Reject dstack access combined with inactivity_duration

The persistent server connection counts as SSH activity, so a dev
environment with both dstack access and inactivity_duration would never
be detected as inactive. Reject the combination at configuration time
instead of silently breaking inactivity_duration.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Andrey Cheptsov

src/dstack/_internal/core/models/configurations.py

@@ -786,6 +786,13 @@ def validate_entrypoint(cls, v: Optional[str]) -> Optional[str]:
             raise ValueError("entrypoint is not supported for dev-environment")
         return v
 
+    @root_validator
+    def validate_dstack_and_inactivity_duration(cls, values):
+        if values.get("dstack") and values.get("inactivity_duration") is not None:
+            # The persistent server connection counts as activity, so inactivity is never detected
+            raise ValueError("`dstack` is not supported together with `inactivity_duration`")
+        return values
+
 
 class TaskConfigurationParams(CoreModel):
     nodes: Annotated[int, Field(description="Number of nodes", ge=1)] = 1

src/tests/_internal/core/models/test_configurations.py

@@ -37,6 +37,20 @@ def test_server_access_not_supported_for_service(self):
                 }
             )
 
+    def test_server_access_not_supported_with_inactivity_duration(self):
+        with pytest.raises(ConfigurationError, match="inactivity_duration"):
+            parse_run_configuration(
+                {
+                    "type": "dev-environment",
+                    "dstack": True,
+                    "inactivity_duration": "1h",
+                }
+            )
+
+    def test_server_access_and_inactivity_duration_allowed_separately(self):
+        parse_run_configuration({"type": "dev-environment", "dstack": True})
+        parse_run_configuration({"type": "dev-environment", "inactivity_duration": "1h"})
+
     def test_server_access_allows_unresolved_passthrough_env(self):
         parsed = parse_run_configuration(
             {