If you can answer the question "which files is this change going to touch?" with a precise list from the prompt alone, proceed. If not, ask.

Worked example. If the user says "Move my project to Opus 4.6. I want adaptive thinking everywhere it makes sense." you do not know whether "my project" means the whole working directory, just src/, just the production code, or something else - the everywhere makes the intent clear (update every call site within scope) but the scope itself is still not defined. Do not start editing. Respond with:

Before I start editing, can you confirm the scope? I can migrate:

1. Every .py file in the working directory
2. Just the files under src/ (production code)
3. A specific subdirectory or list of files you name

Which one?

Then wait for the answer. The same applies to "Migrate to Opus 4.7" and bare "Help me upgrade to Sonnet 4.6" - ask before editing.

Sizing the scope question (large repos). Before asking, get a per-directory count so the user can pick concretely:

rg -l "<old-model-id>" --type-not md | cut -d/ -f1 | sort | uniq -c | sort -rn

Present the breakdown in your scope question (e.g. "Found 217 references across 3 directories: api/ (130), api-go/ (62), routing/ (25). Which to migrate?"). Also confirm git status is clean before surveying - unexpected modifications mean a concurrent process; stop and investigate before proceeding.

---

Step 1: Classify each file

Not every file that contains the old model ID is a caller of the API. Before editing, classify each file into one of these buckets - the right action differs:

#	Bucket	What it looks like	Action
1	Calls the API/SDK	client.messages.create(model=...), anthropic.Anthropic(), request payloads	Swap the model ID and apply the breaking-change checklist for the target version (below).
2	Defines or serves the model	Model registries, OpenAPI specs, routing/queue configs, model-policy enums, generated catalogs	The old entry stays (the model is still served). Ask whether to (a) add the new model alongside, (b) leave alone, or (c) retire the old model - never blind-replace. If you can't ask, default to (a): add the new model alongside and flag it - replacing would de-register a model that's still in production.
3	References the ID as an opaque string	UI fallback constants, capability-gate substring checks, generic test fixtures, label parsers, env defaults	Usually swap the string and verify any parser/regex/substring match handles the new ID - but check the sub-cases below first.
4	Suffixed variant ID	claude-<model>-<suffix> like -fast, -1024k, -200k, [1m], dated snapshots	These are deployment/routing identifiers, not the public model ID. Do not assume a new-model equivalent exists. Verify in the registry first; if absent, leave the string alone and flag it. Exception: -fast strings (e.g. claude-opus-4-6-fast) are handled by the Fast Mode section below, which rewrites them to Opus 4.8 plus speed="fast" and the fast-mode-2026-02-01 beta rather than leaving them in place.

Bucket 3 sub-cases - before swapping a string reference, check:

• Capability gate (e.g. if 'opus-4-6' in model_id: enables a feature) -> add the new ID alongside, don't replace. The old model is still served and still has the capability, so replacing would silently disable the feature for any old-model traffic that still flows through. If you know no old-model traffic will hit this gate (single-caller codebase fully migrating), replacing is fine; if unsure, add alongside.
• Registry-assert test (e.g. assert "claude-X" in supported_models, test_X_has_N_clusters) -> add an assertion for the new model alongside; keep the old one. The old model is still served, so its assertion stays valid - but the registry should also include the new model, so assert that too. Heuristic: if the test references multiple model versions in a list, it's a registry test; if one model in a struct compared only to itself, it's a generic fixture.
• Frozen / generated snapshot -> regenerate, don't hand-edit.
• Coupled to a definer (e.g. an integration test that passes model authorization via a shared conftest seed list, or asserts on a billing-tier / rate-limit-group enum or a generated SKU/pricing catalog) -> verify the definer has a new-model entry first. If not, add a seed entry (reusing the nearest existing tier as a placeholder); if you can't confidently do that, ask the user how to populate the definer. Do not skip the test. Swapping without populating the definer will make the test fail at runtime.

When migrating tests specifically: breaking parameters (temperature, top_p, budget_tokens) are usually absent - test fixtures rarely set sampling params on placeholder models. The breaking-change scan is still required, but expect mostly clean results.

Find intentionally-flagged sync points first. Many codebases tag spots that must change at every model launch with comment markers like MODEL LAUNCH, KEEP IN SYNC, @model-update, or similar. Grep for whatever convention the repo uses before the broad model-ID grep - those markers point at the load-bearing changes.

---

Per-SDK Syntax Reference

Code examples in this guide are Python. The same fields exist in every official Anthropic SDK - Stainless generates all 7 from the same OpenAPI spec, so JSON field names map 1:1 with only case-convention differences. Use the rows below to translate the Python examples to the SDK you are migrating.

Verify type and method names against the SDK source before writing them into customer code. WebFetch the relevant repository from the SDK source-code table in shared/live-sources.md (one row per SDK) and confirm the exact symbol - particularly for typed SDKs (Go, Java, C#) where union/builder names can differ from the JSON shape. Do not guess type names that aren't in the table below or in <lang>/claude-api/README.md.

thinking - budget_tokens -> adaptive

SDK	Before	After
Python	thinking={"type": "enabled", "budget_tokens": N}	thinking={"type": "adaptive"}
TypeScript	thinking: { type: 'enabled', budget_tokens: N }	thinking: { type: 'adaptive' }
Go	Thinking: anthropic.ThinkingConfigParamOfEnabled(N)	Thinking: anthropic.ThinkingConfigParamUnion{OfAdaptive: &anthropic.ThinkingConfigAdaptiveParam{}}
Ruby	thinking: { type: "enabled", budget_tokens: N }	thinking: { type: "adaptive" }
Java	.thinking(ThinkingConfigEnabled.builder().budgetTokens(N).build())	.thinking(ThinkingConfigAdaptive.builder().build())
C#	Thinking = new ThinkingConfigEnabled { BudgetTokens = N }	Thinking = new ThinkingConfigAdaptive()
PHP	thinking: ['type' => 'enabled', 'budget_tokens' => N]	thinking: ['type' => 'adaptive']

Sampling parameters - temperature / top_p / top_k

(Remove the field entirely on Opus 4.7; on Claude 4.x keep at most one of temperature or top_p.)

SDK	Field(s) to remove
Python	temperature=..., top_p=..., top_k=...
TypeScript	temperature: ..., top_p: ..., top_k: ...
Go	Temperature: anthropic.Float(...), TopP: anthropic.Float(...), TopK: anthropic.Int(...)
Ruby	temperature: ..., top_p: ..., top_k: ...
Java	.temperature(...), .topP(...), .topK(...)
C#	Temperature = ..., TopP = ..., TopK = ...
PHP	temperature: ..., topP: ..., topK: ...

