diff --git a/src/frontmatter/file-io.ts b/src/frontmatter/file-io.ts
new file mode 100644
index 0000000..483c632
--- /dev/null
+++ b/src/frontmatter/file-io.ts
@@ -0,0 +1,77 @@
+// File I/O functions for frontmatter parsing.
+//
+// ⚠️ These functions depend on `node:fs/promises` and are only available in
+// Node.js-compatible runtimes. Consumers targeting Deno, Bun, or browsers
+// should use `parseFrontmatter` directly with their own file-reading mechanism.
+
+import { readFile, readdir } from 'node:fs/promises';
+import { join, extname } from 'node:path';
+import { parseFrontmatter } from './parse.js';
+import type { TaskInput as TaskInputType } from '../schema/task.js';
+
+/**
+ * Read a markdown file and parse its YAML frontmatter into a validated
+ * `TaskInput` object.
+ *
+ * Delegates to `parseFrontmatter` for parsing and validation.
+ * Throws the underlying Node.js error for I/O failures (ENOENT, EACCES, etc.).
+ *
+ * @param filePath — Absolute or relative path to a `.md` file
+ * @returns Validated `TaskInput` object
+ * @throws Node.js system errors for I/O failures
+ * @throws {InvalidInputError} When frontmatter is missing or invalid
+ *
+ * @nodeOnly Uses `node:fs/promises.readFile`
+ */
+export async function parseTaskFile(filePath: string): Promise<TaskInputType> {
+  const content = await readFile(filePath, 'utf-8');
+  return parseFrontmatter(content);
+}
+
+/**
+ * Recursively scan a directory for `.md` files and parse each one that
+ * contains valid `---`-delimited YAML frontmatter.
+ *
+ * - Files without valid frontmatter are silently skipped (no error thrown,
+ *   just omitted from the results).
+ * - Non-`.md` files are ignored.
+ * - Throws the underlying Node.js error for I/O failures (ENOENT, EACCES).
+ * - Uses `parseTaskFile` per file.
+ *
+ * @param dirPath — Absolute or relative path to a directory
+ * @returns Array of validated `TaskInput` objects (one per valid file)
+ * @throws Node.js system errors for I/O failures
+ *
+ * @nodeOnly Uses `node:fs/promises.readdir` and `node:fs/promises.stat`
+ */
+export async function parseTaskDirectory(dirPath: string): Promise<TaskInputType[]> {
+  const results: TaskInputType[] = [];
+  const entries = await readdir(dirPath, { withFileTypes: true });
+
+  for (const entry of entries) {
+    const fullPath = join(dirPath, entry.name);
+
+    if (entry.isDirectory()) {
+      // Recurse into subdirectories
+      const subResults = await parseTaskDirectory(fullPath);
+      results.push(...subResults);
+    } else if (entry.isFile() && extname(entry.name).toLowerCase() === '.md') {
+      // Only process .md files; skip others silently
+      try {
+        const task = await parseTaskFile(fullPath);
+        results.push(task);
+      } catch (err) {
+        // Silently skip files without valid frontmatter (InvalidInputError
+        // from parseFrontmatter means no valid --- delimiters found, or
+        // the YAML/schema is invalid). Rethrow I/O errors.
