fix(setup): pin dev venv to Python 3.12

CHANGELOG.md

@@ -7,6 +7,25 @@
 
 ## [Unreleased]
 
+### Developer Experience
+
+- **`just setup` now pins the Python venv to 3.12.** Setup preferred `python3.12` when
+  it found one but silently fell back to 3.13 or bare `python3`, so a machine on
+  3.13/3.14 built a venv that then failed to install dependencies capped below 3.13
+  (kokoro, misaki, numpy, numba) with `Could not find a version that satisfies the
+  requirement`. Setup now requires 3.12, and recreates a venv that is on the wrong
+  version or whose pip doesn't run, so a stale or half-built one heals on the next
+  `just setup` rather than needing a manual `rm -rf backend/venv`. Reporting 3.12
+  isn't enough for an interpreter to be chosen — it has to actually build a working
+  venv, so setup falls through to the next candidate when one can't (Debian packages
+  `ensurepip` separately as `python3.12-venv`; current Homebrew `[email protected]` bottles
+  carry a `pyexpat` that won't load on macOS 26). If none work it exits with the
+  install command for your platform instead of failing deep inside pip. Windows
+  resolves it through the `py -3.12` launcher, since Windows installers don't create
+  a `python3.12` command.
+- **The 3.12 requirement is now machine-readable.** `backend/pyproject.toml`
+  declares `requires-python = ">=3.12,<3.13"` rather than an open-ended `>=3.12`,
+  so the constraint the docs describe is the one tooling resolves against.
 ### Linux
 
 - **ROCm setup works on Linux AMD systems.** Docker ROCm builds now keep PyTorch

CONTRIBUTING.md

@@ -18,9 +18,11 @@ Thank you for your interest in contributing to Voicebox! This document provides
   curl -fsSL https://bun.sh/install | bash
   ```
 
-- **[Python 3.11+](https://python.org)** - For backend development
+- **[Python 3.12](https://python.org)** - For backend development. Voicebox pins its
+  virtual environment to 3.12 to match CI, because some dependencies have no 3.13
+  release yet. On Debian/Ubuntu also install `python3.12-venv`.
   ```bash
-  python --version  # Should be 3.11 or higher
+  python3.12 --version  # Should be 3.12.x
   ```
 
 - **[Rust](https://rustup.rs)** - For Tauri desktop app (installed automatically by Tauri CLI)
@@ -44,7 +46,7 @@ just dev     # starts backend + desktop app
 ```
 
 `just setup` handles everything automatically, including:
-- Creating a Python virtual environment
+- Creating a Python virtual environment pinned to 3.12 (recreating it if an existing venv is on the wrong version)
 - Installing Python dependencies (with CUDA PyTorch on Windows if an NVIDIA GPU is detected)
 - Installing MLX dependencies on Apple Silicon
 - Installing JavaScript dependencies
@@ -67,7 +69,7 @@ just --list        # see all available commands
 
 #### Windows Notes
 
-The justfile works natively on Windows via PowerShell. No WSL or Git Bash required. On Windows with an NVIDIA GPU, `just setup` automatically installs CUDA-enabled PyTorch for GPU acceleration.
+The justfile works natively on Windows via PowerShell. No WSL or Git Bash required. On Windows with an NVIDIA GPU, `just setup` automatically installs CUDA-enabled PyTorch for GPU acceleration. Python 3.12 is located via the `py -3.12` launcher; install it with `winget install -e --id Python.Python.3.12` if it's missing.
 
 ### Model Downloads
 
@@ -363,7 +365,7 @@ See [docs/content/docs/overview/troubleshooting.mdx](docs/content/docs/overview/
 
 **Quick fixes:**
 
-- **Backend won't start:** Check Python version (3.11+), ensure venv is activated, install dependencies
+- **Backend won't start:** Check Python version (must be 3.12), ensure venv is activated, install dependencies
 - **Tauri build fails:** Ensure Rust is installed, clean build with `cd tauri/src-tauri && cargo clean`
 - **OpenAPI client generation fails:** Ensure backend is running, check `curl http://localhost:17493/openapi.json`
 

README.md

@@ -417,7 +417,7 @@ just dev     # starts backend + desktop app
 
 Install [just](https://github.com/casey/just): `brew install just` or `cargo install just`. Run `just --list` to see all commands.
 