Prefill replacement - structured outputs via output_config.format

SDK	Remove (last assistant turn)	Add
Python	{"role": "assistant", "content": "..."}	output_config={"format": {"type": "json_schema", "schema": SCHEMA}}
TypeScript	{ role: 'assistant', content: '...' }	output_config: { format: { type: 'json_schema', schema: SCHEMA } }
Go	trailing anthropic.MessageParam{Role: "assistant", ...}	OutputConfig: anthropic.OutputConfigParam{Format: anthropic.JSONOutputFormatParam{...}}
Ruby	{ role: "assistant", content: "..." }	output_config: { format: { type: "json_schema", schema: SCHEMA } }
Java	trailing Message.builder().role(ASSISTANT)...	.outputConfig(OutputConfig.builder().format(JsonOutputFormat.builder()...build()).build())
C#	trailing new Message { Role = "assistant", ... }	OutputConfig = new OutputConfig { Format = new JsonOutputFormat { ... } }
PHP	trailing ['role' => 'assistant', 'content' => '...']	outputConfig: ['format' => ['type' => 'json_schema', 'schema' => $SCHEMA]]

thinking.display - opt back into summarized reasoning (Opus 4.7)

SDK	Add
Python	thinking={"type": "adaptive", "display": "summarized"}
TypeScript	thinking: { type: 'adaptive', display: 'summarized' }
Go	Thinking: anthropic.ThinkingConfigParamUnion{OfAdaptive: &anthropic.ThinkingConfigAdaptiveParam{Display: anthropic.ThinkingConfigAdaptiveDisplaySummarized}}
Ruby	thinking: { type: "adaptive", display: "summarized" } (or display_: when constructing the model class directly)
Java	.thinking(ThinkingConfigAdaptive.builder().display(ThinkingConfigAdaptive.Display.SUMMARIZED).build())
C#	Thinking = new ThinkingConfigAdaptive { Display = Display.Summarized }
PHP	thinking: ['type' => 'adaptive', 'display' => 'summarized']

For any field not in these tables, the JSON key in the Python example translates directly: snake_case for Python/TypeScript/Ruby, camelCase named args for PHP, PascalCase struct fields for Go/C#, camelCase builder methods for Java.

---

Explain every change you make

Migration edits often look arbitrary to a user who hasn't read the release notes - a removed temperature, a deleted prefill, a rewritten system-prompt sentence. For each edit, tell the user what you changed and why, tied to the specific API or behavioral change that motivates it. Do this in your summary as you work, not just at the end.

Be especially explicit about system-prompt edits. Users are rightly protective of their prompts, and prompt-tuning changes are judgment calls (not hard API requirements). For any prompt edit:

• Quote the before and after text.
• State the behavioral shift that motivates it (e.g. "Opus 4.7 calibrates response length to task complexity, so I added an explicit length instruction", or "4.6 follows instructions more literally, so 'CRITICAL: YOU MUST use the search tool' will now overtrigger - softened to 'Use the search tool when...'").
• Make clear which prompt edits are optional tuning (tone, length, subagent guidance) versus which code edits are required to avoid a 400 (sampling params, budget_tokens, prefills). Never present an optional prompt change as mandatory.

If you're applying several prompt-tuning edits at once, offer them as a short list the user can accept or decline item-by-item rather than silently rewriting their system prompt.

---

Before You Migrate

1. Confirm the target model ID. Use only the exact strings from shared/models.md - do not append date suffixes to aliases (claude-opus-4-6, not claude-opus-4-6-20251101). Guessing an ID will 404.
2. Check which features your code uses with this checklist:
   • thinking: {type: "enabled", budget_tokens: N} -> migrate to adaptive thinking on Opus 4.6 / Sonnet 4.6 (still functional but deprecated)
   • Assistant-turn prefills (messages ending with role: "assistant") -> must change on Opus 4.6 / Sonnet 4.6 (returns 400)
   • output_format parameter on messages.create() -> must change on all models (deprecated API-wide)
   • max_tokens > ~16000 -> must stream on any model (above ~16K risks SDK HTTP timeouts). When streaming, every current model reaches 128K except Haiku 4.5, which caps at 64K
   • Beta headers effort-2025-11-24, fine-grained-tool-streaming-2025-05-14, interleaved-thinking-2025-05-14 -> GA on 4.6, remove them and switch from client.beta.messages.create to client.messages.create
   • Moving Sonnet 4.5 -> Sonnet 4.6 with no effort set -> 4.6 defaults to high, which may change your latency/cost profile
   • System prompts with CRITICAL, MUST, If in doubt, use X language -> likely to overtrigger on 4.6 (see Prompt-Behavior Changes)
   • Coming from 3.x / 4.0 / 4.1: also check sampling params (temperature + top_p), tool versions (text_editor_20250728), refusal + model_context_window_exceeded stop reasons, trailing-newline tool-param handling
3. Test on a single request first. Run one call against the new model, inspect the response, then roll out.

---

Destination Models (recommended targets)

If you're on...	Migrate to	Why
Claude Mythos Preview (claude-mythos-preview)	{{MYTHOS_ID}} (Project Glasswing successor) or {{FABLE_ID}} (GA)	Same tokenizer family - mostly a model-ID swap; remove thinking config and prefill; see Migrating to {{FABLE_NAME}}
Opus 4.8	{{OPUS_ID}}	The current Opus. Two breaking changes (thinking on by default; disabling thinking capped at high effort) plus prompt re-tuning - see Migrating to {{OPUS_NAME}}
Opus 4.7	{{OPUS_ID}}	Apply the Opus 4.8 section (prompt re-tuning, no new breaking changes), then the {{OPUS_NAME}} section
Opus 4.6	{{OPUS_ID}}	Apply the Opus 4.7 breaking changes, then 4.8 re-tuning, then the {{OPUS_NAME}} section
Opus 4.0 / 4.1 / 4.5 / Opus 3	{{OPUS_ID}}	Apply 4.6 -> 4.7 -> 4.8 -> {{OPUS_NAME}} in order (adaptive thinking, drop sampling params, then re-tune)
Sonnet 4.6	{{SONNET_NEXT_ID}}	Near-Opus quality on agentic and coding work at Sonnet cost; adaptive thinking on by default; see Migrating to {{SONNET_NEXT_NAME}}
Sonnet 4.0 / 4.5 / 3.7 / 3.5	{{SONNET_NEXT_ID}}	Apply the Sonnet 4.6 changes first, then the {{SONNET_NEXT_NAME}} section
Haiku 3 / 3.5	claude-haiku-4-5	Fastest and most cost-effective

Default to the latest Opus for the caller's tier unless they explicitly chose otherwise. The Opus migrations layer: if you're on Opus 4.6 or older, apply each version's section in order up to your target (e.g. 4.5 -> 4.8 means the 4.6, 4.7, and 4.8 sections in sequence). A 4.7 -> 4.8 move has no new breaking changes - see Migrating to Opus 4.8 below.

---

Retired Model Replacements

These models return 404 - update immediately:

Retired model	Retired	Drop-in replacement
claude-3-7-sonnet-20250219	Feb 19, 2026	{{SONNET_NEXT_ID}}
claude-3-5-haiku-20241022	Feb 19, 2026	claude-haiku-4-5
claude-3-opus-20240229	Jan 5, 2026	claude-opus-4-8
claude-3-5-sonnet-20241022	Oct 28, 2025	{{SONNET_NEXT_ID}}
claude-3-5-sonnet-20240620	Oct 28, 2025	{{SONNET_NEXT_ID}}
claude-3-sonnet-20240229	Jul 21, 2025	{{SONNET_NEXT_ID}}
claude-2.1, claude-2.0	Jul 21, 2025	{{SONNET_NEXT_ID}}

Deprecated Models (retiring soon)

Model	Retires	Replacement
claude-3-haiku-20240307	Apr 19, 2026	claude-haiku-4-5
claude-opus-4-20250514	June 15, 2026	claude-opus-4-8
claude-sonnet-4-20250514	June 15, 2026	{{SONNET_NEXT_ID}}

---

Breaking Changes by Source Model

Migrating from Sonnet 4.5 to Sonnet 4.6 (effort default change)

Sonnet 4.5 had no effort parameter; Sonnet 4.6 defaults to high. If you just switch the model string and do nothing else, you may see noticeably higher latency and token usage. Set effort explicitly.

