diff --git a/backend/python/fish-speech/Makefile b/backend/python/fish-speech/Makefile
index ace1ef3ded1d..875f91d7362a 100644
--- a/backend/python/fish-speech/Makefile
+++ b/backend/python/fish-speech/Makefile
@@ -8,8 +8,13 @@ run: fish-speech
 	bash run.sh
 	@echo "fish-speech run."
 
+.PHONY: test-unit
+test-unit:
+	python3 -m unittest -v prepare_upstream_test.py
+	bash run_test.sh
+
 .PHONY: test
-test: fish-speech
+test: fish-speech test-unit
 	@echo "Testing fish-speech..."
 	bash test.sh
 	@echo "fish-speech tested."
diff --git a/backend/python/fish-speech/install.sh b/backend/python/fish-speech/install.sh
index 4d03d344c38a..c80b98903217 100644
--- a/backend/python/fish-speech/install.sh
+++ b/backend/python/fish-speech/install.sh
@@ -44,6 +44,13 @@ fi
 # It requires native portaudio libs which aren't available on all build environments.
 sed -i.bak '/"pyaudio"/d' "${FISH_SPEECH_DIR}/pyproject.toml"
 
+# CUDA 13 has no torch 2.8 wheels, so fish-speech's exact upstream pin would
+# make pip select the CPU-only aarch64 wheel from PyPI. Prepare the cloned tree
+# before resolving it, and use soundfile for reference audio because torchcodec
+# does not publish Linux aarch64 wheels.
+python3 "${backend_dir}/prepare_upstream.py" "${FISH_SPEECH_DIR}" \
+    --cuda-major "${CUDA_MAJOR_VERSION:-}"
+
 # Install fish-speech deps from source (without the package itself since we use PYTHONPATH)
 ensureVenv
 if [ "x${USE_PIP}" == "xtrue" ]; then
diff --git a/backend/python/fish-speech/prepare_upstream.py b/backend/python/fish-speech/prepare_upstream.py
new file mode 100644
index 000000000000..108948b90888
--- /dev/null
+++ b/backend/python/fish-speech/prepare_upstream.py
@@ -0,0 +1,70 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: MIT
+
+import argparse
+from pathlib import Path
+
+
+TORCH_28 = '"torch==2.8.0"'
+TORCH_29 = '"torch==2.9.1"'
+TORCHAUDIO_28 = '"torchaudio==2.8.0"'
+TORCHAUDIO_29 = '"torchaudio==2.9.1"'
+TORCHAUDIO_LOAD = (
+    "        waveform, original_sr = "
+    "torchaudio.load(reference_audio, backend=self.backend)"
+)
+SOUNDFILE_LOAD = "\n".join(
+    (
+        "        import soundfile as _sf",
+        "        import torch as _torch",
+        "",
+        "        data, original_sr = _sf.read(",
+        '            reference_audio, dtype="float32", always_2d=True',
+        "        )",
+        "        waveform = _torch.from_numpy(data.T.copy())",
+    )
+)
+
+
+def patch_cuda13_dependencies(pyproject: Path) -> None:
+    content = pyproject.read_text()
+    if (
+        TORCH_28 not in content
+        and TORCHAUDIO_28 not in content
+        and TORCH_29 in content
+        and TORCHAUDIO_29 in content
+    ):
+        return
+    if TORCH_28 not in content or TORCHAUDIO_28 not in content:
+        raise RuntimeError("fish-speech's torch 2.8 dependency pins have changed")
+
+    content = content.replace(TORCH_28, TORCH_29)
+    content = content.replace(TORCHAUDIO_28, TORCHAUDIO_29)
+    pyproject.write_text(content)
+
+
+def patch_reference_loader(loader: Path) -> None:
+    content = loader.read_text()
+    if TORCHAUDIO_LOAD not in content and content.count(SOUNDFILE_LOAD) == 1:
+        return
+    if content.count(TORCHAUDIO_LOAD) != 1:
+        raise RuntimeError("fish-speech's torchaudio.load call has changed")
+
+    loader.write_text(content.replace(TORCHAUDIO_LOAD, SOUNDFILE_LOAD))
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser()
+    parser.add_argument("source", type=Path)
+    parser.add_argument("--cuda-major")
+    args = parser.parse_args()
+
+    if args.cuda_major == "13":
+        patch_cuda13_dependencies(args.source / "pyproject.toml")
+    patch_reference_loader(
+        args.source / "fish_speech/inference_engine/reference_loader.py"
+    )
+
+
+if __name__ == "__main__":
+    main()
