Update claude-api skill: Claude Fable 5 and Claude Mythos 5 (#1294)

- Add Claude Fable 5 and Claude Mythos 5 to the model tables with pricing,
  context window, and model-selection guidance
- Document Fable-specific API behavior: always-on adaptive thinking (explicit
  disabled returns 400), protected-thinking display and replay rules, new
  tokenizer (~30% more tokens), refusal stop reason with server-side fallbacks
  and SDK fallback middleware, 30-day data-retention requirement
- Add the full Migrating to Claude Fable 5 guide section and checklist,
  including the Mythos Preview migration path
- Refresh skill trigger description, effort/compaction/task-budget notes,
  caching minimums, structured-output and dynamic-filtering support tables

skills/claude-api/SKILL.md

@@ -2,7 +2,7 @@
 name: claude-api
 description: |-
   Reference for the Claude API / Anthropic SDK — model ids, pricing, params, streaming, tool use, MCP, agents, caching, token counting, model migration.
-  TRIGGER — read BEFORE opening the target file; don't skip because it "looks like a one-liner" — whenever: the prompt names Claude/Anthropic in any form (Claude, Anthropic, Opus, Sonnet, Haiku, `anthropic`, `@anthropic-ai`, `claude-*`, `us.anthropic.*`, `[1m]`); the user asks about an LLM (pricing/model choice/limits/caching) — never answer from memory; OR the task is LLM-shaped with provider unstated (agent/MCP/tool-definition/multi-agent/RAG/LLM-judge/computer-use; generate/summarize/extract/classify/rewrite/converse over NL; debugging refusals/cutoffs/streaming/tool-calls/tokens).
+  TRIGGER — read BEFORE opening the target file; don't skip because it "looks like a one-liner" — whenever: the prompt names Claude/Anthropic in any form (Claude, Anthropic, Fable, Opus, Sonnet, Haiku, `anthropic`, `@anthropic-ai`, `claude-*`, `us.anthropic.*`, `[1m]`); the user asks about an LLM (pricing/model choice/limits/caching) — never answer from memory; OR the task is LLM-shaped with provider unstated (agent/MCP/tool-definition/multi-agent/RAG/LLM-judge/computer-use; generate/summarize/extract/classify/rewrite/converse over NL; debugging refusals/cutoffs/streaming/tool-calls/tokens).
   SKIP only when another provider is being worked on (overrides all triggers): OpenAI/GPT/Gemini/Llama/Mistral/Cohere/Ollama named in the query; OR `grep -rE 'openai|langchain_openai|google.generativeai|genai|mistralai|cohere|ollama'` over the project hits (run this grep FIRST if no provider named — don't Read the file).
 license: Complete terms in LICENSE.txt
 ---
@@ -167,17 +167,31 @@ Everything goes through `POST /v1/messages`. Tools and output constraints are fe
 
 ---
 
-## Current Models (cached: 2026-05-26)
+## Current Models (cached: 2026-06-04)
 
 | Model             | Model ID            | Context        | Input $/1M | Output $/1M |
 | ----------------- | ------------------- | -------------- | ---------- | ----------- |
+| Claude Fable 5    | `claude-fable-5`      | 1M             | $10.00     | $50.00      |
+| Claude Mythos 5 (Project Glasswing only) | `claude-mythos-5` | 1M | $10.00     | $50.00      |
 | Claude Opus 4.8   | `claude-opus-4-8`   | 1M             | $5.00      | $25.00      |
 | Claude Opus 4.7   | `claude-opus-4-7`   | 1M             | $5.00      | $25.00      |
 | Claude Opus 4.6   | `claude-opus-4-6`   | 1M             | $5.00      | $25.00      |
 | Claude Sonnet 4.6 | `claude-sonnet-4-6` | 1M             | $3.00      | $15.00      |
 | Claude Haiku 4.5  | `claude-haiku-4-5`  | 200K           | $1.00      | $5.00       |
 