Recommended starting points:

Workload	Start at	Notes
Chat, classification, content generation	low	With thinking: {"type": "disabled"} you'll see similar or better performance vs. Sonnet 4.5 no-thinking
Most applications (balanced)	medium	The default sweet spot for quality vs. cost
Agentic coding, tool-heavy workflows	medium	Pair with adaptive thinking and a generous max_tokens (up to 128K with streaming - Sonnet 4.6's ceiling)
Autonomous multi-step agents, long-horizon loops	high	Scale down to medium if latency/tokens become a concern
Computer-use agents	high + adaptive	Sonnet 4.6's best computer-use accuracy is on adaptive + high

For non-thinking chat workloads specifically:

client.messages.create(
    model="claude-sonnet-4-6",
    max_tokens=8192,
    thinking={"type": "disabled"},
    output_config={"effort": "low"},
    messages=[{"role": "user", "content": "..."}],
)

When to use Opus 4.6 instead: hardest and longest-horizon problems - large code migrations, deep research, extended autonomous work. Sonnet 4.6 wins on fast turnaround and cost efficiency.

Migrating to Opus 4.6 / Sonnet 4.6 (from any older model)

1. Manual extended thinking is deprecated - use adaptive thinking.

thinking: {type: "enabled", budget_tokens: N} (manual extended thinking with a fixed token budget) is deprecated on Opus 4.6 and Sonnet 4.6. Replace it with thinking: {type: "adaptive"}, which lets Claude decide when and how much to think. Adaptive thinking also enables interleaved thinking automatically (no beta header needed).

# Old (still works on older models, deprecated on 4.6)
response = client.messages.create(
    model="claude-sonnet-4-5",
    max_tokens=16000,
    thinking={"type": "enabled", "budget_tokens": 8000},
    messages=[...]
)

# New (Opus 4.6 / Sonnet 4.6)
response = client.messages.create(
    model="claude-opus-4-6",  # or "claude-sonnet-4-6"
    max_tokens=16000,
    thinking={"type": "adaptive"},
    output_config={"effort": "high"},  # optional: low | medium | high | max
    messages=[...]
)

Adaptive thinking is the long-term target, and on internal evaluations it outperforms manual extended thinking. Move when you can.

Transitional escape hatch: manual extended thinking is still functional on Opus 4.6 and Sonnet 4.6 (deprecated, will be removed in a future release). If you need a hard ceiling while migrating - for example, to bound token spend on a runaway workload before you've tuned effort - you can keep budget_tokens around alongside an explicit effort value, then remove it in a follow-up. budget_tokens must be strictly less than max_tokens:

# Transitional only - deprecated, plan to remove
client.messages.create(
    model="claude-sonnet-4-6",
    max_tokens=16384,
    thinking={"type": "enabled", "budget_tokens": 8192},  # must be < max_tokens
    output_config={"effort": "medium"},
    messages=[...],
)

If the user asks for a "thinking budget" on 4.6, the preferred answer is effort - use low, medium, high, or max rather than a token count.

2. Effort parameter (Opus 4.5, Opus 4.6, Sonnet 4.6 only).

Controls thinking depth and overall token spend. Goes inside output_config, not top-level. Default is high. max is supported on Fable 5, Opus 4.6 and later, Sonnet 5, and Sonnet 4.6 - it errors on Sonnet 4.5 and Haiku 4.5.

output_config={"effort": "medium"}  # often the best cost / quality balance

Migrating to the 4.6 family (Opus 4.6 and Sonnet 4.6)

3. Assistant-turn prefills return 400 (Opus 4.6 and Sonnet 4.6).

Prefilled responses on the final assistant turn are no longer supported on either Opus 4.6 or Sonnet 4.6 - both return a 400. Adding assistant messages elsewhere in the conversation (e.g., for few-shot examples) still works. Pick the replacement that matches what the prefill was doing:

Prefill was used for	Replacement
Forcing JSON / YAML / schema output	output_config.format with a json_schema - see example below
Forcing a classification label	Tool with an enum field containing valid labels, or structured outputs
Skipping preambles (Here is the summary:\n)	System prompt instruction: "Respond directly without preamble. Do not start with phrases like 'Here is...' or 'Based on...'."
Steering around bad refusals	Usually no longer needed - 4.6 refuses far more appropriately. Plain user-turn prompting is sufficient.
Continuing an interrupted response	Move continuation into the user turn: "Your previous response was interrupted and ended with [last text]. Continue from there."
Injecting reminders / context hydration	Inject into the user turn instead. For complex agent harnesses, expose context via a tool call or during compaction.

# Old (fails on Opus 4.6 / Sonnet 4.6) - prefill forcing JSON shape
messages=[
    {"role": "user", "content": "Extract the name."},
    {"role": "assistant", "content": "{"name": ""},
]

# New - structured outputs replace the prefill
response = client.messages.create(
    model="claude-opus-4-6",
    max_tokens=1024,
    output_config={"format": {"type": "json_schema", "schema": {...}}},
    messages=[{"role": "user", "content": "Extract the name."}],
)

4. Stream for max_tokens > ~16K (all models); only Haiku 4.5 caps lower, at 64K.

Non-streaming requests hit SDK HTTP timeouts at high max_tokens, regardless of model - stream for anything above ~16K output. The streamable ceiling is 128K for every current model except Haiku 4.5, which caps at 64K.

with client.messages.stream(model="claude-opus-4-6", max_tokens=64000, ...) as stream:
    message = stream.get_final_message()

5. Tool-call JSON escaping may differ (Opus 4.6 and Sonnet 4.6).

Both 4.6 models can produce tool call input fields with Unicode or forward-slash escaping. Always parse with json.loads() / JSON.parse() - never raw-string-match the serialized input.

All models

6. output_format -> output_config.format (API-wide).

The old top-level output_format parameter on messages.create() is deprecated. Use output_config.format instead. This is not 4.6-specific - applies to every model.

---

Beta Headers to Remove on 4.6

Several beta headers that were required on 4.5 are now GA on 4.6 and should be removed. Leaving them in is harmless but misleading; removing them also lets you move from client.beta.messages.create(...) back to client.messages.create(...).

Header	Status on 4.6	Action
effort-2025-11-24	Effort parameter is GA	Remove
fine-grained-tool-streaming-2025-05-14	GA	Remove
interleaved-thinking-2025-05-14	Adaptive thinking enables interleaved thinking automatically	Remove when using adaptive thinking; still functional on Sonnet 4.6 with manual extended thinking, but that path is deprecated
token-efficient-tools-2025-02-19	Built in to all Claude 4+ models	Remove (no effect)
output-128k-2025-02-19	Built in to Claude 4+ models	Remove (no effect)

Once you remove all of these and finish moving to adaptive thinking, you can switch the SDK call site from the beta namespace back to the regular one:

# Before
response = client.beta.messages.create(
    model="claude-opus-4-5",
    betas=["interleaved-thinking-2025-05-14", "effort-2025-11-24"],
    ...
)

# After
response = client.messages.create(
    model="claude-opus-4-6",
    thinking={"type": "adaptive"},
    output_config={"effort": "high"},
    ...
)

---

Additional Changes When Coming from 3.x / 4.0 / 4.1 -> 4.6

If you're jumping from Opus 4.1, Sonnet 4, Sonnet 3.7, or an older Claude 3.x model directly to 4.6, apply everything above plus the items in this section. Users already on Opus 4.5 / Sonnet 4.5 can skip this.

1. Sampling parameters: temperature OR top_p, not both.

Passing both will error on every Claude 4+ model:

# Old (3.x only - errors on 4+)
client.messages.create(temperature=0.7, top_p=0.9, ...)

# New
client.messages.create(temperature=0.7, ...)  # or top_p, not both

2. Update tool versions.

Legacy tool versions are not supported on 4+. Both the type and the name field change - text_editor_20250728 and str_replace_based_edit_tool are a pair; updating one without the other 400s. Also remove the undo_edit command from your text-editor integration:

Old	New
text_editor_20250124 + str_replace_editor	text_editor_20250728 + str_replace_based_edit_tool
code_execution_* (earlier versions)	code_execution_20260521
undo_edit command	(no longer supported - delete call sites)

# Before
tools = [{"type": "text_editor_20250124", "name": "str_replace_editor"}]

# After - BOTH fields change
tools = [{"type": "text_editor_20250728", "name": "str_replace_based_edit_tool"}]

3. Handle the refusal stop reason.

Claude 4+ can return stop_reason: "refusal" on the response. If your code only handles end_turn / tool_use / max_tokens, add a branch:

if response.stop_reason == "refusal":
    # Surface the refusal to the user; do not retry with the same prompt
    ...

