Fix double rendering of exception tracebacks in stdlib integration

CHANGELOG.md

@@ -15,6 +15,9 @@ You can find our backwards-compatibility policy [here](https://github.com/hynek/
 
 ## [Unreleased](https://github.com/hynek/structlog/compare/26.1.0...HEAD)
 
+### Fixed
+
+- `structlog.stdlib.BoundLogger` now avoids double-rendering exception information when a processor has already rendered it, preventing duplicate traceback output for `exception` events.
 
 ## [26.1.0](https://github.com/hynek/structlog/compare/25.5.0...26.1.0) - 2026-06-06
 

src/structlog/stdlib.py

@@ -283,7 +283,22 @@ def _proxy_to_logger(
         if event_args:
             event_kw["positional_args"] = event_args
 
-        return super()._proxy_to_logger(method_name, event=event, **event_kw)
+        try:
+            args, kw = self._process_event(method_name, event, event_kw)
+
+            # If a processor already rendered the exception, suppress stdlib's automatic rendering.
+            is_rendered = kw.pop(
+                "_structlog_exception_already_rendered", False
+            ) or kw.get("extra", {}).pop(
+                "_structlog_exception_already_rendered", False
+            )
+
+            if method_name == "exception" and is_rendered:
+                kw["exc_info"] = False
+
+            return getattr(self._logger, method_name)(*args, **kw)
+        except DropEvent:
+            return None
 
     # Pass-through attributes and methods to mimic the stdlib's logger
     # interface.

tests/test_exc_info_double_render.py

@@ -0,0 +1,45 @@
+import logging
+
+import structlog
+
+from structlog.stdlib import BoundLogger, LoggerFactory, render_to_log_kwargs
+
+
+def test_exception_no_double_render_with_in_band_signaling(caplog):
+    """
+    Ensure that the stdlib BoundLogger suppresses exc_info when a processor
+    has already rendered the exception, preventing duplicate tracebacks.
+    """
+
+    def mock_format_exc_info(logger, name, event_dict):
+        event_dict["exception"] = "Mocked Traceback"
+        event_dict["_structlog_exception_already_rendered"] = True
+        return event_dict
+
+    structlog.configure(
+        processors=[
+            mock_format_exc_info,
+            structlog.stdlib.add_log_level,
+            render_to_log_kwargs,
+        ],
+        logger_factory=LoggerFactory(),
+        wrapper_class=BoundLogger,
+    )
+
+    logger = structlog.get_logger("test_logger")
+
+    with caplog.at_level(logging.ERROR):
+        try:
+            raise ValueError("test error")
+        except ValueError:
+            logger.exception("An error occurred")
+
+    assert len(caplog.records) == 1
+    record = caplog.records[0]
+
+    # Verify the underlying logger was called without exc_info
+    assert record.exc_info is False or record.exc_info is None
+
+    # Verify the message and extra data are intact
+    assert record.msg == "An error occurred"
+    assert record.__dict__.get("exception") == "Mocked Traceback"