How can I fix the 'Cloud Run service `app` does not exist' error during Firebase deployment?

Below is an explanation and a recommended fix.

What’s Happening

Firebase Hosting is trying to route all incoming requests to a backend by using a rewrite rule. Your current rewrite specifies a function as a Cloud Run service with the following structure:

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"rewrites": [
  {
    "source": "**",
    "function": {
      "functionId": "app",
      "region": "us-central1"
    }
  }
]

The error

Cloud Run service `app` does not exist in region `us-central1` in this project.

indicates that Firebase Hosting expected to find a Cloud Run service named “app” in that region—but it couldn’t. This happens even though your function deployed successfully (with “firebase deploy --only functions”) because the Hosting rewrite format is interpreted differently.

Likely Causes

1. Incorrect Rewrite Format
   When using Cloud Functions with Firebase Hosting, a rewrite typically points to a function by its name without wrapping it in a sub‑object. For example, your rewrite can simply be:

   JSONCopied
   {
     "source": "**",
     "function": "app"
   }

2. Misplaced Configuration Key
   Your configuration block in firebase.json is under "config" rather than "hosting". Firebase Hosting expects its configuration to be defined under the "hosting" key. Placing it under the wrong key might cause the CLI or the backend to interpret the rewrite in an unexpected manner.

Recommended Fixes

1. Move the Hosting Settings Under the Correct Key
   Ensure your Firebase configuration is structured as follows:

   JSONCopied
   {
     "hosting": {
       "public": "static",
       "ignore": [
         "firebase.json",
         "**/.*",
         "**/node_modules/**",
         "**/*.map"
       ],
       "rewrites": [
         {
           "source": "**",
           "function": "app"
         }
       ],
       "cleanUrls": true,
       "site": "toyoura-yutaka"
     }
   }

   This tells Firebase Hosting to forward all routes to the Cloud Function named app as expected.

2. Redeploy Your Function (if needed)
   After correcting the configuration, run:

   BASHCopied
   firebase deploy --only hosting,functions

   This ensures that your updated rewrite is applied and that the function is accessible the way Firebase Hosting expects.

Why the Error Occurred

Even though your function deploy succeeded, the rewrite format that includes a nested "functionId" and "region" is typically used when explicitly referencing a Cloud Run service. Since your function was deployed as a Cloud Function (even though under the hood Cloud Functions use Cloud Run), the custom object format caused Firebase Hosting to look for a native Cloud Run service named app in us-central1—which wasn’t there.

Following the recommendations above should resolve the error by aligning your configuration with standard Firebase Hosting rewrite rules for Cloud Functions.