diff --git a/agent/transports/__init__.py b/agent/transports/__init__.py
index b606da7fecaea..d7e5e067ba62a 100644
--- a/agent/transports/__init__.py
+++ b/agent/transports/__init__.py
@@ -6,6 +6,8 @@
     result = transport.normalize_response(raw_response)
 """
 
+import threading
+
 from agent.transports.types import (
     NormalizedResponse,
     ToolCall,
@@ -16,6 +18,7 @@
 
 _REGISTRY: dict = {}
 _discovered: bool = False
+_discover_lock = threading.Lock()
 
 
 def register_transport(api_mode: str, transport_cls: type) -> None:
@@ -49,20 +52,28 @@ def get_transport(api_mode: str):
 def _discover_transports() -> None:
     """Import all transport modules to trigger auto-registration."""
     global _discovered
-    _discovered = True
-    try:
-        import agent.transports.anthropic  # noqa: F401
-    except ImportError:
-        pass
-    try:
-        import agent.transports.codex  # noqa: F401
-    except ImportError:
-        pass
-    try:
-        import agent.transports.chat_completions  # noqa: F401
-    except ImportError:
-        pass
-    try:
-        import agent.transports.bedrock  # noqa: F401
-    except ImportError:
-        pass
+    if _discovered:
+        return
+
+    with _discover_lock:
+        if _discovered:
+            return
+
+        try:
+            import agent.transports.anthropic  # noqa: F401
+        except ImportError:
+            pass
+        try:
+            import agent.transports.codex  # noqa: F401
+        except ImportError:
+            pass
+        try:
+            import agent.transports.chat_completions  # noqa: F401
+        except ImportError:
+            pass
+        try:
+            import agent.transports.bedrock  # noqa: F401
+        except ImportError:
+            pass
+
+        _discovered = True
diff --git a/tests/agent/transports/test_transport_discovery_concurrency.py b/tests/agent/transports/test_transport_discovery_concurrency.py
new file mode 100644
index 0000000000000..21bd263a7d2a9
--- /dev/null
+++ b/tests/agent/transports/test_transport_discovery_concurrency.py
@@ -0,0 +1,94 @@
+"""Concurrency regressions for transport discovery."""
+
+import builtins
+from concurrent.futures import ThreadPoolExecutor
+import threading
+
+import pytest
+
+import agent.transports as transports
+
+
+class _ObservedLock:
+    """Expose when the second caller reaches lock acquisition."""
+
+    def __init__(self) -> None:
+        self._lock = threading.Lock()
+        self._state_lock = threading.Lock()
+        self._attempts = 0
+        self.second_attempted = threading.Event()
+
+    def __enter__(self) -> "_ObservedLock":
+        with self._state_lock:
+            self._attempts += 1
+            if self._attempts == 2:
+                self.second_attempted.set()
+        self._lock.acquire()
+        return self
+
+    def __exit__(self, *exc_info: object) -> None:
+        self._lock.release()
+
+
+def test_discovery_is_published_after_one_serialized_import_sweep(monkeypatch) -> None:
+    target_imports = (
+        "agent.transports.anthropic",
+        "agent.transports.codex",
+        "agent.transports.chat_completions",
+        "agent.transports.bedrock",
+    )
+    observed_lock = _ObservedLock()
+    first_import_started = threading.Event()
+    release_first_import = threading.Event()
+    imported: list[str] = []
+    original_import = builtins.__import__
+
+    def controlled_import(name: str, *args: object, **kwargs: object) -> object:
+        if name in target_imports:
+            imported.append(name)
+            if name == target_imports[0]:
+                first_import_started.set()
+                assert release_first_import.wait(timeout=5)
+        return original_import(name, *args, **kwargs)
+
+    monkeypatch.setattr(transports, "_discovered", False)
+    monkeypatch.setattr(transports, "_discover_lock", observed_lock)
+    monkeypatch.setattr(builtins, "__import__", controlled_import)
+
+    pool = ThreadPoolExecutor(max_workers=2)
+    try:
+        first = pool.submit(transports._discover_transports)
+        assert first_import_started.wait(timeout=5)
+        assert transports._discovered is False
+
+        second = pool.submit(transports._discover_transports)
+        assert observed_lock.second_attempted.wait(timeout=5)
+        assert second.done() is False
+        assert imported == [target_imports[0]]
+
+        release_first_import.set()
+        first.result(timeout=5)
+        second.result(timeout=5)
+    finally:
+        release_first_import.set()
+        pool.shutdown(wait=True)
+
+    assert transports._discovered is True
+    assert imported == list(target_imports)
+
+
+def test_failed_discovery_is_not_published(monkeypatch) -> None:
+    original_import = builtins.__import__
+
+    def fail_first_transport(name: str, *args: object, **kwargs: object) -> object:
+        if name == "agent.transports.anthropic":
+            raise RuntimeError("transport import failed")
+        return original_import(name, *args, **kwargs)
+
+    monkeypatch.setattr(transports, "_discovered", False)
+    monkeypatch.setattr(builtins, "__import__", fail_first_transport)
+
+    with pytest.raises(RuntimeError, match="transport import failed"):
+        transports._discover_transports()
+
+    assert transports._discovered is False