diff --git a/src/memory/__tests__/cross-process-lock.test.ts b/src/memory/__tests__/cross-process-lock.test.ts
new file mode 100644
index 0000000000..0d8765d4a0
--- /dev/null
+++ b/src/memory/__tests__/cross-process-lock.test.ts
@@ -0,0 +1,55 @@
+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 cross-process exclusion (#1819, #3286).
+ *
+ * The in-process mutation queue (#4555) serialises one instance's mutations,
+ * but the server is stdio-only, so every client is its own process with its
+ * own queue. Two instances on one file are a faithful stand-in: nothing but
+ * the sidecar lock file separates them, exactly as with two processes.
+ */
+describe('KnowledgeGraphManager cross-process exclusion', () => {
+  let testDir: string;
+  let testFilePath: string;
+
+  beforeEach(async () => {
+    testDir = await fs.mkdtemp(path.join(os.tmpdir(), 'mcp-memory-xproc-'));
+    testFilePath = path.join(testDir, 'memory.jsonl');
+  });
+
+  afterEach(async () => {
+    await fs.rm(testDir, { recursive: true, force: true });
+  });
+
+  const entity = (name: string) => ({ name, entityType: 'thing', observations: [] as string[] });
+
+  it('keeps every entity when two instances write the same file concurrently', async () => {
+    const a = new KnowledgeGraphManager(testFilePath);
+    const b = new KnowledgeGraphManager(testFilePath);
+    await Promise.all(
+      Array.from({ length: 20 }, (_, i) => (i % 2 ? b : a).createEntities([entity(`e${i}`)])),
+    );
+    const names = (await a.readGraph()).entities.map(e => e.name).sort();
+    expect(names).toEqual(Array.from({ length: 20 }, (_, i) => `e${i}`).sort());
+  });
+
+  it('removes its lock file after each mutation', async () => {
+    const manager = new KnowledgeGraphManager(testFilePath);
+    await manager.createEntities([entity('x')]);
+    await expect(fs.stat(`${testFilePath}.lock`)).rejects.toMatchObject({ code: 'ENOENT' });
+  });
+
+  it('reclaims a lock left behind by a crashed process', async () => {
+    await fs.mkdir(testDir, { recursive: true });
+    await fs.writeFile(`${testFilePath}.lock`, '');
+    const old = new Date(Date.now() - 60_000);
+    await fs.utimes(`${testFilePath}.lock`, old, old);
+    const manager = new KnowledgeGraphManager(testFilePath);
+    await manager.createEntities([entity('x')]);
+    expect((await manager.readGraph()).entities).toHaveLength(1);
+  });
+});
diff --git a/src/memory/index.ts b/src/memory/index.ts
index d9f814877b..214fd2f5ad 100644
--- a/src/memory/index.ts
+++ b/src/memory/index.ts
@@ -95,7 +95,8 @@ export class KnowledgeGraphManager {
   private mutationQueue: Promise<unknown> = Promise.resolve();
 
   private async withLock<T>(operation: () => Promise<T>): Promise<T> {
-    const result = this.mutationQueue.then(operation, operation);
+    const guarded = () => this.withFileLock(operation);
+    const result = this.mutationQueue.then(guarded, guarded);
     // Always resolve the queue itself, even if this operation failed, so a
     // single failed mutation doesn't permanently wedge every call after it.
     // The failure still propagates normally to whoever awaited `result`.
@@ -106,6 +107,56 @@ export class KnowledgeGraphManager {
     return result;
   }
 
+  // Cross-process exclusion. The queue above serialises this instance's
+  // mutations, but the server is stdio-only, so every client is its own
+  // process: two editor windows or two git worktrees sharing MEMORY_FILE_PATH
+  // each run their own queue and still overwrite each other's load→mutate→save
+  // (#1819, #3286). Measured on this commit: 20 writes split across two
+  // processes keep exactly 10 — each process serialises its own half and the
+  // last one to write the file wins. A sidecar lock file created with O_EXCL is
+  // atomic on POSIX and Windows and needs no dependency. A lock older than
+  // LOCK_STALE_MS is treated as left behind by a crashed process.
+  private static readonly LOCK_STALE_MS = 30_000;
+  private static readonly LOCK_RETRY_MS = 15;
+  private static readonly LOCK_TIMEOUT_MS = 10_000;
+
+  private get lockFilePath(): string {
+    return `${this.memoryFilePath}.lock`;
+  }
+
+  private async withFileLock<T>(operation: () => Promise<T>): Promise<T> {
+    await fs.mkdir(path.dirname(this.memoryFilePath), { recursive: true }).catch(() => {});
+    const deadline = Date.now() + KnowledgeGraphManager.LOCK_TIMEOUT_MS;
+    for (;;) {
+      let handle: fs.FileHandle;
+      try {
+        handle = await fs.open(this.lockFilePath, 'wx');
+      } catch (error) {
+        if ((error as NodeJS.ErrnoException).code !== 'EEXIST') throw error;
+        try {
+          const stat = await fs.stat(this.lockFilePath);
+          if (Date.now() - stat.mtimeMs > KnowledgeGraphManager.LOCK_STALE_MS) {
+            await fs.unlink(this.lockFilePath).catch(() => {});
+            continue;
+          }
+        } catch {
+          continue; // the holder released between open and stat; retry now
+        }
+        if (Date.now() > deadline) {
+          throw new Error(`Timed out waiting for lock ${this.lockFilePath}`);
+        }
+        await new Promise(resolve => setTimeout(resolve, KnowledgeGraphManager.LOCK_RETRY_MS));
+        continue;
+      }
+      try {
+        return await operation();
+      } finally {
+        await handle.close().catch(() => {});
+        await fs.unlink(this.lockFilePath).catch(() => {});
+      }
+    }
+  }
+
   private async loadGraph(): Promise<KnowledgeGraph> {
     try {
       const data = await fs.readFile(this.memoryFilePath, "utf-8");