diff --git a/crates/nu-plugin-test-support/src/plugin_test.rs b/crates/nu-plugin-test-support/src/plugin_test.rs
index 18872af44d..3f9326a33b 100644
--- a/crates/nu-plugin-test-support/src/plugin_test.rs
+++ b/crates/nu-plugin-test-support/src/plugin_test.rs
@@ -8,7 +8,8 @@ use nu_plugin::{Plugin, PluginCommand, PluginCustomValue, PluginSource};
 use nu_protocol::{
     debugger::WithoutDebug,
     engine::{EngineState, Stack, StateWorkingSet},
-    report_error_new, Example, LabeledError, PipelineData, ShellError, Span, Value,
+    report_error_new, CustomValue, Example, IntoSpanned as _, LabeledError, PipelineData,
+    ShellError, Span, Value,
 };
 
 use crate::{diff::diff_by_line, fake_register::fake_register};
@@ -345,4 +346,17 @@ impl PluginTest {
             _ => Ok(a == b),
         }
     }
+
+    /// This implements custom value comparison with `plugin.custom_value_to_base_value()` to behave
+    /// as similarly as possible to comparison in the engine.
+    pub fn custom_value_to_base_value(
+        &self,
+        val: &dyn CustomValue,
+        span: Span,
+    ) -> Result<Value, ShellError> {
+        let mut serialized = PluginCustomValue::serialize_from_custom_value(val, span)?;
+        serialized.set_source(Some(self.source.clone()));
+        let persistent = self.source.persistent(None)?.get_plugin(None)?;
+        persistent.custom_value_to_base_value(serialized.into_spanned(span))
+    }
 }