diff --git a/actions/setup/js/mount_mcp_as_cli.cjs b/actions/setup/js/mount_mcp_as_cli.cjs
index 32a1c764d05..3acaf78eaf5 100644
--- a/actions/setup/js/mount_mcp_as_cli.cjs
+++ b/actions/setup/js/mount_mcp_as_cli.cjs
@@ -609,7 +609,9 @@ async function main() {
     }
     core.info(`  Found ${tools.length} tool(s)`);
 
-    // Cache the tool list
+    // Cache the tool list. This file only contains tool name/description/schema
+    // metadata returned by the gateway; it does not contain the API key or any
+    // other secret, so world-readable permissions (0o644) are acceptable here.
     try {
       fs.writeFileSync(toolsFile, JSON.stringify(tools, null, 2), { mode: 0o644 });
     } catch (err) {
@@ -618,13 +620,31 @@ async function main() {
 
     // Write the CLI wrapper script using the container-accessible URL
     const scriptPath = path.join(CLI_BIN_DIR, name);
+    let scriptFd;
     try {
-      fs.writeFileSync(scriptPath, generateCLIWrapperScript(name, containerUrl, toolsFile, apiKey, bridgeScript), { mode: 0o755 });
+      // Owner-only permissions: the wrapper script embeds the plaintext gateway API key,
+      // so it must not be world- or group-readable (matches chmod 600 used elsewhere for
+      // this same credential, e.g. convert_gateway_config_copilot.sh).
+      // Note: writeFileSync(mode) only applies when creating a new file; for existing files,
+      // force mode with fchmodSync so prior permissive modes (e.g., 0o755) are corrected.
+      scriptFd = fs.openSync(scriptPath, "w", 0o700);
+      fs.fchmodSync(scriptFd, 0o700);
+      fs.writeFileSync(scriptFd, generateCLIWrapperScript(name, containerUrl, toolsFile, apiKey, bridgeScript), "utf8");
+      fs.closeSync(scriptFd);
+      scriptFd = undefined;
       mountedServers.push(name);
       mountedServerTools.push({ name, tools });
       core.info(`  ✓ Mounted as: ${scriptPath}`);
     } catch (err) {
       core.warning(`  Failed to write CLI wrapper for ${name}: ${getErrorMessage(err)}`);
+    } finally {
+      if (scriptFd !== undefined) {
+        try {
+          fs.closeSync(scriptFd);
+        } catch {
+          // Ignore close errors in cleanup path; main error already reported above.
+        }
+      }
     }
   }
 
diff --git a/actions/setup/js/mount_mcp_as_cli.test.cjs b/actions/setup/js/mount_mcp_as_cli.test.cjs
index d8b60858c19..8bc37491be5 100644
--- a/actions/setup/js/mount_mcp_as_cli.test.cjs
+++ b/actions/setup/js/mount_mcp_as_cli.test.cjs
@@ -1,6 +1,7 @@
 // @ts-check
-import { describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 import fs from "fs";
+import http from "http";
 import os from "os";
 import path from "path";
 
@@ -328,3 +329,93 @@ describe("mount_mcp_as_cli.cjs", () => {
     expect(warnings[1]).toContain("stopping empty tools/list retries");
   });
 });
+
+describe("mount_mcp_as_cli.cjs main() file permissions", () => {
+  /** @type {string | undefined} */
+  let tempDir;
+  /** @type {http.Server | undefined} */
+  let server;
+
+  afterEach(async () => {
+    if (server) {
+      await new Promise(resolve => server.close(resolve));
+      server = undefined;
+    }
+    if (tempDir) {
+      // The bin directory is locked to 0o555 by main(); restore write permissions
+      // so the temp directory can be removed during cleanup.
+      const binDir = path.join(tempDir, "gh-aw/mcp-cli/bin");
+      if (fs.existsSync(binDir)) {
+        fs.chmodSync(binDir, 0o755);
+      }
+      fs.rmSync(tempDir, { recursive: true, force: true });
+      tempDir = undefined;
+    }
+    delete process.env.RUNNER_TEMP;
+    delete process.env.MCP_GATEWAY_API_KEY;
+    delete process.env.MCP_GATEWAY_DOMAIN;
+    delete process.env.MCP_GATEWAY_PORT;
+    delete global.core;
+    vi.resetModules();
+  });
+
+  it("rewrites an existing 0o755 CLI wrapper script to owner-only permissions (0o700)", async () => {
+    // Minimal fake MCP server that answers initialize / notifications/initialized / tools/list
+    server = http.createServer((req, res) => {
+      let data = "";
+      req.on("data", chunk => (data += chunk));
+      req.on("end", () => {
+        const parsed = JSON.parse(data);
+        if (parsed.method === "tools/list") {
+          res.writeHead(200, { "Content-Type": "application/json" });
+          res.end(JSON.stringify({ jsonrpc: "2.0", id: parsed.id, result: { tools: [{ name: "echo" }] } }));
+        } else {
+          res.writeHead(200, { "Content-Type": "application/json" });
+          res.end(JSON.stringify({ jsonrpc: "2.0", id: parsed.id, result: {} }));
+        }
+      });
+    });
+    await new Promise(resolve => server.listen(0, "127.0.0.1", resolve));
+    const address = server.address();
+    const port = typeof address === "object" && address ? address.port : 0;
+
+    tempDir = fs.mkdtempSync(path.join(os.tmpdir(), "mcp-cli-mount-test-"));
+    process.env.RUNNER_TEMP = tempDir;
+    process.env.MCP_GATEWAY_API_KEY = "super-secret-gateway-key";
+    delete process.env.MCP_GATEWAY_DOMAIN;
+    delete process.env.MCP_GATEWAY_PORT;
+
+    const manifestDir = path.join(tempDir, "gh-aw/mcp-cli");
+    fs.mkdirSync(manifestDir, { recursive: true });
+    fs.writeFileSync(path.join(manifestDir, "manifest.json"), JSON.stringify({ servers: [{ name: "testserver", url: `http://127.0.0.1:${port}/mcp` }] }), "utf8");
+
+    const binDir = path.join(tempDir, "gh-aw/mcp-cli/bin");
+    fs.mkdirSync(binDir, { recursive: true });
+    const scriptPath = path.join(binDir, "testserver");
+    fs.writeFileSync(scriptPath, "#!/usr/bin/env bash\necho preexisting\n", { mode: 0o755 });
+    fs.chmodSync(scriptPath, 0o755);
+
+    const infos = [];
+    const warnings = [];
+    global.core = {
+      info: msg => infos.push(msg),
+      warning: msg => warnings.push(msg),
+      addPath: () => {},
+      setOutput: () => {},
+    };
+
+    vi.resetModules();
+    const mod = await import("./mount_mcp_as_cli.cjs?t=" + Date.now());
+
+    await mod.main();
+
+    expect(fs.existsSync(scriptPath)).toBe(true);
+
+    const mode = fs.statSync(scriptPath).mode & 0o777;
+    expect(mode).toBe(0o700);
+    expect(mode).not.toBe(0o755);
+
+    const scriptContent = fs.readFileSync(scriptPath, "utf8");
+    expect(scriptContent).toContain("super-secret-gateway-key");
+  });
+});