diff --git a/packages/squad-sdk/src/agents/history-shadow.ts b/packages/squad-sdk/src/agents/history-shadow.ts
index ce5e8ea04..696940d52 100644
--- a/packages/squad-sdk/src/agents/history-shadow.ts
+++ b/packages/squad-sdk/src/agents/history-shadow.ts
@@ -11,6 +11,81 @@ import * as fs from 'fs/promises';
 import * as path from 'path';
 import { ConfigurationError } from '../adapter/errors.js';
 
+// ---------------------------------------------------------------------------
+// Async mutex — prevents read-modify-write races when multiple agents
+// concurrently append to the same history.md (#479).
+//
+// Two layers of protection:
+//   1. In-process async mutex (handles concurrent agents in one Node.js process)
+//   2. Atomic writes via temp-file + rename (prevents partial reads)
+// ---------------------------------------------------------------------------
+
+/**
+ * Per-file async mutex.  Keyed by resolved file path so concurrent calls
+ * targeting the same history.md are serialized, while calls to different
+ * files run in parallel.
+ * @private
+ */
+const fileLocks = new Map<string, Promise<void>>();
+
+/**
+ * Execute `fn` while holding an in-process async mutex for `filePath`.
+ *
+ * Concurrent callers for the same path are queued — each waits for the
+ * previous to finish before starting.  Different paths run in parallel.
+ *
+ * @private
+ */
+async function withFileLock<T>(
+  filePath: string,
+  fn: () => Promise<T>,
+): Promise<T> {
+  // Normalize path to prevent two representations of the same file
+  const key = path.resolve(filePath);
+
+  // Chain: wait for whoever holds the lock right now
+  const prev = fileLocks.get(key) ?? Promise.resolve();
+
+  let releaseLock!: () => void;
+  const gate = new Promise<void>(resolve => {
+    releaseLock = resolve;
+  });
+  fileLocks.set(key, gate);
+
+  // Wait until the previous holder finishes
+  await prev;
+
+  try {
+    return await fn();
+  } finally {
+    releaseLock();
+    // Clean up if we are the last in the chain
+    if (fileLocks.get(key) === gate) {
+      fileLocks.delete(key);
+    }
+  }
+}
+
+/**
+ * Write a file atomically by writing to a temp file then renaming.
+ * Prevents concurrent readers from seeing partial content.
+ * @private
+ */
+async function atomicWriteFile(
+  filePath: string,
+  content: string,
+): Promise<void> {
+  const tmpPath = `${filePath}.${process.pid}.tmp`;
+  try {
+    await fs.writeFile(tmpPath, content, 'utf-8');
+    await fs.rename(tmpPath, filePath);
+  } catch (err) {
+    // Clean up temp file on failure
+    await fs.unlink(tmpPath).catch(() => {});
+    throw err;
+  }
+}
+
 /**
  * Standard history sections that agents maintain.
  */
@@ -147,45 +222,49 @@ export async function appendToHistory(
   try {
     const shadowPath = path.join(teamRoot, '.squad', 'agents', agentName, 'history.md');
     
-    // Read existing history
-    let historyContent: string;
-    try {
-      historyContent = await fs.readFile(shadowPath, 'utf-8');
-    } catch (error) {
-      throw new ConfigurationError(
-        `History shadow not found for agent '${agentName}'. Create it first with createHistoryShadow().`,
-        {
-          agentName,
-          operation: 'appendToHistory',
-          timestamp: new Date(),
-          metadata: { shadowPath },
-        },
-        error instanceof Error ? error : undefined
-      );
-    }
-    
-    // Find section or create it
-    const sectionHeader = `## ${section}`;
-    const sectionRegex = new RegExp(`^${sectionHeader}\\s*$([\\s\\S]*?)(?=^##\\s|\\Z)`, 'm');
-    const match = historyContent.match(sectionRegex);
-    
-    const timestamp = new Date().toISOString().split('T')[0]; // YYYY-MM-DD
-    const entry = `\n### ${timestamp}\n\n${content}\n`;
-    
-    let updatedContent: string;
-    
-    if (match) {
-      // Section exists, append to it
-      const fullMatch = match[0];
-      const sectionContent = match[1];
-      const updatedSection = `${sectionHeader}\n${sectionContent!.trimEnd()}${entry}`;
-      updatedContent = historyContent.replace(fullMatch, updatedSection);
-    } else {
-      // Section doesn't exist, create it at the end
-      updatedContent = historyContent.trimEnd() + `\n\n${sectionHeader}${entry}`;
-    }
-    
-    await fs.writeFile(shadowPath, updatedContent, 'utf-8');
+    // Acquire file lock before the read-modify-write cycle (#479)
+    await withFileLock(shadowPath, async () => {
+      // Read existing history (inside lock to prevent races)
+      let historyContent: string;
+      try {
+        historyContent = await fs.readFile(shadowPath, 'utf-8');
+      } catch (error) {
+        throw new ConfigurationError(
+          `History shadow not found for agent '${agentName}'. Create it first with createHistoryShadow().`,
