diff --git a/pyproject.toml b/pyproject.toml
index 10484348..c2828d37 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -217,6 +217,29 @@ addopts = [
   # don't show frickin captured logs AGAIN in the report..
   '--show-capture=no',
 
+  # sys-level capture. REQUIRED for fork-based spawn
+  # backends (e.g. `subint_forkserver`): default
+  # `--capture=fd` redirects fd 1,2 to temp files, and fork
+  # children inherit those fds — opaque deadlocks happen in
+  # the pytest-capture-machinery ↔ fork-child stdio
+  # interaction. `--capture=sys` only redirects Python-level
+  # `sys.stdout`/`sys.stderr`, leaving fd 1,2 alone.
+  #
+  # Trade-off (vs. `--capture=fd`):
+  # - LOST: per-test attribution of subactor *raw-fd* output
+  #   (C-ext writes, `os.write(2, ...)`, subproc stdout). Not
+  #   zero — those go to the terminal, captured by CI's
+  #   terminal-level capture, just not per-test-scoped in the
+  #   pytest failure report.
+  # - KEPT: Python-level `print()` + `logging` capture per-
+  #   test (tractor's logger uses `sys.stderr`, so tractor
+  #   log output IS still attributed per-test).
+  # - KEPT: user `pytest -s` for debugging (unaffected).
+  #
+  # Full post-mortem in
+  # `ai/conc-anal/subint_forkserver_test_cancellation_leak_issue.md`.
+  '--capture=sys',
+
   # disable `xonsh` plugin
   # https://docs.pytest.org/en/stable/how-to/plugins.html#disabling-plugins-from-autoloading
   # https://docs.pytest.org/en/stable/how-to/plugins.html#deactivating-unregistering-a-plugin-by-name