diff --git a/src/core/validation/validator.ts b/src/core/validation/validator.ts
index 37b43a37d..47071ed47 100644
--- a/src/core/validation/validator.ts
+++ b/src/core/validation/validator.ts
@@ -165,7 +165,7 @@ export class Validator {
           if (!requirementText) {
             issues.push({ level: 'ERROR', path: entryPath, message: `ADDED "${block.name}" is missing requirement text` });
           } else if (!this.containsShallOrMust(requirementText)) {
-            issues.push({ level: 'ERROR', path: entryPath, message: `ADDED "${block.name}" must contain SHALL or MUST` });
+            issues.push({ level: 'ERROR', path: entryPath, message: this.buildMissingShallOrMustMessage('ADDED', block.name) });
           }
           const scenarioCount = this.countScenarios(block.raw);
           if (scenarioCount < 1) {
@@ -186,7 +186,7 @@ export class Validator {
           if (!requirementText) {
             issues.push({ level: 'ERROR', path: entryPath, message: `MODIFIED "${block.name}" is missing requirement text` });
           } else if (!this.containsShallOrMust(requirementText)) {
-            issues.push({ level: 'ERROR', path: entryPath, message: `MODIFIED "${block.name}" must contain SHALL or MUST` });
+            issues.push({ level: 'ERROR', path: entryPath, message: this.buildMissingShallOrMustMessage('MODIFIED', block.name) });
           }
           const scenarioCount = this.countScenarios(block.raw);
           if (scenarioCount < 1) {
@@ -444,6 +444,24 @@ export class Validator {
     return /\b(SHALL|MUST)\b/.test(text);
   }
 
+  /**
+   * Build an error message for a requirement block whose body lacks SHALL/MUST.
+   *
+   * When the SHALL/MUST keyword already appears in the requirement header (e.g.
+   * `### Requirement: The system SHALL ...`) the original generic error
+   * ("must contain SHALL or MUST") is confusing because the keyword is visibly
+   * present in the spec. Per the OpenSpec conventions the keyword has to live
+   * on the requirement body line (the line right after the header), so we point
+   * the author at that exact fix when the keyword is found in the header only.
+   */
+  private buildMissingShallOrMustMessage(action: 'ADDED' | 'MODIFIED', blockName: string): string {
+    const base = `${action} "${blockName}" must contain SHALL or MUST`;
+    if (this.containsShallOrMust(blockName)) {
+      return `${base} in the requirement body, not only in the header. Move the SHALL/MUST statement to the line immediately after the "### Requirement: ..." header.`;
+    }
+    return base;
+  }
+
   private countScenarios(blockRaw: string): number {
     const matches = blockRaw.match(/^####\s+/gm);
     return matches ? matches.length : 0;
diff --git a/test/core/validation.test.ts b/test/core/validation.test.ts
index 972815e51..72ebc2aba 100644
--- a/test/core/validation.test.ts
+++ b/test/core/validation.test.ts
@@ -535,6 +535,92 @@ The system will log all events.
       expect(report.issues.some(i => i.message.includes('must contain SHALL or MUST'))).toBe(true);
     });
 
+    it('should hint the author when ADDED requirement only has SHALL/MUST in the header', async () => {
+      const changeDir = path.join(testDir, 'test-change-shall-in-header-added');
+      const specsDir = path.join(changeDir, 'specs', 'test-spec');
+      await fs.mkdir(specsDir, { recursive: true });
+
+      const deltaSpec = `# Test Spec
+
+## ADDED Requirements
+
+### Requirement: The system SHALL log all errors
+Error handling logic goes here.
+
+#### Scenario: Error occurs
+**Given** an error
+**When** it occurs
+**Then** it is logged`;
+
+      const specPath = path.join(specsDir, 'spec.md');
+      await fs.writeFile(specPath, deltaSpec);
+
+      const validator = new Validator(true);
+      const report = await validator.validateChangeDeltaSpecs(changeDir);
+
+      expect(report.valid).toBe(false);
+      const shallMessage = report.issues.find(i => i.message.includes('must contain SHALL or MUST'));
+      expect(shallMessage?.message).toContain('not only in the header');
+      expect(shallMessage?.message).toContain('### Requirement:');
+    });
+
+    it('should hint the author when MODIFIED requirement only has SHALL/MUST in the header', async () => {
+      const changeDir = path.join(testDir, 'test-change-shall-in-header-modified');
+      const specsDir = path.join(changeDir, 'specs', 'test-spec');
+      await fs.mkdir(specsDir, { recursive: true });
+
+      const deltaSpec = `# Test Spec
+
+## MODIFIED Requirements
+
+### Requirement: The system MUST validate user input
+Please describe how validation should work here.
+
+#### Scenario: Invalid input
+**Given** invalid input
+**When** validation runs
+**Then** an error surfaces`;
+
+      const specPath = path.join(specsDir, 'spec.md');
+      await fs.writeFile(specPath, deltaSpec);
+
+      const validator = new Validator(true);
+      const report = await validator.validateChangeDeltaSpecs(changeDir);
+
+      expect(report.valid).toBe(false);
+      const shallMessage = report.issues.find(i => i.message.includes('must contain SHALL or MUST'));
+      expect(shallMessage?.message).toContain('not only in the header');
+      expect(shallMessage?.message).toContain('### Requirement:');
+    });
+
+    it('should keep the generic SHALL/MUST error when neither header nor body contain the keyword', async () => {
+      const changeDir = path.join(testDir, 'test-change-shall-nowhere');
+      const specsDir = path.join(changeDir, 'specs', 'test-spec');
+      await fs.mkdir(specsDir, { recursive: true });
+
+      const deltaSpec = `# Test Spec
+
+## ADDED Requirements
+
+### Requirement: Logging Feature
+The system will log all events.
+
+#### Scenario: Event occurs
+**Given** an event
+**When** it occurs
+**Then** it is logged`;
+
+      const specPath = path.join(specsDir, 'spec.md');
+      await fs.writeFile(specPath, deltaSpec);
+
+      const validator = new Validator(true);
+      const report = await validator.validateChangeDeltaSpecs(changeDir);
+
+      expect(report.valid).toBe(false);
+      const shallMessage = report.issues.find(i => i.message.includes('must contain SHALL or MUST'));
+      expect(shallMessage?.message).not.toContain('not only in the header');
+    });
+
     it('should handle requirements without metadata fields', async () => {
       const changeDir = path.join(testDir, 'test-change-4');
       const specsDir = path.join(changeDir, 'specs', 'test-spec');