diff --git a/packages/common/src/index.ts b/packages/common/src/index.ts
index cc4607e..fd16251 100644
--- a/packages/common/src/index.ts
+++ b/packages/common/src/index.ts
@@ -2,3 +2,4 @@ export * from './types.js';
 export * from './constants.js';
 export * from './logger.js';
 export * from './wallet.js';
+export * from './json-store.js';
diff --git a/packages/common/src/json-store.test.ts b/packages/common/src/json-store.test.ts
new file mode 100644
index 0000000..d0bb789
--- /dev/null
+++ b/packages/common/src/json-store.test.ts
@@ -0,0 +1,47 @@
+import fs from 'fs';
+import os from 'os';
+import path from 'path';
+import { afterEach, describe, expect, it } from 'vitest';
+import { createJsonWriteQueue, writeJsonSafe } from './json-store.js';
+
+const tempDirs: string[] = [];
+
+function makeTempDir(): string {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'clevercon-json-store-'));
+  tempDirs.push(dir);
+  return dir;
+}
+
+afterEach(() => {
+  for (const dir of tempDirs.splice(0)) {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+describe('writeJsonSafe', () => {
+  it('writes formatted JSON through an atomic temp-file rename', () => {
+    const filePath = path.join(makeTempDir(), 'nested', 'state.json');
+
+    writeJsonSafe(filePath, { ok: true, count: 2 });
+
+    expect(JSON.parse(fs.readFileSync(filePath, 'utf8'))).toEqual({ ok: true, count: 2 });
+    expect(fs.readdirSync(path.dirname(filePath)).filter((name) => name.endsWith('.tmp'))).toEqual(
+      [],
+    );
+  });
+});
+
+describe('createJsonWriteQueue', () => {
+  it('serializes writes and leaves the last queued value on disk', () => {
+    const filePath = path.join(makeTempDir(), 'registry.json');
+    const save = createJsonWriteQueue<Array<{ id: string }>>(filePath);
+
+    save([{ id: 'agent-a' }]);
+    save([{ id: 'agent-a' }, { id: 'agent-b' }]);
+
+    expect(JSON.parse(fs.readFileSync(filePath, 'utf8'))).toEqual([
+      { id: 'agent-a' },
+      { id: 'agent-b' },
+    ]);
+  });
+});
diff --git a/packages/common/src/json-store.ts b/packages/common/src/json-store.ts
new file mode 100644
index 0000000..6ba68b9
--- /dev/null
+++ b/packages/common/src/json-store.ts
@@ -0,0 +1,48 @@
+import fs from 'fs';
+import path from 'path';
+
+function temporaryPathFor(filePath: string): string {
+  const directory = path.dirname(filePath);
+  const basename = path.basename(filePath);
+  const nonce = Math.random().toString(16).slice(2);
+  return path.join(directory, `.${basename}.${process.pid}.${Date.now()}.${nonce}.tmp`);
+}
+
+export function writeJsonSafe(filePath: string, data: unknown): void {
+  const directory = path.dirname(filePath);
+  fs.mkdirSync(directory, { recursive: true });
+
+  const tmpPath = temporaryPathFor(filePath);
+  try {
+    fs.writeFileSync(tmpPath, `${JSON.stringify(data, null, 2)}\n`, 'utf8');
+    fs.renameSync(tmpPath, filePath);
+  } catch (error) {
+    try {
+      if (fs.existsSync(tmpPath)) fs.unlinkSync(tmpPath);
+    } catch {
+      // Preserve the original write/rename error.
+    }
+    throw error;
+  }
+}
+
+export function createJsonWriteQueue<T>(filePath: string): (data: T) => void {
+  const queue: T[] = [];
+  let draining = false;
+
+  return (data: T): void => {
+    queue.push(data);
+
+    if (draining) return;
+
+    draining = true;
+    try {
+      while (queue.length > 0) {
+        const next = queue.shift();
+        writeJsonSafe(filePath, next);
+      }
+    } finally {
+      draining = false;
+    }
+  };
+}
diff --git a/packages/orchestrator/src/activity-store.ts b/packages/orchestrator/src/activity-store.ts
index f98c506..a2e1f5d 100644
--- a/packages/orchestrator/src/activity-store.ts
+++ b/packages/orchestrator/src/activity-store.ts
@@ -6,6 +6,7 @@
 