+          {
+            agentName,
+            operation: 'appendToHistory',
+            timestamp: new Date(),
+            metadata: { shadowPath },
+          },
+          error instanceof Error ? error : undefined
+        );
+      }
+      
+      // Find section or create it
+      const sectionHeader = `## ${section}`;
+      const sectionRegex = new RegExp(`^${sectionHeader}\\s*$([\\s\\S]*?)(?=^##\\s|\\Z)`, 'm');
+      const match = historyContent.match(sectionRegex);
+      
+      const timestamp = new Date().toISOString().split('T')[0]; // YYYY-MM-DD
+      const entry = `\n### ${timestamp}\n\n${content}\n`;
+      
+      let updatedContent: string;
+      
+      if (match) {
+        // Section exists, append to it
+        const fullMatch = match[0];
+        const sectionContent = match[1];
+        const updatedSection = `${sectionHeader}\n${sectionContent!.trimEnd()}${entry}`;
+        updatedContent = historyContent.replace(fullMatch, updatedSection);
+      } else {
+        // Section doesn't exist, create it at the end
+        updatedContent = historyContent.trimEnd() + `\n\n${sectionHeader}${entry}`;
+      }
+      
+      // Atomic write: temp file + rename prevents partial reads
+      await atomicWriteFile(shadowPath, updatedContent);
+    });
     
   } catch (error) {
     if (error instanceof ConfigurationError) {
diff --git a/test/history-shadow.test.ts b/test/history-shadow.test.ts
new file mode 100644
index 000000000..be851cb9c
--- /dev/null
+++ b/test/history-shadow.test.ts
@@ -0,0 +1,208 @@
+/**
+ * Tests for history-shadow.ts — specifically the race condition fix (#479).
+ *
+ * Verifies that:
+ *  1. appendToHistory still works correctly for single callers (regression)
+ *  2. Concurrent appendToHistory calls do not lose data (the race condition)
+ *  3. Lock files are cleaned up after operations complete
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import * as fs from 'fs/promises';
+import * as path from 'path';
+import * as os from 'os';
+import {
+  createHistoryShadow,
+  appendToHistory,
+  readHistory,
+  deleteHistoryShadow,
+  shadowExists,
+} from '@bradygaster/squad-sdk/agents';
+
+let tmpDir: string;
+
+beforeEach(async () => {
+  tmpDir = await fs.mkdtemp(path.join(os.tmpdir(), 'squad-history-test-'));
+});
+
+afterEach(async () => {
+  await fs.rm(tmpDir, { recursive: true, force: true });
+});
+
+describe('history-shadow (#479 race condition fix)', () => {
+
+  describe('appendToHistory — single caller (regression)', () => {
+    it('should append to an existing section', async () => {
+      await createHistoryShadow(tmpDir, 'testbot', 'Initial context');
+
+      await appendToHistory(tmpDir, 'testbot', 'Learnings', 'Learned thing one');
+
+      const history = await readHistory(tmpDir, 'testbot');
+      expect(history.learnings).toContain('Learned thing one');
+    });
+
+    it('should append multiple entries sequentially', async () => {
+      await createHistoryShadow(tmpDir, 'testbot');
+
+      await appendToHistory(tmpDir, 'testbot', 'Learnings', 'Entry A');
+      await appendToHistory(tmpDir, 'testbot', 'Learnings', 'Entry B');
+      await appendToHistory(tmpDir, 'testbot', 'Decisions', 'Decision X');
+
+      const history = await readHistory(tmpDir, 'testbot');
+      expect(history.learnings).toContain('Entry A');
+      expect(history.learnings).toContain('Entry B');
+      expect(history.decisions).toContain('Decision X');
+    });
+
+    it('should create a section if it does not exist', async () => {
+      // Create a minimal history file without a Patterns section
+      const agentDir = path.join(tmpDir, '.squad', 'agents', 'testbot');
+      await fs.mkdir(agentDir, { recursive: true });
+      await fs.writeFile(
+        path.join(agentDir, 'history.md'),
+        '# Testbot — Session History\n\n## Learnings\n\n<!-- empty -->\n',
+        'utf-8',
+      );
+
+      await appendToHistory(tmpDir, 'testbot', 'Patterns', 'Pattern found');
+
+      const history = await readHistory(tmpDir, 'testbot');
+      expect(history.fullContent).toContain('## Patterns');
+      expect(history.fullContent).toContain('Pattern found');
+    });
+  });
+
+  describe('appendToHistory — concurrent callers (race condition)', () => {
+    it('should not lose data when 5 agents append concurrently', async () => {
+      await createHistoryShadow(tmpDir, 'sharedbot', 'Shared context');
+
+      const entries = Array.from({ length: 5 }, (_, i) => `Concurrent entry ${i}`);
+
+      // Fire all appends concurrently — the old code would lose all but the last
+      await Promise.all(
+        entries.map(entry =>
+          appendToHistory(tmpDir, 'sharedbot', 'Learnings', entry),
+        ),
