Agent SDK Knowledge Base (.NET)

You are building AI agents using the Anthropic C# SDK in .NET. There is no .NET Agent SDK — agents must be built manually atop the Messages API. Read the relevant docs below based on what you're building.

Quick Decision: What Pattern Do I Need?

Need	Pattern	Doc
Simplest agent with auto tool loop	IChatClient + UseFunctionInvocation()	06
Full control over each tool call	Manual Messages API loop	06
Stream responses to user in real-time	Streaming agentic loop	07
Production service with DI	DI-friendly agent service	02, 06
Complex reasoning before acting	Extended thinking	08
Choose between SDK packages	SDK comparison	01
Connect to external tool servers	MCP integration	13
Multi-step orchestration	Architecture patterns	12

Reference Documentation

Always Read First

• ${CLAUDE_PLUGIN_ROOT}/docs/01-sdk-landscape.md — Official vs unofficial SDK, feature comparison, selection guidance

Setup & Foundations

• ${CLAUDE_PLUGIN_ROOT}/docs/02-client-setup-di.md — AnthropicClient, IChatClient, DI registration, IOptions, multi-cloud
• ${CLAUDE_PLUGIN_ROOT}/docs/03-messages-api.md — MessageCreateParams, content blocks, StopReason, token usage

Tool Use

• ${CLAUDE_PLUGIN_ROOT}/docs/04-tool-definitions.md — JSON Schema, input_examples, strict mode, tool_choice, description best practices
• ${CLAUDE_PLUGIN_ROOT}/docs/05-tool-execution.md — tool_use/tool_result handling, IChatClient auto-invocation, manual dispatch

Agent Patterns

• ${CLAUDE_PLUGIN_ROOT}/docs/06-agentic-loops.md — 4 loop patterns (auto, manual, streaming, DI service), max-iteration guards
• ${CLAUDE_PLUGIN_ROOT}/docs/12-architecture-patterns.md — Anthropic's 7 patterns with .NET implementation guidance

Streaming & Thinking

• ${CLAUDE_PLUGIN_ROOT}/docs/07-streaming.md — CreateStreaming, MessageContentAggregator, delta processing
• ${CLAUDE_PLUGIN_ROOT}/docs/08-extended-thinking.md — ThinkingParameters, budget tokens, streaming thinking deltas

Reliability & Context

• ${CLAUDE_PLUGIN_ROOT}/docs/09-system-prompts.md — Agent prompt engineering, role definition, chain-of-thought, auto-generated tool prompts
• ${CLAUDE_PLUGIN_ROOT}/docs/10-error-handling-retries.md — Exception hierarchy, built-in retries, Polly, graceful degradation
• ${CLAUDE_PLUGIN_ROOT}/docs/11-context-management.md — Token counting, sliding window, summarization compaction

Advanced

• ${CLAUDE_PLUGIN_ROOT}/docs/13-advanced-features.md — MCP integration, programmatic tool calling, Bedrock/Foundry, Semantic Kernel

Critical Rules (Common Mistakes)

1. There is no .NET Agent SDK — Don't look for one. Build agentic loops manually using the Messages API or use UseFunctionInvocation() with IChatClient.
2. Always add max-iteration guards — Every agentic loop must have a maximum iteration count (e.g., 10-25). Infinite loops burn tokens and money.
3. Always check StopReason — end_turn means done, tool_use means execute tools and continue. Ignoring this breaks the loop.
4. Return ALL tool_results together — When Claude makes parallel tool calls, execute all tools and return all results in a single user message. Never send partial results.
5. Prefer UseFunctionInvocation() — For most agents, IChatClient with UseFunctionInvocation() handles the tool loop automatically. Only use manual loops when you need per-iteration control.
6. Write 3-4 sentence tool descriptions — This is the single most important factor in tool performance. Include what it does, when to use it, and limitations.
7. No string interpolation in system prompts — Prevents prompt injection. Use parameterized tool inputs instead.
8. Dispose streaming resources — IAsyncEnumerable streams must be properly consumed or disposed. Use await foreach or ConfigureAwait(false).
9. Extended thinking needs streaming — In the official SDK, extended thinking is only available via CreateStreaming, not the synchronous Create method.
10. Register AnthropicClient and IChatClient as Singleton — Both are thread-safe. Scoped/transient registration wastes resources.
11. Never put API keys in code — Use environment variables (ANTHROPIC_API_KEY), user secrets, or a vault.
12. Capture tool errors as tool_result — When a tool throws, catch the exception and return it as a tool_result with is_error: true. Don't let tool failures crash the loop.