diff --git a/README.md b/README.md
index 01b46e6..4ea511b 100644
--- a/README.md
+++ b/README.md
@@ -103,19 +103,62 @@ Filters you can use:
 ## Building and flashing firmware
 Prereqs: Pico SDK + CMake toolchain set up.
 
-### One-shot build + flash (picotool)
+### Using `build.py`
+
+`build.py` configures CMake, builds the firmware, checks that both output formats
+were created, and flashes the ELF with `picotool`.
+
+Before running it:
+
+1. Install the Pico SDK, CMake toolchain, and `picotool`.
+2. Connect the Pico in BOOTSEL mode.
+3. From the repository root, run:
+
 ```sh
 python3 build.py
 ```
-- Requires `picotool` on your `PATH` (or set `PICOTOOL_PATH=/path/to/picotool`) and a connected Pico in BOOTSEL mode to automatically flash.
-- Set `ELF_PATH` to override the default `build/switch-pico.elf`.
+
+The generated files are:
+
+- `build/switch-pico.elf`, which `build.py` passes to `picotool`.
+- `build/switch-pico.uf2`, which can also be copied to the Pico manually.
+
+To customize the controller grip color while building, pass one of these mutually
+exclusive options:
+
+```sh
+# Use a random color for both grips
+python3 build.py --random-grip-color
+
+# Use a specific six-digit RGB color for both grips
+python3 build.py --grip-color FF00AA
+```
+
+Both options update `controller_color_config.h` before building. With no color
+option, that file is left unchanged. Run `python3 build.py --help` to see the
+available command-line options.
+
+If the tools or artifacts are in non-default locations, use these environment
+variables:
+
+```sh
+PICOTOOL_PATH=/path/to/picotool \
+ELF_PATH=/path/to/switch-pico.elf \
+UF2_PATH=/path/to/switch-pico.uf2 \
+python3 build.py
+```
+
+`PICOTOOL_PATH` selects the flashing tool, `ELF_PATH` selects the ELF that is
+checked and flashed, and `UF2_PATH` selects the UF2 that is checked after the
+build. Their defaults are `picotool` from `PATH`, `build/switch-pico.elf`, and
+`build/switch-pico.uf2`, respectively.
 
 ### Manual build
 ```sh
 cmake -S . -B build -DSWITCH_PICO_LOG=OFF
 cmake --build build -j
 ```
-This produces a `.uf2` you can flash (typically `build/switch-pico.uf2`).
+This produces both `build/switch-pico.elf` and a flashable `build/switch-pico.uf2`.
 
 ### Manual UF2 flashing (BOOTSEL, no tools)
 If you already have a built (or use the pre-built one in `firmware/`) `.uf2`, you can flash it without rebuilding:
@@ -131,11 +174,6 @@ Flash alternatives: bootsel + drag-drop or `picotool load`.
 Flags:
 - `SWITCH_PICO_LOG`: enable/disable UART logging on the Pico.
 
-### Changing controller colours
-`build.py` can optionally update the **grip** colours in `controller_color_config.h` before building/flashing (default leaves the file unchanged):
-- Random grip colours: `python3 build.py --random-grip-color`
-- Set grip colours: `python3 build.py --grip-color FF00AA`
-
 ## Python bridge (recommended)
 Works on macOS, Windows, Linux. Uses SDL2 + pyserial.
 
diff --git a/build.py b/build.py
index dabefe8..5d96461 100644
--- a/build.py
+++ b/build.py
@@ -14,6 +14,7 @@ CONFIG_FILE = SCRIPT_DIR / "controller_color_config.h"
 BUILD_DIR = SCRIPT_DIR / "build"
 
 ELF_PATH = Path(os.environ.get("ELF_PATH", BUILD_DIR / "switch-pico.elf")).expanduser()
+UF2_PATH = Path(os.environ.get("UF2_PATH", BUILD_DIR / "switch-pico.uf2")).expanduser()
 
 MACROS = (
     "SWITCH_COLOR_LEFT_GRIP_R",
@@ -120,6 +121,15 @@ def build():
     )
     run_cmd(["cmake", "--build", str(BUILD_DIR)])
 
