docs: clarify how to use S2-Pro with the Python API client (#1175)

* docs: clarify s2-pro python client usage

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Co-authored-by: stablegenius49 <185121704+stablegenius49@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/en/server.md

@@ -40,6 +40,27 @@ Expected response:
 - `POST /v1/vqgan/encode` for VQ encode
 - `POST /v1/vqgan/decode` for VQ decode
 
+### Python client example
+
+The base TTS model is selected when the server starts. In the example above, the server is started with the `checkpoints/s2-pro` weights, so every request sent to `http://127.0.0.1:8080/v1/tts` will use **S2-Pro** automatically. There is no separate per-request `model` field in `tools/api_client.py` for local server calls.
+
+```bash
+python tools/api_client.py \
+  --url http://127.0.0.1:8080/v1/tts \
+  --text "Hello from Fish Speech" \
+  --output s2-pro-demo
+```
+
+If you want to select a saved reference voice, use `--reference_id`. This chooses the **voice reference**, not the base TTS model:
+
+```bash
+python tools/api_client.py \
+  --url http://127.0.0.1:8080/v1/tts \
+  --text "Hello from Fish Speech" \
+  --reference_id my-speaker \
+  --output s2-pro-demo
+```
+
 ## WebUI Inference
 
 For WebUI usage, see:

tools/api_client.py

@@ -15,7 +15,16 @@ from fish_speech.utils.schema import ServeReferenceAudio, ServeTTSRequest
 
 def parse_args():
     parser = argparse.ArgumentParser(
-        description="Send a WAV file and text to a server and receive synthesized audio.",
+        description="Send text to a Fish Speech TTS server and receive synthesized audio.",
+        epilog=(
+            "Model selection note:\n"
+            "  The base TTS model is selected by the server you call. For example, if the\n"
+            "  server was started with checkpoints/s2-pro, this client will use S2-Pro\n"
+            "  automatically. There is no separate per-request --model flag.\n\n"
+            "Examples:\n"
+            '  python tools/api_client.py -u http://127.0.0.1:8080/v1/tts -t "Hello from Fish Speech"\n'
+            '  python tools/api_client.py -u http://127.0.0.1:8080/v1/tts -t "Hello" --reference_id my-speaker'
+        ),
         formatter_class=argparse.RawTextHelpFormatter,
     )
 
@@ -24,7 +33,7 @@ def parse_args():
         "-u",
         type=str,
         default="http://127.0.0.1:8080/v1/tts",
-        help="URL of the server",
+        help="URL of the TTS server. The server decides which base model is loaded.",
     )
     parser.add_argument(
         "--text", "-t", type=str, required=True, help="Text to be synthesized"
@@ -34,7 +43,7 @@ def parse_args():
         "-id",
         type=str,
         default=None,
-        help="ID of the reference model to be used for the speech\n(Local: name of folder containing audios and files)",
+        help="ID of the reference voice to use for synthesis\n(Local: name of folder containing audios and files)",
     )
     parser.add_argument(
         "--reference_audio",