-**Prerequisites:** [Bun](https://bun.sh), [Rust](https://rustup.rs), [Python 3.11+](https://python.org), [Tauri Prerequisites](https://v2.tauri.app/start/prerequisites/), and [Xcode](https://developer.apple.com/xcode/) on macOS.
+**Prerequisites:** [Bun](https://bun.sh), [Rust](https://rustup.rs), [Python 3.12](https://python.org), [Tauri Prerequisites](https://v2.tauri.app/start/prerequisites/), and [Xcode](https://developer.apple.com/xcode/) on macOS.
 
 The repo ships a pre-wired `.mcp.json` at the root — running Claude Code inside this checkout picks up the Voicebox MCP tools automatically once the dev app is running.
 

backend/pyproject.toml

@@ -1,7 +1,12 @@
 [project]
 name = "voicebox-backend"
 version = "0.2.3"
-requires-python = ">=3.12"
+# This is the one place the Python pin is explained; the justfile and docs point here
+# rather than restating it. The upper bound is real, not defensive: requirements.txt
+# currently pulls in kokoro and misaki (which declare requires-python <3.13) plus
+# numpy<2 and numba<0.61 (no 3.13 wheels), so `pip install -r requirements.txt` fails
+# outright on a later interpreter. Recheck those four before raising it.
+requires-python = ">=3.12,<3.13"
 
 # ---------------------------------------------------------------------------
 # Ruff – linter + formatter

docs/content/docs/developer/architecture.mdx

@@ -48,7 +48,7 @@ These two layers communicate via HTTP on `localhost:17493`, with the frontend ma
 
 ### Tech Stack
 
-- **Framework**: FastAPI (Python 3.11+)
+- **Framework**: FastAPI (Python 3.12)
 - **TTS Engines**: Qwen3-TTS, Qwen CustomVoice, LuxTTS, Chatterbox, Chatterbox Turbo, TADA, Kokoro
 - **Transcription**: Whisper (PyTorch or MLX-Whisper)
 - **Inference Backends**: MLX (Apple Silicon), PyTorch (CUDA / ROCm / XPU / DirectML / CPU)

docs/content/docs/developer/setup.mdx

@@ -32,11 +32,12 @@ Ensure you have these installed:
     curl -fsSL https://bun.sh/install | bash
     ```
   </Card>
-  <Card title="Python 3.11+" icon={<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round"><path d="M12 2L2 7l10 5 10-5-10-5z"/><path d="M2 17l10 5 10-5"/><path d="M2 12l10 5 10-5"/></svg>}>
+  <Card title="Python 3.12" icon={<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round"><path d="M12 2L2 7l10 5 10-5-10-5z"/><path d="M2 17l10 5 10-5"/><path d="M2 12l10 5 10-5"/></svg>}>
     [Download Python](https://python.org)
     ```bash
-    python --version
+    python3.12 --version  # Should be 3.12.x
     ```
+    Voicebox pins its venv to 3.12 to match CI, because some dependencies have no 3.13 release yet. On Debian/Ubuntu also install `python3.12-venv`.
   </Card>
   <Card title="Rust" icon={<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round"><path d="m6 9 6 6 6-6"/></svg>}>
     [Install Rust](https://rustup.rs)
@@ -65,7 +66,7 @@ Run `just --list` to see all available commands. Highlights:
 
 | Command | Description |
 |---------|-------------|
-| `just setup` | Full setup (Python venv + JS deps + dev sidecar). Detects Apple Silicon for MLX and NVIDIA/Intel Arc on Windows for accelerated PyTorch. |
+| `just setup` | Full setup (Python venv + JS deps + dev sidecar). Pins the venv to Python 3.12 (recreating a wrong-version one) and detects Apple Silicon for MLX and NVIDIA/Intel Arc on Windows for accelerated PyTorch. |
 | `just setup-python` | Python venv + dependencies only |
 | `just setup-js` | `bun install` only |
 
