diff --git a/packages/cli/package.json b/packages/cli/package.json
index 06dca7d7..038481a2 100644
--- a/packages/cli/package.json
+++ b/packages/cli/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "1.0.9",
+  "version": "1.0.10",
   "type": "module",
   "bin": {
     "spawn": "cli.js"
diff --git a/packages/cli/src/__tests__/update-check.test.ts b/packages/cli/src/__tests__/update-check.test.ts
index 8a378fda..d20f34e2 100644
--- a/packages/cli/src/__tests__/update-check.test.ts
+++ b/packages/cli/src/__tests__/update-check.test.ts
@@ -6,6 +6,9 @@ import path from "node:path";
 import { tryCatch } from "@openrouter/spawn-shared";
 import pkg from "../../package.json";
 
+// Fake install script returned by the mocked curl call — must pass validateInstallScript()
+const FAKE_INSTALL_SCRIPT = "#!/bin/bash\n# fake install script for tests\necho 'installing spawn'\n" + "x".repeat(200);
+
 // ── Test Helpers ───────────────────────────────────────────────────────────────
 
 /** Remove the .update-failed backoff file so it doesn't interfere with tests */
@@ -98,7 +101,9 @@ describe("update-check", () => {
 
       // Mock execFileSync to prevent actual update + re-exec
       const { executor } = await import("../update-check.js");
-      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation(() => Buffer.from(""));
+      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((file: string) =>
+        Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : ""),
+      );
 
       const { checkForUpdates } = await import("../update-check.js");
       await checkForUpdates();
@@ -113,7 +118,9 @@ describe("update-check", () => {
 
       // Mock execFileSync to prevent actual update + re-exec
       const { executor } = await import("../update-check.js");
-      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation(() => Buffer.from(""));
+      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((file: string) =>
+        Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : ""),
+      );
 
       const { checkForUpdates } = await import("../update-check.js");
       await checkForUpdates();
@@ -141,7 +148,9 @@ describe("update-check", () => {
 
       // Mock executor to prevent actual commands
       const { executor } = await import("../update-check.js");
-      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation(() => Buffer.from(""));
+      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((file: string) =>
+        Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : ""),
+      );
 
       const { checkForUpdates } = await import("../update-check.js");
       await checkForUpdates();
@@ -223,7 +232,7 @@ describe("update-check", () => {
             args,
             options,
           });
-          return Buffer.from("");
+          return Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : "");
         },
       );
 
@@ -260,7 +269,7 @@ describe("update-check", () => {
             args,
             options,
           });
-          return Buffer.from("");
+          return Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : "");
         },
       );
 
@@ -300,7 +309,7 @@ describe("update-check", () => {
             args,
             options,
           });
-          return Buffer.from("");
+          return Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : "");
         },
       );
 
@@ -356,7 +365,7 @@ describe("update-check", () => {
 
       const { executor } = await import("../update-check.js");
       let callCount = 0;
-      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((): Buffer => {
+      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((file: string): Buffer => {
         callCount++;
         // First 3 calls succeed (curl, bash, which), 4th call (re-exec) fails
         if (callCount >= 4) {
@@ -366,7 +375,7 @@ describe("update-check", () => {
           });
           throw err;
         }
-        return Buffer.from("");
+        return Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : "");
       });
 
       const { checkForUpdates } = await import("../update-check.js");
@@ -443,7 +452,7 @@ describe("update-check", () => {
           file,
           args,
         });
-        return Buffer.from("");
+        return Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : "");
       });
 
       const { checkForUpdates } = await import("../update-check.js");
@@ -481,7 +490,9 @@ describe("update-check", () => {
       process.env.SPAWN_AUTO_UPDATE = undefined;
       const fetchSpy = spyOn(global, "fetch").mockImplementation(() => Promise.resolve(new Response("1.0.99\n")));
       const { executor } = await import("../update-check.js");
-      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation(() => Buffer.from(""));
+      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((file: string) =>
+        Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : ""),
+      );
 
       const { checkForUpdates } = await import("../update-check.js");
       await checkForUpdates();
@@ -501,7 +512,9 @@ describe("update-check", () => {
       process.env.SPAWN_AUTO_UPDATE = undefined;
       const fetchSpy = spyOn(global, "fetch").mockImplementation(() => Promise.resolve(new Response("1.1.0\n")));
       const { executor } = await import("../update-check.js");
-      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation(() => Buffer.from(""));
+      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((file: string) =>
+        Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : ""),
+      );
 