4. Handle the model_context_window_exceeded stop reason (4.5+).

Distinct from max_tokens: it means the model hit the context window limit, not the requested output cap. Handle both:

if response.stop_reason == "model_context_window_exceeded":
    # Context window exhausted - compact or split the conversation
    ...
elif response.stop_reason == "max_tokens":
    # Requested output cap hit - retry with higher max_tokens or stream
    ...

5. Trailing newlines preserved in tool call string parameters (4.5+).

4.5 and 4.6 preserve trailing newlines that older models stripped. If your tool implementations do exact string matching against tool-call input values (e.g., if name == "foo"), verify they still match when the model sends "foo\n". Normalizing with .rstrip() on the receiving side is usually the simplest fix.

6. Haiku: rate limits reset between generations.

Haiku 4.5 has its own rate-limit pool separate from Haiku 3 / 3.5. If you're ramping traffic as you migrate, check your tier's Haiku 4.5 limits at API rate limits - a quota that comfortably served Haiku 3.5 traffic may need a tier bump for the same volume on 4.5.

---

Prompt-Behavior Changes (Opus 4.5 / 4.6, Sonnet 4.6)

These don't break your code, but prompts that worked on 4.5-and-earlier may over- or under-trigger on 4.6. Tune as needed. For a standing, model-general audit of dated prompt text beyond this migration - skills and tool descriptions included - read shared/prompt-audit.md (or invoke /claude-api prompt-audit).

1. Aggressive instructions cause overtriggering. Opus 4.5 and 4.6 follow the system prompt much more closely than earlier models. Prompts written to overcome the old reluctance are now too aggressive:

Before (worked on 4.0 / 4.5)	After (use on 4.6)
CRITICAL: You MUST use this tool when...	Use this tool when...
Default to using [tool]	Use [tool] when it would improve X
If in doubt, use [tool]	(delete - no longer needed)

If the model is now overtriggering a tool or skill, the fix is almost always to dial back the language, not to add more guardrails.

2. Overthinking and excessive exploration (Opus 4.6). At higher effort settings, Opus 4.6 explores more before answering. If that burns too many thinking tokens, lower effort first (medium is often the sweet spot) before adding prose instructions to constrain reasoning.

3. Overeager subagent spawning (Opus 4.6). Opus 4.6 has a strong preference for delegating to subagents. If you see it spawning a subagent for something a direct grep or read would solve, add guidance: "Use subagents only for parallel or independent workstreams. For single-file reads or sequential operations, work directly."

4. Overengineering (Opus 4.5 / 4.6). Both models may add extra files, abstractions, or defensive error handling beyond what was asked. If you want minimal changes, prompt for it explicitly: "Only make changes directly requested. Don't add helpers, abstractions, or error handling for scenarios that can't happen."

5. LaTeX math output (Opus 4.6). Opus 4.6 defaults to LaTeX (\frac{}{}, $...$) for math and technical content. If you need plain text, instruct it explicitly: "Format all math as plain text - no LaTeX, no $, no \frac{}{}. Use / for division and ^ for exponents."

6. Skipped verbal summaries (4.6 family). The 4.6 models are more concise and may skip the summary paragraph after a tool call, jumping straight to the next action. If you rely on those summaries for visibility, add: "After completing a task that involves tool use, provide a brief summary of what you did."

7. "Think" as a trigger word (Opus 4.5 with thinking disabled). When thinking is off, Opus 4.5 is particularly sensitive to the word think and may reason more than you want. Use consider, evaluate, or reason through instead.

---

Model-ID Rename Quick Reference

Old string (migration source)	New string
claude-opus-4-8	{{OPUS_ID}}
claude-opus-4-7	{{OPUS_ID}}
claude-opus-4-6	{{OPUS_ID}}
claude-opus-4-5	{{OPUS_ID}}
claude-opus-4-1	{{OPUS_ID}}
claude-opus-4-0	{{OPUS_ID}}
claude-mythos-preview	{{MYTHOS_ID}} (Project Glasswing) or {{FABLE_ID}}
claude-sonnet-4-6	{{SONNET_NEXT_ID}}
claude-sonnet-4-5	{{SONNET_NEXT_ID}}
claude-sonnet-4-0	{{SONNET_NEXT_ID}}

Older aliases (claude-opus-4-7, claude-opus-4-6, claude-opus-4-5, claude-sonnet-4-6, claude-sonnet-4-5, etc.) are still active and can be pinned if you need time before upgrading - see shared/models.md for the full legacy list.

Amazon Bedrock model IDs

If the code uses the AnthropicBedrockMantle client (Python anthropic[bedrock], TypeScript @anthropic-ai/bedrock-sdk, Java BedrockMantleBackend, Go bedrock.NewMantleClient, etc.) or targets https://bedrock-mantle.{region}.api.aws/anthropic, it is running on Claude in Amazon Bedrock. All breaking changes in this guide apply unchanged there - it serves the same Messages API shape - but model IDs carry an anthropic. provider prefix:

First-party ID	Bedrock ID
claude-opus-4-8	anthropic.claude-opus-4-8
{{OPUS_ID}}	anthropic.{{OPUS_ID}}
claude-opus-4-7	anthropic.claude-opus-4-7
{{SONNET_NEXT_ID}}	anthropic.{{SONNET_NEXT_ID}}
claude-haiku-4-5	anthropic.claude-haiku-4-5

When migrating a Bedrock file, apply the same rename-table row as first-party, then keep/add the anthropic. prefix. Do not generate a first-party claude-* ID for a Bedrock client - it will 400.

Skip for Bedrock: the code_execution_* tool-version checklist item and the Task Budgets section - neither is available on Bedrock (see shared/platform-availability.md for the per-feature table). Everything else in this guide - effort, adaptive/extended thinking, output_config.format, thinking.display, fine-grained tool streaming, token counting - is available on Bedrock.

Out of scope: the legacy Amazon Bedrock integration (InvokeModel / Converse APIs with ARN-versioned IDs like anthropic.claude-3-5-sonnet-20241022-v2:0) uses a different request shape and model-ID format. This guide does not cover it; WebFetch the Bedrock page in shared/live-sources.md if the user is migrating between the two Bedrock integrations.

Claude Platform on AWS

If the code uses AnthropicAWS / AnthropicAws / anthropicaws.NewClient / AnthropicAwsClient (or targets https://aws-external-anthropic.{region}.api.aws), it is running on Claude Platform on AWS - Anthropic-operated, same-day API parity. Model IDs are bare first-party strings; apply the rename table above verbatim and every breaking-change section in this guide unchanged. There is nothing to skip. Do not add an anthropic. prefix (that's Amazon Bedrock, a separate offering). See shared/claude-platform-on-aws.md for client/auth details.

---

Migration Checklist

Every item is tagged: [BLOCKS] items cause a 400 error, infinite loop, silent timeout, or wrong tool selection if missed - apply these as code edits, not as suggestions. [TUNE] items are quality/cost adjustments.

