diff --git a/src/backend/models/api/measurement.py b/src/backend/models/api/measurement.py
index 4b5980e..7aabeb3 100644
--- a/src/backend/models/api/measurement.py
+++ b/src/backend/models/api/measurement.py
@@ -12,6 +12,11 @@ class MeasurementCreate(BaseModel):
     value: float
     lot_number: Optional[str] = Field(None, max_length=100)
     serial_number: Optional[str] = Field(None, max_length=100)
+    # "camera" is deliberately absent from this pattern, even though the ORM
+    # enum and the database allow it. Do not add it here: a camera
+    # measurement must be produced by vision_service.execute_task, never by a
+    # client posting JSON straight to this endpoint - there is no image, no
+    # graph and no engine_version behind a value that arrived this way.
     input_method: str = Field("manual", pattern="^(usb_caliper|manual)$")
     input_duration_ms: Optional[int] = Field(None, ge=0)
     # The production this belongs to, when one is open at the station.