 import fs from 'fs';
 import path from 'path';
+import { writeJsonSafe } from '@clevercon/common';
 
 const __dirname = path.dirname(path.resolve(process.argv[1]));
 const DATA_DIR = path.join(__dirname, '..', '..', '..', 'data');
@@ -41,7 +42,7 @@ function load(): Log {
   try {
     fs.mkdirSync(DATA_DIR, { recursive: true });
     if (!fs.existsSync(LOG_PATH)) {
-      fs.writeFileSync(LOG_PATH, '[]', 'utf8');
+      writeJsonSafe(LOG_PATH, []);
     }
     cache = JSON.parse(fs.readFileSync(LOG_PATH, 'utf8')) as Log;
   } catch {
@@ -54,7 +55,7 @@ function save(log: Log): void {
   fs.mkdirSync(DATA_DIR, { recursive: true });
   // Keep at most 1000 entries globally
   const trimmed = log.slice(-1000);
-  fs.writeFileSync(LOG_PATH, JSON.stringify(trimmed, null, 2), 'utf8');
+  writeJsonSafe(LOG_PATH, trimmed);
   cache = trimmed;
 }
 
diff --git a/packages/orchestrator/src/orchestrator-store.ts b/packages/orchestrator/src/orchestrator-store.ts
index e95e457..638ea4e 100644
--- a/packages/orchestrator/src/orchestrator-store.ts
+++ b/packages/orchestrator/src/orchestrator-store.ts
@@ -8,6 +8,7 @@
 
 import fs from 'fs';
 import path from 'path';
+import { writeJsonSafe } from '@clevercon/common';
 
 const __dirname = path.dirname(path.resolve(process.argv[1]));
 // process.argv[1] = .../packages/orchestrator/src/server.ts → go up 3 levels to workspace root
@@ -34,7 +35,7 @@ function load(): Store {
   try {
     fs.mkdirSync(DATA_DIR, { recursive: true });
     if (!fs.existsSync(STORE_PATH)) {
-      fs.writeFileSync(STORE_PATH, '{}', 'utf8');
+      writeJsonSafe(STORE_PATH, {});
     }
     cache = JSON.parse(fs.readFileSync(STORE_PATH, 'utf8')) as Store;
   } catch {
@@ -45,7 +46,7 @@ function load(): Store {
 
 function save(store: Store): void {
   fs.mkdirSync(DATA_DIR, { recursive: true });
-  fs.writeFileSync(STORE_PATH, JSON.stringify(store, null, 2), 'utf8');
+  writeJsonSafe(STORE_PATH, store);
   cache = store;
 }
 
diff --git a/packages/orchestrator/src/task-results.ts b/packages/orchestrator/src/task-results.ts
index 6168291..f149bdd 100644
--- a/packages/orchestrator/src/task-results.ts
+++ b/packages/orchestrator/src/task-results.ts
@@ -5,7 +5,7 @@
 
 import fs from 'fs';
 import path from 'path';
-import type { TaskResult } from '@clevercon/common';
+import { writeJsonSafe, type TaskResult } from '@clevercon/common';
 
