Receiver: return JSON for 401/404 instead of stdlib HTML

The default BaseHTTPRequestHandler error page is HTML, which makes the
receiver's rejection responses inconsistent with tango's own JSON-shaped
API and harder to inspect programmatically (e.g. via simulate's
response_body field).

Now responds with `{"ok": false, "error": "<code>"}` and Content-Type
application/json on 401 (invalid_signature) and 404 (not_found), and
`{"ok": true}` on 200 — explicit Content-Length on all paths.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vdavez

tango/webhooks/receiver.py

@@ -114,7 +114,7 @@ def log_message(self, format: str, *args: Any) -> None:  # noqa: A002
 
             def do_POST(self) -> None:  # noqa: N802 (stdlib API)
                 if self.path != receiver.path:
-                    self.send_error(404, "Not Found")
+                    self._write_json(404, {"ok": False, "error": "not_found"})
                     return
                 length = int(self.headers.get("Content-Length", "0") or 0)
                 body = self.rfile.read(length) if length > 0 else b""
@@ -130,7 +130,7 @@ def do_POST(self) -> None:  # noqa: N802 (stdlib API)
                 )
                 if require and not verified:
                     self._record(body, signature, verified=False)
-                    self.send_error(401, "Invalid signature")
+                    self._write_json(401, {"ok": False, "error": "invalid_signature"})
                     return
 
                 forward_status: int | None = None
@@ -145,10 +145,15 @@ def do_POST(self) -> None:  # noqa: N802 (stdlib API)
                     forward_status=forward_status,
                     forward_error=forward_error,
                 )
-                self.send_response(200)
+                self._write_json(200, {"ok": True})
+
+            def _write_json(self, status: int, body: dict[str, Any]) -> None:
+                payload = json.dumps(body).encode("utf-8")
+                self.send_response(status)
                 self.send_header("Content-Type", "application/json")
+                self.send_header("Content-Length", str(len(payload)))
                 self.end_headers()
-                self.wfile.write(b'{"ok": true}')
+                self.wfile.write(payload)
 
             def _record(
                 self,

tests/test_webhooks_receiver.py

@@ -32,6 +32,8 @@ def test_receiver_records_verified_delivery() -> None:
     with WebhookReceiver(secret=SECRET).run() as rx:
         resp = _post_signed(rx.url, body, SECRET)
         assert resp.status_code == 200
+        assert resp.headers["content-type"] == "application/json"
+        assert resp.json() == {"ok": True}
         assert rx.deliveries[0].verified is True
         assert rx.deliveries[0].body_bytes == body
         assert rx.deliveries[0].body_json == PAYLOAD
@@ -47,6 +49,8 @@ def test_receiver_rejects_bad_signature_with_401() -> None:
             timeout=5.0,
         )
         assert resp.status_code == 401
+        assert resp.headers["content-type"] == "application/json"
+        assert resp.json() == {"ok": False, "error": "invalid_signature"}
         # The bad delivery is still recorded, marked unverified, so devs
         # can debug what arrived.
         assert len(rx.deliveries) == 1
@@ -73,6 +77,8 @@ def test_receiver_404s_on_unknown_path() -> None:
         wrong = rx.url.replace("/tango/webhooks", "/elsewhere")
         resp = httpx.post(wrong, content=b"{}", timeout=5.0)
         assert resp.status_code == 404
+        assert resp.headers["content-type"] == "application/json"
+        assert resp.json() == {"ok": False, "error": "not_found"}
 
 
 def test_receiver_invokes_on_delivery_callback() -> None: