diff --git a/AGENTS.md b/AGENTS.md
index 0eb8262..9785d7c 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -138,7 +138,7 @@ Use `.env.example` as the source of truth. Never commit `.env.local`, API keys,
 
 - Text and Vision use `TEXT_*` and `VISION_*`; default protocol is `openai_compatible`, with native `anthropic` and `google` available via `TEXT_PROVIDER` / `VISION_PROVIDER`.
 - Image uses `IMAGE_*`; supported protocols are `runware`, `openai_compatible`, native `openai`, and native `google`. When `IMAGE_PROVIDER` is unset, Runware is inferred from `*.runware.ai` URLs and otherwise falls back to OpenAI-compatible image generations.
-- TTS uses Xiaomi MiMo protocol and is optional: blank config means silent mode.
+- TTS supports Xiaomi MiMo (voicedesign + voiceclone) or StepFun (preset voices auto-selected by keyword scoring), inferred from `TTS_BASE_URL` (host containing `stepfun.com` → StepFun, otherwise → MiMo). `CharacterVoice` is a discriminated union on `provider`; synth dispatches on the voice's own tag so a session may carry both shapes through a provider switch. Blank config means silent mode.
 - `MOCK_IMAGE=true` skips image generation and returns a placeholder for cheap local iteration.
 - `NEXT_PUBLIC_IMAGE_PROXY_URL` and `NEXT_PUBLIC_IMAGE_PROXY_ALLOWED_HOSTS` opt into browser-side image proxying for allowed hosts.
 - Analytics uses optional Umami `NEXT_PUBLIC_UMAMI_*` values and must stay content-free/privacy-preserving.
diff --git a/app/api/beat-audio/route.ts b/app/api/beat-audio/route.ts
index 7815d9b..fe881f2 100644
--- a/app/api/beat-audio/route.ts
+++ b/app/api/beat-audio/route.ts
@@ -13,9 +13,12 @@ export async function POST(req: Request) {
     return NextResponse.json({ error: "Invalid JSON" }, { status: 400 });
   }
 
-  if (!body.beat?.id || !body.beat?.line || !body.voice?.referenceAudioBase64) {
+  // Accept either provider's voice shape — xiaomi carries referenceAudioBase64,
+  // stepfun carries voiceId. We only check the discriminator + the line text;
+  // shape-specific validation lives in each provider's synth function.
+  if (!body.beat?.id || !body.beat?.line || !body.voice?.provider) {
     return NextResponse.json(
-      { error: "beat.id, beat.line and voice.referenceAudioBase64 are required" },
+      { error: "beat.id, beat.line and voice.provider are required" },
       { status: 400 },
     );
   }
diff --git a/lib/tts-client/index.ts b/lib/tts-client/index.ts
index eb83fd2..c4c6515 100644
--- a/lib/tts-client/index.ts
+++ b/lib/tts-client/index.ts
@@ -1 +1,38 @@
-export { xiaomiProvision as provisionVoice, xiaomiSynthesize as synthesize } from "./xiaomi";
+import type { CharacterVoice, TtsConfig } from "@infiplot/types";
+import { stepfunProvision, stepfunSynthesize } from "./stepfun";
+import { xiaomiProvision, xiaomiSynthesize } from "./xiaomi";
+
+// Provider auto-detection by base URL — mirrors the image client convention
+// of inferring Runware from *.runware.ai and falling back otherwise. Keeps
+// the BYO client flow unchanged: TTS_PROVIDER env var stays unused, and
+// browser-side keys (Xiaomi only today) keep working through the xiaomi path.
+function isStepfun(cfg: TtsConfig): boolean {
+  return /(^|[./])stepfun\.com\b/i.test(cfg.baseUrl);
+}
+
+export async function provisionVoice(
+  cfg: TtsConfig,
+  description: string,
+): Promise<CharacterVoice> {
+  return isStepfun(cfg)
+    ? stepfunProvision(cfg, description)
+    : xiaomiProvision(cfg, description);
+}
+
+// Dispatch by the voice's own provider tag, not by the current config. A
+// session can outlive a provider switch (e.g. .env.local flip mid-game), and
+// each voice must be synthesized via the protocol that minted it. The cfg
+// still needs to point at the matching provider's endpoint; mismatch surfaces
+// as a transparent network error, which `synthesizeBeat` already swallows.
