feat: use template/module config variable's default value as its field placeholder if present

site/src/pages/TemplateBuilder/BaseTemplateParametersStep.tsx

@@ -11,6 +11,7 @@ import {
 	TemplateBuilderTitle,
 } from "#/pages/TemplateBuilder/TemplateBuilderHeader";
 import type { ConfigurationFieldDefinition } from "./ConfigurationField";
+import { defaultPlaceholder } from "./defaultPlaceholder";
 import { TemplateConfiguration } from "./TemplateConfiguration";
 
 interface BaseTemplateParametersStepProps {
@@ -53,7 +54,9 @@ function variableToField(
 		label: variable.name,
 		description: variable.description || undefined,
 		required: variable.required,
-		placeholder: variable.required ? "Required" : "Optional",
+		placeholder:
+			defaultPlaceholder(variable.default) ??
+			(variable.required ? "Required" : "Optional"),
 		field: {
 			name: variable.name,
 			id,

site/src/pages/TemplateBuilder/ModuleSettingsStep.tsx

@@ -12,6 +12,7 @@ import {
 	TemplateBuilderTitle,
 } from "#/pages/TemplateBuilder/TemplateBuilderHeader";
 import type { ConfigurationFieldDefinition } from "./ConfigurationField";
+import { defaultPlaceholder } from "./defaultPlaceholder";
 import { ModuleConfiguration } from "./ModuleConfiguration";
 
 interface ModuleSettingsStepProps {
@@ -51,7 +52,9 @@ function variableToField(
 		label: variable.name,
 		description: variable.description || undefined,
 		required: variable.required,
-		placeholder: variable.required ? "Required" : "Optional",
+		placeholder:
+			defaultPlaceholder(variable.default) ??
+			(variable.required ? "Required" : "Optional"),
 		field: {
 			name: variable.name,
 			id,

site/src/pages/TemplateBuilder/defaultPlaceholder.test.ts

@@ -0,0 +1,28 @@
+import { defaultPlaceholder } from "./defaultPlaceholder";
+
+describe("defaultPlaceholder", () => {
+	it("formats numeric defaults, including zero", () => {
+		expect(defaultPlaceholder(13337)).toBe("13337");
+		expect(defaultPlaceholder(0)).toBe("0");
+	});
+
+	it("formats boolean defaults", () => {
+		expect(defaultPlaceholder(false)).toBe("false");
+		expect(defaultPlaceholder(true)).toBe("true");
+	});
+
+	it("returns non-empty string defaults unchanged", () => {
+		expect(defaultPlaceholder("v0.10.0")).toBe("v0.10.0");
+	});
+
+	it("returns undefined for empty or absent defaults", () => {
+		expect(defaultPlaceholder("")).toBeUndefined();
+		expect(defaultPlaceholder(undefined)).toBeUndefined();
+		expect(defaultPlaceholder(null)).toBeUndefined();
+	});
+
+	it("returns undefined for object and array defaults", () => {
+		expect(defaultPlaceholder({ key: "value" })).toBeUndefined();
+		expect(defaultPlaceholder(["a", "b"])).toBeUndefined();
+	});
+});

site/src/pages/TemplateBuilder/defaultPlaceholder.ts

@@ -0,0 +1,22 @@
+/**
+ * Coerces a module variable's `default` into placeholder text.
+ *
+ * `TemplateBuilderModuleVariable.default` is generated as
+ * `Record<string, string>` because every `json.RawMessage` field maps to that
+ * type, but the actual wire value is a JSON scalar (string, number, or
+ * boolean). The parameter is typed `unknown` so the generated value is accepted
+ * without a cast.
+ *
+ * An empty-string default returns `undefined` so callers fall back to their
+ * existing Required/Optional hint. Object or array defaults also return
+ * `undefined` since they have no meaningful placeholder representation.
+ */
+export function defaultPlaceholder(value: unknown): string | undefined {
+	if (typeof value === "number" || typeof value === "boolean") {
+		return String(value);
+	}
+	if (typeof value === "string" && value !== "") {
+		return value;
+	}
+	return undefined;
+}