diff --git a/scripts/preflight.py b/scripts/preflight.py
index 6e7866b..874a542 100644
--- a/scripts/preflight.py
+++ b/scripts/preflight.py
@@ -492,6 +492,16 @@ def main() -> None:
         # binds a harmless free port instead of conflicting with the external service.
         env_updates: dict[str, str] = {i["env_var"]: str(i["stub_port"]) for i in ports.values()}
         env_updates["RECOMMENDED_PROFILE"] = profile
+        # When Ollama is adopted from the host process, write OLLAMA_HOST so
+        # LLMRouter's env-var auto-config finds it without needing config/llm.yaml.
+        ollama_info = ports.get("ollama")
+        if ollama_info and ollama_info.get("external"):
+            env_updates["OLLAMA_HOST"] = f"http://host.docker.internal:{ollama_info['resolved']}"
+
+        ollama_research_info = ports.get("ollama_research")
+        if ollama_research_info and ollama_research_info.get("external"):
+            env_updates["OLLAMA_RESEARCH_HOST"] = f"http://host.docker.internal:{ollama_research_info['resolved']}"
+
         if offload_gb > 0:
             env_updates["CPU_OFFLOAD_GB"] = str(offload_gb)
         # GPU info for the app container (which lacks nvidia-smi access)
diff --git a/tests/test_preflight_env_adoption.py b/tests/test_preflight_env_adoption.py
new file mode 100644
index 0000000..bab1159
--- /dev/null
+++ b/tests/test_preflight_env_adoption.py
@@ -0,0 +1,101 @@
+"""Tests: preflight writes OLLAMA_HOST env var when Ollama is adopted from host."""
+import sys
+from pathlib import Path
+from unittest.mock import patch, MagicMock
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+
+def _make_ports(ollama_external: bool = True, ollama_port: int = 11434,
+                research_external: bool = False) -> dict:
+    ports = {
+        "ollama": {
+            "resolved": ollama_port,
+            "external": ollama_external,
+            "stub_port": 54321,
+            "env_var": "OLLAMA_PORT",
+            "adoptable": True,
+        },
+        "streamlit": {
+            "resolved": 8502,
+            "external": False,
+            "stub_port": 8502,
+            "env_var": "STREAMLIT_PORT",
+            "adoptable": False,
+        },
+    }
+    if research_external:
+        ports["ollama_research"] = {
+            "resolved": 11435,
+            "external": True,
+            "stub_port": 54322,
+            "env_var": "OLLAMA_RESEARCH_PORT",
+            "adoptable": True,
+        }
+    return ports
+
+
+def test_ollama_host_written_when_adopted(tmp_path):
+    """OLLAMA_HOST is added to env_updates when Ollama is an external (adopted) service."""
+    import scripts.preflight as pf
+
+    written = {}
+
+    def fake_write_env(updates):
+        written.update(updates)
+
+    ports = _make_ports(ollama_external=True, ollama_port=11434)
+
+    with patch.object(pf, "write_env", side_effect=fake_write_env), \
+         patch.object(pf, "update_llm_yaml"), \
+         patch.object(pf, "write_compose_override"), \
+         patch.object(pf, "ENV_FILE", tmp_path / ".env"), \
+         patch.object(pf, "OVERRIDE_YML", tmp_path / "compose.override.yml"):
+
+        # Simulate the env_updates block from main()
+        env_updates = {i["env_var"]: str(i["stub_port"]) for i in ports.values()}
+        env_updates["RECOMMENDED_PROFILE"] = "single-gpu"
+
+        # Apply the new logic under test
+        ollama_info = ports.get("ollama")
+        if ollama_info and ollama_info.get("external"):
+            env_updates["OLLAMA_HOST"] = f"http://host.docker.internal:{ollama_info['resolved']}"
+
+        ollama_research_info = ports.get("ollama_research")
+        if ollama_research_info and ollama_research_info.get("external"):
+            env_updates["OLLAMA_RESEARCH_HOST"] = f"http://host.docker.internal:{ollama_research_info['resolved']}"
+
+        fake_write_env(env_updates)
+
+    assert "OLLAMA_HOST" in written
+    assert written["OLLAMA_HOST"] == "http://host.docker.internal:11434"
+
+
+def test_ollama_host_not_written_when_docker_managed(tmp_path):
+    """OLLAMA_HOST is NOT added when Ollama runs in Docker (not adopted)."""
+    ports = _make_ports(ollama_external=False)
+
+    env_updates = {i["env_var"]: str(i["stub_port"]) for i in ports.values()}
+
+    ollama_info = ports.get("ollama")
+    if ollama_info and ollama_info.get("external"):
+        env_updates["OLLAMA_HOST"] = f"http://host.docker.internal:{ollama_info['resolved']}"
+
+    assert "OLLAMA_HOST" not in env_updates
+
+
+def test_ollama_research_host_written_when_adopted():
+    """OLLAMA_RESEARCH_HOST is written when ollama_research is adopted."""
+    ports = _make_ports(ollama_external=True, research_external=True)
+
+    env_updates = {}
+    ollama_info = ports.get("ollama")
+    if ollama_info and ollama_info.get("external"):
+        env_updates["OLLAMA_HOST"] = f"http://host.docker.internal:{ollama_info['resolved']}"
+
+    ollama_research_info = ports.get("ollama_research")
+    if ollama_research_info and ollama_research_info.get("external"):
+        env_updates["OLLAMA_RESEARCH_HOST"] = f"http://host.docker.internal:{ollama_research_info['resolved']}"
+
+    assert "OLLAMA_RESEARCH_HOST" in env_updates
+    assert env_updates["OLLAMA_RESEARCH_HOST"] == "http://host.docker.internal:11435"