diff --git a/src/frontmatter/index.ts b/src/frontmatter/index.ts
index 02f0be1..a893198 100644
--- a/src/frontmatter/index.ts
+++ b/src/frontmatter/index.ts
@@ -1,4 +1,4 @@
 // Frontmatter submodule — parse and serialize
 
-export { parseFrontmatter, parseTaskFile, parseTaskDirectory } from './parse.js';
+export { splitFrontmatter, parseFrontmatter, parseTaskFile, parseTaskDirectory } from './parse.js';
 export { serializeFrontmatter } from './serialize.js';
\ No newline at end of file
diff --git a/src/frontmatter/parse.ts b/src/frontmatter/parse.ts
index 4033e56..20fa93c 100644
--- a/src/frontmatter/parse.ts
+++ b/src/frontmatter/parse.ts
@@ -1,5 +1,78 @@
 // YAML/frontmatter parsing + typebox validation
 
+/**
+ * Split a markdown string with `---`-delimited YAML frontmatter into its
+ * data and content parts.
+ *
+ * Rules (per architecture spec):
+ * 1. Opening `---` must appear at the very start of the string (optional BOM
+ *    or leading whitespace on line 1 is stripped first, but the opening
+ *    delimiter must still be the first non-whitespace content).
+ * 2. Only exactly three dashes (`---`) qualify as a delimiter — four or more
+ *    (`----`) are NOT delimiters.
+ * 3. The closing delimiter must be `\n---` (i.e. a newline followed by three
+ *    dashes). A closing `---` without a preceding newline is not valid.
+ * 4. Content body begins after the closing `---` plus its trailing newline.
+ *
+ * @returns `{ data, content }` when valid frontmatter is found, or `null`
+ *          when the input has no valid frontmatter block.
+ */
+export function splitFrontmatter(
+  markdown: string,
+): { data: string; content: string } | null {
+  // Strip optional UTF-8 BOM
+  const input = markdown.replace(/^\uFEFF/, '');
+
+  // Opening delimiter: must be `---` at start of string (after BOM removal),
+  // optionally preceded by whitespace on the first line. The `---` must be
+  // exactly 3 dashes — 4+ dashes are NOT a valid opening.
+  const openingMatch = /^[ \t]*(---)(?!\-)/.exec(input);
+  if (!openingMatch) return null;
+
+  // The opening delimiter ends at the end of its line (consume the newline)
+  const afterOpening = input.indexOf('\n', openingMatch.index + openingMatch[0].length);
+  if (afterOpening === -1) return null;
+
+  // Search for closing delimiter: `\n---` with exactly 3 dashes (not 4+).
+  // The data section starts right after the opening delimiter's newline.
+  const dataStart = afterOpening + 1;
+
+  // Helper: check if `pos` starts a valid closing `---` line (exactly 3 dashes,
+  // not 4+), and if so, return the parsed result.
+  const tryClosing = (closingPos: number, dataEnd: number): { data: string; content: string } | null => {
+    if (
+      input[closingPos] === '-' &&
+      input[closingPos + 1] === '-' &&
+      input[closingPos + 2] === '-' &&
+      input[closingPos + 3] !== '-'
+    ) {
+      const data = input.slice(dataStart, dataEnd);
+      // Content starts after the closing `---` plus its trailing newline.
+      // If there's no trailing newline, content is empty.
+      const afterClosing = input.indexOf('\n', closingPos + 3);
+      const content = afterClosing === -1 ? '' : input.slice(afterClosing + 1);
+      return { data, content };
+    }
+    return null;
+  };
+
+  // Check immediately after opening newline (handles `---\n---` with empty data)
+  const immediate = tryClosing(dataStart, dataStart);
+  if (immediate !== null) return immediate;
+
+  // Scan for newlines and check each as a potential closing delimiter
+  for (let i = dataStart; i < input.length; i++) {
+    if (input[i] !== '\n') continue;
+
+    const lineStart = i + 1;
+    const result = tryClosing(lineStart, i);
+    if (result !== null) return result;
+  }
+
+  // No valid closing delimiter found
+  return null;
+}
+
 export function parseFrontmatter(_input: string): unknown {
   // Stub — implementation pending
   return {};
diff --git a/tasks/implementation/frontmatter/splitter.md b/tasks/implementation/frontmatter/splitter.md
index 6232cd5..fb72d4a 100644
--- a/tasks/implementation/frontmatter/splitter.md
+++ b/tasks/implementation/frontmatter/splitter.md
@@ -1,7 +1,7 @@
 ---
 id: frontmatter/splitter
 name: Implement frontmatter delimiter splitter (~40 lines)
-status: pending
+status: completed
 depends_on:
   - setup/project-init
 scope: single
@@ -22,15 +22,15 @@ Per [frontmatter.md](../../../docs/architecture/frontmatter.md), the splitter:
 
 ## Acceptance Criteria
 
-- [ ] `splitFrontmatter(markdown: string): { data: string; content: string } | null`
-- [ ] Opening `---` must be at the start of the file (or after optional BOM/whitespace on first line)
-- [ ] `----` (4+ dashes) is NOT a valid delimiter — only exact `---`
-- [ ] Closing delimiter requires `\n---` (newline before dashes)
-- [ ] Returns `null` if no valid frontmatter found
-- [ ] Returns `{ data: "", content: "" }` if frontmatter is present but empty (e.g., `---\n---`)
-- [ ] Content body starts after the closing `---` + newline
-- [ ] Handles edge cases: no closing delimiter (returns null), file with only `---\n---`, file with no `---` at all
-- [ ] Unit tests: standard frontmatter, no frontmatter, empty frontmatter, multi-line content, dashes in content body (shouldn't be treated as delimiters), 4+ dashes ignored
+- [x] `splitFrontmatter(markdown: string): { data: string; content: string } | null`
+- [x] Opening `---` must be at the start of the file (or after optional BOM/whitespace on first line)
+- [x] `----` (4+ dashes) is NOT a valid delimiter — only exact `---`
+- [x] Closing delimiter requires `\n---` (newline before dashes)
+- [x] Returns `null` if no valid frontmatter found
+- [x] Returns `{ data: "", content: "" }` if frontmatter is present but empty (e.g., `---\n---`)
+- [x] Content body starts after the closing `---` + newline
+- [x] Handles edge cases: no closing delimiter (returns null), file with only `---\n---`, file with no `---` at all
+- [x] Unit tests: standard frontmatter, no frontmatter, empty frontmatter, multi-line content, dashes in content body (shouldn't be treated as delimiters), 4+ dashes ignored
 
 ## References
 
@@ -38,8 +38,13 @@ Per [frontmatter.md](../../../docs/architecture/frontmatter.md), the splitter:
 
 ## Notes
 
-> To be filled by implementation agent
+Self-contained `splitFrontmatter` function implemented with no external dependencies. Uses regex for opening delimiter match and manual scan for closing delimiter to enforce exact 3-dash rule. Handles BOM stripping and empty frontmatter.
 
 ## Summary
 
-> To be filled on completion
\ No newline at end of file
+Implemented the `splitFrontmatter` function in `src/frontmatter/parse.ts` per architecture spec.
+- Modified: `src/frontmatter/parse.ts` (added `splitFrontmatter` function, ~65 lines including JSDoc)
+- Modified: `src/frontmatter/index.ts` (exported `splitFrontmatter`)
+- Modified: `test/frontmatter.test.ts` (18 comprehensive tests)
+- Tests: 18 splitFrontmatter tests + 4 existing placeholder tests, all passing (22 total)
+- TypeScript check: passing
\ No newline at end of file
diff --git a/test/frontmatter.test.ts b/test/frontmatter.test.ts
index 4f560f7..d86580d 100644
--- a/test/frontmatter.test.ts
+++ b/test/frontmatter.test.ts
@@ -1,7 +1,180 @@
 import { describe, it, expect } from 'vitest';
+import { splitFrontmatter } from '../src/frontmatter/parse.js';
 
-describe('Frontmatter', () => {
-  it('placeholder — parse and serialize', () => {
-    expect(true).toBe(true);
+describe('splitFrontmatter', () => {
+  // ─── Standard frontmatter ────────────────────────────────────────────
+
+  it('extracts YAML data and markdown content from standard frontmatter', () => {
+    const input = `---
+title: Hello
+status: pending
+---
+# Heading
+
+Some content here.
+`;
+
+    const result = splitFrontmatter(input);
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('title: Hello\nstatus: pending');
+    expect(result!.content).toBe('# Heading\n\nSome content here.\n');
+  });
+
+  it('handles multi-line YAML data', () => {
+    const input = `---
+title: My Task
+depends_on:
+  - task-a
+  - task-b
+---
+Content here`;
+
+    const result = splitFrontmatter(input);
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('title: My Task\ndepends_on:\n  - task-a\n  - task-b');
+    expect(result!.content).toBe('Content here');
+  });
+
+  // ─── Empty frontmatter ───────────────────────────────────────────────
+
+  it('returns empty data and content for "---\\n---"', () => {
+    const result = splitFrontmatter('---\n---');
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('');
+    expect(result!.content).toBe('');
+  });
+
+  it('returns empty data with trailing content after empty frontmatter', () => {
+    const result = splitFrontmatter('---\n---\nSome content');
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('');
+    expect(result!.content).toBe('Some content');
+  });
+
+  // ─── No frontmatter ──────────────────────────────────────────────────
+
+  it('returns null when there is no frontmatter at all', () => {
+    const result = splitFrontmatter('Hello world\nNo frontmatter here');
+    expect(result).toBeNull();
+  });
+
+  it('returns null when file starts with text (no opening ---)', () => {
+    const result = splitFrontmatter('Some text\n---\nMore text');
+    expect(result).toBeNull();
+  });
+
+  it('returns null when opening --- exists but no closing delimiter', () => {
+    const result = splitFrontmatter('---\ntitle: Hello\nNo closing delimiter');
+    expect(result).toBeNull();
+  });
+
+  // ─── 4+ dashes are NOT delimiters ────────────────────────────────────
+
+  it('returns null when opening delimiter is 4+ dashes (----)', () => {
+    const result = splitFrontmatter('----\ntitle: Hello\n----\nContent');
+    expect(result).toBeNull();
+  });
+
+  it('does not treat ---- as a closing delimiter', () => {
+    const input = `---
+title: Hello
+----
+Content after four dashes`;
+    const result = splitFrontmatter(input);
+    // No valid closing delimiter found (---- doesn't count)
+    expect(result).toBeNull();
+  });
+
+  it('does not treat 5 dashes as a closing delimiter', () => {
+    const input = `---
+title: Hello
+-----
+Content`;
+    const result = splitFrontmatter(input);
+    expect(result).toBeNull();
+  });
+
+  // ─── Dashes in content body (shouldn't be treated as delimiters) ─────
+
+  it('ignores --- in the content body after valid frontmatter', () => {
+    const input = `---
+title: Hello
+---
+Some text
+
+---
+More text
+`;
+
+    const result = splitFrontmatter(input);
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('title: Hello');
+    expect(result!.content).toBe('Some text\n\n---\nMore text\n');
+  });
+
+  it('handles horizontal rule (---) in content', () => {
+    const input = `---
+title: Hello
+---
+Paragraph above
+
+---
+
+Paragraph below
+`;
+
+    const result = splitFrontmatter(input);
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('title: Hello');
+    expect(result!.content).toBe('Paragraph above\n\n---\n\nParagraph below\n');
+  });
+
+  // ─── BOM handling ─────────────────────────────────────────────────────
+
+  it('handles UTF-8 BOM at start of file', () => {
+    const input = '\uFEFF---\ntitle: Hello\n---\nContent';
+    const result = splitFrontmatter(input);
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('title: Hello');
+    expect(result!.content).toBe('Content');
+  });
+
+  // ─── Edge cases ──────────────────────────────────────────────────────
+
+  it('file with only "---\\n---"', () => {
+    const result = splitFrontmatter('---\n---');
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('');
+    expect(result!.content).toBe('');
+  });
+
+  it('content body starts after closing ---\\n', () => {
+    const input = `---
+key: value
+---
+Body starts here`;
+    const result = splitFrontmatter(input);
+    expect(result).not.toBeNull();
+    expect(result!.content).toBe('Body starts here');
+  });
+
+  it('content is empty string when closing --- is at end of file with no trailing newline', () => {
+    const result = splitFrontmatter('---\nkey: value\n---');
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('key: value');
+    expect(result!.content).toBe('');
+  });
+
+  it('handles leading whitespace on first line before ---', () => {
+    const input = '  ---\ntitle: Hello\n---\nContent';
+    const result = splitFrontmatter(input);
+    expect(result).not.toBeNull();
+    expect(result!.data).toBe('title: Hello');
+    expect(result!.content).toBe('Content');
+  });
+
+  it('returns null if only opening --- with no newline', () => {
+    const result = splitFrontmatter('---');
+    expect(result).toBeNull();
   });
 });
\ No newline at end of file