diff --git a/Overlord-Client/cmd/agent/handlers/agent_update.go b/Overlord-Client/cmd/agent/handlers/agent_update.go
index effb4a1..6f9972f 100644
--- a/Overlord-Client/cmd/agent/handlers/agent_update.go
+++ b/Overlord-Client/cmd/agent/handlers/agent_update.go
@@ -2,6 +2,8 @@ package handlers
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"log"
@@ -15,7 +17,7 @@ import (
 	"overlord-client/cmd/agent/wire"
 )
 
-func HandleAgentUpdate(ctx context.Context, env *agentRuntime.Env, cmdID string, sourcePath string) error {
+func HandleAgentUpdate(ctx context.Context, env *agentRuntime.Env, cmdID string, sourcePath string, expectedHash string) error {
 	sourcePath = strings.TrimSpace(sourcePath)
 	if sourcePath == "" {
 		return wire.WriteMsg(ctx, env.Conn, wire.CommandResult{Type: "command_result", CommandID: cmdID, OK: false, Message: "missing update path"})
@@ -29,6 +31,10 @@ func HandleAgentUpdate(ctx context.Context, env *agentRuntime.Env, cmdID string,
 		return wire.WriteMsg(ctx, env.Conn, wire.CommandResult{Type: "command_result", CommandID: cmdID, OK: false, Message: err.Error()})
 	}
 
+	if err := verifyFileHash(sourceAbs, expectedHash); err != nil {
+		return wire.WriteMsg(ctx, env.Conn, wire.CommandResult{Type: "command_result", CommandID: cmdID, OK: false, Message: fmt.Sprintf("integrity check failed: %v", err)})
+	}
+
 	if err := wire.WriteMsg(ctx, env.Conn, wire.CommandResult{Type: "command_result", CommandID: cmdID, OK: true}); err != nil {
 		return err
 	}
@@ -139,3 +145,46 @@ func samePath(left string, right string) bool {
 	}
 	return samePathByOS(left, right)
 }
+
+func verifyFileHash(filePath string, expectedHash string) error {
+	if expectedHash == "" {
+		return nil
+	}
+	f, err := os.Open(filePath)
+	if err != nil {
+		return fmt.Errorf("failed to open file for hash verification: %w", err)
+	}
+	defer f.Close()
+	h := sha256.New()
+	if _, err := io.Copy(h, f); err != nil {
+		return fmt.Errorf("failed to compute file hash: %w", err)
+	}
+	actualHash := hex.EncodeToString(h.Sum(nil))
+	if actualHash != expectedHash {
+		return fmt.Errorf("hash mismatch: expected %s, got %s", expectedHash, actualHash)
+	}
+	return nil
+}
+
+func backupExecutable(targetPath string) error {
+	info, err := os.Stat(targetPath)
+	if err != nil || !info.Mode().IsRegular() {
+		return nil
+	}
+	backupPath := targetPath + ".bak"
+	src, err := os.Open(targetPath)
+	if err != nil {
+		return fmt.Errorf("failed to open executable for backup: %w", err)
+	}
+	defer src.Close()
+	dst, err := os.OpenFile(backupPath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, info.Mode().Perm())
+	if err != nil {
+		return fmt.Errorf("failed to create backup file: %w", err)
+	}
+	defer dst.Close()
+	if _, err := io.Copy(dst, src); err != nil {
+		return fmt.Errorf("failed to write backup: %w", err)
+	}
+	log.Printf("agent_update: backed up %q to %q", targetPath, backupPath)
+	return nil
+}
diff --git a/Overlord-Client/cmd/agent/handlers/agent_update_unix.go b/Overlord-Client/cmd/agent/handlers/agent_update_unix.go
index 07b2ee8..97f1bb1 100644
--- a/Overlord-Client/cmd/agent/handlers/agent_update_unix.go
+++ b/Overlord-Client/cmd/agent/handlers/agent_update_unix.go
@@ -5,6 +5,7 @@ package handlers
 
 import (
 	"fmt"
+	"log"
 	"os"
 	"path/filepath"
 
@@ -43,9 +44,14 @@ func runAgentUpdate(sourcePath string, enablePersistence bool) error {
 		}
 	}
 
