diff --git a/conductor/code_styleguides/python.md b/conductor/code_styleguides/python.md
index 5982a9a5..d255ebc4 100644
--- a/conductor/code_styleguides/python.md
+++ b/conductor/code_styleguides/python.md
@@ -67,6 +67,8 @@ is processed by AI agents, while preserving readability for human review.
 - **No empty `__init__.py` files.**
 - **Minimal blank lines.** Token-efficient density is preferred over visual padding.
 - **Short variable names are acceptable** in tight scopes (loop vars, lambdas). Use descriptive names for module-level and class attributes.
+- **No diagnostic noise in production code (Added 2026-06-09).** `sys.stderr.write(f"[XYZ_DIAG] ...")` lines added to `src/*.py` for one-time debugging are technical debt the moment they ship. The project's production code should not contain `[XYZ_DIAG]` markers, `print(...debug...)` calls, or any other ad-hoc debug instrumentation. The right place for diagnostic output during a one-time investigation is `tests/artifacts/<test_name>.diag.log` (a log file) or a standalone `/tmp/diag_<name>.py` script. If you must instrument a production function for a single test run, the diag lines are part of the same atomic commit as the fix — they do not live uncommitted in the working tree. If you "revert everything," that means the diag lines are also reverted.
+- **Test files ARE allowed to be diagnostic.** `tests/test_*.py` may use `print(..., file=sys.stderr)` freely for test output. The rule against diagnostic noise applies to `src/*.py` only.
 
 ## 10. Anti-OOP Conventions