Smarter partial merging of prompt components

Author: mrubens

src/shared/__tests__/modes-empty-prompt-component.spec.ts

@@ -40,7 +40,7 @@ describe("getModeSelection with empty promptComponent", () => {
 		expect(result.baseInstructions).toBe("Custom instructions")
 	})
 
-	it("should use promptComponent when it has partial content", () => {
+	it("should merge promptComponent with built-in mode when it has partial content", () => {
 		const architectMode = modes.find((m) => m.slug === "architect")!
 
 		// Test with promptComponent that only has customInstructions
@@ -49,8 +49,49 @@ describe("getModeSelection with empty promptComponent", () => {
 		}
 		const result = getModeSelection("architect", partialPromptComponent, [])
 
-		// Should use promptComponent since it has some content
-		expect(result.roleDefinition).toBe("") // No roleDefinition in promptComponent
-		expect(result.baseInstructions).toBe("Only custom instructions")
+		// Should merge: use promptComponent's customInstructions but fall back to built-in roleDefinition
+		expect(result.roleDefinition).toBe(architectMode.roleDefinition) // Falls back to built-in
+		expect(result.baseInstructions).toBe("Only custom instructions") // Uses promptComponent
+	})
+
+	it("should merge promptComponent with built-in mode when it only has roleDefinition", () => {
+		const debugMode = modes.find((m) => m.slug === "debug")!
+
+		// Test with promptComponent that only has roleDefinition
+		const partialPromptComponent: PromptComponent = {
+			roleDefinition: "Custom debug role",
+		}
+		const result = getModeSelection("debug", partialPromptComponent, [])
+
+		// Should merge: use promptComponent's roleDefinition but fall back to built-in customInstructions
+		expect(result.roleDefinition).toBe("Custom debug role") // Uses promptComponent
+		expect(result.baseInstructions).toBe(debugMode.customInstructions) // Falls back to built-in
+	})
+
+	it("should handle promptComponent with both roleDefinition and customInstructions", () => {
+		// Test with promptComponent that has both properties
+		const fullPromptComponent: PromptComponent = {
+			roleDefinition: "Full custom role",
+			customInstructions: "Full custom instructions",
+		}
+		const result = getModeSelection("architect", fullPromptComponent, [])
+
+		// Should use promptComponent values for both
+		expect(result.roleDefinition).toBe("Full custom role")
+		expect(result.baseInstructions).toBe("Full custom instructions")
+	})
+
+	it("should fall back to default mode when built-in mode is not found", () => {
+		const defaultMode = modes[0] // First mode is the default
+
+		// Test with non-existent mode
+		const partialPromptComponent: PromptComponent = {
+			customInstructions: "Custom instructions for unknown mode",
+		}
+		const result = getModeSelection("non-existent-mode", partialPromptComponent, [])
+
+		// Should merge with default mode
+		expect(result.roleDefinition).toBe(defaultMode.roleDefinition) // Falls back to default mode
+		expect(result.baseInstructions).toBe("Custom instructions for unknown mode") // Uses promptComponent
 	})
 })

src/shared/__tests__/modes.spec.ts

@@ -443,10 +443,11 @@ describe("getModeSelection", () => {
 		expect(selection.baseInstructions).toBe(newCustomMode.customInstructions)
 	})
 
-	test("should return empty strings if slug does not exist in custom, prompt, or built-in modes", () => {
+	test("should fall back to default mode if slug does not exist in custom, prompt, or built-in modes", () => {
 		const selection = getModeSelection("non-existent-mode", undefined, customModesList)
-		expect(selection.roleDefinition).toBe("")
-		expect(selection.baseInstructions).toBe("")
+		const defaultMode = modes[0] // First mode is the default
+		expect(selection.roleDefinition).toBe(defaultMode.roleDefinition)
+		expect(selection.baseInstructions).toBe(defaultMode.customInstructions || "")
 	})
 
 	test("customMode's properties are used if customMode exists, ignoring promptComponent's properties", () => {

src/shared/modes.ts

@@ -183,23 +183,29 @@ export function findModeBySlug(slug: string, modes: readonly ModeConfig[] | unde
 /**
  * Get the mode selection based on the provided mode slug, prompt component, and custom modes.
  * If a custom mode is found, it takes precedence over the built-in modes.
- * If no custom mode is found, the built-in mode is used.
+ * If no custom mode is found, the built-in mode is used with partial merging from promptComponent.
  * If neither is found, the default mode is used.
  */
 export function getModeSelection(mode: string, promptComponent?: PromptComponent, customModes?: ModeConfig[]) {
 	const customMode = findModeBySlug(mode, customModes)
 	const builtInMode = findModeBySlug(mode, modes)
 
-	const modeToUse = customMode || promptComponent || builtInMode
+	// If we have a custom mode, use it entirely
+	if (customMode) {
+		return {
+			roleDefinition: customMode.roleDefinition || "",
+			baseInstructions: customMode.customInstructions || "",
+			description: customMode.description || "",
+		}
+	}
 
-	const roleDefinition = modeToUse?.roleDefinition || ""
-	const baseInstructions = modeToUse?.customInstructions || ""
-	const description = (customMode || builtInMode)?.description || ""
+	// Otherwise, use built-in mode as base and merge with promptComponent
+	const baseMode = builtInMode || modes[0] // fallback to default mode
 
 	return {
-		roleDefinition,
-		baseInstructions,
-		description,
+		roleDefinition: promptComponent?.roleDefinition || baseMode.roleDefinition || "",
+		baseInstructions: promptComponent?.customInstructions || baseMode.customInstructions || "",
+		description: baseMode.description || "",
 	}
 }