diff --git a/__tests__/installer-targets.test.ts b/__tests__/installer-targets.test.ts
index 697f8e976..b08f0857b 100644
--- a/__tests__/installer-targets.test.ts
+++ b/__tests__/installer-targets.test.ts
@@ -793,6 +793,38 @@ describe('Installer targets — partial-state idempotency', () => {
     expect(body).toContain('  telegram:\n  - hermes-telegram');
   });
 
+  it('hermes: install matches existing sibling list-item indent when creating new cli block (issue #507)', () => {
+    const hermes = getTarget('hermes')!;
+    const config = path.join(tmpHome, '.hermes', 'config.yaml');
+    fs.mkdirSync(path.dirname(config), { recursive: true });
+    // platform_toolsets exists but has no cli key; siblings use 2-space (PyYAML) item style
+    const original = [
+      'platform_toolsets:',
+      '  discord:',
+      '  - hermes-discord',
+      '  telegram:',
+      '  - hermes-telegram',
+      '',
+    ].join('\n');
+    fs.writeFileSync(config, original);
+
+    hermes.install('global', { autoAllow: true });
+    const body = fs.readFileSync(config, 'utf-8');
+
+    // New cli block must use the same 2-space item indent as discord/telegram
+    expect(body).toContain('  cli:\n  - hermes-cli\n  - mcp-codegraph');
+    // Existing sections untouched
+    expect(body).toContain('  discord:\n  - hermes-discord');
+    expect(body).toContain('  telegram:\n  - hermes-telegram');
+    // No 4-space item indent leaked in
+    expect(body).not.toMatch(/^ {4}- hermes-cli/m);
+    expect(body).not.toMatch(/^ {4}- mcp-codegraph/m);
+
+    // Idempotent
+    const second = hermes.install('global', { autoAllow: true });
+    expect(second.files[0]?.action).toBe('unchanged');
+  });
+
   it('opencode: uninstall removes only mcp.codegraph, preserves comments and siblings', () => {
     const opencode = getTarget('opencode')!;
     const dir = path.join(tmpHome, '.config', 'opencode');
diff --git a/src/installer/targets/hermes.ts b/src/installer/targets/hermes.ts
index eacab6d3d..b55c7d7d8 100644
--- a/src/installer/targets/hermes.ts
+++ b/src/installer/targets/hermes.ts
@@ -305,6 +305,14 @@ function removeCodeGraphMcpServer(content: string): string {
   return joinLines(lines);
 }
 
+function detectItemIndent(lines: string[], range: LineRange): string {
+  for (let i = range.start + 1; i < range.end; i++) {
+    const m = (lines[i] ?? '').match(/^( +)- /);
+    if (m && m[1]) return m[1];
+  }
+  return '    ';
+}
+
 function upsertCodeGraphToolset(content: string): string {
   const lines = splitLines(content);
   const parent = topLevelRange(lines, 'platform_toolsets');
@@ -318,7 +326,8 @@ function upsertCodeGraphToolset(content: string): string {
   }
 
   if (!cli) {
-    lines.splice(parent.end, 0, '  cli:', '    - hermes-cli', '    - mcp-codegraph');
+    const itemIndent = detectItemIndent(lines, parent);
+    lines.splice(parent.end, 0, '  cli:', `${itemIndent}- hermes-cli`, `${itemIndent}- mcp-codegraph`);
     return joinLines(lines);
   }