-**ALWAYS use `claude-opus-4-8` unless the user explicitly names a different model.** This is non-negotiable. Do not use `claude-sonnet-4-6`, `claude-sonnet-4-5`, or any other model unless the user literally says "use sonnet" or "use haiku". Never downgrade for cost — that's the user's decision, not yours.
+**ALWAYS use `claude-opus-4-8` unless the user explicitly names a different model.** This is non-negotiable. Do not use `claude-sonnet-4-6`, `claude-sonnet-4-5`, or any other model unless the user literally says "use sonnet" or "use haiku". Never downgrade for cost — that's the user's decision, not yours. Use `claude-fable-5` only when the user explicitly asks for Claude Fable 5, "fable", or Anthropic's most capable model — it has different API behavior than the Opus family (see below) and pricing that exceeds Opus-tier.
+
+### Claude Fable 5 (`claude-fable-5`) — most capable widely released model
+
+Claude Fable 5 is Anthropic's most capable widely released model, for the most demanding reasoning and long-horizon agentic work. **Claude Mythos 5** (`claude-mythos-5`) offers the same capabilities, pricing, and API surface through Project Glasswing (participation is the only way to access it), succeeding the invitation-only Claude Mythos Preview (`claude-mythos-preview`) — everything below applies to both models. 1M context window (the maximum is also the default), 128K max output. Key API differences from Opus-tier — see `shared/model-migration.md` → Migrating to Claude Fable 5 for details:
+
+- **Thinking is always on** — omit the `thinking` parameter entirely (or send `{type: "adaptive"}`). Any other explicit configuration is rejected: `{type: "disabled"}` and `{type: "enabled", budget_tokens: N}` both return a 400. Control depth with `output_config.effort` (supports `low` through `xhigh` and `max`).
+- **Protected thinking = the raw chain of thought, not the summary** — responses carry regular `thinking` blocks (not `redacted_thinking`): `display: "summarized"` returns a readable summary, `"omitted"` (the default) leaves the `thinking` field as an empty string; the raw chain of thought is never exposed on any model. Replay rules: pass thinking blocks back exactly as received on the same model (including empty-text blocks — the API rejects *modified* blocks, not read ones); a **different** model **silently ignores** them (not an error), but ignored blocks still bill input tokens — strip them when switching models for good.
+- **New tokenizer** — the same content tokenizes to roughly 30% more tokens than on Opus-tier models. Don't reuse token counts or `max_tokens` settings measured on other models; re-baseline with `count_tokens`.
+- **`refusal` stop reason** — safety classifiers may decline a request (HTTP 200, `stop_reason: "refusal"`, with a `stop_details` category). A pre-output refusal has an empty `content` array and is not billed at all; a mid-stream refusal bills the already-streamed output — discard the partial output. Always check `stop_reason` before reading `content`. To retry on another model: the beta `fallbacks` parameter (Claude API and Claude Platform on AWS) retries server-side in one round trip; the GA SDKs' `BetaRefusalFallbackMiddleware` + `BetaFallbackState` handle client-side retry everywhere else (incl. Bedrock/Vertex); fallback credit refunds the cache-switch cost of client-side retries. See the migration guide's refusal section.
+- **No assistant prefill** — same as the rest of the 4.6+ family.
+- **30-day data retention required** — Claude Fable 5 is not available under zero data retention; requests from an org whose retention configuration doesn't meet the requirement return `400 invalid_request_error`.
+- **Longer turns, different prompting** — single requests on hard tasks can run many minutes (plan timeouts/streaming/progress UX); effort sweeps should include low/medium for routine work; prompts written for prior models are often too prescriptive and reduce output quality. See `shared/model-migration.md` → Migrating to Claude Fable 5 → Behavioral shifts (prompt-tunable) for the recommended prompt snippets (anti-overplanning, no-tidying, grounded progress claims, boundaries, async sub-agents, memory, `send_to_user`).
 
 **CRITICAL: Use only the exact model ID strings from the table above — they are complete as-is. Do not append date suffixes.** For example, use `claude-sonnet-4-6`, never `claude-sonnet-4-6-20251114` or any other date-suffixed variant you might recall from training data. If the user requests an older model not in the table (e.g., "opus 4.5", "sonnet 3.7"), read `shared/models.md` for the exact ID — do not construct one yourself.
 
@@ -189,13 +203,13 @@ A note: if any of the model strings above look unfamiliar to you, that's to be e
 
