claude-code/src/utils/__tests__/systemPrompt.test.ts

import { describe, expect, test } from "bun:test";
import { buildEffectiveSystemPrompt } from "../systemPrompt";

const defaultPrompt = ["You are a helpful assistant.", "Follow instructions."];

function buildPrompt(overrides: Record<string, unknown> = {}) {
  return buildEffectiveSystemPrompt({
    mainThreadAgentDefinition: undefined,
    toolUseContext: { options: {} as any },
    customSystemPrompt: undefined,
    defaultSystemPrompt: defaultPrompt,
    appendSystemPrompt: undefined,
    ...overrides,
  });
}

describe("buildEffectiveSystemPrompt", () => {
  test("returns default system prompt when no overrides", () => {
    const result = buildPrompt();
    expect(Array.from(result)).toEqual(defaultPrompt);
  });

  test("overrideSystemPrompt replaces everything", () => {
    const result = buildPrompt({ overrideSystemPrompt: "override" });
    expect(Array.from(result)).toEqual(["override"]);
  });

  test("customSystemPrompt replaces default", () => {
    const result = buildPrompt({ customSystemPrompt: "custom" });
    expect(Array.from(result)).toEqual(["custom"]);
  });

  test("appendSystemPrompt is appended after main prompt", () => {
    const result = buildPrompt({ appendSystemPrompt: "appended" });
    expect(Array.from(result)).toEqual([...defaultPrompt, "appended"]);
  });

  test("agent definition replaces default prompt", () => {
    const agentDef = {
      getSystemPrompt: () => "agent prompt",
      agentType: "custom",
    } as any;
    const result = buildPrompt({ mainThreadAgentDefinition: agentDef });
    expect(Array.from(result)).toEqual(["agent prompt"]);
  });

  test("agent definition with append combines both", () => {
    const agentDef = {
      getSystemPrompt: () => "agent prompt",
      agentType: "custom",
    } as any;
    const result = buildPrompt({
      mainThreadAgentDefinition: agentDef,
      appendSystemPrompt: "extra",
    });
    expect(Array.from(result)).toEqual(["agent prompt", "extra"]);
  });

  test("override takes precedence over agent and custom", () => {
    const agentDef = {
      getSystemPrompt: () => "agent prompt",
      agentType: "custom",
    } as any;
    const result = buildPrompt({
      mainThreadAgentDefinition: agentDef,
      customSystemPrompt: "custom",
      appendSystemPrompt: "extra",
      overrideSystemPrompt: "override",
    });
    expect(Array.from(result)).toEqual(["override"]);
  });

  test("returns array of strings", () => {
    const result = buildPrompt();
    expect(Array.isArray(result)).toBe(true);
    for (const item of result) {
      expect(typeof item).toBe("string");
    }
  });

  test("custom + append combines both", () => {
    const result = buildPrompt({
      customSystemPrompt: "custom",
      appendSystemPrompt: "extra",
    });
    expect(Array.from(result)).toEqual(["custom", "extra"]);
  });
});