diff --git a/server-rs/crates/platform-audio/src/background_music_prompt.rs b/server-rs/crates/platform-audio/src/background_music_prompt.rs
new file mode 100644
index 000000000..ba5d43e73
--- /dev/null
+++ b/server-rs/crates/platform-audio/src/background_music_prompt.rs
@@ -0,0 +1,76 @@
+use crate::{AudioError, SUNO_GPT_DESCRIPTION_PROMPT_MAX_CHARS};
+
+const BACKGROUND_MUSIC_GENERATION_PROMPT_FIELD: &str = "gpt_description_prompt";
+const BACKGROUND_MUSIC_COMPLETION_PROMPT_FIELD: &str = "currentPrompt";
+const BACKGROUND_MUSIC_GENERATION_MIN_EFFECTIVE_CHARS: usize = 1;
+const BACKGROUND_MUSIC_COMPLETION_MIN_EFFECTIVE_CHARS: usize = 2;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct ValidatedBackgroundMusicPrompt<'a> {
+    /// The exact caller-provided string. Validation never trims or normalizes it.
+    pub prompt: &'a str,
+    pub char_count: usize,
+    pub effective_char_count: usize,
+}
+
+/// Counts Unicode scalar values, matching TypeScript `Array.from(prompt).length`.
+pub fn background_music_prompt_char_count(prompt: &str) -> usize {
+    prompt.chars().count()
+}
+
+/// Counts code points outside the Unicode `White_Space` property.
+///
+/// U+0085 is whitespace, while U+200B and U+FEFF remain effective characters.
+pub fn background_music_prompt_effective_char_count(prompt: &str) -> usize {
+    prompt
+        .chars()
+        .filter(|character| !character.is_whitespace())
+        .count()
+}
+
+pub fn validate_background_music_generation_prompt(
+    prompt: &str,
+) -> Result<ValidatedBackgroundMusicPrompt<'_>, AudioError> {
+    validate_background_music_prompt(
+        prompt,
+        BACKGROUND_MUSIC_GENERATION_PROMPT_FIELD,
+        BACKGROUND_MUSIC_GENERATION_MIN_EFFECTIVE_CHARS,
+    )
+}
+
+pub fn validate_background_music_completion_prompt(
+    prompt: &str,
+) -> Result<ValidatedBackgroundMusicPrompt<'_>, AudioError> {
+    validate_background_music_prompt(
+        prompt,
+        BACKGROUND_MUSIC_COMPLETION_PROMPT_FIELD,
+        BACKGROUND_MUSIC_COMPLETION_MIN_EFFECTIVE_CHARS,
+    )
+}
+
+fn validate_background_music_prompt<'a>(
+    prompt: &'a str,
+    field: &'static str,
+    min_effective_chars: usize,
+) -> Result<ValidatedBackgroundMusicPrompt<'a>, AudioError> {
+    let char_count = background_music_prompt_char_count(prompt);
+    let effective_char_count = background_music_prompt_effective_char_count(prompt);
+
+    if char_count > SUNO_GPT_DESCRIPTION_PROMPT_MAX_CHARS {
+        return Err(AudioError::invalid_request(format!(
+            "{field} 超过 {} 字符",
+            SUNO_GPT_DESCRIPTION_PROMPT_MAX_CHARS
+        )));
+    }
+    if effective_char_count < min_effective_chars {
+        return Err(AudioError::invalid_request(format!(
+            "{field} 至少需要 {min_effective_chars} 个有效字符"
+        )));
+    }
+
+    Ok(ValidatedBackgroundMusicPrompt {
+        prompt,
+        char_count,
+        effective_char_count,
+    })
+}
diff --git a/server-rs/crates/platform-audio/src/lib.rs b/server-rs/crates/platform-audio/src/lib.rs
index a686612a7..cb6a3b5dc 100644
--- a/server-rs/crates/platform-audio/src/lib.rs
+++ b/server-rs/crates/platform-audio/src/lib.rs
@@ -1,3 +1,4 @@
+mod background_music_prompt;
 mod client;
 mod download;
 mod error;
@@ -6,6 +7,11 @@ mod request;
 mod response;
 mod types;
 
+pub use background_music_prompt::{
+    ValidatedBackgroundMusicPrompt, background_music_prompt_char_count,
+    background_music_prompt_effective_char_count, validate_background_music_completion_prompt,
+    validate_background_music_generation_prompt,
+};
 pub use client::{
     build_vector_engine_audio_http_client, resolve_audio_task_download_urls,
     submit_background_music_task, submit_editor_background_music_task,
diff --git a/server-rs/crates/platform-audio/tests/vector_engine_audio.rs b/server-rs/crates/platform-audio/tests/vector_engine_audio.rs
index bcdee944d..0aabc1c75 100644
--- a/server-rs/crates/platform-audio/tests/vector_engine_audio.rs
