diff --git a/.claude/rules/CLAUDE.md b/.claude/rules/CLAUDE.md
index daced9bd..241dc736 100644
--- a/.claude/rules/CLAUDE.md
+++ b/.claude/rules/CLAUDE.md
@@ -63,3 +63,7 @@ Strong success criteria let you loop independently. Weak criteria ("make it work
 ---
 
 **These guidelines are working if:** fewer unnecessary changes in diffs, fewer rewrites due to overcomplication, and clarifying questions come before implementation rather than after mistakes.
+
+For current-firmware merges, board customization retention, ESP-IDF build/flash,
+and external-app compatibility verification, also follow
+`.claude/skills/tactility-firmware-development/SKILL.md`.
diff --git a/.claude/skills/tactility-firmware-development/SKILL.md b/.claude/skills/tactility-firmware-development/SKILL.md
new file mode 100644
index 00000000..64318b4c
--- /dev/null
+++ b/.claude/skills/tactility-firmware-development/SKILL.md
@@ -0,0 +1,97 @@
+---
+name: tactility-firmware-development
+description: Use when merging current Tactility firmware, preserving local ES3C28P/ES3C35P customizations, building/flashing ESP32 firmware, or validating external apps against it.
+---
+
+# Tactility firmware development
+
+## Scope and success criteria
+
+Use this skill for firmware upgrades, especially when upstream changes the app
+loader, dashboard API, SDK export, or device model. Before editing, record the
+active branch, `git status --short`, selected `Devices/<id>`, serial port, and
+the expected device identity. Preserve user work and local board
+customizations; do not reset or overwrite a dirty tree.
+
+Success is not merely a successful flash: the board must boot, mount storage,
+return `/api/sysinfo`, expose required internal apps, and accept an external
+app rebuilt against this exact firmware.
+
+## Upgrade and customization audit
+
+1. Fetch and compare the intended upstream branch before merging. Treat
+   `upstream/main` as distinct from experimental release/IDF branches unless
+   the task explicitly asks for one of those branches.
+2. Merge without discarding local work (for example, `git merge --autostash
+   upstream/main` after inspecting status). Resolve conflicts by preserving
+   required ES3C28P and ES3C35P device definitions, partition selection,
+   display/font customizations, and product features.
+3. Compare the pre-upgrade customization commit against the resulting worktree.
+   Check new app registrations, CMake/source inclusion, settings pages,
+   web-server endpoints, and board-specific `sdkconfig` entries—not just files
+   that happen to conflict.
+4. Commit the resulting firmware customization as a focused, reviewable
+   commit after `git diff --check`.
+
+## Multi-binary external-app compatibility
+
+Upstream app packaging changed to `bin/<platform>/<binary>.elf`. The old
+`elf/<platform>.elf` archive layout produces a loader `Not executable` error.
+
+- Manifest v0.2: `bin/esp32s3/app.elf`
+- Manifest v0.3: `bin/esp32s3/<app.0.binary>.elf`
+
+When this change arrives, update the companion app tool and rebuild every app
+from a fresh SDK exported from this firmware. Do not attempt to repair a
+packaged ELF on the SD card: its layout and ABI must both be regenerated.
+
+## Build and flash
+
+Use the installed IDF 5.5 environment; host virtual environments can corrupt
+both Python dependencies and IDF tooling:
+
+```zsh
+unset VIRTUAL_ENV PYTHONPATH PYTHONHOME
+export IDF_PYTHON_ENV_PATH=/Users/adolforeyna/.espressif/python_env/idf5.5_py3.9_env
+source /Users/adolforeyna/esp/esp-idf/export.sh
+
+# Select/check the intended board before this step.
+python device.py <device-id>
+idf.py build
+idf.py -p /dev/cu.usbmodemXXXX flash
+```
+
+`Tactility/CMakeLists.txt` uses a non-configure-dependent source glob. After
+adding a new `.c`/`.cpp` file, run `idf.py reconfigure build`; otherwise a
+linker error for a newly referenced symbol may only mean CMake has not
+discovered the source file yet.
+
+## Boot and feature acceptance
+
+Capture serial at 115200 after flash or reset. Confirm the board name, SD-card
+mount, HTTP-server start, Wi-Fi address, and any feature-specific startup logs.
+Then query the resolved device:
+
+```zsh
+curl -fsS http://<ip>/api/sysinfo
+curl -fsS http://<ip>/api/apps
+```
+
+For MCP settings, verify the `McpSettings` internal app appears in `/api/apps`.
+When enabled, verify MCP stream startup logs. For a feature restored from an
+older customization, validate its registration path and persisted settings,
+not only its source files.
+
+## External-app acceptance gate
+
+Use the companion app skill's exact-firmware wrapper:
+
+```zsh
+cd /path/to/tactility_apps
+scripts/build_current_firmware_app.sh Apps/MyApp --firmware /path/to/tactility
+```
+
+Inspect the resulting tar member path, install through port-80 dashboard API,
+launch it, and read serial. Required evidence is the loader's `Loading
+.../bin/...`, an ELF entry address, `Task started`, and app-specific startup
+logs. HTTP 200 or a package simply appearing in `/api/apps` is insufficient.