diff --git a/docs/specs/issue-4-bundle-required-utils.md b/docs/specs/issue-4-bundle-required-utils.md
new file mode 100644
index 0000000..80edf41
--- /dev/null
+++ b/docs/specs/issue-4-bundle-required-utils.md
@@ -0,0 +1,102 @@
+---
+issue_number: 4
+issue_title: "[#1] feat: bundle required utils (stringifyError, jsonOutput, ToolOutput)"
+repo: "beogip/code-first-agents-tool"
+labels: [enhancement]
+plan_level: "lean"
+depth: "medium"
+branch_name: "feat/4-bundle-required-utils"
+created_at: "2026-05-23T12:00:00Z"
+---
+
+# Implementation Plan: #4 — [#1] feat: bundle required utils (stringifyError, jsonOutput, ToolOutput)
+
+## Files
+
+| # | Action | Path | Purpose |
+|---|--------|------|---------|
+| 1 | modify | `src/index.ts` | Re-export `jsonOutput` + `stringifyError` from `./utils.ts` |
+| 2 | create | `tests/utils.test.ts` | Dedicated unit tests for the 3 utils exports |
+| 3 | verify | `src/utils.ts` | Already exists on epic branch; verify contents match ACs |
+
+## Codebase Context
+
+- `src/utils.ts` already exists on `feat/1-implement-code-first-agentstool-package` with `ToolOutput`, `jsonOutput`, `stringifyError` — created as part of PR #9 (issue #3)
+- `tool-class.ts` imports `{ jsonOutput, type ToolOutput }` from `"./utils.ts"`
+- `envelopes.ts` imports `{ stringifyError }` from `"./utils.ts"`
+- `biome.json` has `noConsole: "warn"` — `jsonOutput` already carries a `biome-ignore` comment
+- `tsconfig.json` has `verbatimModuleSyntax: true` — type-only imports must use `import type`
+- Build command: `bun build src/index.ts --outdir dist --target bun`
+- Package has zero kael.factory references in any `src/` file
+
+## Steps
+
+1. **Create branch from epic branch**
+   Create `feat/4-bundle-required-utils` from `feat/1-implement-code-first-agentstool-package` HEAD (4d80253).
+   **Done when:** `git log --oneline -1` shows commit `4d80253` as the starting point.
+
+2. **Verify `src/utils.ts` exports match acceptance criteria**
+   Confirm file exports `ToolOutput` type, `jsonOutput` function, and `stringifyError` function.
+   **Done when:** All 3 exports are present and match the kael.factory originals (minus unused helpers).
+
+3. **Update `src/index.ts` to re-export `jsonOutput` and `stringifyError`**
+   Add `export { jsonOutput, stringifyError } from "./utils.ts"` alongside the existing `export type { ToolOutput }` line.
+   **Done when:** `index.ts` exports all 3 utils items and `bunx tsc --noEmit` passes.
+
+4. **Create `tests/utils.test.ts` with unit tests**
+   Test `stringifyError` with Error, string, null, undefined, number, and plain object inputs.
+   Test `jsonOutput` by mocking `process.exit` and capturing `console.log` to verify JSON serialization.
+   **Done when:** `bun test` passes with all new tests green.
+
+## Interfaces
+
+- `ToolOutput`: `{ ok: boolean; message: string; [key: string]: unknown }` — already defined in `src/utils.ts`, no changes needed.
+
+## Function Design
+
+- `src/utils.ts`: `jsonOutput(data: ToolOutput): never` — serialize ToolOutput envelope to stdout as JSON and terminate with exit code 0
+- `src/utils.ts`: `stringifyError(err: unknown): string` — coerce an unknown caught value to a human-readable string, preserving Error.message when available
+
+## Acceptance Criteria (EARS)
+
+- **AC-1.** `src/utils.ts` shall export `stringifyError` function, `jsonOutput` function, and `ToolOutput` type. [from issue]
+- **AC-2.** When `tool-class.ts` and `envelopes.ts` reference utils, they shall import from `./utils.ts` (local path, not kael.factory). [from issue]
+- **AC-3.** The package shall have zero runtime dependency on kael.factory. [from issue]
+
+## Out of Scope
+
+- Adding `jsonError`, `jsonEscape`, `fail`, or other utils from kael.factory not mentioned in the issue
+- Modifying `tool-class.ts` or `envelopes.ts` import paths (already correct on epic branch)
+
+## Edge Cases + Error Handling
+
+| # | Scenario | Source | Handling |
+|---|----------|--------|----------|
+| 1 | `stringifyError` receives `null` | [inferred] | `String(null)` returns `"null"` |
+| 2 | `stringifyError` receives `undefined` | [inferred] | `String(undefined)` returns `"undefined"` |
+| 3 | `stringifyError` receives non-Error object | [inferred] | `String(obj)` returns `"[object Object]"` |