+++ b/server-rs/crates/platform-audio/tests/vector_engine_audio.rs
@@ -1,13 +1,96 @@
 use platform_audio::{
     AudioTaskKind, BackgroundMusicTaskRequest, EditorBackgroundMusicTaskRequest,
     EditorSoundEffectTaskRequest, SUNO_DEFAULT_MODEL, VIDU_AUDIO_MODEL, VIDU_PROMPT_MAX_CHARS,
-    audio_mime_to_extension, build_background_music_task_body,
+    audio_mime_to_extension, background_music_prompt_char_count,
+    background_music_prompt_effective_char_count, build_background_music_task_body,
     build_editor_background_music_task_body, build_editor_sound_effect_task_body,
     build_sound_effect_task_body, extract_audio_urls, is_failed_task_status,
     is_pending_task_status, normalize_audio_mime_type, normalize_task_status,
+    validate_background_music_completion_prompt, validate_background_music_generation_prompt,
 };
 use serde_json::json;
 
+#[test]
+fn background_music_prompt_counts_unicode_code_points_and_effective_characters() {
+    let prompt = " \t\n\u{00a0}\u{2003}\u{200b}😀";
+
+    assert_eq!(background_music_prompt_char_count(prompt), 7);
+    assert_eq!(background_music_prompt_effective_char_count(prompt), 2);
+    assert_eq!(background_music_prompt_char_count("\r\n"), 2);
+    assert_eq!(background_music_prompt_effective_char_count("\r\n"), 0);
+    assert_eq!(background_music_prompt_char_count("e\u{0301}"), 2);
+    assert_eq!(background_music_prompt_effective_char_count("e\u{0301}"), 2);
+    assert_eq!(background_music_prompt_char_count("👩‍💻"), 3);
+    assert_eq!(background_music_prompt_effective_char_count("👩‍💻"), 3);
+    assert_eq!(background_music_prompt_char_count("\u{0085}"), 1);
+    assert_eq!(background_music_prompt_effective_char_count("\u{0085}"), 0);
+    assert_eq!(background_music_prompt_char_count("\u{feff}"), 1);
+    assert_eq!(background_music_prompt_effective_char_count("\u{feff}"), 1);
+}
+
+#[test]
+fn background_music_generation_prompt_requires_one_effective_character_without_cleaning() {
+    for prompt in ["", " \t\r\n", "\u{00a0}\u{2003}"] {
+        let error = validate_background_music_generation_prompt(prompt)
+            .expect_err("Unicode-whitespace-only prompts should be rejected");
+        assert!(error.message().contains("至少需要 1 个有效字符"));
+    }
+
+    for prompt in ["乐", "\u{200b}", "😀"] {
+        let validated = validate_background_music_generation_prompt(prompt)
+            .expect("one non-whitespace code point should be accepted for generation");
+        assert_eq!(validated.prompt, prompt);
+        assert_eq!(validated.char_count, 1);
+        assert_eq!(validated.effective_char_count, 1);
+    }
+
+    let prompt = "  音\n\u{200b}😀  ";
+    let validated = validate_background_music_generation_prompt(prompt)
+        .expect("valid prompt should retain every original code point");
+    assert_eq!(validated.prompt, prompt);
+    assert_eq!(validated.char_count, 8);
+    assert_eq!(validated.effective_char_count, 3);
+}
+
+#[test]
+fn background_music_completion_prompt_requires_two_effective_characters() {
+    for prompt in ["", "乐", "乐 \n", "\u{200b}", "😀"] {
+        let error = validate_background_music_completion_prompt(prompt)
+            .expect_err("completion should require two effective characters");
+        assert!(error.message().starts_with("currentPrompt "));
+        assert!(error.message().contains("至少需要 2 个有效字符"));
+    }
+
+    for prompt in ["音乐", "\u{200b}乐", "😀🎵", "👩‍💻"] {
+        let validated = validate_background_music_completion_prompt(prompt)
+            .expect("completion should accept at least two effective code points");
+        assert_eq!(validated.prompt, prompt);
+        assert!(validated.effective_char_count >= 2);
+    }
+}
+
+#[test]
+fn background_music_prompt_validation_accepts_200_and_rejects_201_code_points() {
+    let max_prompt = "😀".repeat(200);
+    let generation = validate_background_music_generation_prompt(&max_prompt)
+        .expect("200 code points should be accepted for generation");
+    let completion = validate_background_music_completion_prompt(&max_prompt)
