diff --git a/reme4/components/job/base_job.py b/reme4/components/job/base_job.py
index 0d2432cf..860849f7 100644
--- a/reme4/components/job/base_job.py
+++ b/reme4/components/job/base_job.py
@@ -29,33 +29,36 @@ class BaseJob(BaseComponent):
         self.description = description
         self.parameters = parameters or {}
         self.step_configs = steps or []
+        # Resolved at start: (cls, params) pairs. Steps are re-instantiated per call so they stay
+        # stateless across runs and concurrent invocations don't share mutable step state.
         self.step_specs: list[tuple[type["BaseStep"], dict]] = []
 
     async def _start(self) -> None:
-        """Resolve step configs into (cls, params) pairs; defer instantiation to __call__."""
         if self.app_context is None:
             raise RuntimeError(f"app_context must be provided for job '{self.name}'")
-        for raw in self.step_configs:
-            config = raw if isinstance(raw, ComponentConfig) else ComponentConfig(**raw)
-            if not config.backend:
-                raise ValueError("Step is missing the required 'backend' field")
-            step_cls = R.get(ComponentEnum.STEP, config.backend)
-            if not step_cls:
-                raise ValueError(f"Unregistered backend '{config.backend}' of type '{ComponentEnum.STEP}'")
-            params = config.model_dump()
-            params["app_context"] = self.app_context
-            self.step_specs.append((step_cls, params))
+        self.step_specs = [self._resolve_step(raw) for raw in self.step_configs]
 
     async def _close(self) -> None:
-        """Release all step specs."""
         self.step_specs.clear()
 
+    def _resolve_step(self, raw: ComponentConfig | dict) -> tuple[type["BaseStep"], dict]:
+        """Validate a step config and look up its class via the registry."""
+        config = raw if isinstance(raw, ComponentConfig) else ComponentConfig(**raw)
+        if not config.backend:
+            raise ValueError("Step is missing the required 'backend' field")
+        step_cls = R.get(ComponentEnum.STEP, config.backend)
+        if not step_cls:
+            raise ValueError(f"Unregistered backend '{config.backend}' of type '{ComponentEnum.STEP}'")
+        params = config.model_dump()
+        params["app_context"] = self.app_context
+        return step_cls, params
+
     def _build_steps(self) -> list["BaseStep"]:
-        """Instantiate fresh step instances from stored specs."""
+        # dict(params) copies kwargs so steps cannot mutate the shared spec.
         return [step_cls(**dict(params)) for step_cls, params in self.step_specs]
 
     async def __call__(self, **kwargs) -> Response:
-        """Execute all steps in order and return the final response."""
+        """Run all steps in order, capturing any failure into the response."""
         context = RuntimeContext(**kwargs)
         try:
             for step in self._build_steps():