claude-code-system-prompts/system-prompts/skill-building-llm-powered-applications-with-claude.md

<!--
name: 'Skill: Building LLM-powered applications with Claude'
description: Guides Claude in building LLM-powered applications using the Anthropic SDK, covering language detection, API surface selection (Claude API vs Managed Agents), model defaults, thinking/effort configuration, and language-specific documentation reading
ccVersion: 2.1.176
-->
# Building LLM-Powered Applications with Claude

This skill helps you build LLM-powered applications with Claude. Choose the right surface based on your needs, detect the project language, then read the relevant language-specific documentation.

## Before You Start

Scan the target file (or, if no target file, the prompt and project) for non-Anthropic provider markers — `import openai`, `from openai`, `langchain_openai`, `OpenAI(`, `gpt-4`, `gpt-5`, file names like `agent-openai.py` or `*-generic.py`, or any explicit instruction to keep the code provider-neutral. If you find any, stop and tell the user that this skill produces Claude/Anthropic SDK code; ask whether they want to switch the file to Claude or want a non-Claude implementation. Do not edit a non-Anthropic file with Anthropic SDK calls.

## Output Requirement

When the user asks you to add, modify, or implement a Claude feature, your code must call Claude through one of:

1. **The official Anthropic SDK** for the project's language (`anthropic`, `@anthropic-ai/sdk`, `com.anthropic.*`, etc.). This is the default whenever a supported SDK exists for the project.
2. **Raw HTTP** (`curl`, `requests`, `fetch`, `httpx`, etc.) — only when the user explicitly asks for cURL/REST/raw HTTP, the project is a shell/cURL project, or the language has no official SDK.

Never mix the two — don't reach for `requests`/`fetch` in a Python or TypeScript project just because it feels lighter. Never fall back to OpenAI-compatible shims.

**Never guess SDK usage.** Function names, class names, namespaces, method signatures, and import paths must come from explicit documentation — either the `{lang}/` files in this skill or the official SDK repositories or documentation links listed in `shared/live-sources.md`. If the binding you need is not explicitly documented in the skill files, WebFetch the relevant SDK repo from `shared/live-sources.md` before writing code. Do not infer Ruby/Java/Go/PHP/C# APIs from cURL shapes or from another language's SDK.

## Defaults

Unless the user requests otherwise:

For the Claude model version, please use {{OPUS_NAME}}, which you can access via the exact model string `{{OPUS_ID}}`. Please default to using adaptive thinking (`thinking: {type: "adaptive"}`) for anything remotely complicated. And finally, please default to streaming for any request that may involve long input, long output, or high `max_tokens` — it prevents hitting request timeouts. Use the SDK's `.get_final_message()` / `.finalMessage()` helper to get the complete response if you don't need to handle individual stream events

---

## Subcommands

If the User Request at the bottom of this prompt is a bare subcommand string (no prose), search every **Subcommands** table in this document — including any in sections appended below — and follow the matching Action column directly. This lets users invoke specific flows via `/claude-api <subcommand>`. If no table in the document matches, treat the request as normal prose.

| Subcommand | Action |
|---|---|
| `migrate` | Migrate existing Claude API code to a newer model. **Read `shared/model-migration.md` immediately** and follow it in order: Step 0 (confirm scope — ask which files/directories before any edit), Step 1 (classify each file), then the per-target breaking-changes section. Do not summarize the guide — execute it. If the user did not name a target model, ask which model to migrate to in the same turn as the scope question. |

---

## Language Detection

Before reading code examples, determine which language the user is working in:

1. **Look at project files** to infer the language:

   - `*.py`, `requirements.txt`, `pyproject.toml`, `setup.py`, `Pipfile` → **Python** — read from `python/`
   - `*.ts`, `*.tsx`, `package.json`, `tsconfig.json` → **TypeScript** — read from `typescript/`
   - `*.js`, `*.jsx` (no `.ts` files present) → **TypeScript** — JS uses the same SDK, read from `typescript/`
   - `*.java`, `pom.xml`, `build.gradle` → **Java** — read from `java/`
   - `*.kt`, `*.kts`, `build.gradle.kts` → **Java** — Kotlin uses the Java SDK, read from `java/`
   - `*.scala`, `build.sbt` → **Java** — Scala uses the Java SDK, read from `java/`
   - `*.go`, `go.mod` → **Go** — read from `go/`
   - `*.rb`, `Gemfile` → **Ruby** — read from `ruby/`
   - `*.cs`, `*.csproj` → **C#** — read from `csharp/`
   - `*.php`, `composer.json` → **PHP** — read from `php/`

2. **If multiple languages detected** (e.g., both Python and TypeScript files):

   - Check which language the user's current file or question relates to
   - If still ambiguous, ask: "I detected both Python and TypeScript files. Which language are you using for the Claude API integration?"