diff --git a/backend/python/fish-speech/prepare_upstream_test.py b/backend/python/fish-speech/prepare_upstream_test.py
new file mode 100644
index 000000000000..7a1df73c7464
--- /dev/null
+++ b/backend/python/fish-speech/prepare_upstream_test.py
@@ -0,0 +1,136 @@
+# SPDX-License-Identifier: MIT
+
+import importlib.util
+import sys
+import tempfile
+import types
+import unittest
+from pathlib import Path
+
+
+MODULE_PATH = Path(__file__).with_name("prepare_upstream.py")
+
+
+def load_prepare_upstream():
+    if not MODULE_PATH.exists():
+        raise AssertionError("prepare_upstream.py is missing")
+    spec = importlib.util.spec_from_file_location("prepare_upstream", MODULE_PATH)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+class FakeAudioData:
+    @property
+    def T(self):
+        return self
+
+    def copy(self):
+        return "channels-first"
+
+
+class PrepareUpstreamTests(unittest.TestCase):
+    def test_cuda13_dependencies_follow_available_pytorch_wheels(self):
+        prepare_upstream = load_prepare_upstream()
+
+        with tempfile.TemporaryDirectory() as tmp:
+            pyproject = Path(tmp) / "pyproject.toml"
+            pyproject.write_text(
+                'dependencies = [\n    "torch==2.8.0",\n    "torchaudio==2.8.0",\n]\n'
+                'stable = [\n    "torch==2.8.0",\n    "torchaudio",\n]\n'
+            )
+
+            prepare_upstream.patch_cuda13_dependencies(pyproject)
+
+            self.assertEqual(
+                pyproject.read_text(),
+                'dependencies = [\n    "torch==2.9.1",\n    "torchaudio==2.9.1",\n]\n'
+                'stable = [\n    "torch==2.9.1",\n    "torchaudio",\n]\n',
+            )
+
+    def test_reference_audio_uses_soundfile_without_torchcodec(self):
+        prepare_upstream = load_prepare_upstream()
+
+        with tempfile.TemporaryDirectory() as tmp:
+            loader = Path(tmp) / "reference_loader.py"
+            loader.write_text(
+                "class ReferenceLoader:\n"
+                "    def load_audio(self, reference_audio):\n"
+                "        waveform, original_sr = torchaudio.load(reference_audio, backend=self.backend)\n"
+                "        return waveform, original_sr\n"
+            )
+            prepare_upstream.patch_reference_loader(loader)
+
+            calls = []
+            fake_soundfile = types.SimpleNamespace(
+                read=lambda source, **kwargs: (
+                    calls.append((source, kwargs)) or FakeAudioData(),
+                    24000,
+                )
+            )
+            fake_torch = types.SimpleNamespace(
+                from_numpy=lambda data: ("tensor", data),
+            )
+            previous_soundfile = sys.modules.get("soundfile")
+            previous_torch = sys.modules.get("torch")
+            sys.modules["soundfile"] = fake_soundfile
+            sys.modules["torch"] = fake_torch
+            try:
+                namespace = {"torchaudio": None}
+                exec(compile(loader.read_text(), str(loader), "exec"), namespace)
+                instance = namespace["ReferenceLoader"]()
+                instance.backend = "soundfile"
+
+                waveform, sample_rate = instance.load_audio("voice.wav")
+            finally:
+                if previous_soundfile is None:
+                    del sys.modules["soundfile"]
+                else:
+                    sys.modules["soundfile"] = previous_soundfile
+                if previous_torch is None:
+                    del sys.modules["torch"]
+                else:
+                    sys.modules["torch"] = previous_torch
+
+            self.assertEqual(waveform, ("tensor", "channels-first"))
+            self.assertEqual(sample_rate, 24000)
+            self.assertEqual(
+                calls,
+                [("voice.wav", {"dtype": "float32", "always_2d": True})],