+        if (err instanceof Error && err.name === 'InvalidInputError') {
+          continue;
+        }
+        throw err;
+      }
+    }
+    // Non-.md files and other entry types are silently skipped
+  }
+
+  return results;
+}
\ No newline at end of file
diff --git a/src/frontmatter/index.ts b/src/frontmatter/index.ts
index a893198..9bd11df 100644
--- a/src/frontmatter/index.ts
+++ b/src/frontmatter/index.ts
@@ -1,4 +1,5 @@
-// Frontmatter submodule — parse and serialize
+// Frontmatter submodule — parse, serialize, and file I/O
 
-export { splitFrontmatter, parseFrontmatter, parseTaskFile, parseTaskDirectory } from './parse.js';
-export { serializeFrontmatter } from './serialize.js';
\ No newline at end of file
+export { splitFrontmatter, parseFrontmatter } from './parse.js';
+export { serializeFrontmatter } from './serialize.js';
+export { parseTaskFile, parseTaskDirectory } from './file-io.js';
\ No newline at end of file
diff --git a/src/frontmatter/parse.ts b/src/frontmatter/parse.ts
index d3e4b64..b3433e5 100644
--- a/src/frontmatter/parse.ts
+++ b/src/frontmatter/parse.ts
@@ -134,12 +134,3 @@ export function parseFrontmatter(markdown: string): TaskInputType {
   return cleaned as TaskInputType;
 }
 
-export function parseTaskFile(_input: string): unknown {
-  // Stub — implementation pending
-  return {};
-}
-
-export function parseTaskDirectory(_dir: string): unknown[] {
-  // Stub — implementation pending
-  return [];
-}
\ No newline at end of file
diff --git a/src/frontmatter/serialize.ts b/src/frontmatter/serialize.ts
index 1e96a63..535b883 100644
--- a/src/frontmatter/serialize.ts
+++ b/src/frontmatter/serialize.ts
@@ -1,6 +1,36 @@
-// TaskInput → markdown with frontmatter
+// TaskInput → markdown with YAML frontmatter
 
-export function serializeFrontmatter(_input: unknown): string {
-  // Stub — implementation pending
-  return '';
+import { stringify as yamlStringify } from 'yaml';
+import type { TaskInput as TaskInputType } from '../schema/task.js';
+
+/**
+ * Serialize a `TaskInput` object into a `---`-delimited markdown string.
+ *
+ * - Uses `yaml.stringify()` for the data portion (YAML 1.2 output).
+ * - Includes all `TaskInput` fields in the YAML (per schema convention).
+ * - Nullable fields set to `null` produce explicit `null` in YAML.
+ * - Absent (undefined) fields are omitted from the YAML output.
+ * - Appends optional body content (default: empty string) after the
+ *   closing `---`.
+ *
+ * @param task — The `TaskInput` object to serialize
+ * @param body — Optional markdown body content after the frontmatter
+ * @returns A complete markdown string with frontmatter and optional body
+ */
+export function serializeFrontmatter(task: TaskInputType, body?: string): string {
+  // Build a clean object: include all defined fields, omit undefined ones.
+  // This ensures `yaml.stringify()` only writes keys that are present,
+  // keeping the output compact and matching what `parseFrontmatter` expects.
+  const data: Record<string, unknown> = {};
+
+  for (const [key, value] of Object.entries(task)) {
+    if (value !== undefined) {
+      data[key] = value;
+    }
+  }
+
+  const yaml = yamlStringify(data, { lineWidth: 0 });
+  const content = body ?? '';
+
+  return `---\n${yaml}---\n${content}`;
 }
\ No newline at end of file
diff --git a/tasks/implementation/frontmatter/file-io-and-serialize.md b/tasks/implementation/frontmatter/file-io-and-serialize.md
index 4a94370..418573d 100644
--- a/tasks/implementation/frontmatter/file-io-and-serialize.md
+++ b/tasks/implementation/frontmatter/file-io-and-serialize.md
@@ -1,7 +1,7 @@
 ---
 id: frontmatter/file-io-and-serialize
 name: Implement parseTaskFile, parseTaskDirectory, and serializeFrontmatter
-status: pending
+status: completed
 depends_on:
   - frontmatter/parsing
   - schema/input-schemas
@@ -22,23 +22,23 @@ Per [frontmatter.md](../../../docs/architecture/frontmatter.md):
 
 ## Acceptance Criteria
 
-- [ ] `parseTaskFile(filePath: string): Promise<TaskInput>`:
+- [x] `parseTaskFile(filePath: string): Promise<TaskInput>`:
   - Reads file using `node:fs/promises.readFile`
   - Delegates to `parseFrontmatter` for parsing and validation
   - Throws underlying Node.js error for I/O failures (ENOENT, EACCES, etc.)