+export async function synthesize(
+  cfg: TtsConfig,
+  voice: CharacterVoice,
+  text: string,
+  delivery?: string,
+  signal?: AbortSignal,
+): Promise<{ audioBase64: string; mimeType: string }> {
+  if (voice.provider === "stepfun") {
+    return stepfunSynthesize(cfg, voice, text, delivery, signal);
+  }
+  return xiaomiSynthesize(cfg, voice, text, delivery, signal);
+}
diff --git a/lib/tts-client/stepfun.ts b/lib/tts-client/stepfun.ts
new file mode 100644
index 0000000..00238ac
--- /dev/null
+++ b/lib/tts-client/stepfun.ts
@@ -0,0 +1,183 @@
+import type { CharacterVoice, TtsConfig } from "@infiplot/types";
+
+// StepFun TTS uses an OpenAI-compatible /v1/audio/speech endpoint with PRESET
+// voice IDs only — there is no "design a new voice from text description"
+// equivalent to Xiaomi MiMo's voicedesign. We therefore translate the LLM's
+// Chinese voiceDescription into a preset voice ID by keyword matching
+// (gender + age + tone), with a deterministic hash-based spread across the
+// top-N candidates so multiple similar characters don't collapse onto the
+// same voice. Provision is a pure function — no network call needed.
+
+const OUTPUT_FORMAT = "mp3";
+const OUTPUT_MIME = "audio/mpeg";
+
+type PresetVoice = {
+  id: string;
+  gender: "male" | "female";
+  age: "teen" | "young" | "middle";
+  /** Keywords (中文 or English) that, when present in the LLM's voice
+   *  description, boost this preset's score. Drawn from StepFun's published
+   *  voice name + recommended scenario. */
+  tones: string[];
+};
+
+// Full catalog from StepFun's docs (32 presets across step-tts-mini /
+// step-tts-2 / stepaudio-2.5-tts). Adding more later is safe — the scorer
+// degrades gracefully when an unknown id is picked.
+const PRESET_VOICES: PresetVoice[] = [
+  { id: "cixingnansheng", gender: "male", age: "young", tones: ["磁性", "成熟", "narrative"] },
+  { id: "wenrounansheng", gender: "male", age: "young", tones: ["温柔", "gentle", "supportive"] },
+  { id: "wenrougongzi", gender: "male", age: "young", tones: ["温柔", "公子", "tender"] },
+  { id: "yuanqinansheng", gender: "male", age: "teen", tones: ["元气", "energetic", "阳光"] },
+  { id: "zhengpaiqingnian", gender: "male", age: "young", tones: ["正派", "正气", "earnest"] },
+  { id: "shuangkuainansheng", gender: "male", age: "young", tones: ["爽快", "干脆", "brisk"] },
+  { id: "boyinnansheng", gender: "male", age: "middle", tones: ["播音", "broadcast", "稳重"] },
+  { id: "ruyananshi", gender: "male", age: "middle", tones: ["儒雅", "斯文", "refined"] },
+  { id: "shenchennanyin", gender: "male", age: "middle", tones: ["深沉", "低沉", "deep"] },
+  { id: "qingniandaxuesheng", gender: "male", age: "young", tones: ["大学生", "青年", "student"] },
+  { id: "zixinnansheng", gender: "male", age: "young", tones: ["自信", "confident"] },
+  { id: "elegantgentle-female", gender: "female", age: "young", tones: ["气质", "温婉", "professional"] },
+  { id: "livelybreezy-female", gender: "female", age: "teen", tones: ["活力", "轻快", "upbeat"] },
+  { id: "jingdiannvsheng", gender: "female", age: "middle", tones: ["经典", "classic", "成熟"] },
+  { id: "wenroushunv", gender: "female", age: "middle", tones: ["温柔", "熟女", "mature"] },
+  { id: "tianmeinvsheng", gender: "female", age: "young", tones: ["甜美", "sweet"] },
+  { id: "qingchunshaonv", gender: "female", age: "teen", tones: ["清纯", "少女", "pure"] },
+  { id: "yuanqishaonv", gender: "female", age: "teen", tones: ["元气", "少女", "活力", "energetic"] },