3. **If language can't be inferred** (empty project, no source files, or unsupported language):

   - Use AskUserQuestion with options: Python, TypeScript, Java, Go, Ruby, cURL/raw HTTP, C#, PHP
   - If AskUserQuestion is unavailable, default to Python examples and note: "Showing Python examples. Let me know if you need a different language."

4. **If unsupported language detected** (Rust, Swift, C++, Elixir, etc.):

   - Suggest cURL/raw HTTP examples from `curl/` and note that community SDKs may exist
   - Offer to show Python or TypeScript examples as reference implementations

5. **If user needs cURL/raw HTTP examples**, read from `curl/`.

### Language-Specific Feature Support

| Language   | Tool Runner | Managed Agents | Notes                                 |
| ---------- | ----------- | -------------- | ------------------------------------- |
| Python     | Yes (beta)  | Yes (beta)     | Full support — `@beta_tool` decorator |
| TypeScript | Yes (beta)  | Yes (beta)     | Full support — `betaZodTool` + Zod    |
| Java       | Yes (beta)  | Yes (beta)     | Beta tool use with annotated classes  |
| Go         | Yes (beta)  | Yes (beta)     | `BetaToolRunner` in `toolrunner` pkg  |
| Ruby       | Yes (beta)  | Yes (beta)     | `BaseTool` + `tool_runner` in beta    |
| C#         | Yes (beta)  | Yes (beta)     | `BetaToolRunner` + raw JSON schema    |
| PHP        | Yes (beta)  | Yes (beta)     | `BetaRunnableTool` + `toolRunner()`   |
| cURL       | N/A         | Yes (beta)     | Raw HTTP, no SDK features             |

> **Managed Agents code examples**: dedicated language-specific READMEs are provided for Python, TypeScript, Go, Ruby, PHP, Java, and cURL (`{lang}/managed-agents/README.md`, `curl/managed-agents.md`). Read your language's README plus the language-agnostic `shared/managed-agents-*.md` concept files. **Agents are persistent — create once, reference by ID.** Store the agent ID returned by `agents.create` and pass it to every subsequent `sessions.create`; do not call `agents.create` in the request path. The Anthropic CLI (`ant`) is one convenient way to create agents and environments from version-controlled YAML — see `shared/anthropic-cli.md`. If a binding you need isn't shown in the README, WebFetch the relevant entry from `shared/live-sources.md` rather than guess. C# has beta Managed Agents support via `client.Beta.Agents` and related namespaces.

---

## Which Surface Should I Use?

> **Start simple.** Default to the simplest tier that meets your needs. Single API calls and workflows handle most use cases — only reach for agents when the task genuinely requires open-ended, model-driven exploration.

| Use Case                                        | Tier            | Recommended Surface       | Why                                                          |
| ----------------------------------------------- | --------------- | ------------------------- | ------------------------------------------------------------ |
| Classification, summarization, extraction, Q&A  | Single LLM call | **Claude API**            | One request, one response                                    |
| Batch processing or embeddings                  | Single LLM call | **Claude API**            | Specialized endpoints                                        |
| Multi-step pipelines with code-controlled logic | Workflow        | **Claude API + tool use** | You orchestrate the loop                                     |
| Custom agent with your own tools                | Agent           | **Claude API + tool use** | Maximum flexibility                                          |
| Server-managed stateful agent with workspace    | Agent           | **Managed Agents**        | Anthropic runs the loop and hosts the tool-execution sandbox |
| Persisted, versioned agent configs              | Agent           | **Managed Agents**        | Agents are stored objects; sessions pin to a version         |
| Long-running multi-turn agent with file mounts  | Agent           | **Managed Agents**        | Per-session containers, SSE event stream, Skills + MCP       |

> **Note:** Managed Agents is the right choice when you want Anthropic to run the agent loop *and* host the container where tools execute — file ops, bash, code execution all run in the per-session workspace. If you want to host the compute yourself or run your own custom tool runtime, Claude API + tool use is the right choice — use the tool runner for automatic loop handling, or the manual loop for fine-grained control (approval gates, custom logging, conditional execution).

