StartFrame is not reaching serialize method of FrameSerializer

[rohitkhatri]

pipecat version

0.0.94

Python version

3.13.2

Operating System

macOS 15.6.1 (24G90)

Issue description

I'm writing a custom serializer for a third party service and I need to send a session.ready event when the pipeline starts therefore I'm trying to put a condition with StartFrame but It's not reaching in serializer method of Serializer

if  isinstance(frame, StartFrame):
    print("StartFrame received")
    return '{"type": "session.ready"}'

Reproduction steps

Just try to put the StartFrame condition in a custom serializer like below

if  isinstance(frame, StartFrame):
    print("StartFrame received")
    return '{"type": "session.ready"}'

And the condition should be executed

Expected behavior

The serialize method in Serializer class should receive the StartFrame

Actual behavior

The StartFrame is not received in the serialize method of Serializer class

Logs

[markbackman]

Serializers aren't FrameProcessors, so they don't have access to Frames. Instead, there's an on_pipeline_started event handler that you should be able to use to get access to the same timing information.

[rohitkhatri]

I got your point, but If something is related to a provider like twilio which needs to receive an event called session.ready to finalize the connection, what is the right place to implement.

Currently I'm doing it in the @transport.event_handler("on_client_connected") listener where I'm directly accessing the websocket using the runner_args.websocket.send_json({"type": "session.ready"}) but my only concern is that It's related to a provider and If I change the provider later to something else, I should be changing just the serializer and It should work.

Is there a better way to handle this?