diff --git a/src/core/specs-apply.ts b/src/core/specs-apply.ts
index 88142ec00..29bc1ed55 100644
--- a/src/core/specs-apply.ts
+++ b/src/core/specs-apply.ts
@@ -47,6 +47,11 @@ export interface SpecsApplyOutput {
   noChanges: boolean;
 }
 
+interface ScenarioBlock {
+  name: string;
+  raw: string;
+}
+
 // -----------------------------------------------------------------------------
 // Public API
 // -----------------------------------------------------------------------------
@@ -283,7 +288,8 @@ export async function buildUpdatedSpec(
   // MODIFIED
   for (const mod of plan.modified) {
     const key = normalizeRequirementName(mod.name);
-    if (!nameToBlock.has(key)) {
+    const currentBlock = nameToBlock.get(key);
+    if (!currentBlock) {
       throw new Error(`${specName} MODIFIED failed for header "### Requirement: ${mod.name}" - not found`);
     }
     // Replace block with provided raw (ensure header line matches key)
@@ -293,6 +299,12 @@ export async function buildUpdatedSpec(
         `${specName} MODIFIED failed for header "### Requirement: ${mod.name}" - header mismatch in content`
       );
     }
+    const missingScenarios = findMissingCurrentScenarios(currentBlock, mod);
+    if (missingScenarios.length > 0) {
+      throw new Error(
+        `${specName} MODIFIED failed for header "### Requirement: ${mod.name}" - current spec contains scenario(s) not present in the modified block: ${missingScenarios.map(name => `"${name}"`).join(', ')}. Refresh the change spec before archiving to avoid dropping scenarios.`
+      );
+    }
     nameToBlock.set(key, mod);
   }
 
@@ -376,6 +388,41 @@ export function buildSpecSkeleton(specFolderName: string, changeName: string): s
   return `# ${titleBase} Specification\n\n## Purpose\nTBD - created by archiving change ${changeName}. Update Purpose after archive.\n\n## Requirements\n`;
 }
 
+function findMissingCurrentScenarios(current: RequirementBlock, incoming: RequirementBlock): string[] {
+  const incomingScenarioNames = new Set(parseScenarioBlocks(incoming.raw).map((scenario) => scenario.name));
+  return parseScenarioBlocks(current.raw)
+    .filter((scenario) => !incomingScenarioNames.has(scenario.name))
+    .map((scenario) => scenario.name);
+}
+
+function parseScenarioBlocks(requirementRaw: string): ScenarioBlock[] {
+  const lines = requirementRaw.replace(/\r\n?/g, '\n').split('\n');
+  const scenarios: ScenarioBlock[] = [];
+  let index = 0;
+
+  while (index < lines.length) {
+    const headerMatch = lines[index].match(/^####\s*Scenario:\s*(.+)\s*$/);
+    if (!headerMatch) {
+      index++;
+      continue;
+    }
+
+    const start = index;
+    const name = headerMatch[1].trim();
+    index++;
+    while (index < lines.length && !/^####\s*Scenario:\s*(.+)\s*$/.test(lines[index])) {
+      index++;
+    }
+
+    scenarios.push({
+      name,
+      raw: lines.slice(start, index).join('\n').trimEnd(),
+    });
+  }
+
+  return scenarios;
+}
+
 /**
  * Apply all delta specs from a change to main specs.
  *
diff --git a/test/core/archive.test.ts b/test/core/archive.test.ts
index 1d0f75e14..a40d16bbb 100644
--- a/test/core/archive.test.ts
+++ b/test/core/archive.test.ts
@@ -561,6 +561,84 @@ new text
       await expect(fs.access(changeDir)).resolves.not.toThrow();
     });
 
+    it('should abort stale MODIFIED blocks that would drop current scenarios (issue #1246)', async () => {
+      const mainSpecDir = path.join(tempDir, 'openspec', 'specs', 'stale-modified');
+      await fs.mkdir(mainSpecDir, { recursive: true });
+      const mainSpecPath = path.join(mainSpecDir, 'spec.md');
+      const baseSpec = `# stale-modified Specification
+
+## Purpose
+Stale modified purpose.
+
+## Requirements
+
+### Requirement: Shared Rule
+The system SHALL support the shared rule.
+
+#### Scenario: Existing behavior
+- **WHEN** the original behavior runs
+- **THEN** it succeeds`;
+      await fs.writeFile(mainSpecPath, baseSpec);
+
+      const changeA = 'modify-shared-a';
+      const changeADir = path.join(tempDir, 'openspec', 'changes', changeA);
+      const changeASpecDir = path.join(changeADir, 'specs', 'stale-modified');
+      await fs.mkdir(changeASpecDir, { recursive: true });
+      await fs.writeFile(path.join(changeASpecDir, 'spec.md'), `# Stale Modified - Change A
+
+## MODIFIED Requirements
+
+### Requirement: Shared Rule
+The system SHALL support the shared rule.
+
+#### Scenario: Existing behavior
+- **WHEN** the original behavior runs
+- **THEN** it succeeds
+
+#### Scenario: Behavior from A
+- **WHEN** change A behavior runs
+- **THEN** it succeeds`);
+
+      const changeB = 'modify-shared-b';
+      const changeBDir = path.join(tempDir, 'openspec', 'changes', changeB);
+      const changeBSpecDir = path.join(changeBDir, 'specs', 'stale-modified');
+      await fs.mkdir(changeBSpecDir, { recursive: true });
+      await fs.writeFile(path.join(changeBSpecDir, 'spec.md'), `# Stale Modified - Change B
+
+## MODIFIED Requirements
+
+### Requirement: Shared Rule
+The system SHALL support the shared rule.
+
+#### Scenario: Existing behavior
+- **WHEN** the original behavior runs
+- **THEN** it succeeds
+
+#### Scenario: Behavior from B
+- **WHEN** change B behavior runs
+- **THEN** it succeeds`);
+
+      await archiveCommand.execute(changeA, { yes: true, noValidate: true });
+      await archiveCommand.execute(changeB, { yes: true, noValidate: true });
+
+      const updated = await fs.readFile(mainSpecPath, 'utf-8');
+      expect(updated).toContain('#### Scenario: Existing behavior');
+      expect(updated).toContain('#### Scenario: Behavior from A');
+      expect(updated).not.toContain('#### Scenario: Behavior from B');
+      expect(console.log).toHaveBeenCalledWith(
+        expect.stringContaining(
+          'stale-modified MODIFIED failed for header "### Requirement: Shared Rule" - current spec contains scenario(s) not present in the modified block: "Behavior from A"'
+        )
+      );
+      expect(console.log).toHaveBeenCalledWith('Aborted. No files were changed.');
+
+      await expect(fs.access(changeBDir)).resolves.not.toThrow();
+      const archiveDir = path.join(tempDir, 'openspec', 'changes', 'archive');
+      const archives = await fs.readdir(archiveDir);
+      expect(archives.some(a => a.includes(changeA))).toBe(true);
+      expect(archives.some(a => a.includes(changeB))).toBe(false);
+    });
+
     it('should abort with a structural error when target spec hides requirements outside ## Requirements', async () => {
       const changeName = 'hidden-requirement-target';
       const changeDir = path.join(tempDir, 'openspec', 'changes', changeName);