Advanced Configuration

The BuiltInAgent accepts a full set of configuration options to control model behavior, tool calling, and more.

Multi-step tool calling#

By default, the agent performs a single generation step. Set maxSteps to allow the agent to call tools and then continue reasoning:

src/copilotkit.ts

const agent = new BuiltInAgent({
  model: "openai:gpt-5.4-mini",
  maxSteps: 5, 
  tools: [searchDocs, createTicket],
});

With maxSteps: 5, the agent can call a tool, process the result, call another tool, and so on — up to 5 iterations. This is essential for workflows where the agent needs to chain multiple tool calls.

Tool choice#

Control how the agent selects tools:

const agent = new BuiltInAgent({
  model: "openai:gpt-5.4-mini",
  toolChoice: "auto",       // Let the model decide (default)
  // toolChoice: "required", // Force the model to call a tool
  // toolChoice: "none",     // Disable tool calling
  // toolChoice: { type: "tool", toolName: "searchDocs" }, // Force a specific tool
});

System prompt#

Customize the agent's system prompt:

const agent = new BuiltInAgent({
  model: "openai:gpt-5.4-mini",
  prompt: "You are a customer support agent for Acme Corp. Be concise and helpful. Always check the knowledge base before answering.", 
});

Generation parameters#

Fine-tune the model's output:

const agent = new BuiltInAgent({
  model: "openai:gpt-5.4-mini",
  temperature: 0.7,        // Creativity (0 = deterministic, 1+ = creative)
  topP: 0.9,               // Nucleus sampling
  topK: 40,                // Top-K sampling (provider-dependent)
  maxOutputTokens: 4096,   // Maximum tokens in the response
  presencePenalty: 0.1,    // Penalize repeated topics
  frequencyPenalty: 0.1,   // Penalize repeated tokens
  stopSequences: ["END"],  // Stop generation at these sequences
  seed: 42,                // Deterministic output (provider-dependent)
  maxRetries: 3,           // Retry on transient failures
});

Not all parameters are supported by every provider. For example, topK is supported by Google but not OpenAI. Unsupported parameters are ignored.

Provider-specific options#

Pass options specific to a model provider using providerOptions:

// OpenAI reasoning models (o3, o4-mini) with reasoning effort
const agent = new BuiltInAgent({
  model: "openai:o3",
  providerOptions: { 
    openai: { reasoningEffort: "high" },
  },
});

// Anthropic with extended thinking
const agent = new BuiltInAgent({
  model: "anthropic:claude-sonnet-4-5",
  providerOptions: { 
    anthropic: { thinking: { type: "enabled", budgetTokens: 10000 } },
  },
});

Overridable properties#

Allow the frontend to override specific configuration at runtime. This is useful when you want users to switch models or adjust behavior without redeploying:

const agent = new BuiltInAgent({
  model: "openai:gpt-5.4-mini",
  temperature: 0.5,
  overridableProperties: ["model", "temperature", "prompt"], 
});

The full list of overridable properties: model, toolChoice, maxOutputTokens, temperature, topP, topK, presencePenalty, frequencyPenalty, stopSequences, seed, maxRetries, prompt, providerOptions

Message forwarding#

Control whether system and developer messages from the conversation are forwarded to the LLM:

const agent = new BuiltInAgent({
  model: "openai:gpt-5.4-mini",
  forwardSystemMessages: true,    // Forward system-role messages
  forwardDeveloperMessages: true, // Forward developer-role messages (as system messages)
});

Full configuration reference#

Property	Type	Default	Description
`model`	`string \| LanguageModel`	—	Model specifier or AI SDK instance
`apiKey`	`string`	env var	API key for the provider
`maxSteps`	`number`	`1`	Max tool-calling iterations
`toolChoice`	`"auto" \| "required" \| "none" \| { type: "tool", toolName: string }`	`"auto"`	How tools are selected
`maxOutputTokens`	`number`	—	Max tokens in response
`temperature`	`number`	—	Sampling temperature
`topP`	`number`	—	Nucleus sampling
`topK`	`number`	—	Top-K sampling
`presencePenalty`	`number`	—	Presence penalty
`frequencyPenalty`	`number`	—	Frequency penalty
`stopSequences`	`string[]`	—	Stop sequences
`seed`	`number`	—	Random seed
`maxRetries`	`number`	—	Retry count
`prompt`	`string`	—	System prompt
`tools`	`ToolDefinition[]`	`[]`	Server-side tools
`mcpServers`	`MCPClientConfig[]`	`[]`	MCP server connections (agent-managed, per-request)
`mcpClients`	`MCPClientProvider[]`	`[]`	User-managed MCP clients (persistent, user controls lifecycle)
`overridableProperties`	`string[]`	`[]`	Properties the frontend can override
`providerOptions`	`Record<string, any>`	—	Provider-specific options
`forwardSystemMessages`	`boolean`	`false`	Forward system messages
`forwardDeveloperMessages`	`boolean`	`false`	Forward developer messages

Need even more control?

If BuiltInAgent's configuration options don't cover your needs — for example, if you want to use TanStack AI, a custom LLM backend, or async initialization — see Custom Agent.