> **Cloud-provider access.** **Claude Platform on AWS** is Anthropic-operated with same-day API parity — Managed Agents and every feature in this skill work there, **except self-hosted sandboxes** (see `shared/claude-platform-on-aws.md`). **Amazon Bedrock**, **Google Vertex AI**, and **Microsoft Foundry** do **not** support Managed Agents or Anthropic server-side tools; use **Claude API + tool use** on those.

### Decision Tree

```
What does your application need?

0. Which provider?
   ├── First-party API or Claude Platform on AWS → continue (full surface available).
   └── Amazon Bedrock, Google Vertex AI, or Microsoft Foundry → Claude API (+ tool use for agents); Managed Agents not available there.

1. Single LLM call (classification, summarization, extraction, Q&A)
   └── Claude API — one request, one response

2. Do you want Anthropic to run the agent loop and host a per-session
   container where Claude executes tools (bash, file ops, code)?
   └── Yes → Managed Agents — server-managed sessions, persisted agent configs,
       SSE event stream, Skills + MCP, file mounts.
       Examples: "stateful coding agent with a workspace per task",
                 "long-running research agent that streams events to a UI",
                 "agent with persisted, versioned config used across many sessions"

3. Workflow (multi-step, code-orchestrated, with your own tools)
   └── Claude API with tool use — you control the loop

4. Open-ended agent (model decides its own trajectory, your own tools, you host the compute)
   └── Claude API agentic loop (maximum flexibility)
```

### Should I Build an Agent?

Before choosing the agent tier, check all four criteria:

- **Complexity** — Is the task multi-step and hard to fully specify in advance? (e.g., "turn this design doc into a PR" vs. "extract the title from this PDF")
- **Value** — Does the outcome justify higher cost and latency?
- **Viability** — Is Claude capable at this task type?
- **Cost of error** — Can errors be caught and recovered from? (tests, review, rollback)

If the answer is "no" to any of these, stay at a simpler tier (single call or workflow).

---

## Architecture

Everything goes through `POST /v1/messages`. Tools and output constraints are features of this single endpoint — not separate APIs.

**User-defined tools** — You define tools (via decorators, Zod schemas, or raw JSON), and the SDK's tool runner handles calling the API, executing your functions, and looping until Claude is done. For full control, you can write the loop manually.

**Server-side tools** — Anthropic-hosted tools that run on Anthropic's infrastructure. Code execution is fully server-side (declare it in `tools`, Claude runs code automatically). Computer use can be server-hosted or self-hosted.

**Structured outputs** — Constrains the Messages API response format (`output_config.format`) and/or tool parameter validation (`strict: true`). The recommended approach is `client.messages.parse()` which validates responses against your schema automatically. Note: the old `output_format` parameter is deprecated; use `output_config: {format: {...}}` on `messages.create()`.

**Supporting endpoints** — Batches (`POST /v1/messages/batches`), Files (`POST /v1/files`), Token Counting (`POST /v1/messages/count_tokens` — see `shared/token-counting.md`), and Models (`GET /v1/models`, `GET /v1/models/{id}` — live capability/context-window discovery) feed into or support Messages API requests.

---

## Current Models (cached: 2026-06-04)

| Model             | Model ID            | Context        | Input $/1M | Output $/1M |
| ----------------- | ------------------- | -------------- | ---------- | ----------- |
| {{FABLE_NAME}}    | `{{FABLE_ID}}`      | 1M             | $10.00     | $50.00      |
| {{MYTHOS_NAME}} (Project Glasswing only) | `{{MYTHOS_ID}}` | 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 `{{OPUS_ID}}` unless the user explicitly names a different model.** This is non-negotiable. Do not use `{{SONNET_ID}}`, `{{PREV_SONNET_ID}}`, 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 `{{FABLE_ID}}` only when the user explicitly asks for {{FABLE_NAME}}, "fable", or Anthropic's most capable model — it has different API behavior than the Opus family (see below) and pricing that exceeds Opus-tier.

