diff --git a/site/src/types/preview.ts b/site/src/types/preview.ts
index f423a4e..945c7e1 100644
--- a/site/src/types/preview.ts
+++ b/site/src/types/preview.ts
@@ -89,6 +89,9 @@ export interface SessionInputs {
     readonly User: WorkspaceOwner;
 }
 
+// From types/value.go
+export const UnknownStringValue = "??";
+
 // From types/parameter.go
 export const ValidationMonotonicDecreasing = "decreasing";
 
diff --git a/types/value.go b/types/value.go
index ab8b2a4..4cc269b 100644
--- a/types/value.go
+++ b/types/value.go
@@ -9,6 +9,10 @@ import (
 	"github.com/zclconf/go-cty/cty"
 )
 
+const (
+	UnknownStringValue = "??"
+)
+
 type NullHCLString struct {
 	Value string `json:"value"`
 	Valid bool   `json:"valid"`
@@ -94,7 +98,7 @@ func (s HCLString) AsString() string {
 		return *s.Source
 	}
 
-	return "??"
+	return UnknownStringValue
 }
 
 func (s HCLString) IsKnown() bool {
diff --git a/types/value_test.go b/types/value_test.go
new file mode 100644
index 0000000..099b0bd
--- /dev/null
+++ b/types/value_test.go
@@ -0,0 +1,93 @@
+package types_test
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/zclconf/go-cty/cty"
+
+	"github.com/coder/preview/types"
+)
+
+func TestSafeHCLString(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name  string
+		input types.HCLString
+
+		asString string
+		known    bool
+		valid    bool
+	}{
+		{
+			name:     "empty",
+			asString: types.UnknownStringValue,
+		},
+		{
+			name:     "string_literal",
+			input:    types.StringLiteral("hello world"),
+			asString: "hello world",
+			known:    true,
+			valid:    true,
+		},
+		{
+			name: "number",
+			input: types.HCLString{
+				Value: cty.NumberIntVal(1),
+			},
+			asString: "1",
+			known:    true,
+			valid:    true,
+		},
+		{
+			name: "bool",
+			input: types.HCLString{
+				Value: cty.BoolVal(true),
+			},
+			asString: "true",
+			known:    true,
+			valid:    true,
+		},
+		// Crazy ideas
+		{
+			name: "null",
+			input: types.HCLString{
+				Value: cty.NullVal(cty.NilType),
+			},
+			asString: types.UnknownStringValue,
+			known:    false,
+			valid:    false,
+		},
+		{
+			name: "empty_string_list",
+			input: types.HCLString{
+				Value: cty.ListValEmpty(cty.String),
+			},
+			asString: types.UnknownStringValue,
+			known:    false,
+			valid:    false,
+		},
+		{
+			name: "dynamic",
+			input: types.HCLString{
+				Value: cty.DynamicVal,
+			},
+			asString: types.UnknownStringValue,
+			known:    false,
+			valid:    false,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			require.Equal(t, tc.asString, tc.input.AsString())
+			require.Equal(t, tc.known, tc.input.IsKnown(), "known")
+			require.Equal(t, tc.valid, tc.input.Valid(), "valid")
+
+			_, err := json.Marshal(tc.input)
+			require.NoError(t, err)
+		})
+	}
+}