Validate compiled Suno output

Sources/MusicAssistantCore/Services/PromptCompiler/SunoOutputValidation.swift

@@ -0,0 +1,29 @@
+import Foundation
+
+public protocol SunoOutputValidating: Sendable {
+    func validate(_ output: CompiledSunoOutput) throws
+}
+
+public enum SunoOutputValidationError: Error, Equatable, LocalizedError, Sendable {
+    case missingContent
+
+    public var errorDescription: String? {
+        switch self {
+        case .missingContent:
+            return "Add lyrics or a style prompt before sending the song to Suno."
+        }
+    }
+}
+
+public struct CompiledSunoOutputValidator: SunoOutputValidating {
+    public init() {}
+
+    public func validate(_ output: CompiledSunoOutput) throws {
+        let lyrics = output.lyricsText.trimmingCharacters(in: .whitespacesAndNewlines)
+        let style = output.stylePrompt.trimmingCharacters(in: .whitespacesAndNewlines)
+
+        guard !lyrics.isEmpty || !style.isEmpty else {
+            throw SunoOutputValidationError.missingContent
+        }
+    }
+}

Tests/MusicAssistantCoreTests/SunoOutputValidationTests.swift

@@ -0,0 +1,19 @@
+import MusicAssistantCore
+import XCTest
+
+final class SunoOutputValidationTests: XCTestCase {
+    func testValidatorRejectsOutputWithoutSunoContent() {
+        let output = CompiledSunoOutput(lyricsText: " \n ", stylePrompt: "\t")
+
+        XCTAssertThrowsError(try CompiledSunoOutputValidator().validate(output)) { error in
+            XCTAssertEqual(error as? SunoOutputValidationError, .missingContent)
+        }
+    }
+
+    func testValidatorAllowsLyricsOrAStylePrompt() throws {
+        let validator = CompiledSunoOutputValidator()
+
+        XCTAssertNoThrow(try validator.validate(CompiledSunoOutput(lyricsText: "Lyrics", stylePrompt: "")))
+        XCTAssertNoThrow(try validator.validate(CompiledSunoOutput(lyricsText: "", stylePrompt: "Cinematic folk")))
+    }
+}

docs/TASKS.md

@@ -102,7 +102,7 @@ requirement is missing and blocks implementation, record it in
     appropriate.
 -   [x] Generate style prompt from genre blend, instrumentation, vocals,
     tempo, harmony, emotion and production instructions.
--   [ ] Validate output before handoff.
+-   [x] Validate output before handoff.
 -   [ ] Keep compiler independent from UI and OpenAI client.
 
 ## Phase 8 --- Suno Handoff