@@ -214,8 +215,8 @@ This installs dependencies for:
 ```bash
 cd backend
 
-# Create virtual environment
-python -m venv venv
+# Create virtual environment (use Python 3.12 — see Prerequisites)
+python3.12 -m venv venv
 
 # Activate virtual environment
 source venv/bin/activate  # macOS/Linux
@@ -277,7 +278,7 @@ bun run tauri dev
 
 <AccordionGroup>
   <Accordion title="Backend won't start">
-    - Check Python version (must be 3.11+)
+    - Check Python version (must be 3.12)
     - Ensure virtual environment is activated: `source backend/venv/bin/activate`
     - Verify all dependencies are installed: `pip install -r requirements.txt`
     - Check if port 17493 is available

docs/content/docs/overview/troubleshooting.mdx

@@ -292,13 +292,19 @@ This is expected behavior. The first generation downloads the selected TTS engin
 
 <AccordionGroup>
   <Accordion title="Python Version">
-    Ensure Python 3.11 or higher:
+    Ensure Python 3.12:
 
     ```bash
-    python --version
+    python3.12 --version
     ```
 
-    If not, install Python 3.11+ and recreate the virtual environment.
+    Voicebox pins its venv to Python 3.12 because some dependencies have no 3.13
+    release yet. `just setup` recreates a venv that was built on the wrong version,
+    so re-running it is usually the fix.
+
+    If `just setup` can't find a usable 3.12, install one: `uv python install 3.12`
+    works anywhere, or use `brew install [email protected]` on macOS and `sudo apt install
+    python3.12 python3.12-venv` on Debian/Ubuntu.
   </Accordion>
 
   <Accordion title="Virtual Environment">

justfile

@@ -17,8 +17,11 @@ pip := if os() == "windows" { venv_bin / "pip.exe" } else { venv_bin / "pip" }
 # Shell selection: use powershell on Windows, bash elsewhere
 set windows-shell := ["powershell", "-NoProfile", "-Command"]
 
-# Detect best python for venv creation (platform-aware)
-system_python := if os() == "windows" { "python" } else { `command -v python3.12 2>/dev/null || command -v python3.13 2>/dev/null || echo python3` }
+# Pin the venv to one version instead of following whatever `python3` happens to be;
+# a newer interpreter fails `pip install` with "no matching distribution". See
+# requires-python in backend/pyproject.toml for which dependencies set the ceiling.
+# Keep in sync with that and with actions/setup-python in .github/workflows.
+python_version := "3.12"
 
 # ─── Setup ────────────────────────────────────────────────────────────
 
@@ -32,14 +35,66 @@ setup: setup-python setup-js
 setup-python:
     #!/usr/bin/env bash
     set -euo pipefail
-    if [ ! -d "{{ venv }}" ]; then
-        echo "Creating Python virtual environment..."
-        PY_MINOR=$({{ system_python }} -c "import sys; print(sys.version_info[1])")
-        if [ "$PY_MINOR" -gt 13 ]; then
-            echo "Warning: Python 3.$PY_MINOR detected. ML packages may not be compatible."
-            echo "Recommended: brew install [email protected]"
+
+    is_pinned() {
+        "$1" -c 'import sys; sys.exit(sys.version_info[:2] != tuple(map(int, "{{ python_version }}".split("."))))' 2>/dev/null
+    }
+
+    # Testing for an executable file is not enough: console scripts hard-code the
+    # venv's absolute path in their shebang, so a venv that was built elsewhere and
+    # moved into place has a pip that exists, is executable, and still dies with
+    # "cannot execute: required file not found". Running it is the only real check.
+    pip_works() {
+        "{{ pip }}" --version >/dev/null 2>&1
+    }
+
+    # Every interpreter on PATH that could be the pinned version, plus uv's, which
+    # keeps its Pythons off PATH. More than one can match, and the first is not
+    # necessarily the good one.
+    candidates() {
+        type -aP "python{{ python_version }}" python3 python 2>/dev/null || true
+        if command -v uv >/dev/null 2>&1; then
+            uv python find "{{ python_version }}" 2>/dev/null || true
         fi
-        {{ system_python }} -m venv {{ venv }}
+    }
+
+    # Reuse the venv only if it is on the pinned version and has a working pip: a
+    # wrong-version venv resolves dependencies against the wrong Python, and a
+    # broken pip means an earlier `-m venv` was interrupted or the venv was moved.
