diff --git a/src/connectrpc/_protocol.py b/src/connectrpc/_protocol.py
index 7009f29..eb69a2d 100644
--- a/src/connectrpc/_protocol.py
+++ b/src/connectrpc/_protocol.py
@@ -6,7 +6,9 @@
 from http import HTTPStatus
 from typing import TYPE_CHECKING, Protocol, TypeVar, cast
 
+from google.protobuf import symbol_database
 from google.protobuf.any_pb2 import Any
+from google.protobuf.json_format import MessageToDict
 
 from ._compression import Compression
 from .code import Code
@@ -157,19 +159,29 @@ def to_http_status(self) -> ExtendedHTTPStatus:
     def to_dict(self) -> dict:
         data: dict = {"code": self.code.value, "message": self.message}
         if self.details:
-            details: list[dict[str, str]] = []
+            details: list[dict] = []
             for detail in self.details:
                 if detail.type_url.startswith("type.googleapis.com/"):
                     detail_type = detail.type_url[len("type.googleapis.com/") :]
                 else:
                     detail_type = detail.type_url
-                details.append(
-                    {
-                        "type": detail_type,
-                        # Connect requires unpadded base64
-                        "value": b64encode(detail.value).decode("utf-8").rstrip("="),
-                    }
-                )
+                detail_dict: dict = {
+                    "type": detail_type,
+                    # Connect requires unpadded base64
+                    "value": b64encode(detail.value).decode("utf-8").rstrip("="),
+                }
+                # Try to produce debug info, but expect failure when we don't
+                # have descriptors for the message type.
+                debug = None
+                try:
+                    msg_instance = symbol_database.Default().GetSymbol(detail_type)()
+                    if detail.Unpack(msg_instance):
+                        debug = MessageToDict(msg_instance)
+                except Exception:
+                    debug = None
+                if debug is not None:
+                    detail_dict["debug"] = debug
+                details.append(detail_dict)
             data["details"] = details
         return data
 
diff --git a/test/test_details.py b/test/test_details.py
index b6d5ee2..5c35f40 100644
--- a/test/test_details.py
+++ b/test/test_details.py
@@ -3,10 +3,13 @@
 from typing import NoReturn
 
 import pytest
+from google.protobuf.any_pb2 import Any as AnyPb
+from google.protobuf.duration_pb2 import Duration
 from google.protobuf.struct_pb2 import Struct, Value
 from pyqwest import Client, SyncClient
 from pyqwest.testing import ASGITransport, WSGITransport
 
+from connectrpc._protocol import ConnectWireError
 from connectrpc.code import Code
 from connectrpc.errors import ConnectError, pack_any
 
@@ -81,3 +84,49 @@ async def make_hat(self, request, ctx) -> NoReturn:
     s1 = Struct()
     assert exc_info.value.details[1].Unpack(s1)
     assert s1.fields["color"].string_value == "red"
+
+
+def test_error_detail_debug_field() -> None:
+    """Debug field is populated when proto descriptors are available."""
+    wire_error = ConnectWireError.from_exception(
+        ConnectError(
+            Code.RESOURCE_EXHAUSTED,
+            "Resource exhausted",
+            details=[Struct(fields={"animal": Value(string_value="bear")})],
+        )
+    )
+    data = wire_error.to_dict()
+    assert len(data["details"]) == 1
+    detail = data["details"][0]
+    assert "debug" in detail
+    # Struct uses proto-JSON well-known type mapping: becomes a plain JSON object
+    assert detail["debug"] == {"animal": "bear"}
+
+
+def test_error_detail_debug_field_well_known_type() -> None:
+    """Debug field uses proto-JSON well-known type representation (e.g. Duration as string)."""
+    wire_error = ConnectWireError.from_exception(
+        ConnectError(
+            Code.RESOURCE_EXHAUSTED, "Resource exhausted", details=[Duration(seconds=1)]
+        )
+    )
+    data = wire_error.to_dict()
+    assert len(data["details"]) == 1
+    detail = data["details"][0]
+    assert "debug" in detail
+    # Duration uses proto-JSON well-known type mapping: serializes as "1s"
+    assert detail["debug"] == "1s"
+
+
+def test_error_detail_debug_field_absent_for_unknown_type() -> None:
+    """Debug field is omitted when no descriptor is available for the type."""
+    unknown_detail = AnyPb(
+        type_url="type.googleapis.com/completely.Unknown.Message", value=b"\x08\x01"
+    )
+    wire_error = ConnectWireError(
+        code=Code.INTERNAL, message="test", details=[unknown_detail]
+    )
+    data = wire_error.to_dict()
+    assert len(data["details"]) == 1
+    detail = data["details"][0]
+    assert "debug" not in detail