+  { id: "linjiajiejie", gender: "female", age: "young", tones: ["邻家", "姐姐"] },
+  { id: "jilingshaonv", gender: "female", age: "teen", tones: ["机灵", "灵动", "少女"] },
+  { id: "ruanmengnvsheng", gender: "female", age: "teen", tones: ["软萌", "可爱", "稚嫩", "甜软"] },
+  { id: "youyanvsheng", gender: "female", age: "young", tones: ["优雅", "elegant"] },
+  { id: "lengyanyujie", gender: "female", age: "middle", tones: ["冷艳", "御姐", "高冷"] },
+  { id: "shuangkuaijiejie", gender: "female", age: "young", tones: ["爽快", "姐姐", "干脆"] },
+  { id: "wenjingxuejie", gender: "female", age: "young", tones: ["文静", "学姐", "安静"] },
+  { id: "linjiameimei", gender: "female", age: "teen", tones: ["邻家", "妹妹"] },
+  { id: "zhixingjiejie", gender: "female", age: "young", tones: ["知性", "姐姐", "聪慧"] },
+  { id: "ganliannvsheng", gender: "female", age: "middle", tones: ["干练", "sharp", "professional"] },
+  { id: "qinhenvsheng", gender: "female", age: "young", tones: ["亲和", "warm", "亲切"] },
+  { id: "huolinvsheng", gender: "female", age: "young", tones: ["活力", "lively", "活泼"] },
+  { id: "qinqienvsheng", gender: "female", age: "middle", tones: ["亲切", "温暖"] },
+  { id: "wenrounvsheng", gender: "female", age: "young", tones: ["温柔", "tender", "柔和"] },
+];
+
+// Cheap deterministic 32-bit hash — used only to spread similar descriptions
+// across the top-N candidate voices so two "温柔女声" characters don't collide.
+function hashStr(s: string): number {
+  let h = 5381;
+  for (let i = 0; i < s.length; i++) h = ((h << 5) + h + s.charCodeAt(i)) | 0;
+  return Math.abs(h);
+}
+
+function detectGender(desc: string): "male" | "female" {
+  // Female signals (broader cast — galgame skews toward female NPCs).
+  if (/女性|女声|少女|姐姐|妹妹|熟女|御姐|阿姨|奶奶|女孩|姑娘|大妈|女子|女生|女士|她|小姐/.test(desc)) {
+    return "female";
+  }
+  if (/男性|男声|少年|青年|大叔|哥哥|弟弟|男人|男孩|大爷|爷爷|男子|男生|先生|他|公子|师傅/.test(desc)) {
+    return "male";
+  }
+  // No strong signal: default female (matches the catalog's center of mass).
+  return "female";
+}
+
+function detectAge(desc: string): "teen" | "young" | "middle" {
+  if (/中年|熟女|大叔|大妈|阿姨|奶奶|爷爷|老师|师傅|御姐|经理|总监|教授|博士|总裁|长辈|父亲|母亲|爸爸|妈妈/.test(desc)) {
+    return "middle";
+  }
+  if (/少女|少年|学生|高中|初中|妹妹|弟弟|小学|童年|稚嫩|十几岁|十六|十七|十八|未成年/.test(desc)) {
+    return "teen";
+  }
+  return "young";
+}
+
+/** Map an LLM-written 中文 voice description to a StepFun preset voice ID.
+ *  Pure function — exported for tests and for the synthesis-time sanity log.
+ */
+export function pickStepfunVoiceId(description: string, salt = ""): string {
+  const desc = description.toLowerCase();
+  const gender = detectGender(desc);
+  const age = detectAge(desc);
+
+  const scored = PRESET_VOICES
+    .filter((v) => v.gender === gender)
+    .map((v) => {
+      let score = 0;
+      if (v.age === age) score += 4;
+      for (const tone of v.tones) {
+        if (desc.includes(tone.toLowerCase())) score += 2;
+      }
+      return { v, score };
+    })
+    .sort((a, b) => b.score - a.score);
+
+  // Catalog can't be filtered to zero; this guards against a future edit
+  // that prunes the table too aggressively.
+  if (scored.length === 0) return PRESET_VOICES[0]!.id;
+
+  // Pick from the top 3 (or fewer) deterministically by hashing the
+  // description + an optional salt (charName) so two characters that share
+  // archetype keywords don't collapse onto the identical preset.
