diff --git a/.github/workflows/all.yml b/.github/workflows/all.yml
index 72325145..b59893e6 100644
--- a/.github/workflows/all.yml
+++ b/.github/workflows/all.yml
@@ -351,7 +351,6 @@ jobs:
           CCACHE_IGNOREOPTIONS: "-fno-strict-overflow -fwrapv -W* -arch x86_64 arm64 -dynamic -fno-common -g -I/usr/local/opt/*"
           CCACHE_LOGFILE: ${{ github.workspace }}/.ccache_log
           USE_ASAN: "1"
-          USE_PORTABLE_SIMD: "1" # Use portable SIMD to avoid "Illegal instruction" on different runner CPUs
           DISABLE_LTO: "1" # Speeds up un-cacheable link step which doesn't really increase performance in tests anyways
           CC: ${{ matrix.cc }}
           CXX: ${{ matrix.cxx }}
@@ -798,7 +797,6 @@ jobs:
         env:
           DEBUG: "0"
           USE_ASAN: "1"
-          USE_PORTABLE_SIMD: "1" # Use portable SIMD to avoid "Illegal instruction" on different runner CPUs
           CCACHE_DIR: ${{ github.workspace }}/.ccache
           CCACHE_BASEDIR: ${{ github.workspace }}
           CCACHE_DEBUGDIR: ${{ github.workspace }}/.ccache_debug
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 00000000..a6aa1a65
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,97 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Pedalboard is a Python library (with C++ internals) for audio processing: reading, writing, rendering, and adding effects. Built by Spotify's Audio Intelligence Lab using pybind11 bindings over the JUCE framework.
+
+## Build Commands
+
+```bash
+# Clone with submodules (required for vendors/JUCE)
+git clone --recurse-submodules --shallow-submodules git@github.com:spotify/pedalboard.git
+
+# Install build deps and build
+pip install pybind11 scikit-build-core
+pip install --no-build-isolation -ve .
+
+# Fast debug build with ccache (macOS)
+rm -rf build && CC="ccache clang" CXX="ccache clang++" DEBUG=1 python3 -m pip install -e .
+
+# Fast debug build with ccache (Linux, GCC)
+rm -rf build && CC="ccache gcc" CXX="scripts/ccache_g++" DEBUG=1 python3 setup.py build -j8 develop
+```
+
+Build environment variables: `DEBUG=1`, `DISABLE_LTO`, `USE_MARCH_NATIVE`, `USE_ASAN`, `USE_TSAN`, `USE_MSAN`.
+
+## Testing
+
+```bash
+# Run all tests
+pytest tests/
+
+# Run a single test file
+pytest tests/test_python_interface.py -v
+
+# Run a single test
+pytest tests/test_python_interface.py::test_no_transforms
+
+# Via tox (runs coverage)
+tox
+```
+
+Test dependencies are in `test-requirements.txt`. CI parallelizes tests via `NUM_TEST_WORKERS` and `TEST_WORKER_INDEX` env vars (see `tests/conftest.py`).
+
+## Linting and Formatting
+
+```bash
+# Python lint
+ruff check pedalboard
+
+# Python format check
+ruff format --check pedalboard --diff
+
+# Type checking
+pyright pedalboard tests/test_*.py
+
+# C++ formatting (clang-format 14, LLVM style)
+# Applied to pedalboard/ C++ files, excluding vendors/
+```
+
+Ruff config is in `pyproject.toml`: line-length 100, target Python 3.10+.
+
+## Architecture
+
+**Python layer** (`pedalboard/`):
+- `__init__.py` — re-exports from `pedalboard_native` (the compiled C++ module)
+- `_pedalboard.py` — `Pedalboard` class (effect chain container) and `load_plugin()` for VST3/AU
+- `io/` — `AudioFile` (read/write WAV, FLAC, MP3, OGG, AIFF) and `AudioStream` (live audio)
+
+**C++ native module** (`pedalboard_native`):
+- `pedalboard/python_bindings.cpp` — main pybind11 entry point
+- `pedalboard/plugins/*.h` — individual effect plugin implementations (Compressor, Reverb, Delay, Chorus, etc.), each wrapping a JUCE processor via `JucePlugin<T>` template
+- `pedalboard/plugin_templates/` — reusable plugin wrappers (Resample, ForceMono, etc.)