 ## Thinking & Effort (Quick Reference)
 
-**Opus 4.8 / 4.7 — Adaptive thinking only:** Use `thinking: {type: "adaptive"}`. `thinking: {type: "enabled", budget_tokens: N}` returns a 400 — adaptive is the only on-mode. `{type: "disabled"}` and omitting `thinking` both work. Sampling parameters (`temperature`, `top_p`, `top_k`) are also removed and will 400. Opus 4.8 keeps the same request surface as 4.7 (no new breaking changes) — see `shared/model-migration.md` → Migrating to Opus 4.8 for the behavioral re-tuning, and → Migrating to Opus 4.7 for the full breaking-change list when coming from 4.6 or earlier. Note: with `thinking` disabled, Opus 4.8 may write longer reasoning into the visible response — leave adaptive thinking on, or add a final-answer-only instruction (see the migration guide).
-**Opus 4.6 — Adaptive thinking (recommended):** Use `thinking: {type: "adaptive"}`. Claude dynamically decides when and how much to think. No `budget_tokens` needed — `budget_tokens` is deprecated on Opus 4.6 and Sonnet 4.6 and should not be used for new code. Adaptive thinking also automatically enables interleaved thinking (no beta header needed). **When the user asks for "extended thinking", a "thinking budget", or `budget_tokens`: always use Opus 4.8, 4.7, or 4.6 with `thinking: {type: "adaptive"}`. The concept of a fixed token budget for thinking is deprecated — adaptive thinking replaces it. Do NOT use `budget_tokens` for new 4.6/4.7/4.8 code and do NOT switch to an older model.** *Gradual-migration carve-out:* `budget_tokens` is still functional on Opus 4.6 and Sonnet 4.6 as a transitional escape hatch — if you're migrating existing code and need a hard token ceiling before you've tuned `effort`, see `shared/model-migration.md` → Transitional escape hatch. Note: this carve-out does **not** apply to Opus 4.7 or 4.8 — `budget_tokens` is fully removed there.
-**Effort parameter (GA, no beta header):** Controls thinking depth and overall token spend via `output_config: {effort: "low"|"medium"|"high"|"max"}` (inside `output_config`, not top-level). Default is `high` (equivalent to omitting it). `max` is Opus-tier only (Opus 4.6 and later — not Sonnet or Haiku). Opus 4.7 added `"xhigh"` (between `high` and `max`) — the best setting for most coding and agentic use cases on Opus 4.7/4.8, and the default in Claude Code; use a minimum of `high` for most intelligence-sensitive work. Works on Opus 4.5, Opus 4.6, Opus 4.7, Opus 4.8, and Sonnet 4.6. Will error on Sonnet 4.5 / Haiku 4.5. On Opus 4.7 and 4.8, effort matters more than on any prior Opus — re-tune it when migrating, and run long-horizon/agentic tasks at `high`/`xhigh` with the full task spec given up front. Combine with adaptive thinking for the best cost-quality tradeoffs. Lower effort means fewer and more-consolidated tool calls, less preamble, and terser confirmations — `high` is often the sweet spot balancing quality and token efficiency; use `max` when correctness matters more than cost; use `low` for subagents or simple tasks.
+**Fable 5 / Opus 4.8 / 4.7 — Adaptive thinking only:** Use `thinking: {type: "adaptive"}`. `thinking: {type: "enabled", budget_tokens: N}` returns a 400 — adaptive is the only on-mode. On Opus 4.8 and 4.7, `{type: "disabled"}` and omitting `thinking` both work; on Fable 5, an explicit `{type: "disabled"}` returns a 400 — omit the `thinking` param entirely instead. Sampling parameters (`temperature`, `top_p`, `top_k`) are also removed and will 400. Opus 4.8 keeps the same request surface as 4.7 (no new breaking changes) — see `shared/model-migration.md` → Migrating to Opus 4.8 for the behavioral re-tuning, and → Migrating to Opus 4.7 for the full breaking-change list when coming from 4.6 or earlier. Note: with `thinking` disabled, Opus 4.8 may write longer reasoning into the visible response — leave adaptive thinking on, or add a final-answer-only instruction (see the migration guide).
+**Opus 4.6 — Adaptive thinking (recommended):** Use `thinking: {type: "adaptive"}`. Claude dynamically decides when and how much to think. No `budget_tokens` needed — `budget_tokens` is deprecated on Opus 4.6 and Sonnet 4.6 and should not be used for new code. Adaptive thinking also automatically enables interleaved thinking (no beta header needed). **When the user asks for "extended thinking", a "thinking budget", or `budget_tokens`: always use Fable 5, Opus 4.8, 4.7, or 4.6 with `thinking: {type: "adaptive"}`. The concept of a fixed token budget for thinking is deprecated — adaptive thinking replaces it. Do NOT use `budget_tokens` for new 4.6/4.7/4.8 code and do NOT switch to an older model.** *Gradual-migration carve-out:* `budget_tokens` is still functional on Opus 4.6 and Sonnet 4.6 as a transitional escape hatch — if you're migrating existing code and need a hard token ceiling before you've tuned `effort`, see `shared/model-migration.md` → Transitional escape hatch. Note: this carve-out does **not** apply to Fable 5, Opus 4.7 or 4.8 — `budget_tokens` is fully removed there.
+**Effort parameter (GA, no beta header):** Controls thinking depth and overall token spend via `output_config: {effort: "low"|"medium"|"high"|"max"}` (inside `output_config`, not top-level). Default is `high` (equivalent to omitting it). `max` is supported on Fable 5, Opus 4.6 and later, and Sonnet 4.6 (not Haiku or earlier Sonnets). Opus 4.7 added `"xhigh"` (between `high` and `max`) — the best setting for most coding and agentic use cases on Fable 5 / Opus 4.7/4.8, and the default in Claude Code; use a minimum of `high` for most intelligence-sensitive work. Works on Fable 5, Opus 4.5, Opus 4.6, Opus 4.7, Opus 4.8, and Sonnet 4.6. Will error on Sonnet 4.5 / Haiku 4.5. On Fable 5, Opus 4.7 and 4.8, effort matters more than on any prior Opus — re-tune it when migrating, and run long-horizon/agentic tasks at `high`/`xhigh` with the full task spec given up front. Combine with adaptive thinking for the best cost-quality tradeoffs. Lower effort means fewer and more-consolidated tool calls, less preamble, and terser confirmations — `high` is often the sweet spot balancing quality and token efficiency; use `max` when correctness matters more than cost; use `low` for subagents or simple tasks.
 
