diff --git a/bot_bottle/sidecar_init.py b/bot_bottle/sidecar_init.py
index 8bfa984..2b303a6 100644
--- a/bot_bottle/sidecar_init.py
+++ b/bot_bottle/sidecar_init.py
@@ -245,7 +245,12 @@ class _Supervisor:
                             except ProcessLookupError:
                                 pass
 
-        return all(p.poll() is not None for _, p in self.procs)
+        done = all(p.poll() is not None for _, p in self.procs)
+        if done:
+            for _, p in self.procs:
+                if p.stdout is not None:
+                    p.stdout.close()
+        return done
 
     def exit_code(self) -> int:
         """Positive child failures win; otherwise report success.
@@ -335,6 +340,8 @@ class _Supervisor:
                     except ProcessLookupError:
                         pass
                     p.wait()
+            if p.stdout is not None:
+                p.stdout.close()
             self._logged_dead.discard(daemon_name)
             new_proc = _spawn(spec)
             self.procs[idx] = (spec, new_proc)
diff --git a/docs/prds/0043-sidecar-pipe-lifecycle.md b/docs/prds/0043-sidecar-pipe-lifecycle.md
new file mode 100644
index 0000000..f5b0632
--- /dev/null
+++ b/docs/prds/0043-sidecar-pipe-lifecycle.md
@@ -0,0 +1,74 @@
+# PRD 0043: Sidecar Pipe Lifecycle Cleanup
+
+- **Status:** Active
+- **Author:** didericis-codex
+- **Created:** 2026-06-02
+- **Issue:** #140
+
+## Summary
+
+Close the unclosed child stdout pipe file descriptors that `sidecar_init.py`
+leaks during restart and shutdown paths, eliminating `ResourceWarning` noise
+and tightening the process lifecycle.
+
+## Problem
+
+Unit tests for `sidecar_init.py` pass, but restart and shutdown cases emit
+`ResourceWarning: unclosed file <_io.BufferedReader …>` for child stdout pipes,
+originating around lines 141 and 273. The warnings indicate the restart path
+leaks pipe file descriptors: a pipe opened for a stopped or replaced child is
+not explicitly closed before the next child is spawned or before the supervisor
+exits.
+
+## Goals / Success Criteria
+
+- `python3 -m unittest tests.unit.test_sidecar_init` produces no
+  `ResourceWarning` output.
+- Pipe file descriptors for stopped or replaced child processes are explicitly
+  closed in the restart path.
+- Pipe file descriptors for all children are explicitly closed in the shutdown
+  path.
+- No change to the external signal or exit-code contract from PRD 0034.
+
+## Non-goals
+
+- No changes to restart or shutdown policy (coalescing, ordering, timeout).
+- No changes to egress, pipelock, git-gate, or supervise daemon argv.
+- No new runtime dependencies.
+
+## Scope
+
+In scope:
+
+- `bot_bottle/sidecar_init.py` pipe open/close lifecycle in `_Supervisor`.
+- Unit tests in `tests/unit/test_sidecar_init.py` asserting no leaked pipes.
+
+Out of scope:
+
+- Changing how pumping threads read from pipes.
+- Integration tests that start a live sidecar container.
+
+## Design
+
+Audit every code path in `_Supervisor` where a child process is stopped,
+replaced, or reaches end-of-life, and ensure the corresponding stdout pipe is
+explicitly closed before spawning a replacement or exiting the supervisor loop.
+
+Where a pumping thread holds a reference to the pipe, coordinate closure so the
+thread sees EOF and exits cleanly rather than blocking indefinitely.
+
+## Testing Strategy
+
+- Enable `ResourceWarning` as an error in test setUp:
+  `warnings.simplefilter("error", ResourceWarning)`.
+- Run existing restart and shutdown test cases under this stricter setting.
+- Add tests for restart-then-shutdown if not already covered.
+
+Run:
+
+- `python3 -m unittest tests.unit.test_sidecar_init`
+- `python3 -m unittest discover -s tests/unit`
+
+## Open Questions
+
+None.
diff --git a/tests/unit/test_sidecar_init.py b/tests/unit/test_sidecar_init.py
index 88f0b9b..1256af0 100644
--- a/tests/unit/test_sidecar_init.py
+++ b/tests/unit/test_sidecar_init.py
@@ -14,6 +14,7 @@ import subprocess
 import sys
 import time
 import unittest
+import warnings
 from pathlib import Path
 from unittest.mock import patch
 
@@ -135,6 +136,10 @@ class TestSupervisor(unittest.TestCase):
     We don't go through `main()` because main installs signal
     handlers process-wide, which collides with the test runner."""
 
+    def setUp(self):
+        warnings.simplefilter("error", ResourceWarning)
+        self.addCleanup(warnings.resetwarnings)
+
     def _drive(self, sup: _Supervisor, max_wait_s: float = 6.0) -> int:
         deadline = time.monotonic() + max_wait_s
         while not sup.tick():