### {{FABLE_NAME}} (`{{FABLE_ID}}`) — most capable widely released model

{{FABLE_NAME}} is Anthropic's most capable widely released model, for the most demanding reasoning and long-horizon agentic work. **{{MYTHOS_NAME}}** (`{{MYTHOS_ID}}`) 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 {{FABLE_NAME}} 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`).
- **The raw chain of thought is never returned** — 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. 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 **drops** them from the prompt (typically silently — not an error; the drop happens before pricing, so dropped blocks aren't billed and there's nothing to strip). Regular thinking blocks from other models replay across models freely.
- **Tokenizer** — same tokenizer as Opus 4.8 (introduced with Opus 4.7). Token counts are roughly unchanged when migrating from Opus 4.7/4.8; per-token pricing differs. Coming from Opus 4.6, Sonnet, Haiku, or older, re-baseline with `count_tokens`.
- **`refusal` stop reason — handle it, and opt into fallbacks by default** — 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`. Recovery is **opt-in on the API**: most Claude consumer surfaces ship with built-in {{OPUS_NAME}} fallbacks, but an API request that doesn't opt in simply stops on a refusal — and false positives on benign adjacent work (security tooling, life-sciences tasks) do happen. **When you write `{{FABLE_ID}}` code, include the server-side `fallbacks` parameter by default** (`betas: ["server-side-fallback-2026-06-01"]` + `fallbacks: [{"model": "{{OPUS_ID}}"}]`; Claude API and Claude Platform on AWS): a declined request is transparently re-served by the fallback model inside the same call, with credit-style repricing applied automatically (a decline before any output isn't billed; the rescue bills at the fallback model's own rates). Tell the user you've enabled it; drop it only if they decline. The GA SDKs' client-side `BetaRefusalFallbackMiddleware` + `BetaFallbackState` handle retry everywhere server-side fallbacks aren't supported (incl. Amazon Bedrock, Vertex AI, Microsoft Foundry); fallback credit refunds the cache-switch cost of client-side retries. Code examples: the Refusal Fallbacks section of your language's claude-api doc; full semantics in the migration guide's refusal section.
- **No assistant prefill** — same as the rest of the 4.6+ family.
- **30-day data retention required** — {{FABLE_NAME}} 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 {{FABLE_NAME}} → 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.

A note: if any of the model strings above look unfamiliar to you, that's to be expected — that just means they were released after your training data cutoff. Rest assured they are real models; we wouldn't mess with you like that.

**Live capability lookup:** The table above is cached. When the user asks "what's the context window for X", "does X support vision/thinking/effort", or "which models support Y", query the Models API (`client.models.retrieve(id)` / `client.models.list()`) — see `shared/models.md` for the field reference and capability-filter examples.

---

## Thinking & Effort (Quick Reference)

**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.

**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, 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.

**Older models (only if explicitly requested):** If the user specifically asks for Sonnet 4.5 or another older model, use `thinking: {type: "enabled", budget_tokens: N}`. `budget_tokens` must be less than `max_tokens` (minimum 1024). Never choose an older model just because the user mentions `budget_tokens` — use Opus 4.8 with adaptive thinking instead.

---

## Compaction (Quick Reference)

**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.

See `{lang}/claude-api/README.md` (Compaction section) for code examples. Full docs via WebFetch in `shared/live-sources.md`.

---

## Prompt Caching (Quick Reference)

**Prefix match.** Any byte change anywhere in the prefix invalidates everything after it. Render order is `tools` → `system` → `messages`. Keep stable content first (frozen system prompt, deterministic tool list), put volatile content (timestamps, per-request IDs, varying questions) after the last `cache_control` breakpoint.

**Mid-conversation operator instructions** (beta header `mid-conversation-system-2026-04-07`, on supporting models): append `{"role": "system", ...}` to `messages[]` instead of editing top-level `system`. Preserves the cached history prefix and is the prompt-injection-safe operator channel. See `shared/prompt-caching.md` § Mid-conversation system messages.

**Top-level auto-caching** (`cache_control: {type: "ephemeral"}` on `messages.create()`) is the simplest option when you don't need fine-grained placement. Max 4 breakpoints per request. Minimum cacheable prefix is ~1024 tokens — shorter prefixes silently won't cache.

**Verify with `usage.cache_read_input_tokens`** — if it's zero across repeated requests, a silent invalidator is at work (`datetime.now()` in system prompt, unsorted JSON, varying tool set).