+    missing_artifacts = [path for path in (ELF_PATH, UF2_PATH) if not path.is_file()]
+    if missing_artifacts:
+        missing = ", ".join(str(path) for path in missing_artifacts)
+        sys.stderr.write(f"Error: Build did not produce required artifact(s): {missing}\n")
+        sys.exit(1)
+
+    print(f"Built ELF: {ELF_PATH}")
+    print(f"Built UF2: {UF2_PATH}")
+
 def flash():
     picotool = resolve_picotool()
     if not ELF_PATH.exists():
diff --git a/uv.lock b/uv.lock
index c8f27d1..ec7899d 100644
--- a/uv.lock
+++ b/uv.lock
@@ -1,5 +1,5 @@
 version = 1
-revision = 2
+revision = 3
 requires-python = ">=3.9"
 resolution-markers = [
     "python_full_version >= '3.10'",
@@ -468,7 +468,7 @@ resolution-markers = [
     "python_full_version < '3.10'",
 ]
 dependencies = [
-    { name = "mdurl", marker = "python_full_version < '3.10'" },
+    { name = "mdurl" },
 ]
 sdist = { url = "https://files.pythonhosted.org/packages/38/71/3b932df36c1a044d397a1f92d1cf91ee0a503d91e470cbd670aa66b07ed0/markdown-it-py-3.0.0.tar.gz", hash = "sha256:e3f60a94fa066dc52ec76661e37c851cb232d92f9886b15cb560aaada2df8feb", size = 74596, upload-time = "2023-06-03T06:41:14.443Z" }
 wheels = [
@@ -483,7 +483,7 @@ resolution-markers = [
     "python_full_version >= '3.10'",
 ]
 dependencies = [
-    { name = "mdurl", marker = "python_full_version >= '3.10'" },
+    { name = "mdurl" },
 ]
 sdist = { url = "https://files.pythonhosted.org/packages/5b/f5/4ec618ed16cc4f8fb3b701563655a69816155e79e24a17b651541804721d/markdown_it_py-4.0.0.tar.gz", hash = "sha256:cb0a2b4aa34f932c007117b194e945bd74e0ec24133ceb5bac59009cda1cb9f3", size = 73070, upload-time = "2025-08-11T12:57:52.854Z" }
 wheels = [
@@ -804,16 +804,16 @@ wheels = [
 
 [[package]]
 name = "pysdl3"
-version = "0.9.11b0"
+version = "0.9.11b1"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "aiohttp" },
     { name = "packaging" },
     { name = "requests" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/de/2a/86e2b1cc7ac5a186ac3bad262cfbe55313268b6a8f7a3e61a4212fa4af09/pysdl3-0.9.11b0.tar.gz", hash = "sha256:e87dce397221c4943763bb45ec8725deb07b2bf5636d5da875e9d0ae34504068", size = 1421769, upload-time = "2026-03-15T19:28:04.222Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/19/71/1de90f3baf555003b97e8c09db2f40376aa44105a2f2f98e239260cd07b4/pysdl3-0.9.11b1.tar.gz", hash = "sha256:1608b1e979fd6693b321af38e1d70a8a1df8141753096ade3477ff2b437c6ed0", size = 1421849, upload-time = "2026-05-06T23:50:21.42Z" }
 wheels = [
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+    { url = "https://files.pythonhosted.org/packages/a5/5f/290b001a4f46a3811caf103bd98fb23bae0c700ab231fde8e1d6bfe2ecdf/pysdl3-0.9.11b1-py3-none-any.whl", hash = "sha256:2dd0bfe859ae93564b38c97f718d1310dbb4ea898fe85baea499f03012b1a152", size = 99721, upload-time = "2026-05-06T23:50:19.819Z" },
 ]
 
 [[package]]
@@ -897,9 +897,9 @@ resolution-markers = [
     "python_full_version < '3.10'",
 ]
 dependencies = [
-    { name = "idna", marker = "python_full_version < '3.10'" },
-    { name = "multidict", marker = "python_full_version < '3.10'" },
-    { name = "propcache", marker = "python_full_version < '3.10'" },
+    { name = "idna" },
+    { name = "multidict" },
+    { name = "propcache" },
 ]
 sdist = { url = "https://files.pythonhosted.org/packages/57/63/0c6ebca57330cd313f6102b16dd57ffaf3ec4c83403dcb45dbd15c6f3ea1/yarl-1.22.0.tar.gz", hash = "sha256:bebf8557577d4401ba8bd9ff33906f1376c877aa78d1fe216ad01b4d6745af71", size = 187169, upload-time = "2025-10-06T14:12:55.963Z" }
 wheels = [
@@ -1042,9 +1042,9 @@ resolution-markers = [
     "python_full_version >= '3.10'",
 ]
 dependencies = [
-    { name = "idna", marker = "python_full_version >= '3.10'" },
-    { name = "multidict", marker = "python_full_version >= '3.10'" },
-    { name = "propcache", marker = "python_full_version >= '3.10'" },
+    { name = "idna" },
+    { name = "multidict" },
+    { name = "propcache" },
 ]
 sdist = { url = "https://files.pythonhosted.org/packages/23/6e/beb1beec874a72f23815c1434518bfc4ed2175065173fb138c3705f658d4/yarl-1.23.0.tar.gz", hash = "sha256:53b1ea6ca88ebd4420379c330aea57e258408dd0df9af0992e5de2078dc9f5d5", size = 194676, upload-time = "2026-03-01T22:07:53.373Z" }
 wheels = [