diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 3abc593..a70e65c 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -1,8 +1,7 @@
 name: Release
 on:
   pull_request:
-    branches:
-      - main
+    branches: [main]
     types: [closed]
 permissions:
   contents: write
@@ -11,10 +10,16 @@ jobs:
     if: github.event.pull_request.merged == true
     runs-on: ubuntu-latest
     steps:
+      - name: Create GitHub App token
+        id: app-token
+        uses: actions/create-github-app-token@v2
+        with:
+          app-id: ${{ vars.RELEASE_APP_ID }}
+          private-key: ${{ secrets.RELEASE_APP_PRIVATE_KEY }}
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
-          token: ${{ secrets.PERSONAL_ACCESS_TOKEN }}
+          token: ${{ steps.app-token.outputs.token }}
       - name: Install uv
         uses: astral-sh/setup-uv@v6
       - name: Install the project
diff --git a/packages/asyncfast/src/asyncfast/__init__.py b/packages/asyncfast/src/asyncfast/__init__.py
index 26dbe82..2c74dcd 100644
--- a/packages/asyncfast/src/asyncfast/__init__.py
+++ b/packages/asyncfast/src/asyncfast/__init__.py
@@ -54,6 +54,12 @@
 _PARAMETER_PATTERN = re.compile(r"{(.*)}")
 
 
+class InvalidChannelDefinitionError(ValueError):
+    """
+    Raised when a channel or message handler is defined with an invalid shape.
+    """
+
+
 async def _send_message(send: AMGISendCallable, message: Mapping[str, Any]) -> None:
     message_send_event: MessageSendEvent = {
         "type": "message.send",
@@ -323,13 +329,17 @@ def _add_channel(
                 elif _is_message(message_sender_type):
                     messages = [message_sender_type]
 
-        assert len(payloads) <= 1, "Channel must have no more than 1 payload"
+        if len(payloads) > 1:
+            raise InvalidChannelDefinitionError(
+                "Channel must have no more than 1 payload"
+            )
 
         payload = payloads[0] if len(payloads) == 1 else None
 
-        assert (
-            len(message_senders) <= 1
-        ), "Channel must have no more than 1 message sender"
+        if len(message_senders) > 1:
+            raise InvalidChannelDefinitionError(
+                "Channel must have no more than 1 message sender"
+            )
 
         message_sender = message_senders[0] if len(message_senders) == 1 else None
 
diff --git a/packages/asyncfast/tests_asyncfast/test_errors.py b/packages/asyncfast/tests_asyncfast/test_errors.py
new file mode 100644
index 0000000..310beeb
--- /dev/null
+++ b/packages/asyncfast/tests_asyncfast/test_errors.py
@@ -0,0 +1,29 @@
+from typing import Any
+
+import pytest
+from asyncfast import AsyncFast
+from asyncfast import InvalidChannelDefinitionError
+from asyncfast import MessageSender
+
+
+async def test_only_one_payload() -> None:
+    app = AsyncFast()
+
+    with pytest.raises(InvalidChannelDefinitionError):
+
+        @app.channel("topic")
+        async def topic_handler(payload1: int, payload2: int) -> None:
+            pass  # pragma: no cover
+
+
+async def test_only_one_message_sender() -> None:
+    app = AsyncFast()
+
+    with pytest.raises(InvalidChannelDefinitionError):
+
+        @app.channel("topic")
+        async def topic_handler(
+            message_sender1: MessageSender[dict[str, Any]],
+            message_sender2: MessageSender[dict[str, Any]],
+        ) -> None:
+            pass  # pragma: no cover