+  const top = scored.slice(0, Math.min(3, scored.length));
+  const idx = hashStr(description + "|" + salt) % top.length;
+  return top[idx]!.v.id;
+}
+
+// Provision is synchronous / no network — StepFun has no voicedesign equivalent.
+// We mirror xiaomiProvision's async signature so the router stays uniform.
+export async function stepfunProvision(
+  cfg: TtsConfig,
+  description: string,
+): Promise<CharacterVoice> {
+  const voiceId = pickStepfunVoiceId(description);
+  return {
+    provider: "stepfun",
+    voiceId,
+    model: cfg.speechModel,
+    mimeType: OUTPUT_MIME,
+  };
+}
+
+export async function stepfunSynthesize(
+  cfg: TtsConfig,
+  voice: CharacterVoice,
+  text: string,
+  _delivery?: string,
+  signal?: AbortSignal,
+): Promise<{ audioBase64: string; mimeType: string }> {
+  if (voice.provider !== "stepfun") {
+    throw new Error(
+      `stepfunSynthesize received non-stepfun voice (provider="${voice.provider}")`,
+    );
+  }
+
+  // Strip trailing slash so /v1 + /audio/speech doesn't double up.
+  const base = cfg.baseUrl.replace(/\/$/, "");
+  const url = `${base}/audio/speech`;
+
+  const body = {
+    model: voice.model || cfg.speechModel,
+    input: text,
+    voice: voice.voiceId,
+    response_format: OUTPUT_FORMAT,
+  };
+
+  const res = await fetch(url, {
+    method: "POST",
+    headers: {
+      "Content-Type": "application/json",
+      Authorization: `Bearer ${cfg.apiKey}`,
+    },
+    body: JSON.stringify(body),
+    signal,
+  });
+
+  if (!res.ok) {
+    const txt = await res.text();
+    throw new Error(`StepFun TTS ${res.status}: ${txt.slice(0, 300)}`);
+  }
+
+  const ab = await res.arrayBuffer();
+  // Buffer is fine here — TTS routes run on runtime="nodejs". Falls back to
+  // btoa+chunks if we ever target Edge.
+  const audioBase64 = Buffer.from(ab).toString("base64");
+  return { audioBase64, mimeType: OUTPUT_MIME };
+}
diff --git a/lib/tts-client/xiaomi.ts b/lib/tts-client/xiaomi.ts
index 6faa116..4708807 100644
--- a/lib/tts-client/xiaomi.ts
+++ b/lib/tts-client/xiaomi.ts
@@ -79,6 +79,11 @@ export async function xiaomiSynthesize(
   delivery?: string,
   signal?: AbortSignal,
 ): Promise<{ audioBase64: string; mimeType: string }> {
+  if (voice.provider !== "xiaomi") {
+    throw new Error(
+      `xiaomiSynthesize received non-xiaomi voice (provider="${voice.provider}")`,
+    );
+  }
   const url = joinUrl(cfg.baseUrl, "/chat/completions");
 
   // The free-form delivery direction rides in the `user` (director) message,
diff --git a/lib/types/index.ts b/lib/types/index.ts
index 47e81ab..c0a1989 100644
--- a/lib/types/index.ts
+++ b/lib/types/index.ts
@@ -160,12 +160,24 @@ export type WriterPlan = {
 //  Characters & voices (TTS)
 // ──────────────────────────────────────────────────────────────────────
 
-export type CharacterVoice = {
-  provider: "xiaomi";
-  /** Xiaomi MiMo design output stored as reference audio for later clones. */
-  referenceAudioBase64: string;
-  mimeType: string;
-};
+export type CharacterVoice =
+  | {
+      provider: "xiaomi";
+      /** Xiaomi MiMo design output stored as reference audio for later clones. */
+      referenceAudioBase64: string;
+      mimeType: string;
+    }
+  | {
+      provider: "stepfun";
+      /** StepFun preset voice ID (e.g. "cixingnansheng"). Selected by keyword
+       *  matching against the LLM-written voiceDescription — no network call
+       *  on provision (StepFun has no voicedesign endpoint), so this carries
+       *  only the picked preset, not a clip. */
+      voiceId: string;
+      /** TTS model used at synth time (step-tts-mini / step-tts-2 / stepaudio-2.5-tts). */
+      model: string;
+      mimeType: string;
+    };
 
 export type Character = {
   name: string;