diff --git a/plugins/model-providers/openrouter/__init__.py b/plugins/model-providers/openrouter/__init__.py
index d1bf10de11d..1b464b42e82 100644
--- a/plugins/model-providers/openrouter/__init__.py
+++ b/plugins/model-providers/openrouter/__init__.py
@@ -43,6 +43,8 @@ class OpenRouterProfile(ProviderProfile):
         self, *, session_id: str | None = None, **context: Any
     ) -> dict[str, Any]:
         body: dict[str, Any] = {}
+        if session_id:
+            body["session_id"] = session_id
         prefs = context.get("provider_preferences")
         if prefs:
             body["provider"] = prefs
diff --git a/tests/providers/test_provider_profiles.py b/tests/providers/test_provider_profiles.py
index df96a80fd80..7a2bb081593 100644
--- a/tests/providers/test_provider_profiles.py
+++ b/tests/providers/test_provider_profiles.py
@@ -98,6 +98,11 @@ class TestOpenRouterProfile:
         body = p.build_extra_body(provider_preferences={"allow": ["anthropic"]})
         assert body["provider"] == {"allow": ["anthropic"]}
 
+    def test_extra_body_session_id(self):
+        p = get_provider_profile("openrouter")
+        body = p.build_extra_body(session_id="test-session-123")
+        assert body["session_id"] == "test-session-123"
+
     def test_extra_body_no_prefs(self):
         p = get_provider_profile("openrouter")
         body = p.build_extra_body()