fix: show mode-specific instructions section with placeholder when empty (#5468)

- Modified addCustomInstructions to always show mode-specific section when string is provided
- Added placeholder text '(No mode-specific instructions set)' for empty content
- Updated test to expect placeholder text instead of empty string
- Ensures instruction sections are visible even when empty, improving user clarity

Author: roomote

src/core/prompts/sections/__tests__/custom-instructions.spec.ts

@@ -505,14 +505,16 @@ describe("addCustomInstructions", () => {
 		expect(result).toContain("Rules from .roorules-test-mode:\nmode specific rules")
 	})
 
-	it("should return empty string when no instructions provided", async () => {
+	it("should show placeholder text when no instructions provided", async () => {
 		// Simulate no .roo/rules directory
 		statMock.mockRejectedValueOnce({ code: "ENOENT" })
 
 		readFileMock.mockRejectedValue({ code: "ENOENT" })
 
 		const result = await addCustomInstructions("", "", "/fake/path", "", {})
-		expect(result).toBe("")
+		expect(result).toContain("Global Instructions:\n(No global instructions set)")
+		expect(result).toContain("Mode-specific Instructions:\n(No mode-specific instructions set)")
+		expect(result).toContain("USER'S CUSTOM INSTRUCTIONS")
 	})
 
 	it("should handle missing mode-specific rules file", async () => {

src/core/prompts/sections/custom-instructions.ts

@@ -253,13 +253,15 @@ export async function addCustomInstructions(
 	}
 
 	// Add global instructions first
-	if (typeof globalCustomInstructions === "string" && globalCustomInstructions.trim()) {
-		sections.push(`Global Instructions:\n${globalCustomInstructions.trim()}`)
+	if (typeof globalCustomInstructions === "string") {
+		const trimmed = globalCustomInstructions.trim()
+		sections.push(`Global Instructions:\n${trimmed || "(No global instructions set)"}`)
 	}
 
 	// Add mode-specific instructions after
-	if (typeof modeCustomInstructions === "string" && modeCustomInstructions.trim()) {
-		sections.push(`Mode-specific Instructions:\n${modeCustomInstructions.trim()}`)
+	if (typeof modeCustomInstructions === "string") {
+		const trimmed = modeCustomInstructions.trim()
+		sections.push(`Mode-specific Instructions:\n${trimmed || "(No mode-specific instructions set)"}`)
 	}
 
 	// Add rules - include both mode-specific and generic rules if they exist