fix: read MCPgError from __context__ too when surfacing tool errors

CHANGELOG.md

@@ -17,7 +17,8 @@ adheres to [Semantic Versioning](https://semver.org/).
   `ToolError` with a generic "Error executing tool <name>". MCPg's own errors (`MCPgError` subclasses, e.g. an
   unconfigured NL→SQL provider that names the env var to set) are re-raised as `ToolError` by the call wrapper, and the
   audit log records their text; the heavy-diagnostics gate raises `ToolError`. Unexpected exceptions keep the SDK's
-  generic message.
+  generic message. The wrapper reads the `MCPgError` from the exception's `__cause__` or, failing that, its `__context__`
+  (CodeRabbit on #356).
 
 ## [0.8.3] - 2026-10-14
 

src/mcpg/server.py

@@ -211,7 +211,7 @@ async def call_tool(
                 # MCPgError messages are written for the client ("provider
                 # 'openai' is not configured ...; set OPENAI_API_KEY"), so they
                 # go back as a ToolError, and the audit log gets the real text.
-                domain_error = exc if isinstance(exc, MCPgError) else exc.__cause__
+                domain_error = exc if isinstance(exc, MCPgError) else (exc.__cause__ or exc.__context__)
                 if isinstance(domain_error, MCPgError):
                     tool_exc = ToolError(str(domain_error))
                     audit.record(audit.AuditEvent(tool=name, arguments=arguments, status="error", error=str(tool_exc)))

tests/unit/test_server.py

@@ -511,3 +511,34 @@ def test_dynamic_session_intent_middleware_absent_by_default() -> None:
     server = create_server(settings)
 
     assert not any(isinstance(m, DynamicSessionIntentMiddleware) for m in server._lowlevel_server.middleware)
+
+
+async def test_call_tool_surfaces_mcpg_error_found_only_in_exception_context() -> None:
+    """An MCPgError reached through ``__context__`` (an implicit raise inside an
+    except block, no ``from``) is surfaced like one reached through ``__cause__``:
+    the client gets its message as a ToolError, not the SDK's generic text."""
+    from mcp.server.mcpserver.exceptions import ToolError, UnexpectedToolError
+
+    from mcpg.errors import MCPgError
+
+    server = create_server(_SETTINGS)
+
+    async def boom(self: Any, name: str, arguments: dict[str, Any], context: Any = None) -> Any:
+        try:
+            raise MCPgError("provider 'x' is not configured; set X_API_KEY")
+        except MCPgError:
+            raise UnexpectedToolError(f"Error executing tool {name}")  # noqa: B904 - context only
+
+    events: list[Any] = []
+    with (
+        patch.object(MCPServer, "call_tool", boom),
+        patch.object(server_mod.audit, "record", side_effect=events.append),
+        pytest.raises(ToolError) as exc_info,
+    ):
+        await server.call_tool("some_tool", {})
+    assert "is not configured" in str(exc_info.value)
+    assert "Error executing tool" not in str(exc_info.value)
+    # the audit record carries the domain message, not the SDK's generic text
+    assert [e.status for e in events] == ["error"]
+    assert "is not configured" in events[0].error
+    assert "Error executing tool" not in events[0].error