For each file that calls messages.create() / equivalent SDK method:

• [BLOCKS] Update the model= string to the new alias
• [BLOCKS] Replace budget_tokens with thinking={"type": "adaptive"} (deprecated on Opus 4.6 / Sonnet 4.6)
• [BLOCKS] Move format from top-level output_format into output_config.format
• [BLOCKS] Remove any assistant-turn prefills if targeting Opus 4.6 or Sonnet 4.6 (see the prefill replacement table)
• [BLOCKS] Switch to streaming if max_tokens > ~16000 (otherwise SDK HTTP timeout)
• [TUNE] Verify tool-input handling parses JSON rather than raw-string-matching the serialized input (4.6 may escape Unicode / forward slashes differently; most SDKs already expose block.input as a parsed object)
• [TUNE] Set output_config={"effort": "..."} explicitly - especially when moving Sonnet 4.5 -> Sonnet 4.6 (4.6 defaults to high)
• [TUNE] Remove GA beta headers: effort-2025-11-24, fine-grained-tool-streaming-2025-05-14, token-efficient-tools-2025-02-19, output-128k-2025-02-19; remove interleaved-thinking-2025-05-14 once on adaptive thinking
• [TUNE] Switch client.beta.messages.create(...) -> client.messages.create(...) once all betas are removed
• [TUNE] Review system prompt for aggressive tool language (CRITICAL:, MUST, If in doubt) and dial it back

Extra items when coming from 3.x / 4.0 / 4.1:

• [BLOCKS] Remove either temperature or top_p (passing both 400s on Claude 4+)
• [BLOCKS] Update text-editor tool type to text_editor_20250728
• [BLOCKS] Update text-editor tool name to str_replace_based_edit_tool - changing only the type and keeping name: "str_replace_editor" returns a 400
• [BLOCKS] Update code-execution tool to code_execution_20260521
• [BLOCKS] Delete any undo_edit command call sites
• [TUNE] Add handling for stop_reason == "refusal"
• [TUNE] Add handling for stop_reason == "model_context_window_exceeded" (4.5+)
• [TUNE] Verify tool-param string matching tolerates trailing newlines (preserved on 4.5+)
• [TUNE] If moving to Haiku 4.5: review rate-limit tier (separate pool from Haiku 3.x)

Verification:

• Run one test request and inspect response.stop_reason, response.usage, and whether tool-use / thinking behavior matches expectations

For cached prompts: the render order and hash inputs did not change, so existing cache_control breakpoints keep working. However, changing the model string invalidates the existing cache - the first request on the new model will write the cache fresh.

---

Migrating to Opus 4.7

Model ID claude-opus-4-7 is authoritative as written here. When the user asks to migrate to Opus 4.7, write model="claude-opus-4-7" exactly. Do not WebFetch to verify - this guide is the source of truth for migration target IDs. The corresponding entry exists in shared/models.md.

Claude Opus 4.7 was Anthropic's most capable model at its launch and is now the previous-generation Opus (Opus 4.8 is current - see Migrating to Opus 4.8 below). It is highly autonomous and performs exceptionally well on long-horizon agentic work, knowledge work, vision tasks, and memory tasks. This section summarizes everything that was new at the 4.7 launch and remains the layered breaking-change path for callers coming from Opus 4.6 or older. It is layered on top of the 4.6 migration above - if the caller is jumping from Opus 4.5 or older, apply the 4.6 changes first, then this section, then the 4.8 section.

TL;DR for someone already on Opus 4.6: update the model ID to claude-opus-4-7, strip any remaining budget_tokens and sampling parameters (both 400 on Opus 4.7), give max_tokens extra headroom and re-baseline with count_tokens() against the new model, opt back into thinking.display: "summarized" if reasoning is surfaced to users, and re-tune effort - it matters more on 4.7 than on any prior Opus.

Breaking changes (will 400 on Opus 4.7)

Extended thinking removed.

thinking: {type: "enabled", budget_tokens: N} is no longer supported on Claude Opus 4.7 or later models and returns a 400 error. Switch to adaptive thinking (thinking: {type: "adaptive"}) and use the effort parameter to control thinking depth. Adaptive thinking is off by default on Claude Opus 4.7: requests with no thinking field run without thinking, matching Opus 4.6 behavior. Set thinking: {type: "adaptive"} explicitly to enable it.

# Before (Opus 4.6)
client.messages.create(
    model="claude-opus-4-6",
    max_tokens=64000,
    thinking={"type": "enabled", "budget_tokens": 32000},
    messages=[{"role": "user", "content": "..."}],
)

# After (Opus 4.7)
client.messages.create(
    model="claude-opus-4-7",
    max_tokens=64000,
    thinking={"type": "adaptive"},
    output_config={"effort": "high"},  # or "max", "xhigh", "medium", "low"
    messages=[{"role": "user", "content": "..."}],
)

If the caller wasn't using extended thinking, no change is required - thinking is off by default, or can be set explicitly with thinking={"type": "disabled"}.

Delete budget_tokens plumbing entirely. For the replacement effort value, see Choosing an effort level on Opus 4.7 below - there is no exact 1:1 mapping from budget_tokens.

Sampling parameters removed.

The temperature, top_p, and top_k parameters are no longer accepted on Claude Opus 4.7. Requests that include them return a 400 error. Remove these fields from your request payloads. Prompting is the recommended way to guide model behavior on Claude Opus 4.7. If you were using temperature = 0 for determinism, note that it never guaranteed identical outputs on prior models.

# Before - errors on Opus 4.7
client.messages.create(temperature=0.7, top_p=0.9, ...)

# After
client.messages.create(...)  # no sampling params

• If the intent was determinism - use effort: "low" with a tighter prompt.
• If the intent was creative variance - the prompt replacement depends on the use case; ask the user how they want variance elicited. If you can't ask, add a use-case-appropriate instruction along the lines of "choose something off-distribution and interesting" - e.g. for text generation, "Vary your phrasing and structure across responses"; for frontend/design, use the propose-4-directions approach under Design and frontend coding below.

Choosing an effort level on Opus 4.7

budget_tokens controlled how much to think; effort controls how much to think and act, so there is no exact 1:1 mapping. Use xhigh for best results in coding and agentic use cases, and a minimum of high for most intelligence-sensitive use cases. Experiment with other levels to further tune token usage and intelligence:

Level	Use when	Notes
max	Intelligence-demanding tasks worth testing at the ceiling	Can deliver gains in some use cases but may show diminishing returns from increased token usage; can be prone to overthinking
xhigh	Most coding and agentic use cases	The best setting for these; used as the default in Claude Code
high	Intelligence-sensitive use cases generally	Balances token usage and intelligence; recommended minimum for most intelligence-sensitive work
medium	Cost-sensitive use cases that need to reduce token usage while trading off intelligence
low	Short, scoped tasks and latency-sensitive workloads that are not intelligence-sensitive

Silent default changes (no error, but behavior differs)

Thinking content omitted by default.

Thinking blocks still appear in the response stream on Claude Opus 4.7, but their thinking field is empty unless you explicitly opt in. This is a silent change from Claude Opus 4.6, where the default was to return summarized thinking text. To restore summarized thinking content on Claude Opus 4.7, set thinking.display to "summarized". The block-field name is unchanged - it is still block.thinking on a thinking-type block; do not rename it.

Detect this: any code that reads block.thinking (or equivalent) from a thinking-type block and renders it in a UI, log, or trace. The fix is the request parameter, not the response handling - add display: "summarized" to the thinking parameter:

thinking={"type": "adaptive", "display": "summarized"}  # "display" is new on Opus 4.7; values: "omitted" (default) | "summarized"

The default is "omitted" on Claude Opus 4.7. If thinking content was never surfaced anywhere, no change needed. If your product streams reasoning to users, the new default appears as a long pause before output begins; set display: "summarized" to restore visible progress during thinking.