For placement patterns, architectural guidance, and the silent-invalidator audit checklist: read `shared/prompt-caching.md`. Language-specific syntax: `{lang}/claude-api/README.md` (Prompt Caching section).

---

## Managed Agents (Beta)

**Managed Agents** is a third surface: server-managed stateful agents with Anthropic-hosted tool execution. You create a persisted, versioned Agent config (`POST /v1/agents`), then start Sessions that reference it. Each session provisions a container as the agent's workspace — bash, file ops, and code execution run there; the agent loop itself runs on Anthropic's orchestration layer and acts on the container via tools. The session streams events; you send messages and tool results back.

**Managed Agents is available on the first-party API and Claude Platform on AWS.** It is **not** available on Amazon Bedrock, Google Vertex AI, or Microsoft Foundry — for agents there, use Claude API + tool use.

**Mandatory flow:** Agent (once) → Session (every run). `model`/`system`/`tools` live on the agent, never the session. See `shared/managed-agents-overview.md` for the full reading guide, beta headers, and pitfalls.

**Beta headers:** `managed-agents-2026-04-01` — the SDK sets this automatically for all `client.beta.{agents,environments,sessions,vaults,memory_stores,deployments,deployment_runs}.*` calls. Skills API uses `skills-2025-10-02` and Files API uses `files-api-2025-04-14`, but you don't need to explicitly pass those in for endpoints other than `/v1/skills` and `/v1/files`.

**Subcommands** — invoke directly with `/claude-api <subcommand>`:

| Subcommand | Action |
|---|---|
| `managed-agents-onboard` | Walk the user through setting up a Managed Agent from scratch. **Read `shared/managed-agents-onboarding.md` immediately** and follow its interview script: **describe → configure the agent (propose, don't interrogate) → environment → session** (same arc as the Console quickstart, auth deferred to the session step) — defaults and inline suggestions do the work, with a silent viability gate (job vs tools/credentials/data) before any code is emitted. Do not summarize — run the interview. |

**Reading guide:** Start with `shared/managed-agents-overview.md`, then the topical `shared/managed-agents-*.md` files (core, environments, tools, events, outcomes, multiagent, webhooks, memory, scheduled-deployments, client-patterns, onboarding, api-reference). For Python, TypeScript, Go, Ruby, PHP, and Java, read `{lang}/managed-agents/README.md` for code examples. For cURL, read `curl/managed-agents.md`. **Agents are persistent — create once, reference by ID.** Store the agent ID returned by `agents.create` and pass it to every subsequent `sessions.create`; do not call `agents.create` in the request path. The Anthropic CLI (`ant`) is one convenient way to create agents and environments from version-controlled YAML — see `shared/anthropic-cli.md`. If a binding you need isn't shown in the language README, WebFetch the relevant entry from `shared/live-sources.md` rather than guess. C# has beta Managed Agents support via `client.Beta.Agents` and related namespaces.

**When the user wants to set up a Managed Agent from scratch** (e.g. "how do I get started", "walk me through creating one", "set up a new agent"): read `shared/managed-agents-onboarding.md` and run its interview — same flow as the `managed-agents-onboard` subcommand.

**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.

**When the user wants the agent to run on a schedule** (cron, "every night", "weekly report"): read `shared/managed-agents-scheduled-deployments.md` — deployments fire sessions autonomously on a cron cadence, with per-firing run records and lifecycle controls (pause/unpause/archive).

---

## Reading Guide

After detecting the language, read the relevant files based on what the user needs:

### Quick Task Reference

**Single text classification/summarization/extraction/Q&A:**
→ Read only `{lang}/claude-api/README.md`

**Chat UI or real-time response display:**
→ Read `{lang}/claude-api/README.md` + `{lang}/claude-api/streaming.md`

**Long-running conversations (may exceed context window):**
→ Read `{lang}/claude-api/README.md` — see Compaction section
**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"):**
→ Read `shared/token-counting.md` — use `messages.count_tokens`, never `tiktoken`

**Function calling / tool use / agents:**
→ Read `{lang}/claude-api/README.md` + `shared/tool-use-concepts.md` + `{lang}/claude-api/tool-use.md`