+| 4 | `jsonOutput` calls `process.exit(0)` | [inferred] | Tests must mock `process.exit` to avoid killing the test runner |
+
+## Done Criteria per Feature
+
+| Feature | Done when |
+|---------|-----------|
+| Utils exported from `src/utils.ts` | AC-1 — file exports all 3 items, type-check passes |
+| Local imports in tool modules | AC-2 — grep confirms `tool-class.ts` and `envelopes.ts` import from `./utils.ts` |
+| Zero kael.factory dependency | AC-3 — no kael.factory entry in `package.json`, no kael.factory imports in `src/` |
+
+## Risks
+
+| Risk | Mitigation |
+|------|------------|
+| Worktree is on initial commit, not epic branch | Must create/checkout branch from epic branch HEAD before starting work |
+| PR #9 (issue #3) may have introduced `src/utils.ts` with different API than expected | Verified: exact match with issue #4 ACs — `ToolOutput`, `jsonOutput`, `stringifyError` |
+
+## Test Strategy
+
+- **Unit tests** in `tests/utils.test.ts`:
+  - `stringifyError`: test with `Error`, `string`, `null`, `undefined`, `number`, plain object
+  - `jsonOutput`: mock `process.exit`, spy on `console.log`, verify JSON envelope output
+- **Type check**: `bunx tsc --noEmit` (all files compile cleanly)
+- **Lint**: `bunx biome check .` (no warnings except expected `biome-ignore` for `console.log`)
+- **Full suite**: `bun test` (existing tests from issue #3 + new utils tests must pass)
diff --git a/src/index.ts b/src/index.ts
index e271cf6..7597c13 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -27,3 +27,4 @@ export type {
   ToolMeta,
 } from "./types.ts";
 export type { ToolOutput } from "./utils.ts";
+export { jsonOutput, stringifyError } from "./utils.ts";
diff --git a/tests/utils.test.ts b/tests/utils.test.ts
new file mode 100644
index 0000000..97cd18f
--- /dev/null
+++ b/tests/utils.test.ts
@@ -0,0 +1,85 @@
+import { afterEach, beforeEach, describe, expect, it, mock } from "bun:test";
+import { jsonOutput, stringifyError, type ToolOutput } from "../src/index.ts";
+
+describe("stringifyError", () => {
+  it("extracts message from Error instance", () => {
+    expect(stringifyError(new Error("boom"))).toBe("boom");
+  });
+
+  it("returns string values as-is", () => {
+    expect(stringifyError("raw string")).toBe("raw string");
+  });
+
+  it("coerces null to 'null'", () => {
+    expect(stringifyError(null)).toBe("null");
+  });
+
+  it("coerces undefined to 'undefined'", () => {
+    expect(stringifyError(undefined)).toBe("undefined");
+  });
+
+  it("coerces number to string", () => {
+    expect(stringifyError(42)).toBe("42");
+  });
+
+  it("coerces plain object to '[object Object]'", () => {
+    expect(stringifyError({ key: "val" })).toBe("[object Object]");
+  });
+
+  it("uses custom toString when available", () => {
+    const obj = { toString: () => "custom" };
+    expect(stringifyError(obj)).toBe("custom");
+  });
+});
+
+describe("jsonOutput", () => {
+  const originalExit = process.exit;
+  // biome-ignore lint/suspicious/noConsole: mocking console.log to capture jsonOutput
+  const originalLog = console.log;
+
+  let captured: string;
+  let exitCode: number | undefined;
+
+  beforeEach(() => {
+    captured = "";
+    exitCode = undefined;
+    console.log = mock((...args: unknown[]) => {
+      captured = String(args[0]);
+    });
+    process.exit = mock((code?: number) => {
+      exitCode = code;
+      throw new Error("__exit__");
+    }) as never;
+  });
+
+  afterEach(() => {
+    process.exit = originalExit;
+    console.log = originalLog;
+  });
+
+  it("serializes ToolOutput as JSON to stdout and exits with 0", () => {
+    const data: ToolOutput = { ok: true, message: "done" };
+
+    expect(() => jsonOutput(data)).toThrow("__exit__");
+    expect(exitCode).toBe(0);
+    expect(captured).not.toBe("");
+    expect(JSON.parse(captured)).toEqual({ ok: true, message: "done" });
+  });
+
+  it("includes extra fields in the envelope", () => {
+    const data: ToolOutput = {
+      ok: false,
+      message: "fail",
+      error: "bad_input",
+      detail: "missing field",
+    };
+
+    expect(() => jsonOutput(data)).toThrow("__exit__");
+    expect(exitCode).toBe(0);
+    expect(captured).not.toBe("");
+    const parsed = JSON.parse(captured);
+    expect(parsed.ok).toBe(false);
+    expect(parsed.error).toBe("bad_input");
+    expect(parsed.detail).toBe("missing field");
+  });
+});