diff --git a/src/dvsim/flow/base.py b/src/dvsim/flow/base.py
index 1fdfd93f..7f0d8b7a 100644
--- a/src/dvsim/flow/base.py
+++ b/src/dvsim/flow/base.py
@@ -84,6 +84,10 @@ def __init__(self, flow_cfg_file, hjson_data, args, mk_config) -> None:
         self.scratch_path = ""
         self.scratch_base_path = ""
 
+        # The command line might specify the tool (with --tool). If not, we
+        # leave it as None (allowing an hjson file to populate it)
+        self.tool: str | None = args.tool
+
         # Add exports using 'exports' keyword - these are exported to the child
         # process' environment.
         self.exports = []
diff --git a/src/dvsim/flow/one_shot.py b/src/dvsim/flow/one_shot.py
index c717eea2..df0ee3e8 100644
--- a/src/dvsim/flow/one_shot.py
+++ b/src/dvsim/flow/one_shot.py
@@ -25,7 +25,6 @@ class OneShotCfg(FlowCfg):
 
     def __init__(self, flow_cfg_file, hjson_data, args, mk_config) -> None:
         # Options set from command line
-        self.tool = args.tool
         self.verbose = args.verbose
         self.flist_gen_cmd = ""
         self.flist_gen_opts = []
diff --git a/src/dvsim/job/deploy.py b/src/dvsim/job/deploy.py
index 0d21e641..95327fc8 100644
--- a/src/dvsim/job/deploy.py
+++ b/src/dvsim/job/deploy.py
@@ -117,6 +117,16 @@ def __init__(self, sim_cfg: "FlowCfg") -> None:
 
     def get_job_spec(self) -> "JobSpec":
         """Get the job spec for this deployment."""
+        # At this point, the configuration should have populated its tool field
+        # (either from a command line argument or a value in the hjson that was
+        # loaded. If not, we don't know what to do.
+        if self.sim_cfg.tool is None:
+            msg = (
+                "No tool selected in job configuration. It must either be "
+                "specified in the hjson or passed with the --tool argument."
+            )
+            raise RuntimeError(msg)
+
         return JobSpec(
             name=self.name,
             job_type=self.__class__.__name__,
@@ -951,6 +961,11 @@ def callback(status: JobStatus) -> None:
             if self.dry_run or status != JobStatus.PASSED or not cov_report_path.exists():
                 return
 
+            # At this point, we have finished running a tool, so we know that
+            # self.sim_cfg.tool must have been set.
+            if self.sim_cfg.tool is None:
+                raise RuntimeError("sim_cfg.tool cannot be None now.")
+
             plugin = get_sim_tool_plugin(tool=self._typed_sim_cfg.tool)
 
             results, self.cov_total = plugin.get_cov_summary_table(
diff --git a/src/dvsim/sim/flow.py b/src/dvsim/sim/flow.py
index 664f3163..731a81f5 100644
--- a/src/dvsim/sim/flow.py
+++ b/src/dvsim/sim/flow.py
@@ -80,7 +80,6 @@ class SimCfg(FlowCfg):
 
     def __init__(self, flow_cfg_file, hjson_data, args, mk_config) -> None:
         # Options set from command line
-        self.tool = args.tool
         self.build_opts = []
         self.build_opts.extend(args.build_opts)
         self.en_build_modes = args.build_modes.copy()