diff --git a/agent/process_bootstrap.py b/agent/process_bootstrap.py
index 2fbaa1ad33..85dccfc737 100644
--- a/agent/process_bootstrap.py
+++ b/agent/process_bootstrap.py
@@ -18,6 +18,7 @@ from typing import Any, Optional
 from hermes_bootstrap import _happy_eyeballs_create_connection
 from utils import base_url_hostname, normalize_proxy_url
 from agent.proxy_bypass import first_proxy_env_value, should_bypass_proxy
+from agent.thread_scoped_output import _ThreadRoutingStream
 
 
 _OPENAI_CLS_CACHE = None
@@ -334,7 +335,8 @@ def _install_safe_stdio() -> None:
     """Wrap stdout/stderr so best-effort console output cannot crash the agent."""
     for stream_name in ("stdout", "stderr"):
         stream = getattr(sys, stream_name, None)
-        if stream is not None and not isinstance(stream, _SafeWriter):
+        # The routing proxy already absorbs these errors; wrapping it stacks a new proxy per build (#139687).
+        if stream is not None and not isinstance(stream, (_SafeWriter, _ThreadRoutingStream)):
             setattr(sys, stream_name, _SafeWriter(stream))
 
 
diff --git a/tests/agent/test_thread_scoped_output.py b/tests/agent/test_thread_scoped_output.py
index dc57064d13..7660f92621 100644
--- a/tests/agent/test_thread_scoped_output.py
+++ b/tests/agent/test_thread_scoped_output.py
@@ -130,6 +130,26 @@ def test_temporary_global_redirects_do_not_allocate_new_sinks(monkeypatch):
         sys.stdout, sys.stderr = original_stdout, original_stderr
 
 
+def test_agent_builds_keep_the_installed_routing_proxy(monkeypatch):
+    """Agent builds between silences must not stack wrappers on sys.stdout/stderr (#139687)."""
+    from agent.process_bootstrap import _install_safe_stdio
+
+    monkeypatch.setattr(thread_output, "_installed", {})
+    monkeypatch.setattr(sys, "stdout", io.StringIO())
+    monkeypatch.setattr(sys, "stderr", io.StringIO())
+    _install_safe_stdio()
+    with thread_scoped_silence():
+        pass
+    routers = (sys.stdout, sys.stderr)
+
+    for _ in range(3):
+        _install_safe_stdio()
+        with thread_scoped_silence():
+            pass
+
+    assert (sys.stdout, sys.stderr) == routers
+
+
 def test_silence_survives_redirect_restoring_an_older_proxy(monkeypatch):
     """Silencing is stream-wide, even when a redirect swaps proxy generations."""
     monkeypatch.setattr(thread_output, "_installed", {})