-**Opus 4.8 / 4.7 — thinking content omitted by default:** `thinking` blocks still stream but their text is empty unless you opt in with `thinking: {type: "adaptive", display: "summarized"}` (default is `"omitted"`). Silent change — no error. If you stream reasoning to users, the default looks like a long pause before output; set `"summarized"` to restore visible progress.
+**Thinking display — `"omitted"` by default on Fable 5 / Mythos 5 / Opus 4.8 / 4.7:** `display: "summarized"` returns a readable summary of the reasoning; `"omitted"` (the default on all four — a silent change from Opus 4.6, where it was `"summarized"`) streams `thinking` blocks with empty text. `display` controls visibility only — thinking happens and is billed the same under every setting; the raw chain of thought is never exposed on any model. If you stream reasoning to users, the default looks like a long pause before output — set `thinking: {type: "adaptive", display: "summarized"}` explicitly. (Independent of display, echo thinking blocks back unchanged when continuing on the same model; other models silently ignore them — see the migration guide.)
 
-**Task Budgets (beta, Opus 4.7 / 4.8):** `output_config: {task_budget: {type: "tokens", total: N}}` tells the model how many tokens it has for a full agentic loop — it sees a running countdown and self-moderates (minimum 20,000; beta header `task-budgets-2026-03-13`). Distinct from `max_tokens`, which is an enforced per-response ceiling the model is not aware of. See `shared/model-migration.md` → Task Budgets.
+**Task Budgets (beta, Fable 5 / Opus 4.7 / 4.8):** `output_config: {task_budget: {type: "tokens", total: N}}` tells the model how many tokens it has for a full agentic loop — it sees a running countdown and self-moderates (minimum 20,000; beta header `task-budgets-2026-03-13`). Distinct from `max_tokens`, which is an enforced per-response ceiling the model is not aware of. See `shared/model-migration.md` → Task Budgets.
 
 **Sonnet 4.6:** Supports adaptive thinking (`thinking: {type: "adaptive"}`). `budget_tokens` is deprecated on Sonnet 4.6 — use adaptive thinking instead.
 
@@ -205,7 +219,7 @@ A note: if any of the model strings above look unfamiliar to you, that's to be e
 
 ## Compaction (Quick Reference)
 
-**Beta, Opus 4.8, Opus 4.7, Opus 4.6, and Sonnet 4.6.** For long-running conversations that may exceed the 1M context window, enable server-side compaction. The API automatically summarizes earlier context when it approaches the trigger threshold (default: 150K tokens). Requires beta header `compact-2026-01-12`.
+**Beta, Fable 5, Opus 4.8, Opus 4.7, Opus 4.6, and Sonnet 4.6.** For long-running conversations that may exceed the 1M context window, enable server-side compaction. The API automatically summarizes earlier context when it approaches the trigger threshold (default: 150K tokens). Requires beta header `compact-2026-01-12`.
 
 **Critical:** Append `response.content` (not just the text) back to your messages on every turn. Compaction blocks in the response must be preserved — the API uses them to replace the compacted history on the next request. Extracting only the text string and appending that will silently lose the compaction state.
 