Updated token counting.

Claude Opus 4.7 and Claude Opus 4.6 count tokens differently. The same input text produces a higher token count on Claude Opus 4.7 than on Claude Opus 4.6, and /v1/messages/count_tokens will return a different number of tokens for Claude Opus 4.7 than it did for Claude Opus 4.6. The token efficiency of Claude Opus 4.7 can vary by workload shape. Prompting interventions, task_budget, and effort can help control costs and ensure appropriate token usage. Keep in mind that these controls may trade off model intelligence. Update your max_tokens parameters to give additional headroom, including compaction triggers. Claude Opus 4.7 provides a 1M context window at standard API pricing with no long-context premium.

What else to check:

• Client-side token estimators (tiktoken-style approximations) calibrated against 4.6
• Cost calculators that multiply tokens by a fixed per-token rate
• Rate-limit retry thresholds keyed to measured token counts

Re-baseline by re-running client.messages.count_tokens() against claude-opus-4-7 on a representative sample of the caller's prompts. Do not apply a blanket multiplier. For cost-sensitive workloads, consider reducing effort by one level (e.g. high -> medium). For agentic loops, consider adopting Task Budgets (below).

New feature: Task Budgets (beta)

Opus 4.7 introduces task budgets - tell Claude how many tokens it has for a full agentic loop (thinking + tool calls + final output). The model sees a running countdown and uses it to prioritize work and wrap up gracefully as the budget is consumed.

This is a suggestion the model is aware of, not a hard cap. It is distinct from max_tokens, which remains the enforced per-response limit and is not surfaced to the model. Use task_budget when you want the model to self-moderate; use max_tokens as a hard ceiling to cap usage.

Requires beta header task-budgets-2026-03-13:

client.beta.messages.create(
    betas=["task-budgets-2026-03-13"],
    model="claude-opus-4-7",
    max_tokens=64000,
    thinking={"type": "adaptive"},
    output_config={
        "effort": "high",
        "task_budget": {"type": "tokens", "total": 128000},
    },
    messages=[...],
)

Set a generous budget for open-ended agentic tasks and tighten it for latency-sensitive ones. Minimum task_budget.total is 20,000 tokens. If the budget is too restrictive for the task, the model may complete it less thoroughly, referencing its budget as the constraint. Do not add task_budget during a migration unless you are sure the budget value is right - if you can run the workload and measure, do so; otherwise ask the user for the value rather than guessing. This is the primary lever for offsetting the token-counting shift on agentic workloads.

Capability improvements

High-resolution vision. Opus 4.7 is the first Claude model with high-resolution image support. Maximum image resolution is 2576 pixels on the long edge (up from 1568px on Opus 4.6 and prior). This unlocks gains on vision-heavy workloads, especially computer use and screenshot/artifact/document understanding. Coordinates returned by the model now map 1:1 to actual image pixels, so no scale-factor math is needed.

High-res support is automatic on Opus 4.7 - no beta header, no client-side opt-in required. The model accepts larger inputs and returns pixel-accurate coordinates out of the box.

Token cost. Full-resolution images on Opus 4.7 can use up to ~3× more image tokens than on prior models (up to ~4784 tokens per image, vs. the previous ~1,600-token cap). If the extra fidelity isn't needed, downsample client-side before sending to control cost - but do not add downsampling by default during a migration. If you're not sure whether the pipeline needs the fidelity, ask the user rather than guessing. Use count_tokens() on representative images on Opus 4.7 to re-baseline before reacting to any measured cost shift.

Beyond resolution, Opus 4.7 also improves on low-level perception (pointing, measuring, counting) and natural-image bounding-box localization and detection.

Knowledge work. Meaningful gains on tasks where the model visually verifies its own output - .docx redlining, .pptx editing, and programmatic chart/figure analysis (e.g. pixel-level data transcription via image-processing libraries). If prompts have scaffolding like "double-check the slide layout before returning", try removing it and re-baselining.

Memory. Opus 4.7 is better at writing and using file-system-based memory. If an agent maintains a scratchpad, notes file, or structured memory store across turns, that agent should improve at jotting down notes to itself and leveraging its notes in future tasks.

User-facing progress updates. Opus 4.7 provides more regular, higher-quality interim updates during long agentic traces. If the system prompt has scaffolding like "After every 3 tool calls, summarize progress", try removing it to avoid excessive user-facing text. If the length or contents of Opus 4.7's updates are not well-calibrated to your use case, explicitly describe what these updates should look like in the prompt and provide examples.

Real-time cybersecurity safeguards

Requests that involve prohibited or high-risk topics may lead to refusals.

Fast Mode: {{OPUS_NAME}} / Opus 4.8 only

Fast mode is available on {{OPUS_NAME}} and Opus 4.8. Only surface this if the caller's code actually uses fast mode (e.g. model="claude-opus-4-6-fast", or speed="fast" on an unsupported model); if the word "fast" does not appear in the code, say nothing about Fast Mode.

When you see model="claude-opus-4-6-fast" (or any retired -fast model string), the migration edit is to move the fast-mode traffic onto {{OPUS_NAME}}, the current fast-capable default (Opus 4.8 also works if the caller is staying on that tier):

# Request fast mode on {{OPUS_NAME}}.
client.beta.messages.create(
    model="{{OPUS_ID}}", max_tokens=4096,
    speed="fast", betas=["fast-mode-2026-02-01"],
    messages=[...],
)

That is: switch the model to {{OPUS_NAME}} (or Opus 4.8) and request fast mode the supported way, using the beta client.beta.messages.... endpoint, the fast-mode-2026-02-01 beta flag, and speed="fast" as a top-level request parameter (per-language form in SKILL.md § Fast Mode). Opus 4.7 fast mode has also been removed, so do not land on Opus 4.7 either. Do not leave the code on a retired -fast model string - the failure mode differs by version: claude-opus-4-6-fast is retired and the API silently falls back to standard Opus 4.6 (no error - the caller loses fast-mode speed without noticing); claude-opus-4-7-fast and speed="fast" on Opus 4.7 instead return an API error (hard failure - requests break outright rather than degrading). Either way, migrate to Opus 4.8 fast mode now.

Behavioral shifts (prompt-tunable)

These don't break anything, but prompts tuned for Opus 4.6 may land differently. Opus 4.7 is more steerable than 4.6, so small prompt nudges usually close the gap.

More literal instruction following. Claude Opus 4.7 interprets prompts more literally and explicitly than Claude Opus 4.6, particularly at lower effort levels. It will not silently generalize an instruction from one item to another, and it will not infer requests you didn't make. The upside of this literalism is precision and less thrash. It generally performs better for API use cases with carefully tuned prompts, structured extraction, and pipelines where you want predictable behavior. A prompt and harness review may be especially helpful for migration to Claude Opus 4.7.

Verbosity calibrates to task complexity. Opus 4.7 scales response length to how complex it judges the task to be, rather than defaulting to a fixed verbosity - shorter answers on simple lookups, much longer on open-ended analysis. If the product depends on a particular length or style, tune the prompt explicitly. To reduce verbosity:

"Provide concise, focused responses. Skip non-essential context, and keep examples minimal."

If you see specific kinds of over-verbosity (e.g. over-explaining), add instructions targeting those. Positive examples showing the desired level of concision tend to be more effective than negative examples or instructions telling the model what not to do. Do not assume existing "be concise" instructions should be removed - test first.

Tone and writing style. Opus 4.7 is more direct and opinionated, with less validation-forward phrasing and fewer emoji than Opus 4.6's warmer style. As with any new model, prose style on long-form writing may shift. If the product relies on a specific voice, re-evaluate style prompts against the new baseline. If a warmer or more conversational voice is wanted, specify it:

"Use a warm, collaborative tone. Acknowledge the user's framing before answering."

effort matters more than on any prior Opus. Opus 4.7 respects effort levels more strictly, especially at the low end. At low and medium it scopes work to what was asked rather than going above and beyond - good for latency and cost, but on moderate tasks at low there is some risk of under-thinking.

