Content-length Header Not Being Set On Flask App Engine Response For Served Blob

In my Flask-based Google App Engine server, I am trying to return a response with a 'content-length' header which will contain the final size of a blob being served to the client.

Solution 1:

The Content-Length header is included in the list of headers that can't be modified.

As you and the documentation pointed out, App Engine calculates the Content-Length header from the request data and adds it to the request prior to sending.

You can always set a personalized header, Flask allows it, and GAE will let you use it.

For example, for your response:

response.headers['CL-blob'] = blobstore.BlobInfo(version.blob_key).size