diff --git a/bun.lockb b/bun.lockb
index b48b725..6034660 100755
Binary files a/bun.lockb and b/bun.lockb differ
diff --git a/package.json b/package.json
index 3d6d52d..f93fc8e 100644
--- a/package.json
+++ b/package.json
@@ -37,6 +37,7 @@
     "server-only": "^0.0.1"
   },
   "devDependencies": {
+    "@msgpack/msgpack": "^3.1.3",
     "@types/bun": "^1.3.13",
     "@types/node": "^22.18.13",
     "@types/react": "19.2.2",
diff --git a/src/pipeline/models.ts b/src/pipeline/models.ts
index 54ed81d..3bc1605 100644
--- a/src/pipeline/models.ts
+++ b/src/pipeline/models.ts
@@ -90,6 +90,19 @@ const NEW_MODELS: PipelineModel[] = [
     name: 'TTS-1.5 Mini',
     registered: false,
   },
+  // Fish Audio s2.1-pro, request mode `latency: "balanced"`. The transport
+  // (transports/fish.ts) speaks the realtime msgpack WS; pipelineModels uses
+  // vendorModelId for the `model` header on every request. arenaApiId is
+  // FROZEN ONCE clips are hashed.
+  {
+    id: 'fish-s21-pro',
+    slug: 'fish-s2-1-pro',
+    providerId: 'fish',
+    arenaApiId: 's2-1-pro',
+    vendorModelId: 's2.1-pro',
+    name: 's2.1-pro',
+    registered: false,
+  },
 ];
 
 const registryModels = (): PipelineModel[] =>
diff --git a/src/pipeline/transports/fish.ts b/src/pipeline/transports/fish.ts
new file mode 100644
index 0000000..ac9a57b
--- /dev/null
+++ b/src/pipeline/transports/fish.ts
@@ -0,0 +1,285 @@
+/**
+ * Fish Audio transport (s2.1-pro).
+ *
+ * Synthesis + TTFB ride the same realtime WebSocket:
+ * `wss://api.fish.audio/v1/tts/live`, msgpack-framed (`@msgpack/msgpack`).
+ * Protocol per the Fish docs and the in-house Fish CLI reference
+ * (code/fish/cli/index.js): client sends a `start` frame with the request,
+ * a `text` frame with the line, then `flush` + `stop`; server replies with
+ * `audio` frames carrying mp3 bytes and a terminal `finish` frame. The
+ * Index entry uses `latency: "balanced"` because a local bench (June 2026,
+ * n=20 per mode) measured balanced ~18 ms faster at the median than
+ * `latency: "low"`, despite the name.
+ *
+ * Clone creation (`createClone`) hits the documented multipart
+ * `POST https://api.fish.audio/model` (instant cloning, `train_mode: "fast"`,
+ * private model), per
+ * https://docs.fish.audio/developer-guide/sdk-guide/cookbook/instant-voice-cloning
+ * and verified against code/fish/livekit-demo/fish/src/voice_clone.py. The
+ * returned model id is what the synth `reference_id` points at.
