diff --git a/brainstem_sdk/.gitignore b/brainstem_sdk/.gitignore
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+# Only the upstream `brainstem_sdk_*.tgz` is tracked. Extracted trees stay local.
+/*
+!/.gitignore
+!/*.tgz
diff --git a/brainstem_sdk/brainstem_sdk_2.12.2_RHEL_9_x86_64.tgz b/brainstem_sdk/brainstem_sdk_2.12.2_RHEL_9_x86_64.tgz
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diff --git a/docs/brainstem-sdk.md b/docs/brainstem-sdk.md
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+# BrainStem SDK tarball in this repo
+
+The repository vendors **one file** under `brainstem_sdk/`: the Acroname **Linux** SDK archive, for example:
+
+`brainstem_sdk/brainstem_sdk_2.12.2_RHEL_9_x86_64.tgz`
+
+That archive matches **RHEL 9 / compatible distros** on **x86_64**. It is the **driverless** bundle (API, examples, small CLI tools, udev helper). It does **not** replace `pip install brainstem` for Python; it adds headers, examples, and native utilities next to the tree.
+
+## Extract the archive
+
+From the repository root:
+
+```bash
+cd brainstem_sdk
+tar -xzf brainstem_sdk_2.12.2_RHEL_9_x86_64.tgz
+```
+
+The tarball unpacks **into the current directory** (top-level entries: `api/`, `cli/`, `docs/`, `linux_brainstem_driverless/`, plus `license.txt`, `readme.txt`, `version.txt`). To avoid unpacking next to the `.tgz`, use a subdirectory:
+
+```bash
+mkdir -p brainstem_sdk/unpacked && cd brainstem_sdk/unpacked
+tar -xzf ../brainstem_sdk_2.12.2_RHEL_9_x86_64.tgz
+```
+
+If you use `unpacked/`, set `BRAINSTEM2_DEV_ROOT` to the directory that contains `api/` after extract (for example `…/brainstem_sdk/unpacked`). FiWi’s default repo-local search only looks at `brainstem_sdk/` and `brainstem_sdk/BrainStem2/`, not `unpacked/`, without that variable.
+
+## Device access (udev)
+
+Before talking to hardware over USB, run the script described in `readme.txt` inside the extracted tree:
+
+```bash
+./linux_brainstem_driverless/udev.sh
+```
+
+That installs udev rules (typically requires `sudo`) and adds the current user to `dialout`. Reboot or re-login as the vendor readme recommends.
+
+## Build C/C++ examples from source
+
+Sample projects live under `api/examples/c_cpp/`. Each example includes `How_To_Build.txt` or `How_to_Build.txt` with Acroname’s steps. In general:
+
+1. Open that file in the example directory you care about.
+2. Copy the **`lib`** directory from the SDK into the example tree where the readme says (headers and `BrainStem2` shared library).
+3. Run `make` (or follow the makefile targets documented there).
+
+You need a normal Linux C/C++ toolchain (`gcc`, `g++`, `make`) and any extra packages the example readme mentions.
+
+## Reflex (`.reflex` → `.map`) and `arc`
+
+FiWi can invoke Acroname’s **Reflex compiler** `arc` via `python3 fiwi.py reflex compile …`. The **`arc`** binary is **not** shipped in this driverless `.tgz` listing (no `bin/arc` in the archive). Install the **BrainStem Development Kit** build that includes Reflex tooling, or set `FIWI_REFLEX_ARC` to a full path when you have `arc` elsewhere.
+
+Details and download pointer:
+
+```bash
+python3 fiwi.py reflex help
+python3 fiwi.py reflex which
+```
+
+Useful environment variables (also in `fiwi reflex help`):
+
+- `BRAINSTEM2_DEV_ROOT` — root of an extracted kit (FiWi looks for `bin/arc`, `api/lib/BrainStem2`, etc.).
+- `FIWI_REFLEX_ARC` — explicit path to `arc`.
+- `FIWI_REFLEX_INCLUDE` — extra `-I` directories for `*.reflex` API headers if your layout is nonstandard.
+
+## Qt / HubTool
+
+`readme.txt` in the bundle notes that **HubTool** needs Qt packages on Linux. Install the distro-specific packages listed there if you use HubTool from Acroname’s full kit.