-- [ ] `parseTaskDirectory(dirPath: string): Promise<TaskInput[]>`:
-  - Recursive directory scanning via `node:fs/promises.readdir` with `{ recursive: true }` or manual recursion
+- [x] `parseTaskDirectory(dirPath: string): Promise<TaskInput[]>`:
+  - Recursive directory scanning via `node:fs/promises.readdir` with `{ withFileTypes: true }` + manual recursion
   - Filters for `.md` files only
   - Silently skips files without valid `---`-delimited frontmatter (no error thrown, just omitted from results)
   - Throws underlying Node.js error for I/O failures
   - Uses `parseTaskFile` per file
-- [ ] `serializeFrontmatter(task: TaskInput, body?: string): string`:
+- [x] `serializeFrontmatter(task: TaskInput, body?: string): string`:
   - Constructs `---`-delimited markdown output
-  - Uses `yaml.stringify()` for the `TaskInput` data (excludes `id` from frontmatter? No — per Rust CLI convention, `id` comes from the filename, but in the schema it's part of `TaskInput`. Follow schema: include all `TaskInput` fields in the YAML.)
+  - Uses `yaml.stringify()` for the `TaskInput` data (includes all `TaskInput` fields per schema)
   - Appends body content (default: empty string) after closing `---`
   - Handles nullable fields correctly: `risk: null` → `risk: null` in YAML (explicit null), absent fields → omitted from YAML
-- [ ] File I/O functions documented as Node.js-only in JSDoc comments
-- [ ] Unit tests: parseTaskFile with temp file, parseTaskDirectory with temp dir (including non-.md files, missing frontmatter files), serializeFrontmatter round-trip parseFrontmatter(serializeFrontmatter(task)) ≈ task
+- [x] File I/O functions documented as Node.js-only in JSDoc comments
+- [x] Unit tests: parseTaskFile with temp file, parseTaskDirectory with temp dir (including non-.md files, missing frontmatter files), serializeFrontmatter round-trip parseFrontmatter(serializeFrontmatter(task)) ≈ task
 
 ## References
 
@@ -47,8 +47,14 @@ Per [frontmatter.md](../../../docs/architecture/frontmatter.md):
 
 ## Notes
 
-> To be filled by implementation agent
+All acceptance criteria verified by unit tests and TypeScript type-checker. File I/O functions moved to separate `file-io.ts` module to keep `parse.ts` runtime-agnostic. Stubs removed from `parse.ts`.
 
 ## Summary
 
-> To be filled on completion
\ No newline at end of file
+Implemented `parseTaskFile`, `parseTaskDirectory`, and `serializeFrontmatter`.
+- Created: `src/frontmatter/file-io.ts` (parseTaskFile, parseTaskDirectory — Node.js-only file I/O)
+- Modified: `src/frontmatter/serialize.ts` (replaced stub with full serializeFrontmatter implementation)
+- Modified: `src/frontmatter/parse.ts` (removed parseTaskFile/parseTaskDirectory stubs)
+- Modified: `src/frontmatter/index.ts` (updated exports — file-io functions from file-io.js)
+- Created: `test/frontmatter-fileio-serialize.test.ts` (29 tests)
+- Tests: 29 new + 257 existing = 286 total, all passing; tsc --noEmit clean
\ No newline at end of file
diff --git a/test/frontmatter-fileio-serialize.test.ts b/test/frontmatter-fileio-serialize.test.ts
new file mode 100644
index 0000000..1a4428d
--- /dev/null
+++ b/test/frontmatter-fileio-serialize.test.ts
@@ -0,0 +1,515 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { mkdtemp, mkdir, writeFile, rm } from 'node:fs/promises';
+import { join } from 'node:path';
+import { tmpdir } from 'node:os';
+import { parseTaskFile, parseTaskDirectory } from '../src/frontmatter/file-io.js';
+import { serializeFrontmatter } from '../src/frontmatter/serialize.js';
+import { parseFrontmatter } from '../src/frontmatter/parse.js';
+import type { TaskInput } from '../src/schema/task.js';
+import { InvalidInputError } from '../src/error/index.js';
+
+describe('parseTaskFile', () => {
+  let tempDir: string;
+
+  beforeEach(async () => {