-	if currentErr == nil && !samePath(currentExe, restartPath) {
-		if err := copyExecutableAtomic(sourcePath, currentExe); err != nil {
-			return err
+	if currentErr == nil {
+		if !(startupEnabled && samePath(currentExe, startupPath)) {
+			if err := backupExecutable(currentExe); err != nil {
+				log.Printf("agent_update: backup warning: %v", err)
+			}
+			if err := copyExecutableAtomic(sourcePath, currentExe); err != nil {
+				return err
+			}
 		}
 	}
 	_ = os.Remove(sourcePath)
diff --git a/Overlord-Client/cmd/agent/handlers/agent_update_windows.go b/Overlord-Client/cmd/agent/handlers/agent_update_windows.go
index 75fe199..35ec478 100644
--- a/Overlord-Client/cmd/agent/handlers/agent_update_windows.go
+++ b/Overlord-Client/cmd/agent/handlers/agent_update_windows.go
@@ -46,6 +46,9 @@ func runAgentUpdate(sourcePath string, enablePersistence bool) error {
 	if startupEnabled {
 		// Keep startup binary in sync when persistence/startup is enabled.
 		if !samePath(startupPath, currentExe) {
+			if err := backupExecutable(startupPath); err != nil {
+				log.Printf("agent_update[win]: backup warning for startup path: %v", err)
+			}
 			if err := copyExecutableAtomic(sourcePath, startupPath); err != nil {
 				return err
 			}
@@ -73,6 +76,9 @@ func runAgentUpdate(sourcePath string, enablePersistence bool) error {
 		return launchDeferredUpdateScript(sourcePath, restartPath)
 	}
 
+	if err := backupExecutable(restartPath); err != nil {
+		log.Printf("agent_update[win]: backup warning for restart path: %v", err)
+	}
 	if err := copyExecutableAtomic(sourcePath, restartPath); err != nil {
 		return err
 	}
diff --git a/Overlord-Client/cmd/agent/handlers/command.go b/Overlord-Client/cmd/agent/handlers/command.go
index 1d602e2..f685fd7 100644
--- a/Overlord-Client/cmd/agent/handlers/command.go
+++ b/Overlord-Client/cmd/agent/handlers/command.go
@@ -1630,7 +1630,8 @@ func HandleCommand(ctx context.Context, env *runtime.Env, envelope map[string]in
 	case "agent_update":
 		payload, _ := envelope["payload"].(map[string]interface{})
 		path, _ := payload["path"].(string)
-		return HandleAgentUpdate(ctx, env, cmdID, path)
+		hash, _ := payload["hash"].(string)
+		return HandleAgentUpdate(ctx, env, cmdID, path, hash)
 	case "process_list":
 		return HandleProcessList(ctx, env, cmdID)
 	case "process_kill":
diff --git a/Overlord-Server/src/protocol.ts b/Overlord-Server/src/protocol.ts
index e68bee1..14338b0 100644
--- a/Overlord-Server/src/protocol.ts
+++ b/Overlord-Server/src/protocol.ts
@@ -81,7 +81,8 @@ export type CommandType =
   | "plugin_load_init"
   | "plugin_load_chunk"
   | "plugin_load_finish"
-  | "plugin_unload";
+  | "plugin_unload"
+  | "agent_update";
 
 export type Command = {
   type: "command";
diff --git a/Overlord-Server/src/server/routes/deploy-routes.ts b/Overlord-Server/src/server/routes/deploy-routes.ts
index 22ff4cf..6b73856 100644
--- a/Overlord-Server/src/server/routes/deploy-routes.ts
+++ b/Overlord-Server/src/server/routes/deploy-routes.ts
@@ -1,3 +1,4 @@
+import { createHash } from "crypto";
 import fs from "fs/promises";
 import path from "path";
 import { v4 as uuidv4 } from "uuid";
@@ -298,12 +299,13 @@ export async function handleDeployRoutes(
         );
       }
 
+      const hash = createHash("sha256").update(bytes).digest("hex");
       target.ws.send(
         encodeMessage({
           type: "command",
           commandType: "agent_update",
           id: uuidv4(),
-          payload: { path: destPath },
+          payload: { path: destPath, hash },
         }),
       );