+ */
+import { readFileSync } from 'node:fs';
+import { basename } from 'node:path';
+
+import { decode, encode } from '@msgpack/msgpack';
+
+import { requireEnv } from '../env';
+import { BENCH_TEXT, postFormForJson } from './http';
+import {
+  RateLimitedError,
+  TransportError,
+  type ProviderTransport,
+  type TtfbPlan,
+  type TtfbTrialResult,
+} from './types';
+
+const WS_URL = 'wss://api.fish.audio/v1/tts/live';
+const CLONE_URL = 'https://api.fish.audio/model';
+/** Streaming-optimised mode; see the doc comment above for the local bench. */
+const LATENCY: 'balanced' | 'low' | 'normal' = 'balanced';
+const TRIAL_TIMEOUT_MS = 30_000;
+const SYNTH_TIMEOUT_MS = 60_000;
+
+const apiKey = (): string => requireEnv('FISH_API_KEY');
+
+type ServerEvent =
+  | { event: 'audio'; audio: Uint8Array }
+  | { event: 'finish' }
+  | { event: 'log'; level?: string; message?: string }
+  | { event: string; [key: string]: unknown };
+
+const decodeFrame = (data: ArrayBuffer): ServerEvent | null => {
+  try {
+    const value = decode(new Uint8Array(data));
+    if (typeof value !== 'object' || value === null) return null;
+    return value as ServerEvent;
+  } catch {
+    return null;
+  }
+};
+
+const openWs = (vendorModelId: string): WebSocket => {
+  const ws = new WebSocket(
+    WS_URL,
+    {
+      headers: { Authorization: `Bearer ${apiKey()}`, model: vendorModelId },
+    } as unknown as string[] | undefined,
+  );
+  ws.binaryType = 'arraybuffer';
+  return ws;
+};
+
+const synthOverWs = (
+  vendorModelId: string,
+  providerVoiceId: string,
+  text: string,
+): Promise<Uint8Array> =>
+  new Promise((resolve, reject) => {
+    const ws = openWs(vendorModelId);
+    const chunks: Uint8Array[] = [];
+    let settled = false;
+    const finish = (cb: () => void) => {
+      if (settled) return;
+      settled = true;
+      try {
+        ws.close();
+      } catch {
+        // already closed
+      }
+      cb();
+    };
+    const timer = setTimeout(() => {
+      finish(() =>
+        reject(new TransportError('fish ws synth: no finish within timeout')),
+      );
+    }, SYNTH_TIMEOUT_MS);
+
+    ws.onopen = () => {
+      try {
+        ws.send(
+          encode({
+            event: 'start',
+            request: {
+              text: '',
+              format: 'mp3',
+              latency: LATENCY,
+              reference_id: providerVoiceId,
+            },
+          }),
+        );
+        ws.send(encode({ event: 'text', text: `${text} ` }));
+        ws.send(encode({ event: 'flush' }));
+        ws.send(encode({ event: 'stop' }));
+      } catch (err) {
+        clearTimeout(timer);
+        finish(() => reject(err as Error));
+      }
+    };
+
+    ws.onmessage = (event: MessageEvent) => {
+      if (!(event.data instanceof ArrayBuffer)) return;
+      const msg = decodeFrame(event.data);
+      if (!msg) return;
+      if (msg.event === 'audio') {
+        const audio = (msg as { audio?: unknown }).audio;
+        if (audio instanceof Uint8Array) chunks.push(audio);
+        return;
+      }
+      if (msg.event === 'finish') {
+        clearTimeout(timer);
+        if (chunks.length === 0) {
+          finish(() => reject(new TransportError('fish ws synth: no audio')));
+          return;
+        }
+        const total = chunks.reduce((n, c) => n + c.length, 0);
+        const out = new Uint8Array(total);
+        let off = 0;
+        for (const c of chunks) {
+          out.set(c, off);
+          off += c.length;
+        }
+        finish(() => resolve(out));
+      }
+    };
+
+    ws.onerror = () => {
+      clearTimeout(timer);
+      finish(() => reject(new TransportError('fish ws synth: socket error')));
+    };
+    ws.onclose = (event: CloseEvent) => {
+      clearTimeout(timer);
+      finish(() => {
+        if (chunks.length === 0) {
+          reject(
+            new TransportError(
+              `fish ws synth: closed before finish (code ${event.code} ${event.reason})`.trim(),
+            ),
+          );
+        }
+      });
+    };
+  });
+
+/**
+ * TTFB trial: fresh WS per trial, connectMs covers the handshake before t0,
+ * t0 right after the `text` frame is buffered (matches the in-house Fish
+ * CLI's textSentAt), t1 at the first inbound `audio` frame carrying bytes.
+ * Bench omits `reference_id` so it runs without a clone dependency; once
+ * the arena Clara clone exists locally, pin it here for parity with the
+ * other providers' WS benches.