+    tempDir = await mkdtemp(join(tmpdir(), 'taskgraph-test-'));
+  });
+
+  afterEach(async () => {
+    await rm(tempDir, { recursive: true, force: true });
+  });
+
+  it('reads and parses a valid markdown file with frontmatter', async () => {
+    const filePath = join(tempDir, 'task.md');
+    const content = `---
+id: file-task
+name: File Task
+dependsOn: []
+---
+Some body content`;
+    await writeFile(filePath, content, 'utf-8');
+
+    const result = await parseTaskFile(filePath);
+    expect(result.id).toBe('file-task');
+    expect(result.name).toBe('File Task');
+    expect(result.dependsOn).toEqual([]);
+  });
+
+  it('reads and parses a file with all optional fields', async () => {
+    const filePath = join(tempDir, 'full-task.md');
+    const content = `---
+id: full-file-task
+name: Full File Task
+dependsOn:
+  - task-a
+status: in-progress
+scope: narrow
+risk: high
+impact: component
+level: implementation
+priority: critical
+tags:
+  - urgent
+assignee: alice
+due: "2026-06-01"
+---
+Body here`;
+    await writeFile(filePath, content, 'utf-8');
+
+    const result = await parseTaskFile(filePath);
+    expect(result.id).toBe('full-file-task');
+    expect(result.status).toBe('in-progress');
+    expect(result.risk).toBe('high');
+    expect(result.tags).toEqual(['urgent']);
+  });
+
+  it('throws ENOENT error for non-existent file', async () => {
+    const filePath = join(tempDir, 'does-not-exist.md');
+    await expect(parseTaskFile(filePath)).rejects.toThrow();
+    try {
+      await parseTaskFile(filePath);
+      expect.unreachable('Should have thrown');
+    } catch (err: any) {
+      expect(err.code).toBe('ENOENT');
+    }
+  });
+
+  it('throws InvalidInputError for file with no valid frontmatter', async () => {
+    const filePath = join(tempDir, 'no-frontmatter.md');
+    await writeFile(filePath, 'Just some markdown content\nNo frontmatter', 'utf-8');
+
+    await expect(parseTaskFile(filePath)).rejects.toThrow(InvalidInputError);
+  });
+
+  it('throws InvalidInputError for file with invalid YAML frontmatter', async () => {
+    const filePath = join(tempDir, 'invalid-yaml.md');
+    await writeFile(filePath, `---
+id: bad yaml: [unclosed
+name: Broken
+dependsOn: []
+---`, 'utf-8');
+
+    await expect(parseTaskFile(filePath)).rejects.toThrow(InvalidInputError);
+  });
+
+  it('handles UTF-8 BOM in file', async () => {
+    const filePath = join(tempDir, 'bom-task.md');
+    const content = '\uFEFF---\nid: bom-task\nname: BOM Task\ndependsOn: []\n---\nBody';
+    await writeFile(filePath, content, 'utf-8');
+
+    const result = await parseTaskFile(filePath);
+    expect(result.id).toBe('bom-task');
+  });
+});
+
+describe('parseTaskDirectory', () => {
+  let tempDir: string;
+
+  beforeEach(async () => {
+    tempDir = await mkdtemp(join(tmpdir(), 'taskgraph-dir-test-'));
+  });
+
+  afterEach(async () => {
+    await rm(tempDir, { recursive: true, force: true });
+  });
+
+  it('parses all .md files with valid frontmatter in a directory', async () => {
+    await writeFile(join(tempDir, 'task-a.md'), `---
+id: task-a
+name: Task A
+dependsOn: []
+---`);
+    await writeFile(join(tempDir, 'task-b.md'), `---
+id: task-b
+name: Task B
+dependsOn:
+  - task-a
+---`);
+
+    const results = await parseTaskDirectory(tempDir);
+    expect(results).toHaveLength(2);
+    const ids = results.map(r => r.id).sort();
+    expect(ids).toEqual(['task-a', 'task-b']);
+  });
+
+  it('silently skips non-.md files', async () => {
+    await writeFile(join(tempDir, 'task.md'), `---
+id: task-1
+name: Task 1
+dependsOn: []
+---`);
+    await writeFile(join(tempDir, 'notes.txt'), 'Some notes');
+    await writeFile(join(tempDir, 'config.json'), '{"key": "value"}');
+    await writeFile(join(tempDir, 'script.js'), 'console.log("hi")');
+
+    const results = await parseTaskDirectory(tempDir);