+        .expect("200 code points should be accepted for completion");
+    assert_eq!(generation.prompt, max_prompt);
+    assert_eq!(generation.char_count, 200);
+    assert_eq!(completion.char_count, 200);
+
+    let overlong_prompt = "😀".repeat(201);
+    for error in [
+        validate_background_music_generation_prompt(&overlong_prompt)
+            .expect_err("201 code points should be rejected for generation"),
+        validate_background_music_completion_prompt(&overlong_prompt)
+            .expect_err("201 code points should be rejected for completion"),
+    ] {
+        assert!(error.message().contains("超过 200 字符"));
+    }
+}
+
 #[test]
 fn normalizes_audio_mime_type_from_content_type_and_url() {
     assert_eq!(
diff --git a/server-rs/crates/shared-contracts/src/assets.rs b/server-rs/crates/shared-contracts/src/assets.rs
index 6186d6253..02c8a6b29 100644
--- a/server-rs/crates/shared-contracts/src/assets.rs
+++ b/server-rs/crates/shared-contracts/src/assets.rs
@@ -542,6 +542,19 @@ pub struct EditorBackgroundMusicGenerateRequest {
     pub asset_label: Option<String>,
 }
 
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "camelCase")]
+pub struct BackgroundMusicPromptAssistRequest {
+    pub current_prompt: String,
+}
+
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "camelCase")]
+pub struct BackgroundMusicPromptAssistResponse {
+    pub prompt: String,
+    pub char_count: u32,
+}
+
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
 #[serde(rename_all = "camelCase")]
 pub struct EditorAudioGenerateResponse {
@@ -1442,6 +1455,45 @@ mod tests {
         );
     }
 
+    #[test]
+    fn background_music_prompt_assist_contract_uses_camel_case_shape() {
+        let request = BackgroundMusicPromptAssistRequest {
+            current_prompt: "\n 森林冒险背景音乐 ".to_string(),
+        };
+        let request_payload = serde_json::to_value(&request)
+            .expect("background music prompt assist request should serialize");
+        assert_eq!(
+            request_payload,
+            json!({
+                "currentPrompt": "\n 森林冒险背景音乐 ",
+            })
+        );
+        assert_eq!(
+            serde_json::from_value::<BackgroundMusicPromptAssistRequest>(request_payload)
+                .expect("background music prompt assist request should deserialize"),
+            request
+        );
+
+        let response = BackgroundMusicPromptAssistResponse {
+            prompt: "森林冒险背景音乐，温和推进并适合自然循环".to_string(),
+            char_count: 20,
+        };
+        let response_payload = serde_json::to_value(&response)
+            .expect("background music prompt assist response should serialize");
+        assert_eq!(
+            response_payload,
+            json!({
+                "prompt": "森林冒险背景音乐，温和推进并适合自然循环",
+                "charCount": 20,
+            })
+        );
+        assert_eq!(
+            serde_json::from_value::<BackgroundMusicPromptAssistResponse>(response_payload)
+                .expect("background music prompt assist response should deserialize"),
+            response
+        );
+    }
+
     #[test]
     fn character_workflow_cache_response_keeps_legacy_shape() {
         let payload = serde_json::to_value(CharacterWorkflowCacheSaveResponse {
diff --git a/src/components/image-editor/ImageCanvasBackgroundMusicPromptModel.test.ts b/src/components/image-editor/ImageCanvasBackgroundMusicPromptModel.test.ts
new file mode 100644
index 000000000..0cd600dbd
--- /dev/null
+++ b/src/components/image-editor/ImageCanvasBackgroundMusicPromptModel.test.ts
@@ -0,0 +1,60 @@
+import { describe, expect, it } from 'vitest';
+
+import {
+  BACKGROUND_MUSIC_PROMPT_COMPLETION_MIN_EFFECTIVE_CODE_POINTS,
+  BACKGROUND_MUSIC_PROMPT_GENERATION_MIN_EFFECTIVE_CODE_POINTS,
+  BACKGROUND_MUSIC_PROMPT_MAX_CODE_POINTS,
+  canCompleteBackgroundMusicPrompt,
+  canGenerateBackgroundMusicFromPrompt,
+  countEffectivePromptCodePoints,
+  countPromptCodePoints,
+} from './ImageCanvasBackgroundMusicPromptModel';