 const __dirname = path.dirname(path.resolve(process.argv[1]));
 const DATA_DIR = path.join(__dirname, '..', '..', '..', 'data');
@@ -31,7 +31,7 @@ function load(): Store {
   if (cache) return cache;
   try {
     fs.mkdirSync(DATA_DIR, { recursive: true });
-    if (!fs.existsSync(RESULTS_PATH)) fs.writeFileSync(RESULTS_PATH, '[]', 'utf8');
+    if (!fs.existsSync(RESULTS_PATH)) writeJsonSafe(RESULTS_PATH, []);
     cache = JSON.parse(fs.readFileSync(RESULTS_PATH, 'utf8')) as Store;
   } catch {
     cache = [];
@@ -42,7 +42,7 @@ function load(): Store {
 function save(store: Store): void {
   fs.mkdirSync(DATA_DIR, { recursive: true });
   const trimmed = store.slice(-500); // cap at 500 entries
-  fs.writeFileSync(RESULTS_PATH, JSON.stringify(trimmed, null, 2), 'utf8');
+  writeJsonSafe(RESULTS_PATH, trimmed);
   cache = trimmed;
 }
 
diff --git a/packages/orchestrator/src/vault-ledger.ts b/packages/orchestrator/src/vault-ledger.ts
index 5c296c4..d1da886 100644
--- a/packages/orchestrator/src/vault-ledger.ts
+++ b/packages/orchestrator/src/vault-ledger.ts
@@ -5,6 +5,7 @@
 
 import fs from 'fs';
 import path from 'path';
+import { writeJsonSafe } from '@clevercon/common';
 
 const __dirname = path.dirname(path.resolve(process.argv[1]));
 const DATA_DIR = path.join(__dirname, '..', '..', '..', 'data');
@@ -31,7 +32,7 @@ function load(): Ledger {
   if (cache) return cache;
   try {
     fs.mkdirSync(DATA_DIR, { recursive: true });
-    if (!fs.existsSync(LEDGER_PATH)) fs.writeFileSync(LEDGER_PATH, '[]', 'utf8');
+    if (!fs.existsSync(LEDGER_PATH)) writeJsonSafe(LEDGER_PATH, []);
     cache = JSON.parse(fs.readFileSync(LEDGER_PATH, 'utf8')) as Ledger;
   } catch {
     cache = [];
@@ -42,7 +43,7 @@ function load(): Ledger {
 function save(ledger: Ledger): void {
   fs.mkdirSync(DATA_DIR, { recursive: true });
   const trimmed = ledger.slice(-2000);
-  fs.writeFileSync(LEDGER_PATH, JSON.stringify(trimmed, null, 2), 'utf8');
+  writeJsonSafe(LEDGER_PATH, trimmed);
   cache = trimmed;
 }
 
diff --git a/packages/registry/src/store.ts b/packages/registry/src/store.ts
index 90c855c..7391002 100644
--- a/packages/registry/src/store.ts
+++ b/packages/registry/src/store.ts
@@ -1,18 +1,18 @@
 /**
  * JSON-file-backed persistence for the agent registry.
  *
- * All agents are stored as a single array in `data/registry.json`. Each
- * exported function reads or rewrites the whole file — there is no write
- * locking, so concurrent writers can race (see SECURITY.md "Known limitations").
+ * All agents are stored as a single array in `data/registry.json`. Writes are
+ * serialized through an in-process queue and committed with atomic renames.
  */
 import fs from 'fs';
 import path from 'path';
 import { fileURLToPath } from 'url';
-import type { AgentRecord } from '@clevercon/common';
+import { createJsonWriteQueue, type AgentRecord } from '@clevercon/common';
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
 const DATA_DIR = path.join(__dirname, '..', '..', '..', 'data');
 const REGISTRY_FILE = path.join(DATA_DIR, 'registry.json');
+const writeAgentsQueued = createJsonWriteQueue<AgentRecord[]>(REGISTRY_FILE);
 
 function ensureDataDir(): void {
   if (!fs.existsSync(DATA_DIR)) {
@@ -37,7 +37,7 @@ export function loadAgents(): AgentRecord[] {
 /** Overwrite `data/registry.json` with the given list of agents. */
 export function saveAgents(agents: AgentRecord[]): void {
   ensureDataDir();
-  fs.writeFileSync(REGISTRY_FILE, JSON.stringify(agents, null, 2));
+  writeAgentsQueued(agents);
 }
 
 /** Find a single agent by its `agent_id`, or `undefined` if not registered. */