diff --git a/run_agent.py b/run_agent.py
index cc7603fae..2b022c892 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -6260,6 +6260,10 @@ class AIAgent:
             # falling through to OpenRouter defaults.
             fb_base_url_hint = (fb.get("base_url") or "").strip() or None
             fb_api_key_hint = (fb.get("api_key") or "").strip() or None
+            if not fb_api_key_hint:
+                fb_key_env = (fb.get("key_env") or "").strip()
+                if fb_key_env:
+                    fb_api_key_hint = os.getenv(fb_key_env, "").strip() or None
             # For Ollama Cloud endpoints, pull OLLAMA_API_KEY from env
             # when no explicit key is in the fallback config. Host match
             # (not substring) — see GHSA-76xc-57q6-vm5m.
diff --git a/tests/run_agent/test_provider_fallback.py b/tests/run_agent/test_provider_fallback.py
index e441bfd33..88982437e 100644
--- a/tests/run_agent/test_provider_fallback.py
+++ b/tests/run_agent/test_provider_fallback.py
@@ -155,3 +155,29 @@ class TestFallbackChainAdvancement:
             ]
             assert agent._try_activate_fallback() is True
             assert agent.model == "gpt-4o"
+
+    def test_resolves_key_env_for_fallback_provider(self):
+        fbs = [
+            {
+                "provider": "custom",
+                "model": "fallback-model",
+                "base_url": "https://fallback.example/v1",
+                "key_env": "MY_FALLBACK_KEY",
+            }
+        ]
+        agent = _make_agent(fallback_model=fbs)
+        with (
+            patch.dict("os.environ", {"MY_FALLBACK_KEY": "env-secret"}, clear=False),
+            patch(
+                "agent.auxiliary_client.resolve_provider_client",
+                return_value=(
+                    _mock_client(
+                        base_url="https://fallback.example/v1",
+                        api_key="env-secret",
+                    ),
+                    "fallback-model",
+                ),
+            ) as mock_rpc,
+        ):
+            assert agent._try_activate_fallback() is True
+            assert mock_rpc.call_args.kwargs["explicit_api_key"] == "env-secret"