diff --git a/src/memory/__tests__/file-permissions.test.ts b/src/memory/__tests__/file-permissions.test.ts
new file mode 100644
index 0000000000..eb3b53aae0
--- /dev/null
+++ b/src/memory/__tests__/file-permissions.test.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { promises as fs } from 'fs';
+import path from 'path';
+import os from 'os';
+import { KnowledgeGraphManager } from '../index.js';
+
+/**
+ * Regression tests for the memory file's permission bits.
+ *
+ * saveGraph() writes a temporary file and renames it over the memory file.
+ * The rename replaces the inode, and the temp file is created under the
+ * process umask, so any permission bits the operator set on the memory file
+ * (chmod 600, restored from a 600 backup, written by another tool) are lost
+ * on the next mutation and the file comes back 0644. The filesystem server
+ * (src/filesystem/lib.ts) restores the original bits after the same
+ * temp-file + rename sequence; the memory server must do the same.
+ */
+describe('KnowledgeGraphManager file permissions', () => {
+  let testDir: string;
+  let testFilePath: string;
+
+  // Octal, so a failure reports the modes an operator would have typed.
+  const modeOf = async (file: string): Promise<string> =>
+    ((await fs.stat(file)).mode & 0o777).toString(8);
+
+  beforeEach(async () => {
+    testDir = await fs.mkdtemp(path.join(os.tmpdir(), 'mcp-memory-perms-'));
+    testFilePath = path.join(testDir, 'memory.jsonl');
+  });
+
+  afterEach(async () => {
+    await fs.rm(testDir, { recursive: true, force: true });
+  });
+
+  it.each([
+    { label: 'owner-only (0600)', mode: 0o600 },
+    { label: 'group-readable (0640)', mode: 0o640 },
+  ])('keeps a $label memory file at that mode across a mutation', async ({ mode }) => {
+    const manager = new KnowledgeGraphManager(testFilePath);
+    await manager.createEntities([
+      { name: 'Alice', entityType: 'person', observations: ['works at Acme Corp'] },
+    ]);
+
+    await fs.chmod(testFilePath, mode);
+    expect(await modeOf(testFilePath), 'mode set by the operator').toBe(mode.toString(8));
+
+    await manager.createEntities([
+      { name: 'Bob', entityType: 'person', observations: ['likes programming'] },
+    ]);
+
+    expect(await modeOf(testFilePath), 'mode after a save').toBe(mode.toString(8));
+
+    // The mutation must still have been persisted.
+    const reloaded = await new KnowledgeGraphManager(testFilePath).readGraph();
+    expect(reloaded.entities.map(e => e.name).sort()).toEqual(['Alice', 'Bob']);
+  });
+});
diff --git a/src/memory/index.ts b/src/memory/index.ts
index d9f814877b..b21a553160 100644
--- a/src/memory/index.ts
+++ b/src/memory/index.ts
@@ -183,6 +183,15 @@ export class KnowledgeGraphManager {
     // complete old file or the complete new one, never a partial state.
     // The temp file is kept in the same directory so the rename stays on one
     // filesystem — renaming across mount points fails with EXDEV.
+    //
+    // The rename also replaces the target's inode, and the temp file is
+    // created under the process umask, so a memory file an operator narrowed
+    // with chmod would come back 0644. Capture the existing bits and restore
+    // them after the rename, as src/filesystem/lib.ts does for the same
+    // write pattern. A memory file that does not exist yet has nothing to
+    // preserve.
+    const existingStats = await fs.stat(this.memoryFilePath).catch(() => undefined);
+
     const directory = path.dirname(this.memoryFilePath);
     const tempFilePath = path.join(
       directory,
@@ -197,6 +206,11 @@ export class KnowledgeGraphManager {
       await fs.unlink(tempFilePath).catch(() => {});
       throw error;
     }
+
+    if (existingStats) {
+      // The data is already durable, so a failed chmod must not fail the save.
+      await fs.chmod(this.memoryFilePath, existingStats.mode & 0o777).catch(() => {});
+    }
   }
 
   async createEntities(entities: Entity[]): Promise<Entity[]> {