+      );
+
+      const history = await readHistory(tmpDir, 'sharedbot');
+
+      // Every single entry must be present — no last-write-wins data loss
+      for (const entry of entries) {
+        expect(history.learnings).toContain(entry);
+      }
+    });
+
+    it('should not lose data when appending to different sections concurrently', async () => {
+      await createHistoryShadow(tmpDir, 'multibot', 'Multi-section test');
+
+      await Promise.all([
+        appendToHistory(tmpDir, 'multibot', 'Learnings', 'Learn concurrent'),
+        appendToHistory(tmpDir, 'multibot', 'Decisions', 'Decide concurrent'),
+        appendToHistory(tmpDir, 'multibot', 'Patterns', 'Pattern concurrent'),
+      ]);
+
+      const history = await readHistory(tmpDir, 'multibot');
+      expect(history.learnings).toContain('Learn concurrent');
+      expect(history.decisions).toContain('Decide concurrent');
+      expect(history.fullContent).toContain('Pattern concurrent');
+    });
+
+    it('should handle 10 rapid concurrent appends without data loss', async () => {
+      await createHistoryShadow(tmpDir, 'stressbot', 'Stress test');
+
+      const count = 10;
+      const entries = Array.from({ length: count }, (_, i) => `Stress-entry-${i}`);
+
+      await Promise.all(
+        entries.map(entry =>
+          appendToHistory(tmpDir, 'stressbot', 'Learnings', entry),
+        ),
+      );
+
+      const history = await readHistory(tmpDir, 'stressbot');
+      for (const entry of entries) {
+        expect(history.learnings).toContain(entry);
+      }
+    });
+  });
+
+  describe('lock file cleanup', () => {
+    it('should not leave lock files after successful operation', async () => {
+      await createHistoryShadow(tmpDir, 'cleanbot');
+      await appendToHistory(tmpDir, 'cleanbot', 'Learnings', 'Clean entry');
+
+      const agentDir = path.join(tmpDir, '.squad', 'agents', 'cleanbot');
+      const files = await fs.readdir(agentDir);
+      const lockFiles = files.filter(f => f.endsWith('.lock'));
+
+      expect(lockFiles).toHaveLength(0);
+    });
+
+    it('should not leave temp files after successful operation', async () => {
+      await createHistoryShadow(tmpDir, 'cleanbot2');
+      await appendToHistory(tmpDir, 'cleanbot2', 'Learnings', 'Clean entry 2');
+
+      const agentDir = path.join(tmpDir, '.squad', 'agents', 'cleanbot2');
+      const files = await fs.readdir(agentDir);
+      const tmpFiles = files.filter(f => f.endsWith('.tmp'));
+
+      expect(tmpFiles).toHaveLength(0);
+    });
+
+    it('should not leave lock files after concurrent operations', async () => {
+      await createHistoryShadow(tmpDir, 'cleanbot3');
+
+      await Promise.all(
+        Array.from({ length: 5 }, (_, i) =>
+          appendToHistory(tmpDir, 'cleanbot3', 'Learnings', `Clean concurrent ${i}`),
+        ),
+      );
+
+      const agentDir = path.join(tmpDir, '.squad', 'agents', 'cleanbot3');
+      const files = await fs.readdir(agentDir);
+      const lockFiles = files.filter(f => f.endsWith('.lock'));
+      const tmpFiles = files.filter(f => f.endsWith('.tmp'));
+
+      expect(lockFiles).toHaveLength(0);
+      expect(tmpFiles).toHaveLength(0);
+    });
+  });
+
+  describe('existing API contract (regression)', () => {
+    it('createHistoryShadow creates the file', async () => {
+      const shadowPath = await createHistoryShadow(tmpDir, 'newbot', 'Hello');
+      expect(await shadowExists(tmpDir, 'newbot')).toBe(true);
+      const content = await fs.readFile(shadowPath, 'utf-8');
+      expect(content).toContain('Hello');
+    });
+
+    it('createHistoryShadow does not overwrite existing', async () => {
+      await createHistoryShadow(tmpDir, 'existbot', 'First');
+      await appendToHistory(tmpDir, 'existbot', 'Learnings', 'Important');
+
+      // Call create again — must not overwrite
+      await createHistoryShadow(tmpDir, 'existbot', 'Second');
+      const history = await readHistory(tmpDir, 'existbot');
+      expect(history.learnings).toContain('Important');
+    });
+
+    it('deleteHistoryShadow removes the file', async () => {
+      await createHistoryShadow(tmpDir, 'delbot');
+      expect(await shadowExists(tmpDir, 'delbot')).toBe(true);
+      await deleteHistoryShadow(tmpDir, 'delbot');
+      expect(await shadowExists(tmpDir, 'delbot')).toBe(false);
+    });
+
+    it('readHistory returns empty for non-existent agent', async () => {
+      const history = await readHistory(tmpDir, 'ghost');
+      expect(history.fullContent).toBe('');
+    });
+
+    it('appendToHistory throws if shadow does not exist', async () => {
+      await expect(
+        appendToHistory(tmpDir, 'nobot', 'Learnings', 'Fail'),
+      ).rejects.toThrow(/History shadow not found/);
+    });
+  });
+});