+
+describe('ImageCanvasBackgroundMusicPromptModel', () => {
+  it('counts raw Unicode code points without normalizing the prompt', () => {
+    const combiningText = 'e\u0301';
+    const zwjEmoji = '👨‍👩‍👧‍👦';
+    const prompt = ` \r\n${combiningText}${zwjEmoji}\u200B\uFEFF `;
+
+    expect(countPromptCodePoints('')).toBe(0);
+    expect(countPromptCodePoints('\r\n')).toBe(2);
+    expect(countPromptCodePoints(combiningText)).toBe(2);
+    expect(countPromptCodePoints(zwjEmoji)).toBe(7);
+    expect(countPromptCodePoints(prompt)).toBe(15);
+    expect(prompt).toBe(` \r\n${combiningText}${zwjEmoji}\u200B\uFEFF `);
+  });
+
+  it('counts only non-Unicode-White_Space code points as effective', () => {
+    expect(countEffectivePromptCodePoints(' \t\r\n\u0085\u00A0\u3000')).toBe(0);
+    expect(countEffectivePromptCodePoints('e\u0301')).toBe(2);
+    expect(countEffectivePromptCodePoints('👨‍👩‍👧‍👦')).toBe(7);
+    expect(countEffectivePromptCodePoints('\uFEFF')).toBe(1);
+    expect(countEffectivePromptCodePoints('\u200B')).toBe(1);
+    expect(countEffectivePromptCodePoints('。1A')).toBe(3);
+  });
+
+  it('allows formal generation for 1-200 code points with effective content', () => {
+    expect(BACKGROUND_MUSIC_PROMPT_MAX_CODE_POINTS).toBe(200);
+    expect(BACKGROUND_MUSIC_PROMPT_GENERATION_MIN_EFFECTIVE_CODE_POINTS).toBe(
+      1,
+    );
+    expect(canGenerateBackgroundMusicFromPrompt('')).toBe(false);
+    expect(canGenerateBackgroundMusicFromPrompt(' \r\n\u0085')).toBe(false);
+    expect(canGenerateBackgroundMusicFromPrompt('乐')).toBe(true);
+    expect(canGenerateBackgroundMusicFromPrompt('😀'.repeat(199))).toBe(true);
+    expect(canGenerateBackgroundMusicFromPrompt('😀'.repeat(200))).toBe(true);
+    expect(canGenerateBackgroundMusicFromPrompt('😀'.repeat(201))).toBe(false);
+  });
+
+  it('requires 2 effective code points and at most 200 total for AI completion', () => {
+    expect(BACKGROUND_MUSIC_PROMPT_COMPLETION_MIN_EFFECTIVE_CODE_POINTS).toBe(
+      2,
+    );
+    expect(canCompleteBackgroundMusicPrompt(' \r\n\u0085')).toBe(false);
+    expect(canCompleteBackgroundMusicPrompt('乐')).toBe(false);
+    expect(canCompleteBackgroundMusicPrompt('乐 ')).toBe(false);
+    expect(canCompleteBackgroundMusicPrompt('乐曲')).toBe(true);
+    expect(canCompleteBackgroundMusicPrompt('😀'.repeat(200))).toBe(true);
+    expect(canCompleteBackgroundMusicPrompt('😀'.repeat(201))).toBe(false);
+  });
+});
diff --git a/src/components/image-editor/ImageCanvasBackgroundMusicPromptModel.ts b/src/components/image-editor/ImageCanvasBackgroundMusicPromptModel.ts
new file mode 100644
index 000000000..05d3075a5
--- /dev/null
+++ b/src/components/image-editor/ImageCanvasBackgroundMusicPromptModel.ts
@@ -0,0 +1,35 @@
+export const BACKGROUND_MUSIC_PROMPT_MAX_CODE_POINTS = 200;
+export const BACKGROUND_MUSIC_PROMPT_GENERATION_MIN_EFFECTIVE_CODE_POINTS = 1;
+export const BACKGROUND_MUSIC_PROMPT_COMPLETION_MIN_EFFECTIVE_CODE_POINTS = 2;
+
+const UNICODE_WHITE_SPACE_CODE_POINT = /^\p{White_Space}$/u;
+
+export function countPromptCodePoints(prompt: string) {
+  return Array.from(prompt).length;
+}
+
+export function countEffectivePromptCodePoints(prompt: string) {
+  let effectiveCodePoints = 0;
+  for (const codePoint of prompt) {
+    if (!UNICODE_WHITE_SPACE_CODE_POINT.test(codePoint)) {
+      effectiveCodePoints += 1;
+    }
+  }
+  return effectiveCodePoints;
+}
+
+export function canGenerateBackgroundMusicFromPrompt(prompt: string) {
+  return (
+    countPromptCodePoints(prompt) <= BACKGROUND_MUSIC_PROMPT_MAX_CODE_POINTS &&
+    countEffectivePromptCodePoints(prompt) >=
+      BACKGROUND_MUSIC_PROMPT_GENERATION_MIN_EFFECTIVE_CODE_POINTS
+  );
+}
+
+export function canCompleteBackgroundMusicPrompt(prompt: string) {
+  return (
+    countPromptCodePoints(prompt) <= BACKGROUND_MUSIC_PROMPT_MAX_CODE_POINTS &&
+    countEffectivePromptCodePoints(prompt) >=
+      BACKGROUND_MUSIC_PROMPT_COMPLETION_MIN_EFFECTIVE_CODE_POINTS
+  );
+}