diff --git a/packages/cli/package.json b/packages/cli/package.json
index be1e999f..c6092d7d 100644
--- a/packages/cli/package.json
+++ b/packages/cli/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.27.6",
+  "version": "0.27.7",
   "type": "module",
   "bin": {
     "spawn": "cli.js"
diff --git a/packages/cli/src/shared/agent-setup.ts b/packages/cli/src/shared/agent-setup.ts
index a7ed108b..af65880d 100644
--- a/packages/cli/src/shared/agent-setup.ts
+++ b/packages/cli/src/shared/agent-setup.ts
@@ -299,6 +299,9 @@ export async function offerGithubAuth(runner: CloudRunner, explicitlyRequested?:
   let ghCmd = "curl --proto '=https' -fsSL https://openrouter.ai/labs/spawn/shared/github-auth.sh | bash";
   if (githubToken) {
     const tokenB64 = Buffer.from(githubToken).toString("base64");
+    if (!/^[A-Za-z0-9+/=]+$/.test(tokenB64)) {
+      throw new Error("Unexpected characters in base64 output");
+    }
     ghCmd = `export GITHUB_TOKEN=$(printf '%s' ${shellQuote(tokenB64)} | base64 -d) && ${ghCmd}`;
   }