+ */
+const fishTtfbTrial = (vendorModelId: string): Promise<TtfbTrialResult> =>
+  new Promise((resolve, reject) => {
+    const tConnect = performance.now();
+    const ws = openWs(vendorModelId);
+    let t0 = 0;
+    let connectMs = 0;
+    let settled = false;
+    const finish = (cb: () => void) => {
+      if (settled) return;
+      settled = true;
+      try {
+        ws.close();
+      } catch {
+        // already closed
+      }
+      cb();
+    };
+    const timer = setTimeout(() => {
+      finish(() => reject(new TransportError('fish ws bench: no audio within timeout')));
+    }, TRIAL_TIMEOUT_MS);
+
+    ws.onopen = () => {
+      connectMs = performance.now() - tConnect;
+      try {
+        ws.send(
+          encode({
+            event: 'start',
+            request: { text: '', format: 'mp3', latency: LATENCY },
+          }),
+        );
+        ws.send(encode({ event: 'text', text: `${BENCH_TEXT} ` }));
+        t0 = performance.now();
+        ws.send(encode({ event: 'flush' }));
+        ws.send(encode({ event: 'stop' }));
+      } catch (err) {
+        clearTimeout(timer);
+        finish(() => reject(err as Error));
+      }
+    };
+
+    ws.onmessage = (event: MessageEvent) => {
+      if (!(event.data instanceof ArrayBuffer)) return;
+      const msg = decodeFrame(event.data);
+      if (!msg || t0 === 0) return;
+      if (msg.event === 'audio') {
+        const audio = (msg as { audio?: unknown }).audio;
+        if (audio instanceof Uint8Array && audio.length > 0) {
+          const ttfbMs = performance.now() - t0;
+          clearTimeout(timer);
+          finish(() => resolve({ ttfbMs, connectMs }));
+        }
+      }
+    };
+
+    ws.onerror = () => {
+      clearTimeout(timer);
+      finish(() => reject(new TransportError('fish ws bench: socket error')));
+    };
+    ws.onclose = (event: CloseEvent) => {
+      clearTimeout(timer);
+      finish(() =>
+        reject(
+          new TransportError(
+            `fish ws bench: closed before audio (code ${event.code} ${event.reason})`.trim(),
+          ),
+        ),
+      );
+    };
+  });
+
+export const fish: ProviderTransport = {
+  providerId: 'fish',
+  apiKeyEnv: 'FISH_API_KEY',
+
+  synthesize: async ({ vendorModelId, providerVoiceId, text }) => ({
+    bytes: await synthOverWs(vendorModelId, providerVoiceId, text),
+    format: 'mp3' as const,
+  }),
+
+  createClone: async ({ displayName, sampleFiles }) => {
+    const form = new FormData();
+    form.append('type', 'tts');
+    form.append('title', displayName);
+    form.append('train_mode', 'fast');
+    form.append('visibility', 'private');
+    form.append('enhance_audio_quality', 'true');
+    for (const file of sampleFiles) {
+      form.append(
+        'voices',
+        new Blob([readFileSync(file)], { type: 'audio/wav' }),
+        basename(file),
+      );
+    }
+    const result = await postFormForJson<{ _id: string }>(
+      CLONE_URL,
+      { Authorization: `Bearer ${apiKey()}` },
+      form,
+      'fish create-model',
+    );
+    return result._id;
+  },
+
+  ttfbPlanFor: (vendorModelId): TtfbPlan => ({
+    transport: 'websocket',
+    notes: `wss://api.fish.audio/v1/tts/live (msgpack frames: start/text/flush/stop; audio/finish back), model=${vendorModelId}, latency=${LATENCY}, mp3 chunks, no reference_id (stock voice for the bench)`,
+    trial: async (): Promise<TtfbTrialResult> => {
+      try {
+        return await fishTtfbTrial(vendorModelId);
+      } catch (error) {
+        if (error instanceof RateLimitedError) throw error;
+        return fishTtfbTrial(vendorModelId);
+      }
+    },
+  }),
+};
diff --git a/src/pipeline/transports/index.ts b/src/pipeline/transports/index.ts
index 27547b5..8815b0b 100644
--- a/src/pipeline/transports/index.ts
+++ b/src/pipeline/transports/index.ts
@@ -8,6 +8,7 @@
  */
 import { cartesia } from './cartesia';
 import { elevenlabs } from './elevenlabs';
+import { fish } from './fish';
 import { hume } from './hume';
 import { inworld } from './inworld';
 import { minimax } from './minimax';
@@ -29,6 +30,7 @@ export const TRANSPORTS: Record<string, ProviderTransport> = {
   hume,
   sesame,
   xai,
+  fish,
 };
 
 /** Why the remaining arena providers cannot be measured/generated live. */