diff --git a/index.ts b/index.ts
index 6cb5196..8c301ca 100644
--- a/index.ts
+++ b/index.ts
@@ -6,11 +6,58 @@ import { log } from "./logger.js"
 
 log("Plugin module loaded")
 
+// =============================================================================
+// Global mutex — prevents double startup (server + client both load plugins)
+// =============================================================================
+
+const LOCK_FILE = "/tmp/opencode-matrix-plugin.lock"
+
+function tryAcquireLock(): boolean {
+  try {
+    // Check if an existing lock is stale (process no longer running)
+    if (fs.existsSync(LOCK_FILE)) {
+      const existingPid = parseInt(fs.readFileSync(LOCK_FILE, "utf-8").trim(), 10)
+      if (existingPid && existingPid !== process.pid) {
+        try {
+          process.kill(existingPid, 0) // throws if process doesn't exist
+        } catch {
+          // Process dead, remove stale lock
+          fs.unlinkSync(LOCK_FILE)
+        }
+      } else if (existingPid === process.pid) {
+        // Same process re-entering (shouldn't happen, but be safe)
+        return false
+      } else {
+        return false // Valid lock held by another process
+      }
+    }
+    fs.writeFileSync(LOCK_FILE, String(process.pid), { mode: 0o644 })
+    return true
+  } catch {
+    return false
+  }
+}
+
+function releaseLock(): void {
+  try {
+    fs.unlinkSync(LOCK_FILE)
+  } catch {
+    // ignore
+  }
+}
+
 // =============================================================================
 // Plugin Setup
 // =============================================================================
 
 async function runPlugin(ctx: any, options: any) {
+  // Acquire global lock — only one instance (server or client) starts the bot
+  const locked = tryAcquireLock()
+  if (!locked) {
+    log("Another instance already running (lock held), skipping bot startup")
+    return () => {}
+  }
+
   const eventController = new AbortController()
   let cleanup: (() => void) | undefined
 
@@ -213,15 +260,15 @@ async function runPlugin(ctx: any, options: any) {
       log("Plugin cleaning up...")
       eventController.abort()
       matrix.stop()
+      releaseLock()
     }
 
     return cleanup
   } catch (err) {
     fs.appendFileSync("/tmp/opencode-matrix-plugin/setup-error.log", `[${new Date().toISOString()}] ${String(err)}\n`)
     log(`Setup error: ${String(err)}`)
-    return () => {
-      cleanup?.()
-    }
+    releaseLock()
+    return () => {}
   }
 }