diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8f96d96..6189d6c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- `query`, `mutate` and `request_graphql` given a `gql` document raise a `TypeError` that says to pass the query string, instead of `Object of type DocumentNode is not JSON serializable`.
+
 ## [v6.0.0] - 2026-10-28
 
 - BREAKING: Drop support for Python 3.8 and 3.9, which are end of life. The minimum supported version is now Python 3.10.
diff --git a/python_anvil/api.py b/python_anvil/api.py
index 0916299..5897d57 100644
--- a/python_anvil/api.py
+++ b/python_anvil/api.py
@@ -321,6 +321,12 @@ def request_graphql(
         See the GraphQL reference for every query and mutation:
         https://www.useanvil.com/docs/api/graphql/reference/
         """
+        if not isinstance(query, str):
+            raise TypeError(
+                f"The query must be a string, not {type(query).__name__}. gql "
+                "documents aren't accepted since 6.0.0: pass the string you gave "
+                "gql(), or graphql.print_ast(document) for one built with its DSL."
+            )
         given = (
             json_lib.loads(variables)
             if isinstance(variables, (str, bytes))
@@ -370,7 +376,7 @@ def mutate(
         timeout: float | tuple[float, float] | None = None,
     ) -> dict[str, Any]:
         """Send a GraphQL mutation, as a string or a mutation class. The same as `request_graphql`."""
-        mutation = query if isinstance(query, str) else query.get_mutation()
+        mutation = query.get_mutation() if isinstance(query, BaseQuery) else query
         return self.request_graphql(mutation, variables, timeout=timeout)
 
     def create_etch_packet(
diff --git a/tests/test_compat.py b/tests/test_compat.py
index 9d8f36e..96d74a6 100644
--- a/tests/test_compat.py
+++ b/tests/test_compat.py
@@ -8,6 +8,7 @@
 from unittest import mock
 
 import pytest
+from graphql import parse
 
 from python_anvil import Anvil
 from python_anvil.api_resources.mutations import CreateEtchPacket, ForgeSubmit
@@ -91,6 +92,14 @@ def create_payload(self):
             == "mutation Mine($eid: String!) { mine(eid: $eid) { eid } }"
         )
 
+    def they_refuse_a_gql_document_before_sending(anvil):
+        document = parse("{ currentUser { eid } }")
+        with mock.patch.object(anvil.client, "request") as request:
+            for method in (anvil.query, anvil.mutate, anvil.request_graphql):
+                with pytest.raises(TypeError, match=r"not DocumentNode.*print_ast"):
+                    method(document)
+        request.assert_not_called()
+
     def the_old_module_constants_are_the_mutations():
         assert mutations.create_etch_packet.generate_mutation() == CREATE_ETCH_PACKET
         assert mutations.forge_submit.generate_mutation() == FORGE_SUBMIT