+    expect(results).toHaveLength(1);
+    expect(results[0]!.id).toBe('task-1');
+  });
+
+  it('silently skips .md files without valid frontmatter', async () => {
+    await writeFile(join(tempDir, 'valid.md'), `---
+id: valid-task
+name: Valid
+dependsOn: []
+---`);
+    await writeFile(join(tempDir, 'no-frontmatter.md'), 'Just some text\nNo frontmatter here');
+    await writeFile(join(tempDir, 'invalid-schema.md'), `---
+id: broken
+name: Missing dependsOn
+---`);
+
+    const results = await parseTaskDirectory(tempDir);
+    expect(results).toHaveLength(1);
+    expect(results[0]!.id).toBe('valid-task');
+  });
+
+  it('recursively scans subdirectories', async () => {
+    const subDir = join(tempDir, 'subdir');
+    const deepDir = join(subDir, 'deep');
+    await mkdir(subDir, { recursive: true });
+    await mkdir(deepDir, { recursive: true });
+
+    await writeFile(join(tempDir, 'root.md'), `---
+id: root-task
+name: Root
+dependsOn: []
+---`);
+    await writeFile(join(subDir, 'sub.md'), `---
+id: sub-task
+name: Sub
+dependsOn: []
+---`);
+    await writeFile(join(deepDir, 'deep.md'), `---
+id: deep-task
+name: Deep
+dependsOn: []
+---`);
+
+    const results = await parseTaskDirectory(tempDir);
+    expect(results).toHaveLength(3);
+    const ids = results.map(r => r.id).sort();
+    expect(ids).toEqual(['deep-task', 'root-task', 'sub-task']);
+  });
+
+  it('handles empty directory', async () => {
+    const emptyDir = join(tempDir, 'empty');
+    await mkdir(emptyDir);
+
+    const results = await parseTaskDirectory(emptyDir);
+    expect(results).toEqual([]);
+  });
+
+  it('throws ENOENT for non-existent directory', async () => {
+    const badPath = join(tempDir, 'no-such-dir');
+    await expect(parseTaskDirectory(badPath)).rejects.toThrow();
+    try {
+      await parseTaskDirectory(badPath);
+      expect.unreachable('Should have thrown');
+    } catch (err: any) {
+      expect(err.code).toBe('ENOENT');
+    }
+  });
+
+  it('handles mixed valid, invalid, and non-.md files in subdirectories', async () => {
+    const subDir = join(tempDir, 'mixed');
+    await mkdir(subDir);
+
+    await writeFile(join(tempDir, 'root-valid.md'), `---
+id: root
+name: Root Task
+dependsOn: []
+---`);
+    await writeFile(join(subDir, 'sub-valid.md'), `---
+id: sub
+name: Sub Task
+dependsOn: []
+---`);
+    await writeFile(join(subDir, 'no-fm.md'), 'No frontmatter');
+    await writeFile(join(subDir, 'data.csv'), 'a,b,c');
+
+    const results = await parseTaskDirectory(tempDir);
+    expect(results).toHaveLength(2);
+    const ids = results.map(r => r.id).sort();
+    expect(ids).toEqual(['root', 'sub']);
+  });
+
+  it('skips invalid YAML files (not valid frontmatter) silently', async () => {
+    await writeFile(join(tempDir, 'valid.md'), `---
+id: good
+name: Good
+dependsOn: []
+---`);
+    await writeFile(join(tempDir, 'bad-risk.md'), `---
+id: bad-risk
+name: Bad Risk
+dependsOn: []
+risk: extreme-invalid-value
+---`);
+
+    const results = await parseTaskDirectory(tempDir);
+    expect(results).toHaveLength(1);
+    expect(results[0]!.id).toBe('good');
+  });
+});
+
+describe('serializeFrontmatter', () => {
+  it('serializes a minimal TaskInput with required fields only', () => {
+    const task: TaskInput = {
+      id: 'minimal',
+      name: 'Minimal Task',
+      dependsOn: [],
+    };
+
+    const result = serializeFrontmatter(task);
+    expect(result).toContain('---');
+    expect(result).toContain('id: minimal');
+    expect(result).toContain('name: Minimal Task');
+  });
+
+  it('serializes a TaskInput with all fields populated', () => {
+    const task: TaskInput = {
+      id: 'full',
+      name: 'Full Task',
+      dependsOn: ['task-a', 'task-b'],
+      status: 'in-progress',
+      scope: 'narrow',
+      risk: 'high',
+      impact: 'component',
+      level: 'implementation',
+      priority: 'critical',
+      tags: ['urgent', 'backend'],
+      assignee: 'alice',
+      due: '2026-06-01',
+      created: '2026-04-01',
+      modified: '2026-04-20',
+    };
+
+    const result = serializeFrontmatter(task);
+    expect(result).toContain('id: full');
+    expect(result).toContain('status: in-progress');
+    expect(result).toContain('risk: high');
+    expect(result).toContain('assignee: alice');
+  });
+
+  it('omits undefined fields from YAML output', () => {
+    const task: TaskInput = {
+      id: 'partial',
+      name: 'Partial Task',
+      dependsOn: [],
+      // status, scope, risk, etc. are undefined
+    };
+
+    const result = serializeFrontmatter(task);
+    // The YAML should NOT contain keys for undefined fields
+    expect(result).not.toContain('status:');
+    expect(result).not.toContain('risk:');
+    expect(result).not.toContain('scope:');
+    expect(result).not.toContain('impact:');
+    expect(result).not.toContain('tags:');
+  });
+
+  it('serializes explicit null values as null in YAML', () => {
+    const task: TaskInput = {
+      id: 'null-task',
+      name: 'Null Task',
+      dependsOn: [],
+      risk: null,
+      scope: null,
+      status: null,
+    };
+
+    const result = serializeFrontmatter(task);
+    // YAML null representation (empty value or explicit null)
+    expect(result).toContain('risk:');
+    expect(result).toContain('scope:');
+    expect(result).toContain('status:');
+  });
+
+  it('starts with opening --- delimiter', () => {
+    const task: TaskInput = {
+      id: 'test',
+      name: 'Test',
+      dependsOn: [],
+    };
+
+    const result = serializeFrontmatter(task);
+    expect(result.startsWith('---\n')).toBe(true);
+  });
+
+  it('includes closing --- delimiter before body', () => {
+    const task: TaskInput = {
+      id: 'test',
+      name: 'Test',
+      dependsOn: [],
+    };
+
+    const result = serializeFrontmatter(task);
+    // The closing --- must appear after the YAML content
+    const lines = result.split('\n');
+    // Find the second --- (closing delimiter)
+    const delimiterCount = lines.filter(l => l === '---').length;
+    expect(delimiterCount).toBeGreaterThanOrEqual(2);
+  });
+
+  it('appends body content after closing delimiter', () => {
+    const task: TaskInput = {
+      id: 'body-test',
+      name: 'Body Test',
+      dependsOn: [],
+    };
+
+    const result = serializeFrontmatter(task, '# My Heading\n\nSome content.');
+    expect(result).toContain('# My Heading');
+    expect(result).toContain('Some content.');
+  });
+
+  it('default body is empty string', () => {
+    const task: TaskInput = {
+      id: 'no-body',
+      name: 'No Body',
+      dependsOn: [],
+    };
+
+    const result = serializeFrontmatter(task);
+    // After the closing --- there should be just a trailing newline
+    const closingIndex = result.lastIndexOf('---');
+    const afterClosing = result.slice(closingIndex + 3).trim();
+    expect(afterClosing).toBe('');
+  });
+
+  // ─── Round-trip tests: parseFrontmatter(serializeFrontmatter(task)) ≈ task ──
+
+  describe('round-trip: parseFrontmatter(serializeFrontmatter(task))', () => {
+    it('round-trips a minimal TaskInput', () => {
+      const original: TaskInput = {
+        id: 'rt-minimal',
+        name: 'Round Trip Minimal',
+        dependsOn: [],
+      };
+
+      const serialized = serializeFrontmatter(original);
+      const parsed = parseFrontmatter(serialized);
+
+      expect(parsed.id).toBe(original.id);
+      expect(parsed.name).toBe(original.name);
+      expect(parsed.dependsOn).toEqual(original.dependsOn);
+    });
+
+    it('round-trips a TaskInput with all fields', () => {
+      const original: TaskInput = {
+        id: 'rt-full',
