diff --git a/toolchain/mfc/params/registry.py b/toolchain/mfc/params/registry.py
index 2d79f00157..1d93eae624 100644
--- a/toolchain/mfc/params/registry.py
+++ b/toolchain/mfc/params/registry.py
@@ -233,7 +233,11 @@ def register(self, param: ParamDef) -> None:
         if param.name in self._params:
             existing = self._params[param.name]
             if existing.param_type != param.param_type:
-                raise ValueError(f"Type mismatch for '{param.name}'")
+                raise ValueError(
+                    f"Type mismatch for '{param.name}': "
+                    f"existing type is {existing.param_type!r}, "
+                    f"new type is {param.param_type!r}"
+                )
             existing.tags.update(param.tags)
             for tag in param.tags:
                 self._by_tag[tag].add(param.name)
diff --git a/toolchain/mfc/params_tests/test_registry.py b/toolchain/mfc/params_tests/test_registry.py
index 2f36ca78a3..56d453f335 100644
--- a/toolchain/mfc/params_tests/test_registry.py
+++ b/toolchain/mfc/params_tests/test_registry.py
@@ -38,7 +38,8 @@ def test_register_type_mismatch_raises(self):
         with self.assertRaises(ValueError) as ctx:
             reg.register(ParamDef(name="test", param_type=ParamType.REAL))
 
-        self.assertIn("Type mismatch", str(ctx.exception))
+        self.assertIn("existing type is", str(ctx.exception))
+        self.assertIn("new type is", str(ctx.exception))
 
     def test_get_params_by_tag(self):
         """get_params_by_tag should return params with that tag."""