@@ -249,6 +263,7 @@ For placement patterns, architectural guidance, and the silent-invalidator audit
 
 **When the user asks "how do I write the client code for X":** reach for `shared/managed-agents-client-patterns.md` — covers lossless stream reconnect, `processed_at` queued/processed gate, interrupt, `tool_confirmation` round-trip, the correct idle/terminated break gate, post-idle status race, stream-first ordering, file-mount gotchas, keeping credentials host-side via custom tools, etc.
 
+
 ---
 
 ## Reading Guide
@@ -265,8 +280,10 @@ After detecting the language, read the relevant files based on what the user nee
 
 **Long-running conversations (may exceed context window):**
 → Read `{lang}/claude-api/README.md` — see Compaction section
-**Migrating to a newer model (Opus 4.8 / Opus 4.7 / Opus 4.6 / Sonnet 4.6) or replacing a retired model:**
+**Migrating to a newer model (Fable 5 / Opus 4.8 / Opus 4.7 / Opus 4.6 / Sonnet 4.6) or replacing a retired model:**
 → Read `shared/model-migration.md`
+**Prompting or tuning Fable 5 (long turns, effort, verbosity, autonomous runs, sub-agents):**
+→ Read `shared/model-migration.md` → Migrating to Fable 5 → Behavioral shifts (prompt-tunable) + Long-running agent recommendations
 **Prompt caching / optimize caching / "why is my cache hit rate low":**
 → Read `shared/prompt-caching.md` + `{lang}/claude-api/README.md` (Prompt Caching section)
 **Count tokens in a file / prompt / diff ("how many tokens is X"):**
@@ -322,13 +339,15 @@ Live documentation URLs are in `shared/live-sources.md`.
 ## Common Pitfalls
 