• If shallow reasoning shows up on complex problems, raise effort to high or xhigh rather than prompting around it.
• If effort must stay low for latency, add targeted guidance: "This task involves multi-step reasoning. Think carefully through the problem before responding."
• At xhigh or max, set a large max_tokens so the model has room to think and act across tool calls and subagents. Start at 64K and tune from there. (xhigh is a new effort level on Opus 4.7, between high and max.)

Adaptive-thinking triggering is also steerable. If the model thinks more often than wanted - which can happen with large or complex system prompts - add: "Thinking adds latency and should only be used when it will meaningfully improve answer quality - typically for problems that require multi-step reasoning. When in doubt, respond directly."

Uses tools less often by default. Opus 4.7 tends to use tools less often than 4.6 and to use reasoning more. This produces better results in most cases, but for products that rely on tools (search/retrieval, function-calling, computer-use steps), it can drop tool-use rate. Two levers:

• Raise effort - high or xhigh show substantially more tool usage in agentic search and coding, and are especially useful for knowledge work.
• Prompt for it - be explicit in tool descriptions or the system prompt about when and how to use the tool, and encourage the model to err on the side of using it more often:

"When the answer depends on information not present in the conversation, you MUST call the search tool before answering - do not answer from prior knowledge."

Fewer subagents by default. Opus 4.7 tends to spawn fewer subagents than 4.6. This is steerable - give explicit guidance on when delegation is desirable. For a coding agent, for example:

"Do NOT spawn a subagent for work you can complete directly in a single response (e.g. refactoring a function you can already see). Spawn multiple subagents in the same turn when fanning out across items or reading multiple files."

Design and frontend coding. Opus 4.7 has stronger design instincts than 4.6, with a consistent default house style: warm cream/off-white backgrounds (around #F4F1EA), serif display type (Georgia, Fraunces, Playfair), italic word-accents, and a terracotta/amber accent. This reads well for editorial, hospitality, and portfolio briefs, but will feel off for dashboards, dev tools, fintech, healthcare, or enterprise apps - and it appears in slide decks as well as web UIs.

The default is persistent. Generic instructions ("don't use cream," "make it clean and minimal") tend to shift the model to a different fixed palette rather than producing variety. Two approaches work reliably:

1. Specify a concrete alternative. The model follows explicit specs precisely - give exact hex values, typefaces, and layout constraints.

2. Have the model propose options before building. This breaks the default and gives the user control:

   "Before building, propose 4 distinct visual directions tailored to this brief (each as: bg hex / accent hex / typeface - one-line rationale). Ask the user to pick one, then implement only that direction."

If the caller previously relied on temperature for design variety, use approach (2) - it produces meaningfully different directions across runs.

Opus 4.7 also requires less frontend-design prompting than previous models to avoid generic "AI slop" aesthetics. Where earlier models needed a lengthy anti-slop snippet, Opus 4.7 generates distinctive, creative frontends with a much shorter nudge. This snippet works well alongside the variety approaches above:

"NEVER use generic AI-generated aesthetics like overused font families (Inter, Roboto, Arial, system fonts), cliched color schemes (particularly purple gradients on white or dark backgrounds), predictable layouts and component patterns, and cookie-cutter design that lacks context-specific character. Use unique fonts, cohesive colors and themes, and animations for effects and micro-interactions."

Interactive coding products. Opus 4.7's token usage and behavior can differ between autonomous, asynchronous coding agents with a single user turn and interactive, synchronous coding agents with multiple user turns. Specifically, it tends to use more tokens in interactive settings, primarily because it reasons more after user turns. This can improve long-horizon coherence, instruction following, and coding capabilities in long interactive coding sessions, but also comes with more token usage. To maximize both performance and token efficiency in coding products, use effort: "xhigh" or "high", add autonomous features (like an auto mode), and reduce the number of human interactions required from users.

When limiting required user interactions, specify the task, intent, and relevant constraints upfront in the first human turn. Well-specified, clear, and accurate task descriptions upfront help maximize autonomy and intelligence while minimizing extra token usage after user turns - because Opus 4.7 is more autonomous than prior models, this usage pattern helps to maximize performance. In contrast, ambiguous or underspecified prompts conveyed progressively over multiple user turns tend to reduce token efficiency and sometimes performance.

Code review. Opus 4.7 is meaningfully better at finding bugs than prior models, with both higher recall and precision. However, if a code-review harness was tuned for an earlier model, it may initially show lower recall - this is likely a harness effect, not a capability regression. When a review prompt says "only report high-severity issues," "be conservative," or "don't nitpick," Opus 4.7 follows that instruction more faithfully than earlier models did: it investigates just as thoroughly, identifies the bugs, and then declines to report findings it judges to be below the stated bar. Precision rises, but measured recall can fall even though underlying bug-finding has improved.

Recommended prompt language:

"Report every issue you find, including ones you are uncertain about or consider low-severity. Do not filter for importance or confidence at this stage - a separate verification step will do that. Your goal here is coverage: it is better to surface a finding that later gets filtered out than to silently drop a bug. For each finding, include your confidence level and an estimated severity so a downstream filter can rank them."

This can be used without an actual second step, but moving confidence filtering out of the finding step often helps. If the harness has a separate verification/dedup/ranking stage, tell the model explicitly that its job at the finding stage is coverage, not filtering. If single-pass self-filtering is wanted, be concrete about the bar rather than using qualitative terms like "important" - e.g. "report any bugs that could cause incorrect behavior, a test failure, or a misleading result; only omit nits like pure style or naming preferences." Iterate on prompts against a subset of evals to validate recall or F1 gains.

Computer use. Computer use works across resolutions up to the new 2576px / 3.75MP maximum. Sending images at 1080p provides a good balance of performance and cost. For particularly cost-sensitive workloads, 720p or 1366×768 are lower-cost options with strong performance. Test to find the ideal settings for the use case; experimenting with effort can also help tune behavior.

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Opus 4.7 Migration Checklist

Every item is tagged: [BLOCKS] items cause a 400 error, infinite loop, silent truncation, or empty output if missed - apply these as code edits, not as suggestions. [TUNE] items are quality/cost adjustments - surface them to the user as recommendations.

[BLOCKS] items prefixed with "If..." or "At..." are conditional. Before working through the list, scan the file for the conditions: does it surface thinking text to a UI/log? Does it set output_config.effort to "x-high" or "max"? Is it a security workload? Is it a multi-turn agentic loop? Apply only the items whose condition matches.

• [BLOCKS] Replace thinking: {type: "enabled", budget_tokens: N} with thinking: {type: "adaptive"} + output_config.effort; delete budget_tokens plumbing entirely
• [BLOCKS] Strip temperature, top_p, top_k from request construction
• [BLOCKS] If thinking content is surfaced to users or stored in logs: add thinking.display: "summarized" (otherwise the rendered text is empty)
• [BLOCKS] At output_config.effort of xhigh or max: set max_tokens >= 64000 (otherwise output truncates mid-thought)
• [TUNE] Give max_tokens and compaction triggers extra headroom; re-run count_tokens() against claude-opus-4-7 on representative prompts to re-baseline (no blanket multiplier)
• [TUNE] Re-baseline cost and rate-limit dashboards before reacting to measured shifts
• [TUNE] Re-evaluate effort per route - use xhigh for coding/agentic and a minimum of high for most intelligence-sensitive work; it matters more on 4.7 than any prior Opus
• [TUNE] Multi-turn agentic loops: adopt the API-native Task Budgets (output_config.task_budget, beta task-budgets-2026-03-13, minimum 20k tokens) - this is for capping cumulative spend across a loop; per-turn depth is effort
• [TUNE] Check for ambiguous or underspecified instructions that relied on 4.6 generalizing intent, and update them to be clearer or more precise - 4.7 follows them literally
• [TUNE] Tool-use workloads: add explicit when/how-to-use guidance to tool descriptions (4.7 reaches for tools less often)

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Migrating to Opus 4.8

Model ID claude-opus-4-8 is authoritative as written here. When the user asks to migrate to Opus 4.8, write model="claude-opus-4-8" exactly. Do not WebFetch to verify - this guide is the source of truth for migration target IDs. The corresponding entry exists in shared/models.md.