+    if [ -d "{{ venv }}" ] && ! { is_pinned "{{ python }}" && pip_works; }; then
+        echo "Existing venv is not a working Python {{ python_version }} environment — recreating..."
+        rm -rf "{{ venv }}"
+    fi
+
+    if [ ! -d "{{ venv }}" ]; then
+        # Building the venv is itself the test, so there is no separate probe to keep
+        # in sync. An interpreter can report the right version and still fail inside
+        # `-m venv` — Debian packages ensurepip separately, and Homebrew has shipped a
+        # pyexpat that won't dlopen — so fall through to the next candidate rather
+        # than giving up on the first. Their output is held back until every candidate
+        # has failed, so a run that recovers doesn't look like a broken one.
+        log="$(mktemp)"
+        while IFS= read -r py; do
+            is_pinned "$py" || continue
+            echo "Creating Python virtual environment with $py ..."
+            "$py" -m venv "{{ venv }}" >>"$log" 2>&1 && pip_works && break
+            echo "  ...it reports {{ python_version }} but cannot build a working venv; trying the next."
+            rm -rf "{{ venv }}"
+        done < <(candidates | sed 's|//*|/|g' | awk '!seen[$0]++')
+        pip_works || cat "$log"
+        rm -f "$log"
+    fi
+
+    if ! pip_works; then
+        echo ""
+        echo "ERROR: Voicebox needs a Python {{ python_version }} that can create virtual"
+        echo "       environments (see requires-python in backend/pyproject.toml)."
+        echo "  Any:    uv python install {{ python_version }}"
+        echo "  macOS:  brew install python@{{ python_version }}"
+        echo "  Debian: sudo apt install python{{ python_version }} python{{ python_version }}-venv"
+        echo ""
+        echo "Then re-run: just setup"
+        exit 1
     fi
     echo "Installing Python dependencies..."
     {{ pip }} install --upgrade pip -q
@@ -85,13 +140,40 @@ setup-python:
 
 [windows]
 setup-python:
+    $target = "{{ python_version }}"; \
+    function Test-Py($exe) { \
+        if (-not $exe -or -not (Test-Path $exe)) { return $false }; \
+        $v = & $exe -c "import sys; print('%s.%s' % sys.version_info[:2])" 2>$null; \
+        return ($LASTEXITCODE -eq 0 -and $v -eq $target); \
+    }; \
+    function Test-Pip { \
+        if (-not (Test-Path "{{ pip }}")) { return $false }; \
+        & "{{ pip }}" --version *> $null; \
+        return ($LASTEXITCODE -eq 0); \
+    }; \
+    $py = $null; \
+    if (Get-Command py -ErrorAction SilentlyContinue) { \
+        $p = & py "-$target" -c "import sys; print(sys.executable)" 2>$null; \
+        if ($LASTEXITCODE -eq 0) { $py = $p }; \
+    }; \
+    if (-not $py) { foreach ($n in @("python$target", "python")) { \
+        $c = Get-Command $n -ErrorAction SilentlyContinue; \
+        if ($c -and (Test-Py $c.Source)) { $py = $c.Source; break }; \
+    } }; \
+    if (-not $py) { \
+        Write-Host "ERROR: Voicebox requires Python $target (see requires-python in backend/pyproject.toml)."; \
+        Write-Host "Install it from https://python.org or:  winget install -e --id Python.Python.$target"; \
+        Write-Host "Then re-run: just setup"; \
+        exit 1; \
+    }; \
+    if ((Test-Path "{{ venv }}") -and (-not (Test-Py "{{ python }}") -or -not (Test-Pip))) { \
+        Write-Host "Existing venv is not a working Python $target env - recreating..."; \
+        Remove-Item -Recurse -Force "{{ venv }}"; \
+    }; \
     if (-not (Test-Path "{{ venv }}")) { \
-        Write-Host "Creating Python virtual environment..."; \
-        $pyMinor = & {{ system_python }} -c "import sys; print(sys.version_info[1])"; \
-        if ([int]$pyMinor -gt 13) { \
-            Write-Host "Warning: Python 3.$pyMinor detected. ML packages may not be compatible."; \
-        }; \
-        & {{ system_python }} -m venv {{ venv }}; \
+        Write-Host "Creating Python virtual environment with $py ..."; \
+        & $py -m venv "{{ venv }}"; \
+        if ($LASTEXITCODE -ne 0 -or -not (Test-Pip)) { Write-Host "ERROR: could not create a working venv with $py"; exit 1 }; \
     }
     Write-Host "Installing Python dependencies..."
     & "{{ python }}" -m pip install --upgrade pip -q