       const { checkForUpdates } = await import("../update-check.js");
       await checkForUpdates();
@@ -523,7 +536,9 @@ describe("update-check", () => {
       process.env.SPAWN_AUTO_UPDATE = undefined;
       const fetchSpy = spyOn(global, "fetch").mockImplementation(() => Promise.resolve(new Response("2.0.0\n")));
       const { executor } = await import("../update-check.js");
-      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation(() => Buffer.from(""));
+      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((file: string) =>
+        Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : ""),
+      );
 
       const { checkForUpdates } = await import("../update-check.js");
       await checkForUpdates();
@@ -540,7 +555,9 @@ describe("update-check", () => {
       process.env.SPAWN_AUTO_UPDATE = "1";
       const fetchSpy = spyOn(global, "fetch").mockImplementation(() => Promise.resolve(new Response("1.1.0\n")));
       const { executor } = await import("../update-check.js");
-      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation(() => Buffer.from(""));
+      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((file: string) =>
+        Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : ""),
+      );
 
       const { checkForUpdates } = await import("../update-check.js");
       await checkForUpdates();
@@ -558,7 +575,9 @@ describe("update-check", () => {
       process.env.SPAWN_NO_AUTO_UPDATE = "1";
       const fetchSpy = spyOn(global, "fetch").mockImplementation(() => Promise.resolve(new Response("1.0.99\n")));
       const { executor } = await import("../update-check.js");
-      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation(() => Buffer.from(""));
+      const execFileSyncSpy = spyOn(executor, "execFileSync").mockImplementation((file: string) =>
+        Buffer.from(file === "curl" ? FAKE_INSTALL_SCRIPT : ""),
+      );
 
       const { checkForUpdates } = await import("../update-check.js");
       await checkForUpdates();
diff --git a/packages/cli/src/update-check.ts b/packages/cli/src/update-check.ts
index d05a2b60..083a4b1b 100644
--- a/packages/cli/src/update-check.ts
+++ b/packages/cli/src/update-check.ts
@@ -25,6 +25,7 @@ export const executor = {
 // ── Constants ──────────────────────────────────────────────────────────────────
 
 const FETCH_TIMEOUT = 10000; // 10 seconds
+const MIN_INSTALL_SCRIPT_BYTES = 100; // reject suspiciously small scripts
 const UPDATE_BACKOFF_MS = 60 * 60 * 1000; // 1 hour
 const UPDATE_CHECK_INTERVAL_MS = 60 * 60 * 1000; // 1 hour — skip network check if last success was recent
 
@@ -259,6 +260,39 @@ function reExecWithArgs(): void {
   }
 }
 
+/**
+ * Validate a downloaded install script before execution.
+ *
+ * Checks:
+ * 1. Non-empty and above a minimum size threshold (rejects truncated downloads)
+ * 2. Starts with the expected shebang / header for its platform
+ *
+ * Security note: This is NOT a substitute for cryptographic integrity
+ * verification (SHA256 checksum or code signing). The release pipeline does
+ * not currently publish checksums for the install script, so we rely on
+ * HTTPS (TLS) for transport integrity. These checks catch corruption or
+ * truncation, not a compromised CDN. See GitHub issue #3297.
+ */
+function validateInstallScript(content: string, platform: "unix" | "windows"): void {
+  if (content.length < MIN_INSTALL_SCRIPT_BYTES) {
+    throw new Error(
+      `Install script too small (${content.length} bytes, minimum ${MIN_INSTALL_SCRIPT_BYTES}). ` +
+        "Download may be corrupted or truncated.",
+    );
+  }
+
+  if (platform === "unix") {
+    if (!content.startsWith("#!/")) {
+      throw new Error("Install script missing expected shebang (#!/...). Download may be corrupted.");
+    }
+  } else {
+    // PowerShell scripts should contain recognizable PS content
+    if (!content.includes("$") && !content.includes("function")) {
+      throw new Error("Install script does not appear to be valid PowerShell. Download may be corrupted.");
+    }
+  }
+}
+
 function performAutoUpdate(latestVersion: string, jsonOutput = false): void {
   printUpdateBanner(latestVersion);
 
@@ -295,6 +329,8 @@ function performAutoUpdate(latestVersion: string, jsonOutput = false): void {
       },
     );
     const scriptContent = scriptBytes ? scriptBytes.toString() : "";
+    const platform = isWindows() ? "windows" : "unix";
+    validateInstallScript(scriptContent, platform);
 
     if (isWindows()) {
       // Windows: write to temp file and execute via PowerShell