Property●Since v1.1

systemPrompt

An optional system message for the model.

Use a string for simple, static system prompts. This is the most common use case and works well with template literals for dynamic content. When a string is provided, it's converted to a single text block internally.

Use a SystemMessage when you need advanced features that require structured content:

Anthropic cache control: Use SystemMessage with array content to enable per-block cache control settings (e.g., cache_control: { type: "ephemeral" }). This allows you to have different cache settings for different parts of your system prompt.
Multiple content blocks: When you need multiple text blocks with different metadata or formatting requirements.
Integration with existing code: When working with code that already produces SystemMessage instances.

systemPrompt: string | SystemMessage

Example 1

const agent = createAgent({
  model: "anthropic:claude-sonnet-4-5",
  systemPrompt: "You are a helpful assistant.",
  // ...
});

Example 2

const userRole = "premium";
const agent = createAgent({
  model: "anthropic:claude-sonnet-4-5",
  systemPrompt: `You are a helpful assistant for ${userRole} users.`,
  // ...
});

Example 3

import { SystemMessage } from "@langchain/core/messages";

const agent = createAgent({
  model: "anthropic:claude-sonnet-4-5",
  systemPrompt: new SystemMessage({
    content: [
      {
        type: "text",
        text: "You are a helpful assistant.",
      },
      {
        type: "text",
        text: "Today's date is 2024-06-01.",
        cache_control: { type: "ephemeral" },
      },
    ],
  }),
  // ...
});

Example 4

import { SystemMessage } from "@langchain/core/messages";

const agent = createAgent({
  model: "anthropic:claude-sonnet-4-5",
  systemPrompt: new SystemMessage("You are a helpful assistant."),
  // ...
});

View source on GitHub

systemPrompt

Example 1

Example 2

Example 3

Example 4

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systemPrompt

Example 1

Example 2

Example 3

Example 4