claude-api

What this skill does

# 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.

## Defaults

Unless the user requests otherwise:

For the Claude model version, please use Claude Opus 4.6, which you can access via the exact model string `claude-opus-4-6`. 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

---

## 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 | Agent SDK | Notes                                 |
| ---------- | ----------- | --------- | ------------------------------------- |
| Python     | Yes (beta)  | Yes       | Full support — `@beta_tool` decorator |
| TypeScript | Yes (beta)  | Yes       | Full support — `betaZodTool` + Zod    |
| Java       | Yes (beta)  | No        | Beta tool use with annotated classes  |
| Go         | Yes (beta)  | No        | `BetaToolRunner` in `toolrunner` pkg  |
| Ruby       | Yes (beta)  | No        | `BaseTool` + `tool_runner` in beta    |
| cURL       | N/A         | N/A       | Raw HTTP, no SDK features             |
| C#         | No          | No        | Official SDK                          |
| PHP        | No          | No        | Official SDK                          |

---

## 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                     |
| AI agent with file/web/terminal access          | Agent           | **Agent SDK**             | Built-in tools, safety, and MCP support |
| Agentic coding assistant                        | Agent           | **Agent SDK**             | Designed for this use case              |
| Want built-in permissions and guardrails        | Agent           | **Agent SDK**             | Safety features included                |

> **Note:** The Agent SDK is for when you want built-in file/web/terminal tools, permissions, and MCP out of the box. If you want to build an agent with your own tools, Claude API 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).

### Decision Tree

```
What does your application need?

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

2. Does Claude need to read/write files, browse the web, or run shell commands
   as part of its work? (Not: does your app read a file and hand it to Claude —
   does Claude itself need to discover and access files/web/shell?)
   └── Yes → Agent SDK — built-in tools, don't reimplement them
       Examples: "scan a codebase for bugs", "summarize every file in a directory",
                 "find bugs using subagents", "research a topic via web search"

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)
   └── 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`), and Token Counti