+            )
+
+    def test_reference_loader_drift_fails_the_build(self):
+        prepare_upstream = load_prepare_upstream()
+
+        with tempfile.TemporaryDirectory() as tmp:
+            loader = Path(tmp) / "reference_loader.py"
+            loader.write_text("def load_audio():\n    pass\n")
+
+            with self.assertRaisesRegex(RuntimeError, "torchaudio.load call"):
+                prepare_upstream.patch_reference_loader(loader)
+
+    def test_preparation_can_be_repeated(self):
+        prepare_upstream = load_prepare_upstream()
+
+        with tempfile.TemporaryDirectory() as tmp:
+            pyproject = Path(tmp) / "pyproject.toml"
+            pyproject.write_text(
+                'dependencies = ["torch==2.8.0", "torchaudio==2.8.0"]\n'
+            )
+            loader = Path(tmp) / "reference_loader.py"
+            loader.write_text(
+                "def load_audio(reference_audio):\n"
+                "        waveform, original_sr = torchaudio.load(reference_audio, backend=self.backend)\n"
+            )
+
+            prepare_upstream.patch_cuda13_dependencies(pyproject)
+            prepare_upstream.patch_reference_loader(loader)
+            try:
+                prepare_upstream.patch_cuda13_dependencies(pyproject)
+                prepare_upstream.patch_reference_loader(loader)
+            except RuntimeError as err:
+                self.fail(f"preparation is not idempotent: {err}")
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/backend/python/fish-speech/requirements-cublas13.txt b/backend/python/fish-speech/requirements-cublas13.txt
index c367ab45c8e1..841666316714 100644
--- a/backend/python/fish-speech/requirements-cublas13.txt
+++ b/backend/python/fish-speech/requirements-cublas13.txt
@@ -1,3 +1,3 @@
 --extra-index-url https://download.pytorch.org/whl/cu130
-torch
-torchaudio
+torch==2.9.1+cu130
+torchaudio==2.9.1
diff --git a/backend/python/fish-speech/requirements-l4t13.txt b/backend/python/fish-speech/requirements-l4t13.txt
index c367ab45c8e1..841666316714 100644
--- a/backend/python/fish-speech/requirements-l4t13.txt
+++ b/backend/python/fish-speech/requirements-l4t13.txt
@@ -1,3 +1,3 @@
 --extra-index-url https://download.pytorch.org/whl/cu130
-torch
-torchaudio
+torch==2.9.1+cu130
+torchaudio==2.9.1
diff --git a/backend/python/fish-speech/run.sh b/backend/python/fish-speech/run.sh
index eae121f37b0b..d7a5cc230f1a 100644
--- a/backend/python/fish-speech/run.sh
+++ b/backend/python/fish-speech/run.sh
@@ -6,4 +6,8 @@ else
     source $backend_dir/../common/libbackend.sh
 fi
 
-startBackend $@
+# Editable installs record their build-time absolute source path, which becomes
+# stale when the backend is relocated under /backends at install time.
+export PYTHONPATH="${EDIR}/fish-speech-src${PYTHONPATH:+:${PYTHONPATH}}"
+
+startBackend "$@"
diff --git a/backend/python/fish-speech/run_test.sh b/backend/python/fish-speech/run_test.sh
new file mode 100644
index 000000000000..65affb91f875
--- /dev/null
+++ b/backend/python/fish-speech/run_test.sh
@@ -0,0 +1,27 @@
+#!/bin/bash
+# SPDX-License-Identifier: MIT
+set -euo pipefail
+
+backend_dir=$(cd "$(dirname "$0")" && pwd)
+work=$(mktemp -d)
+trap 'rm -rf "$work"' EXIT
+
+mkdir -p "$work/backend/common" "$work/backend/fish-speech-src"
+cp "$backend_dir/run.sh" "$work/backend/run.sh"
+
+cat > "$work/backend/common/libbackend.sh" <<'EOF'
+EDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
+startBackend() {
+    printf '%s\n' "$PYTHONPATH"
+}
+EOF
+
+actual=$(PYTHONPATH=/existing/path bash "$work/backend/run.sh")
+expected="$work/backend/fish-speech-src:/existing/path"
+
+if [ "$actual" != "$expected" ]; then
+    printf 'expected PYTHONPATH %s, got %s\n' "$expected" "$actual" >&2
+    exit 1
+fi
+
+echo "PASS: relocated fish-speech source is importable"