+- `pedalboard/juce_overrides/` — patched JUCE module inclusions
+
+**Type stubs** (`pedalboard_native/*.pyi`):
+- Generated via `pybind11-stubgen`, post-processed by `scripts/postprocess_type_hints`
+- Update process documented in CONTRIBUTING.md
+
+**Vendor libraries** (`vendors/`): FFTW3, RubberBand, LAME, libgsm, libFLAC — compiled as part of the build.
+
+**Build system**: scikit-build-core (primary, via `pyproject.toml` + `CMakeLists.txt`) with legacy `setup.py` still functional. All `.cpp` and `.mm` files under `pedalboard/` are compiled automatically.
+
+## Platform Notes
+
+- **macOS**: Links Accelerate, AudioToolbox, CoreAudio, and other frameworks. Supports Audio Units.
+- **Linux**: Requires libfreetype-dev, xorg-dev, ALSA. FFTW3 used for FFT (vendored).
+- **Windows**: MSVC required. No AU support.
+- C++17 required. SIMD: AVX on x86, NEON on ARM.
+
+## Key Design Patterns
+
+- Plugin architecture: `Plugin` base → `JucePlugin<juce::dsp::SomeProcessor>` template
+- Container composition: `Pedalboard` extends `Chain` for serial/parallel plugin graphs
+- GIL is released during C++ audio processing for thread safety
+- External plugin hosting: VST3 cross-platform, Audio Units macOS-only
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d9ae8b9f..bfa001c5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -199,13 +199,14 @@ elseif(UNIX AND NOT APPLE)
     if(CMAKE_SYSTEM_PROCESSOR MATCHES "arm|aarch64")
         add_compile_definitions(HAVE_NEON=1)
     else()
-        # Use -march=native for local builds to optimize for the current CPU,
-        # but use a portable baseline for CI builds to avoid "Illegal instruction" errors
-        # when ccache restores objects built on different runner hardware.
-        if(DEFINED ENV{USE_PORTABLE_SIMD})
-            add_compile_options(-mavx)
-        else()
+        # Use -mavx by default for portable binaries (AVX is baseline since
+        # Sandy Bridge, 2011). Set USE_MARCH_NATIVE=1 to optimize for the
+        # local CPU instead (useful for developer builds, but the resulting
+        # binaries may crash on other hardware).
+        if(DEFINED ENV{USE_MARCH_NATIVE})
             add_compile_options(-march=native)
+        else()
+            add_compile_options(-mavx)
         endif()
         add_compile_definitions(HAVE_AVX)
     endif()
diff --git a/setup.py b/setup.py
index b5536e5d..da130fef 100644
--- a/setup.py
+++ b/setup.py
@@ -175,13 +175,14 @@ def ignore_files_matching(files, *matches):
     else:
         # And on x86, ignore the ARM-specific SIMD code (and KCVI; not GCC or Clang compatible).
         fftw_paths = ignore_files_matching(fftw_paths, "neon")
-        # Use -march=native for local builds to optimize for the current CPU,
-        # but use a portable baseline for CI builds to avoid "Illegal instruction" errors
-        # when ccache restores objects built on different runner hardware.
-        if os.getenv("USE_PORTABLE_SIMD"):
-            ALL_CFLAGS.append("-mavx")
-        else:
+        # Use -mavx by default for portable binaries (AVX is baseline since
+        # Sandy Bridge, 2011). Set USE_MARCH_NATIVE=1 to optimize for the
+        # local CPU instead (useful for developer builds, but the resulting
+        # binaries may crash on other hardware).
+        if os.getenv("USE_MARCH_NATIVE"):
             ALL_CFLAGS.append("-march=native")
+        else:
+            ALL_CFLAGS.append("-mavx")
         # Enable SIMD instructions:
         ALL_CFLAGS.extend(
             [