Claude Opus 4.8 is our most capable Opus-tier model - highly autonomous, with state-of-the-art long-horizon agentic execution, knowledge work, and memory. It is layered on top of the Opus 4.7 migration above. If the caller is jumping from Opus 4.6 or older, apply the 4.6 and 4.7 sections first, then this one.

No new breaking changes. Opus 4.8 keeps the same request surface as Opus 4.7. The same calls that already work on 4.7 work unchanged on 4.8 - adaptive thinking only (thinking: {type: "enabled", budget_tokens: N} still 400s; use {type: "adaptive"}), sampling parameters (temperature, top_p, top_k) still rejected, last-assistant-turn prefills still 400, thinking.display still defaults to "omitted", and the low/medium/high/xhigh/max effort levels, Task Budgets (beta), and high-resolution vision all behave as on 4.7. A 4.7 -> 4.8 migration is therefore the model-ID swap plus prompt re-tuning - there is no required code edit beyond the model string.

TL;DR for someone already on Opus 4.7: swap the model ID to claude-opus-4-8. Nothing else is required to avoid an error. Then re-tune prompts for the behavioral shifts: 4.8 narrates more than 4.7 (add a silence-default if you want 4.7-like terseness), writes in a warmer, less hedged voice, is more deliberate and asks more often (add autonomy guidance to claw back ask-rate), and is more conservative about reaching for search, subagents, file-based memory, and custom tools (add explicit "when to use this" triggering). For long-horizon agentic work, give the full task specification up front in one well-specified turn and run at high effort.

No new API breaking changes (inherited from 4.7)

These all carry over from Opus 4.7 unchanged - apply them only if the caller is coming from Opus 4.6 or earlier (see the Migrating to Opus 4.7 section above for the before/after and the SDK-specific syntax):

• thinking: {type: "enabled", budget_tokens: N} -> 400. Use thinking: {type: "adaptive"} + output_config.effort.
• temperature, top_p, top_k -> 400. Remove them; steer with prompting.
• Last-assistant-turn prefills -> 400. Use output_config.format (structured outputs) or a system-prompt instruction.
• thinking.display defaults to "omitted"; set "summarized" if you surface reasoning to users.

If the caller is already on Opus 4.7 and these are clean, there is nothing to change here.

New API feature: mid-session system prompts

You can deliver trusted instructions partway through a session by placing {"role": "system", ...} entries directly in the messages array - without editing the top-level system prompt and invalidating your prompt cache. Use it for things the application learns mid-session: the user delivered async context, a mode toggled (auto-approve enabled), files changed on disk, the remaining token budget dropped.

messages=[
    {"role": "user", "content": [{"type": "tool_result", "tool_use_id": "...", "content": "..."}]},
    {"role": "system", "content": "This project's codebase is Go. Write code in Go."},
]

Phrase these as context, not commands. State the fact and let Claude act on it; avoid override-style language ("ignore what the user said", "regardless of the user's request", "disregard the previous instruction"). Claude is trained to protect users from instructions that appear to work against them, and that protection applies to the system role too. No beta header is required; available on {{PREV_OPUS_NAME}}. For cache-placement details and the older-model <system-reminder> fallback, see shared/prompt-caching.md and shared/agent-design.md.

Capability improvements

Long-horizon agentic execution. Opus 4.8 is state-of-the-art at long, autonomous agentic work - complex refactors and overnight coding runs that complete without human correction. To get the most out of it, give the full task specification up front in a single well-specified initial turn and run at high effort (effort: "high" or "xhigh"). Its long-horizon coherence comes partly from reasoning more at each step; combined with a clear up-front goal, that more-intelligent planning often produces more efficient and more accurate output than prior frontier models. The "clear goal up front" principle maps to two product surfaces: in Claude Code, /goal sets direction for the run; with Managed Agents (CMA), state what "done" looks like via an Outcome (user.define_outcome with a gradeable rubric - the harness runs an iterate -> grade -> revise loop), see shared/managed-agents-outcomes.md.

Effort is a dimension to test, not a fixed setting. On prior models many reached for xhigh reflexively to maximize intelligence. Opus 4.8 has a higher intelligence ceiling, so start at high as the default and iterate rather than defaulting to xhigh. Sweep medium, high, and xhigh on your own eval set and weigh the intelligence <-> latency <-> cost tradeoff per route - the relationship isn't monotonic: higher effort up front often reduces turn count and total cost on agentic work, while for some tasks medium delivers equally good results in less time. Reserve max for extremely hard, latency-insensitive cases. The per-level effort table in the Migrating to Opus 4.7 section above applies unchanged on 4.8.

Writing voice and clarity. Testers consistently describe 4.8's prose as clearer, warmer, and less hedged than prior models, with fewer measurable AI vocal tics - especially at higher effort, where it approaches expert-level prose and structure. This is roughly the opposite direction from the 4.7 shift (4.7 was more clipped, direct, and less validation-forward). If you added style prompts to counter 4.7's terseness or to inject warmth, re-evaluate them against the new baseline before keeping them - they may now overcorrect. 4.8 is also a stronger thought partner: more thoughtful, more willing to push back, and more likely to infer the right answer from context.

Code review and debugging. Stronger real-bug finding and clearer explanations than 4.7 - one-shot fixes where 4.7 needed more, and correctly identifying intermittent flakes rather than declaring "fixed" after one clean run. The 4.7 caveat still applies: if a review harness says "only report high-severity issues" or "be conservative", 4.8 follows it literally and measured recall can drop even though underlying bug-finding improved. Tell the model to report everything and filter downstream (or review a second time) - see the Code review guidance in the 4.7 section for the recommended prompt.

Behavioral shifts (prompt-tunable)

None of these break code, but prompts tuned for Opus 4.7 may land differently. 4.8 follows instructions well, so small, explicit nudges close the gap.

Tool triggering is surface-dependent (search & knowledge). 4.8's tool-triggering is more surface-dependent than in prior models: with a system prompt present it is high-precision / low-recall - web search triggers slightly more often but runs fewer rounds per trigger, while knowledge-retrieval tools (Drive, project knowledge, connected files) trigger less often. It searches when it's confident search is needed and otherwise answers from context, which can lower research depth on tasks that need it. Recover should-search rate with an explicit search-first instruction:

<search_first>
For questions where current information would change the answer (recent events, current roles or prices, version-specific behavior, or anything the user flags as time-sensitive) search before answering rather than answering from memory. For open-ended research requests, begin searching immediately; do not ask a scoping question first unless the request is genuinely ambiguous about what to research.
</search_first>

Under-utilization of subagents, memory, and custom tools. Separately from search, 4.8 is conservative about reaching for capabilities that need an explicit "decide to use this" step - file-based memory, subagent delegation, custom tools. It won't reach for complex or expensive capabilities unless reasonably sure they're needed. This is steerable since 4.8 follows instructions well - say when each capability applies, not just that it exists: