diff --git a/src/frontmatter/parse.ts b/src/frontmatter/parse.ts
index b3433e5..24ec611 100644
--- a/src/frontmatter/parse.ts
+++ b/src/frontmatter/parse.ts
@@ -115,6 +115,17 @@ export function parseFrontmatter(markdown: string): TaskInputType {
     throw new InvalidInputError('', 'YAML frontmatter must be a mapping (object), not a scalar or array');
   }
 
+  // Step 3.5: Normalize known snake_case aliases to camelCase.
+  // The YAML format uses snake_case (e.g., `depends_on`) but our schema
+  // expects camelCase (e.g., `dependsOn`). Without this, `Value.Clean()`
+  // would strip the unknown snake_case key and the field would be silently
+  // lost — causing empty dependency lists and broken graphs.
+  const record = parsed as Record<string, unknown>;
+  if ('depends_on' in record && !('dependsOn' in record)) {
+    record.dependsOn = record.depends_on;
+    delete record.depends_on;
+  }
+
   // Step 4: Clean — strip unknown properties from untrusted input
   const cleaned = Value.Clean(TaskInput, parsed);
 
diff --git a/test/frontmatter.test.ts b/test/frontmatter.test.ts
index 6982fd1..f74b93c 100644
--- a/test/frontmatter.test.ts
+++ b/test/frontmatter.test.ts
@@ -484,4 +484,44 @@ risk: invalid-value
     const result = parseFrontmatter(input);
     expect(result.id).toBe('bom-task');
   });
+
+  // ─── Snake_case compatibility ──────────────────────────────────────────
+
+  it('normalizes depends_on to dependsOn', () => {
+    const input = `---
+id: snake-task
+name: Snake Task
+depends_on:
+  - task-a
+  - task-b
+---
+Body`;
+    const result = parseFrontmatter(input);
+    expect(result.dependsOn).toEqual(['task-a', 'task-b']);
+  });
+
+  it('prefers dependsOn when both depends_on and dependsOn are present', () => {
+    const input = `---
+id: both-task
+name: Both Task
+dependsOn:
+  - from-camel
+depends_on:
+  - from-snake
+---
+Body`;
+    const result = parseFrontmatter(input);
+    expect(result.dependsOn).toEqual(['from-camel']);
+  });
+
+  it('normalizes depends_on with empty array', () => {
+    const input = `---
+id: empty-snake
+name: Empty Snake
+depends_on: []
+---
+Body`;
+    const result = parseFrontmatter(input);
+    expect(result.dependsOn).toEqual([]);
+  });
 });
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