feat(client): surface structured shape errors on TangoValidationError (#46)

400 messages now name the rejected field(s) and reason when the API
returns structured issues, and TangoValidationError gains .issues and
.available_fields accessors over response_data.

Closes #45 (client half; server half tracked in makegov/tango#2586).

Co-authored-by: V. David Zvenyach <dave@zvenyach.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: 3 people

CHANGELOG.md

@@ -7,6 +7,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- `TangoValidationError` now exposes the API's structured validation details
+  directly: `.issues` (the list of `{"path": ..., "reason": ...}` entries the
+  server returns for shape errors) and `.available_fields` (the endpoint's
+  valid field set, when included). Both were previously reachable only by
+  digging through `.response_data`. ([#45](https://github.com/makegov/tango-python/issues/45))
+
+### Changed
+- 400 error messages now name the rejected field(s) and reason when the API
+  returns structured `issues` — e.g.
+  `Invalid request parameters: Invalid shape: tradeoff_process (unknown_field)`
+  instead of just `Invalid request parameters: Invalid shape`. ([#45](https://github.com/makegov/tango-python/issues/45))
+
 ## [1.2.0] - 2026-06-05
 
 ### Added

tango/client.py

@@ -183,6 +183,17 @@ def _request(
                         )
                         if detail:
                             error_msg = f"Invalid request parameters: {detail}"
+                        issues = error_data.get("issues")
+                        if isinstance(issues, list):
+                            rejected = [
+                                f"{issue['path']} ({issue['reason']})"
+                                if issue.get("reason")
+                                else str(issue["path"])
+                                for issue in issues
+                                if isinstance(issue, dict) and issue.get("path")
+                            ]
+                            if rejected:
+                                error_msg = f"{error_msg}: {', '.join(rejected)}"
                 raise TangoValidationError(
                     error_msg,
                     response.status_code,

tango/exceptions.py

@@ -33,7 +33,24 @@ class TangoNotFoundError(TangoAPIError):
 class TangoValidationError(TangoAPIError):
     """Request validation error"""
 
-    pass
+    @property
+    def issues(self) -> list[dict[str, Any]]:
+        """Structured validation issues from the API response.
+
+        For shape errors the API returns entries like
+        ``{"path": "tradeoff_process", "reason": "unknown_field"}``.
+        Empty list when the response carried no structured issues.
+        """
+        val = self.response_data.get("issues")
+        if not isinstance(val, list):
+            return []
+        return [item for item in val if isinstance(item, dict)]
+
+    @property
+    def available_fields(self) -> dict[str, Any] | None:
+        """The endpoint's valid field set, when the API includes one."""
+        val = self.response_data.get("available_fields")
+        return val if isinstance(val, dict) else None
 
 
 class TangoRateLimitError(TangoAPIError):

tests/test_client.py

@@ -1152,6 +1152,55 @@ def test_400_validation_error(self, mock_request):
         assert exc_info.value.status_code == 400
         assert exc_info.value.response_data == {"error": "invalid params"}
 
+    @patch("tango.client.httpx.Client.request")
+    def test_400_structured_shape_error(self, mock_request):
+        """Test 400 with structured shape-error body surfaces issues and available_fields"""
+        body = {
+            "error": "Invalid shape",
+            "issues": [{"path": "fair_opportunity_limited_sources", "reason": "unknown_field"}],
+            "available_fields": {"fields": ["piid", "competition"]},
+        }
+        mock_response = Mock()
+        mock_response.is_success = False
+        mock_response.status_code = 400
+        mock_response.content = b"x"
+        mock_response.json.return_value = body
+        mock_request.return_value = mock_response
+
+        client = TangoClient(api_key="test-key")
+
+        with pytest.raises(TangoValidationError) as exc_info:
+            client.list_agencies()
+
+        err = exc_info.value
+        assert str(err) == (
+            "Invalid request parameters: Invalid shape: "
+            "fair_opportunity_limited_sources (unknown_field)"
+        )
+        assert err.issues == [
+            {"path": "fair_opportunity_limited_sources", "reason": "unknown_field"}
+        ]
+        assert err.available_fields == {"fields": ["piid", "competition"]}
+        assert err.response_data == body
+
+    @patch("tango.client.httpx.Client.request")
+    def test_400_validation_error_issues_accessors_empty(self, mock_request):
+        """Test issues/available_fields accessors on a plain 400 body"""
+        mock_response = Mock()
+        mock_response.is_success = False
+        mock_response.status_code = 400
+        mock_response.content = b'{"error": "invalid params"}'
+        mock_response.json.return_value = {"error": "invalid params"}
+        mock_request.return_value = mock_response
+
+        client = TangoClient(api_key="test-key")
+
+        with pytest.raises(TangoValidationError) as exc_info:
+            client.list_agencies()
+
+        assert exc_info.value.issues == []
+        assert exc_info.value.available_fields is None
+
     @patch("tango.client.httpx.Client.request")
     def test_400_validation_error_no_content(self, mock_request):
         """Test 400 with no content"""