diff --git a/AGENTS.md b/AGENTS.md
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--- /dev/null
+++ b/AGENTS.md
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+# Repository Guidelines
+
+## Project Structure & Module Organization
+`firmware/master` contains the access-point, web server, calibration, and ESP-NOW receiver firmware. `firmware/slave` contains the sensor/transmitter firmware for remote nodes. Source files live under each `src/` directory, and the Master web UI is served from `firmware/master/data/index.html`. Hardware documentation lives in `hardware/schematics` and `hardware/cad`. Top-level docs such as `README.md` and `IMPLEMENTATION_STATUS.md` describe project scope and phase status.
+
+## Build, Test, and Development Commands
+Use PlatformIO from each firmware directory:
+
+```bash
+cd firmware/master && pio run
+cd firmware/master && pio run -t upload
+cd firmware/master && pio device monitor -b 115200
+cd firmware/slave && pio run -e slave2
+```
+
+`pio run` builds the current environment, `-t upload` flashes hardware, and `pio device monitor` opens the serial console. Use named slave environments such as `slave2` to build alternate `NODE_ID` values for multi-node setups.
+
+## Coding Style & Naming Conventions
+The codebase is C++ for Arduino/ESP32 with 4-space indentation and opening braces on the same line. Keep header and implementation pairs aligned (`imu_driver.h` / `imu_driver.cpp`). Use `PascalCase` for classes (`CalibrationManager`), `camelCase` for functions and variables (`readBatteryPercent`), and `UPPER_SNAKE_CASE` for macros and config constants. Preserve the existing comment style: short section banners and targeted inline comments, not redundant narration.
+
+## Testing Guidelines
+There is no automated unit-test suite yet. Treat a clean `pio run` for both Master and Slave as the baseline verification step. For behavior changes, validate on hardware through serial logs at 115200 baud and confirm the Master API/UI flow at `http://192.168.4.1`. Document any manual test coverage in the PR when changing IMU logic, ESP-NOW messaging, calibration, or battery reporting.
+
+## Commit & Pull Request Guidelines
+Recent history uses concise, imperative commit subjects, for example: `Implement Phase 1-4: MVP with differential measurement and median filtering`. Follow that pattern and keep subjects focused on one change set. Pull requests should include a short summary, affected area (`master`, `slave`, `hardware`, or docs), manual validation steps, and screenshots or serial output when UI or device behavior changes. Link the relevant spec, issue, or phase task when applicable.
+
+## Configuration & Hardware Notes
+Do not commit device-specific secrets or ad hoc MAC addresses without explanation. If slave pairing changes, update `firmware/slave/src/config.h` deliberately and mention it in review notes so others can reflash hardware correctly.
diff --git a/firmware/diag_s3/platformio.ini b/firmware/diag_s3/platformio.ini
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+++ b/firmware/diag_s3/platformio.ini
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+[env:esp32-s3]
+platform = espressif32
+board = esp32-s3-devkitc-1
+framework = arduino
+monitor_speed = 115200
+upload_speed = 921600
+build_flags =
+    -D ARDUINO_USB_CDC_ON_BOOT=0
+lib_deps =
+    adafruit/Adafruit MPU6050@^2.2.4
diff --git a/firmware/diag_s3/src/main.cpp b/firmware/diag_s3/src/main.cpp
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+++ b/firmware/diag_s3/src/main.cpp
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+#include <Arduino.h>
+#include <Wire.h>
+#include <Adafruit_MPU6050.h>
+#include <Adafruit_Sensor.h>
+
+static constexpr uint8_t SDA_PIN = 4;
+static constexpr uint8_t SCL_PIN = 5;
+Adafruit_MPU6050 mpu;
+
+void scanI2C() {
+    Serial.println("[I2C] scan start");
+    uint8_t found = 0;
+
+    for (uint8_t addr = 1; addr < 127; addr++) {
+        Wire.beginTransmission(addr);
+        uint8_t err = Wire.endTransmission();
+        if (err == 0) {
+            Serial.printf("[I2C] found device at 0x%02X\n", addr);
+            found++;
+        }
+    }
+
+    if (!found) {
+        Serial.println("[I2C] no devices found");
+    }
+    Serial.println("[I2C] scan done");
+}
+
+void setup() {
+    Serial.begin(115200);
+    delay(500);
+    Serial.println();
+    Serial.println("ESP32-S3 I2C diagnostic");
+    Serial.printf("Pins: SDA=%u SCL=%u\n", SDA_PIN, SCL_PIN);
+    Wire.begin(SDA_PIN, SCL_PIN, 100000);
+    delay(100);
+    scanI2C();
+
+    if (!mpu.begin(0x68, &Wire)) {
+        Serial.println("[MPU] begin failed");
+        return;
+    }
+
+    mpu.setAccelerometerRange(MPU6050_RANGE_2_G);
+    mpu.setGyroRange(MPU6050_RANGE_250_DEG);
+    mpu.setFilterBandwidth(MPU6050_BAND_21_HZ);
+    Serial.println("[MPU] begin ok");
+
+    sensors_event_t accel, gyro, temp;
+    mpu.getEvent(&accel, &gyro, &temp);
+    Serial.printf("[MPU] accel=(%.2f, %.2f, %.2f) temp=%.2f\n",
+                 accel.acceleration.x, accel.acceleration.y, accel.acceleration.z, temp.temperature);
+}
+
+void loop() {
+    static uint32_t last = 0;
+    if (millis() - last > 3000) {
+        last = millis();
+        scanI2C();
+    }
+}