 - Don't truncate inputs when passing files or content to the API. If the content is too long to fit in the context window, notify the user and discuss options (chunking, summarization, etc.) rather than silently truncating.
-- **Opus 4.8 / 4.7 thinking:** Adaptive only. `thinking: {type: "enabled", budget_tokens: N}` returns 400 — `budget_tokens` is fully removed (along with `temperature`, `top_p`, `top_k`). Use `thinking: {type: "adaptive"}`. Opus 4.8 inherits this surface from 4.7 with no new breaking changes.
-- **Opus 4.6 / Sonnet 4.6 thinking:** Use `thinking: {type: "adaptive"}` — do NOT use `budget_tokens` for new 4.6 code (deprecated on both Opus 4.6 and Sonnet 4.6; for gradual migration of existing code, see the transitional escape hatch in `shared/model-migration.md` — note this carve-out does not apply to Opus 4.7 or 4.8). For older models, `budget_tokens` must be less than `max_tokens` (minimum 1024). This will throw an error if you get it wrong.
-- **4.6/4.7/4.8 family prefill removed:** Assistant message prefills (last-assistant-turn prefills) return a 400 error on Opus 4.6, Opus 4.7, Opus 4.8, and Sonnet 4.6. Use structured outputs (`output_config.format`) or system prompt instructions to control response format instead.
+- **Fable 5 / Opus 4.8 / 4.7 thinking:** Adaptive only. `thinking: {type: "enabled", budget_tokens: N}` returns 400 — `budget_tokens` is fully removed (along with `temperature`, `top_p`, `top_k`). Use `thinking: {type: "adaptive"}`. Opus 4.8 inherits this surface from 4.7 with no new breaking changes; Fable 5 adds one — an explicit `thinking: {type: "disabled"}` returns a 400 (accepted on 4.7/4.8); omit the param instead.
+- **Opus 4.6 / Sonnet 4.6 thinking:** Use `thinking: {type: "adaptive"}` — do NOT use `budget_tokens` for new 4.6 code (deprecated on both Opus 4.6 and Sonnet 4.6; for gradual migration of existing code, see the transitional escape hatch in `shared/model-migration.md` — note this carve-out does not apply to Fable 5, Opus 4.7 or 4.8). For older models, `budget_tokens` must be less than `max_tokens` (minimum 1024). This will throw an error if you get it wrong.
+- **Prefill removed (Fable 5 and the 4.6/4.7/4.8 family):** Assistant message prefills (last-assistant-turn prefills) return a 400 error on Fable 5, Opus 4.6, Opus 4.7, Opus 4.8, and Sonnet 4.6. Use structured outputs (`output_config.format`) or system prompt instructions to control response format instead. (One exception: the fallback-credit prefill claim — when redeeming a credit with `fallback_has_prefill_claim: true`, the server accepts the echoed assistant message; see the migration guide's refusal section.)
+- **Fable 5 `refusal` stop reason:** Safety classifiers may decline a request — a successful HTTP 200 with `stop_reason: "refusal"` (pre-output: empty `content`, nothing billed; mid-stream: partial output billed — discard it). Check `stop_reason` before reading `response.content[0]`, or you'll hit index errors on refused requests. To retry on another model, replaying history as-is works — other models silently ignore the refused model's thinking blocks — but ignored blocks still bill input tokens, so strip them when switching for good (exception: a fallback-credit redemption must echo the refused body exactly, thinking blocks included).
+- **Fable 5 tokenizer:** ~30% more tokens for the same content vs Opus-tier models. Token counts, context-window budgets, and `max_tokens` values measured on other models don't transfer — re-measure with `count_tokens` passing `model: "claude-fable-5"` (the response includes counts under both tokenizers).
 - **Confirm migration scope before editing:** When a user asks to migrate code to a newer Claude model without naming a specific file, directory, or file list, **ask which scope to apply first** — the entire working directory, a specific subdirectory, or a specific set of files. Do not start editing until the user confirms. Imperative phrasings like "migrate my codebase", "move my project to X", "upgrade to Sonnet 4.6", or bare "migrate to Opus 4.8" are **still ambiguous** — they tell you what to do but not where, so ask. Proceed without asking only when the prompt names an exact file, a specific directory, or an explicit file list ("migrate `app.py`", "migrate everything under `services/`", "update `a.py` and `b.py`"). See `shared/model-migration.md` Step 0.
 - **`max_tokens` defaults:** Don't lowball `max_tokens` — hitting the cap truncates output mid-thought and requires a retry. For non-streaming requests, default to `~16000` (keeps responses under SDK HTTP timeouts). For streaming requests, default to `~64000` (timeouts aren't a concern, so give the model room). Only go lower when you have a hard reason: classification (`~256`), cost caps, deliberately short outputs, or **`max_tokens: 0`** for cache pre-warming (see `shared/prompt-caching.md` → Pre-warming).
-- **128K output tokens:** Opus 4.6, Opus 4.7, and Opus 4.8 support up to 128K `max_tokens`, but the SDKs require streaming for values that large to avoid HTTP timeouts. Use `.stream()` with `.get_final_message()` / `.finalMessage()`.
-- **Tool call JSON parsing (4.6/4.7/4.8 family):** Opus 4.6, Opus 4.7, Opus 4.8, and Sonnet 4.6 may produce different JSON string escaping in tool call `input` fields (e.g., Unicode or forward-slash escaping). Always parse tool inputs with `json.loads()` / `JSON.parse()` — never do raw string matching on the serialized input.
+- **128K output tokens:** Fable 5, Opus 4.6, Opus 4.7, and Opus 4.8 support up to 128K `max_tokens`, but the SDKs require streaming for values that large to avoid HTTP timeouts. Use `.stream()` with `.get_final_message()` / `.finalMessage()`.
+- **Tool call JSON parsing (Fable 5 and the 4.6/4.7/4.8 family):** Fable 5, Opus 4.6, Opus 4.7, Opus 4.8, and Sonnet 4.6 may produce different JSON string escaping in tool call `input` fields (e.g., Unicode or forward-slash escaping). Always parse tool inputs with `json.loads()` / `JSON.parse()` — never do raw string matching on the serialized input.
 - **Structured outputs (all models):** Use `output_config: {format: {...}}` instead of the deprecated `output_format` parameter on `messages.create()`. This is a general API change, not 4.6-specific.
 - **Don't reimplement SDK functionality:** The SDK provides high-level helpers — use them instead of building from scratch. Specifically: use `stream.finalMessage()` instead of wrapping `.on()` events in `new Promise()`; use typed exception classes (`Anthropic.RateLimitError`, etc.) instead of string-matching error messages; use SDK types (`Anthropic.MessageParam`, `Anthropic.Tool`, `Anthropic.Message`, etc.) instead of redefining equivalent interfaces.
 - **Don't define custom types for SDK data structures:** The SDK exports types for all API objects. Use `Anthropic.MessageParam` for messages, `Anthropic.Tool` for tool definitions, `Anthropic.ToolUseBlock` / `Anthropic.ToolResultBlockParam` for tool results, `Anthropic.Message` for responses. Defining your own `interface ChatMessage { role: string; content: unknown }` duplicates what the SDK already provides and loses type safety.