frames: StartFrame is now a control frame

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- `StartFrame` is now a control frame similar to `EndFrame`.
+
 ### Fixed
 
 - Fixed a `BaseInputTransport` issue that was causing incoming system frames to

src/pipecat/frames/frames.py

@@ -212,14 +212,6 @@ class SystemFrame(Frame):
     pass
 
 
-@dataclass
-class StartFrame(SystemFrame):
-    """This is the first frame that should be pushed down a pipeline."""
-    allow_interruptions: bool = False
-    enable_metrics: bool = False
-    report_only_initial_ttfb: bool = False
-
-
 @dataclass
 class CancelFrame(SystemFrame):
     """Indicates that a pipeline needs to stop right away."""
@@ -306,6 +298,14 @@ class ControlFrame(Frame):
     pass
 
 
+@dataclass
+class StartFrame(ControlFrame):
+    """This is the first frame that should be pushed down a pipeline."""
+    allow_interruptions: bool = False
+    enable_metrics: bool = False
+    report_only_initial_ttfb: bool = False
+
+
 @dataclass
 class EndFrame(ControlFrame):
     """Indicates that a pipeline has ended and frame processors and pipelines