+        name: 'Round Trip Full',
+        dependsOn: ['dep-a', 'dep-b'],
+        status: 'in-progress',
+        scope: 'narrow',
+        risk: 'high',
+        impact: 'component',
+        level: 'implementation',
+        priority: 'critical',
+        tags: ['urgent'],
+        assignee: 'bob',
+        due: '2026-06-01',
+        created: '2026-04-01',
+        modified: '2026-04-20',
+      };
+
+      const serialized = serializeFrontmatter(original);
+      const parsed = parseFrontmatter(serialized);
+
+      expect(parsed.id).toBe(original.id);
+      expect(parsed.name).toBe(original.name);
+      expect(parsed.dependsOn).toEqual(original.dependsOn);
+      expect(parsed.status).toBe(original.status);
+      expect(parsed.scope).toBe(original.scope);
+      expect(parsed.risk).toBe(original.risk);
+      expect(parsed.impact).toBe(original.impact);
+      expect(parsed.level).toBe(original.level);
+      expect(parsed.priority).toBe(original.priority);
+      expect(parsed.tags).toEqual(original.tags);
+      expect(parsed.assignee).toBe(original.assignee);
+      expect(parsed.due).toBe(original.due);
+      expect(parsed.created).toBe(original.created);
+      expect(parsed.modified).toBe(original.modified);
+    });
+
+    it('round-trips nullable fields set to null', () => {
+      const original: TaskInput = {
+        id: 'rt-null',
+        name: 'Round Trip Null',
+        dependsOn: [],
+        risk: null,
+        status: null,
+        assignee: null,
+      };
+
+      const serialized = serializeFrontmatter(original);
+      const parsed = parseFrontmatter(serialized);
+
+      expect(parsed.id).toBe(original.id);
+      expect(parsed.risk).toBeNull();
+      expect(parsed.status).toBeNull();
+      expect(parsed.assignee).toBeNull();
+      // Fields that were undefined in original should be undefined after round-trip
+      expect(parsed.scope).toBeUndefined();
+      expect(parsed.impact).toBeUndefined();
+    });
+
+    it('round-trips with body content preserved separately', () => {
+      const task: TaskInput = {
+        id: 'rt-body',
+        name: 'Round Trip Body',
+        dependsOn: [],
+      };
+      const body = '# Implementation Notes\n\nSome details here.';
+
+      const serialized = serializeFrontmatter(task, body);
+      // The serialized string should contain the body
+      expect(serialized).toContain('# Implementation Notes');
+
+      // Parse only the frontmatter portion (parseFrontmatter ignores body)
+      const parsed = parseFrontmatter(serialized);
+      expect(parsed.id).toBe('rt-body');
+    });
+
+    it('round-trips with empty dependsOn array', () => {
+      const original: TaskInput = {
+        id: 'rt-empty-deps',
+        name: 'Empty Deps',
+        dependsOn: [],
+      };
+
+      const serialized = serializeFrontmatter(original);
+      const parsed = parseFrontmatter(serialized);
+
+      expect(parsed.dependsOn).toEqual([]);
+    });
+
+    it('round-trips with populated dependsOn array', () => {
+      const original: TaskInput = {
+        id: 'rt-deps',
+        name: 'With Deps',
+        dependsOn: ['alpha', 'beta', 'gamma'],
+      };
+
+      const serialized = serializeFrontmatter(original);
+      const parsed = parseFrontmatter(serialized);
+
+      expect(parsed.dependsOn).toEqual(['alpha', 'beta', 'gamma']);
+    });
+
+    it('round-trips with tags array', () => {
+      const original: TaskInput = {
+        id: 'rt-tags',
+        name: 'With Tags',
+        dependsOn: [],
+        tags: ['urgent', 'backend', 'security'],
+      };
+
+      const serialized = serializeFrontmatter(original);
+      const parsed = parseFrontmatter(serialized);
+
+      expect(parsed.tags).toEqual(['urgent', 'backend', 'security']);
+    });
+  });
+});
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