diff --git a/src/otari/_base.py b/src/otari/_base.py
index af33103..d1b7b0d 100644
--- a/src/otari/_base.py
+++ b/src/otari/_base.py
@@ -252,7 +252,13 @@ def _url_encode(value: str) -> str:
 
 
 def extract_detail(error: ApiException) -> str:
-    """Pull the gateway's human-readable detail from an ``ApiException`` body."""
+    """Pull the gateway's human-readable detail from an ``ApiException`` body.
+
+    Recognizes the FastAPI/gateway ``{"detail": "..."}`` shape and the
+    OpenAI-style ``{"error": {"message": "..."}}`` / ``{"error": "..."}``
+    shapes, mirroring ``extract_detail`` in the Rust SDK (``core.rs``) and
+    ``detailFromObject`` in the TS SDK (``mapError.ts``).
+    """
     body = error.body
     if isinstance(body, (bytes, bytearray)):
         body = body.decode("utf-8", "replace")
@@ -265,8 +271,12 @@ def extract_detail(error: ApiException) -> str:
             detail = parsed.get("detail") or parsed.get("message") or parsed.get("error")
             if isinstance(detail, str):
                 return detail
+            if isinstance(detail, dict):
+                nested = detail.get("message")
+                if isinstance(nested, str):
+                    return nested
             if detail is not None:
-                return str(detail)
+                return json.dumps(detail)
         return body
     return error.reason or "An error occurred"
 
diff --git a/tests/unit/test_errors.py b/tests/unit/test_errors.py
index 8336683..fe6266b 100644
--- a/tests/unit/test_errors.py
+++ b/tests/unit/test_errors.py
@@ -5,6 +5,10 @@
 
 from __future__ import annotations
 
+import json
+
+from otari._base import extract_detail
+from otari._client.exceptions import ApiException
 from otari.errors import (
     AuthenticationError,
     BatchNotCompleteError,
@@ -175,3 +179,45 @@ def test_str_with_provider_name(self) -> None:
             provider="anthropic",
         )
         assert str(err) == "[gateway] not supported"
+
+
+class TestExtractDetailOpenAIEnvelope:
+    """extract_detail must unwrap the OpenAI-style nested ``error`` envelope.
+
+    Mirrors the TS SDK's ``detailFromObject`` (mozilla-ai/otari-sdk-ts,
+    PR #41 / issue #40) and the Rust SDK's ``extract_detail`` in
+    ``otari-sdk-rust/src/core.rs``, which both already handle this shape.
+    """
+
+    def test_nested_openai_error_object_unwraps_to_message(self) -> None:
+        body = json.dumps({"error": {"message": "context length exceeded"}})
+        err = ApiException(status=400, body=body, reason="Bad Request")
+        assert extract_detail(err) == "context length exceeded"
+
+    def test_fastapi_detail_shape_unchanged(self) -> None:
+        body = json.dumps({"detail": "boom"})
+        err = ApiException(status=400, body=body, reason="Bad Request")
+        assert extract_detail(err) == "boom"
+
+    def test_flat_error_string_unchanged(self) -> None:
+        body = json.dumps({"error": "flat string"})
+        err = ApiException(status=400, body=body, reason="Bad Request")
+        assert extract_detail(err) == "flat string"
+
+    def test_top_level_message_unchanged(self) -> None:
+        body = json.dumps({"message": "top level message"})
+        err = ApiException(status=400, body=body, reason="Bad Request")
+        assert extract_detail(err) == "top level message"
+
+    def test_nested_error_object_without_message_falls_back_to_json(self) -> None:
+        body = json.dumps({"error": {"code": 400}})
+        err = ApiException(status=400, body=body, reason="Bad Request")
+        result = extract_detail(err)
+        # Must be valid JSON round-tripping to the nested object, not a
+        # Python repr (which would contain single-quoted keys).
+        assert "'" not in result
+        assert json.loads(result) == {"code": 400}
+
+    def test_non_json_body_returned_verbatim(self) -> None:
+        err = ApiException(status=500, body="plain text failure", reason="Server Error")
+        assert extract_detail(err) == "plain text failure"