fix: cap rendered webhook prompts

maintainer edit: prevent custom webhook templates from enqueueing the full request body into the agent/session path. Reuses the existing prompt cap and adds regression coverage for {{ body }}.

nanobot/webhooks.py

@@ -363,7 +363,7 @@ def _render_prompt(route: WebhookRouteConfig, context: dict[str, Any]) -> str:
         raise WebhookError(400, f"webhook prompt template failed: {exc}") from exc
     if not rendered.strip():
         raise WebhookError(400, "webhook prompt template rendered empty content")
-    return rendered
+    return truncate_text(rendered, _DEFAULT_PROMPT_MAX_CHARS)
 
 
 def _render_thread(route: WebhookRouteConfig, context: dict[str, Any]) -> str:

tests/test_webhooks.py

@@ -117,6 +117,37 @@ async def test_generic_webhook_accepts_hmac_signature() -> None:
     assert "untrusted external data" in msg.content
 
 
+@pytest.mark.asyncio
+async def test_custom_prompt_is_truncated_after_rendering() -> None:
+    bus = MessageBus()
+    router = WebhookRouter(
+        WebhooksConfig(
+            routes={
+                "big": WebhookRouteConfig(
+                    auth="none",
+                    to="websocket:ops",
+                    prompt="{{ body }}",
+                )
+            }
+        ),
+        bus,
+    )
+    body = b"a" * 1_048_576
+
+    response = await router.handle(
+        method="POST",
+        path="/webhooks/big",
+        headers={},
+        body=body,
+    )
+
+    assert response is not None
+    assert response.status == 202
+    msg = await bus.consume_inbound()
+    assert len(msg.content) < len(body)
+    assert msg.content.endswith("\n... (truncated)")
+
+
 @pytest.mark.asyncio
 async def test_webhook_without_delivery_id_is_not_deduped() -> None:
     bus = MessageBus()