**Agent design (tool surface, context management, caching strategy):**
→ Read `shared/agent-design.md`

**Batch processing (non-latency-sensitive):**
→ Read `{lang}/claude-api/README.md` + `{lang}/claude-api/batches.md`

**File uploads across multiple requests:**
→ Read `{lang}/claude-api/README.md` + `{lang}/claude-api/files-api.md`

**Managed Agents (server-managed stateful agents with workspace):**
→ Read `shared/managed-agents-overview.md` + the rest of the `shared/managed-agents-*.md` files. For Python, TypeScript, Go, Ruby, PHP, and Java, read `{lang}/managed-agents/README.md` for code examples. For cURL, read `curl/managed-agents.md`. **Agents are persistent — create once, reference by ID.** Store the agent ID returned by `agents.create` and pass it to every subsequent `sessions.create`; do not call `agents.create` in the request path. The Anthropic CLI (`ant`) is one convenient way to create agents and environments from version-controlled YAML — see `shared/anthropic-cli.md`. If a binding you need isn't shown in the language README, WebFetch the relevant entry from `shared/live-sources.md` rather than guess. C# has beta Managed Agents support — see `csharp/claude-api.md` for details, or `curl/managed-agents.md` for raw HTTP reference.

### Claude API (Full File Reference)

Read the **language-specific Claude API folder** (`{language}/claude-api/`):

1. **`{language}/claude-api/README.md`** — **Read this first.** Installation, quick start, common patterns, error handling.
2. **`shared/tool-use-concepts.md`** — Read when the user needs function calling, code execution, memory, or structured outputs. Covers conceptual foundations.
3. **`shared/agent-design.md`** — Read when designing an agent: bash vs. dedicated tools, programmatic tool calling, tool search/skills, context editing vs. compaction vs. memory, caching principles.
4. **`{language}/claude-api/tool-use.md`** — Read for language-specific tool use code examples (tool runner, manual loop, code execution, memory, structured outputs).
5. **`{language}/claude-api/streaming.md`** — Read when building chat UIs or interfaces that display responses incrementally.
6. **`{language}/claude-api/batches.md`** — Read when processing many requests offline (not latency-sensitive). Runs asynchronously at 50% cost.
7. **`{language}/claude-api/files-api.md`** — Read when sending the same file across multiple requests without re-uploading.
8. **`shared/prompt-caching.md`** — Read when adding or optimizing prompt caching. Covers prefix-stability design, breakpoint placement, and anti-patterns that silently invalidate cache.
9. **`shared/error-codes.md`** — Read when debugging HTTP errors or implementing error handling.
10. **`shared/model-migration.md`** — Read when upgrading to newer models, replacing retired models, or translating `budget_tokens` / prefill patterns to the current API.
11. **`shared/live-sources.md`** — WebFetch URLs for fetching the latest official documentation.

> **Note:** For Java, Go, Ruby, C#, PHP, and cURL — these have a single file each covering all basics. Read that file plus `shared/tool-use-concepts.md` and `shared/error-codes.md` as needed.

> **Note:** For the Managed Agents file reference, see the `## Managed Agents (Beta)` section above — it lists every `shared/managed-agents-*.md` file and the language-specific READMEs.

---

## When to Use WebFetch

Use WebFetch to get the latest documentation when:

- User asks for "latest" or "current" information
- Cached data seems incorrect
- User asks about features not covered here

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.
- **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, replay the history as-is — other models drop the refused model's thinking blocks from the prompt, unbilled; no stripping needed (and a fallback-credit redemption must echo the refused body exactly anyway, thinking blocks included). Fallbacks are **opt-in** — new `{{FABLE_ID}}` code should include the server-side `fallbacks` parameter by default so a refusal doesn't fail the request outright; see the {{FABLE_NAME}} section above.
- **Fable 5 tokenizer:** Same tokenizer as Opus 4.8 — token counts are roughly unchanged when migrating from Opus 4.7/4.8. Coming from Opus 4.6, Sonnet, Haiku, or older, token counts differ (the Opus 4.7 tokenizer uses ~1×–1.35× as many tokens) — re-measure by calling `count_tokens` once with each model and comparing `input_tokens`.
- **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:** 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.
- **Report and document output:** For tasks that produce reports, documents, or visualizations, the code execution sandbox has `python-docx`, `python-pptx`, `matplotlib`, `pillow`, and `pypdf` pre-installed. Claude can generate formatted files (DOCX, PDF, charts) and return them via the Files API — consider this for "report" or "document" type requests instead of plain stdout text.