diff --git a/src/specify_cli/workflows/steps/prompt/__init__.py b/src/specify_cli/workflows/steps/prompt/__init__.py
index 5ec99b794d..c8f8a32fd6 100644
--- a/src/specify_cli/workflows/steps/prompt/__init__.py
+++ b/src/specify_cli/workflows/steps/prompt/__init__.py
@@ -160,4 +160,16 @@ def validate(self, config: dict[str, Any]) -> list[str]:
             errors.append(
                 f"Prompt step {config.get('id', '?')!r} is missing 'prompt' field."
             )
+        elif not isinstance(config["prompt"], str):
+            # execute() str()-coerces prompt and dispatches it to the
+            # integration CLI, so a null or list 'prompt' would send the Python
+            # repr ('None', "['review', 'this']") to the model as instructions —
+            # silently wrong, with no error. Reject non-strings at validation,
+            # mirroring the shell-step 'run' and command-step input/options type
+            # checks. An expression like "{{ ... }}" is still a str, so it stays
+            # valid.
+            errors.append(
+                f"Prompt step {config.get('id', '?')!r}: 'prompt' must be a "
+                f"string, got {type(config['prompt']).__name__}."
+            )
         return errors
diff --git a/tests/test_workflows.py b/tests/test_workflows.py
index 8fefad740e..cf95fa5b7a 100644
--- a/tests/test_workflows.py
+++ b/tests/test_workflows.py
@@ -1336,6 +1336,21 @@ def test_validate_missing_prompt(self):
         errors = step.validate({"id": "test"})
         assert any("missing 'prompt'" in e for e in errors)
 
+    @pytest.mark.parametrize("bad_prompt", [None, ["review", "this"], 42, {"a": 1}])
+    def test_validate_rejects_non_string_prompt(self, bad_prompt):
+        """A non-string 'prompt' must be rejected at validation.
+
+        execute() str()-coerces prompt and dispatches it to the integration
+        CLI, so a null or list prompt would otherwise send the Python repr to
+        the model as instructions — silently wrong. Mirrors the shell-step
+        'run' type check.
+        """
+        from specify_cli.workflows.steps.prompt import PromptStep
+
+        step = PromptStep()
+        errors = step.validate({"id": "p", "prompt": bad_prompt})
+        assert any("'prompt' must be a string" in e for e in errors)
+
     def test_validate_valid(self):
         from specify_cli.workflows.steps.prompt import PromptStep
 
@@ -1343,6 +1358,16 @@ def test_validate_valid(self):
         errors = step.validate({"id": "test", "prompt": "do something"})
         assert errors == []
 
+    def test_validate_accepts_expression_prompt(self):
+        """A '{{ ... }}' expression prompt is a str, so it stays valid."""
+        from specify_cli.workflows.steps.prompt import PromptStep
+
+        step = PromptStep()
+        errors = step.validate(
+            {"id": "p", "prompt": "Review {{ inputs.file }}"}
+        )
+        assert errors == []
+
 
 class TestShellStep:
     """Test the shell step type."""