diff --git a/src/labthings_fastapi/webthing_subprotocol.py b/src/labthings_fastapi/webthing_subprotocol.py
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+++ b/src/labthings_fastapi/webthing_subprotocol.py
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+"""WebThing WebSocket subprotocol models.
+
+This module defines models for the messages sent over websockets, which are aligned with
+the ``webthingprotocol`` subprotocol as set out in the `community group draft report`_.
+
+.. _community group draft report: https://w3c.github.io/web-thing-protocol/
+
+(c) Richard Bowman October 2026, released under MIT license
+"""
+
+from datetime import datetime
+from typing import Any, Literal
+from uuid import UUID, uuid4
+
+from pydantic import BaseModel, Field
+
+from labthings_fastapi.middleware.url_for import URLFor
+
+
+class WebsocketMessage(BaseModel):
+    """A base model for all websocket messages.
+
+    This will auto-generate the messageID and timestamp if they
+    are not supplied.
+    """
+
+    thingID: str | URLFor
+    messageID: UUID = Field(default_factory=uuid4)
+    messageType: Literal["request", "response", "notification"]
+    operation: str
+    correlationID: UUID | None = None
+    timestamp: datetime | None = Field(default_factory=datetime.now)
+
+
+class ObservePropertyMessage(WebsocketMessage):
+    """A base model for messages related to observing a property."""
+
+    name: str
+    operation: Literal["observeproperty"] = "observeproperty"
+
+
+class ObservePropertyNotification(ObservePropertyMessage):
+    """A model for property change messages."""
+
+    messageType: Literal["notification"] = "notification"
+    value: Any
+
+
+class ObserveActionMessage(WebsocketMessage):
+    """A base model for messages related to observing an action."""
+
+    name: str
+    operation: Literal["observeaction"] = "observeaction"
+
+
+class ObserveActionNotification(ObserveActionMessage):
+    """A model for action notification messages.
+
+    This is not part of the webthingprotocol draft, so should be considered
+    at risk of summary removal.
+    """
+
+    messageType: Literal["notification"] = "notification"
+    actionID: UUID
+    state: Literal["pending", "running", "completed", "failed"]
+
+
+class ActionStatus(BaseModel):
+    """The status of an action invocation."""
+
+    actionID: UUID
+    state: Literal["pending", "running", "completed", "failed"]
+    output: Any | None = None
+    error: "ProblemDetails | None" = None
+    timeRequested: datetime | None = None
+    timeEnded: datetime | None = None
+
+
+class ProblemDetails(BaseModel):
+    """Details of an error.
+
+    This follows RFC9457_.
+
+    .. _RFC9457: https://datatracker.ietf.org/doc/html/rfc9457
+    """
+
+    status: int
+    type: str
+    title: str
+    detail: str | None = None
+
+
+class ObservationErrorResponse(WebsocketMessage):
+    """A websocket error response for observing an action or property."""
+
+    messageType: Literal["response"] = "response"
+    operation: Literal["observeaction", "observeproperty"]
+    name: str
+    error: ProblemDetails