diff --git a/src/mcp/client/session.py b/src/mcp/client/session.py
index 35cae2d314..247ad5302b 100644
--- a/src/mcp/client/session.py
+++ b/src/mcp/client/session.py
@@ -78,6 +78,17 @@ def _clamp_inbound_ttl(raw: dict[str, Any]) -> None:
         raw["ttlMs"] = 0
 
 
+def _default_inbound_complete_result(raw: dict[str, Any], version: str) -> dict[str, Any]:
+    if (
+        version not in MODERN_PROTOCOL_VERSIONS
+        or "resultType" in raw
+        or "inputRequests" in raw
+        or "requestState" in raw
+    ):
+        return raw
+    return {"resultType": "complete", **raw}
+
+
 @cache
 def _wire_fields(target: type[BaseModel] | UnionType) -> frozenset[str]:
     """Top-level wire keys `target` declares (its members', for a union)."""
@@ -587,6 +598,7 @@ async def send_request(
         _clamp_inbound_ttl(raw)
         # Literal fallback covers pre-handshake and stateless; matches runner.py.
         version = self._negotiated_version or "2025-11-25"
+        raw = _default_inbound_complete_result(raw, version)
         try:
             _methods.validate_server_result(method, version, raw)
         except KeyError:
diff --git a/tests/client/test_result_type_compat.py b/tests/client/test_result_type_compat.py
new file mode 100644
index 0000000000..a6cc5a60a7
--- /dev/null
+++ b/tests/client/test_result_type_compat.py
@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+import pytest
+
+from mcp.client.session import _default_inbound_complete_result
+from mcp_types import methods
+
+
+def test_modern_missing_discriminator_is_a_complete_result_for_the_client() -> None:
+    original = {"content": [{"type": "text", "text": "ok"}]}
+    normalized = _default_inbound_complete_result(original, "2026-07-28")
+    assert original == {"content": [{"type": "text", "text": "ok"}]}
+    assert normalized["resultType"] == "complete"
+    methods.validate_server_result("tools/call", "2026-07-28", normalized)
+
+
+@pytest.mark.parametrize("body", [
+    {"resultType": "input_required", "inputRequests": {"confirm": {}}},
+    {"resultType": "task", "content": []},
+    {"inputRequests": {"confirm": {}}},
+    {"requestState": "sealed"},
+    {"resultType": None, "content": []},
+])
+def test_explicit_or_pending_result_types_are_never_downgraded(body: dict) -> None:
+    assert _default_inbound_complete_result(body, "2026-07-28") == body
+
+
+def test_legacy_wire_is_never_rewritten() -> None:
+    body = {"content": []}
+    assert _default_inbound_complete_result(body, "2025-11-25") is body