diff --git a/switch_input.h b/switch_input.h
new file mode 100644
index 0000000..93b65e0
--- /dev/null
+++ b/switch_input.h
@@ -0,0 +1,72 @@
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#define SWITCH_PRO_HAT_UP        0x00
+#define SWITCH_PRO_HAT_UPRIGHT   0x01
+#define SWITCH_PRO_HAT_RIGHT     0x02
+#define SWITCH_PRO_HAT_DOWNRIGHT 0x03
+#define SWITCH_PRO_HAT_DOWN      0x04
+#define SWITCH_PRO_HAT_DOWNLEFT  0x05
+#define SWITCH_PRO_HAT_LEFT      0x06
+#define SWITCH_PRO_HAT_UPLEFT    0x07
+#define SWITCH_PRO_HAT_NOTHING   0x08
+
+#define SWITCH_PRO_MASK_Y       (1U << 0)
+#define SWITCH_PRO_MASK_B       (1U << 1)
+#define SWITCH_PRO_MASK_A       (1U << 2)
+#define SWITCH_PRO_MASK_X       (1U << 3)
+#define SWITCH_PRO_MASK_L       (1U << 4)
+#define SWITCH_PRO_MASK_R       (1U << 5)
+#define SWITCH_PRO_MASK_ZL      (1U << 6)
+#define SWITCH_PRO_MASK_ZR      (1U << 7)
+#define SWITCH_PRO_MASK_MINUS   (1U << 8)
+#define SWITCH_PRO_MASK_PLUS    (1U << 9)
+#define SWITCH_PRO_MASK_L3      (1U << 10)
+#define SWITCH_PRO_MASK_R3      (1U << 11)
+#define SWITCH_PRO_MASK_HOME    (1U << 12)
+#define SWITCH_PRO_MASK_CAPTURE (1U << 13)
+
+#define SWITCH_PRO_JOYSTICK_MIN 0x0000
+#define SWITCH_PRO_JOYSTICK_MID 0x7FFF
+#define SWITCH_PRO_JOYSTICK_MAX 0xFFFF
+
+typedef struct {
+    int16_t accel_x;
+    int16_t accel_y;
+    int16_t accel_z;
+    int16_t gyro_x;
+    int16_t gyro_y;
+    int16_t gyro_z;
+} SwitchImuSample;
+
+typedef struct {
+    bool dpad_up;
+    bool dpad_down;
+    bool dpad_left;
+    bool dpad_right;
+
+    bool button_a;
+    bool button_b;
+    bool button_x;
+    bool button_y;
+    bool button_l;
+    bool button_r;
+    bool button_zl;
+    bool button_zr;
+    bool button_plus;
+    bool button_minus;
+    bool button_home;
+    bool button_capture;
+    bool button_l3;
+    bool button_r3;
+
+    uint16_t lx;
+    uint16_t ly;
+    uint16_t rx;
+    uint16_t ry;
+
+    uint8_t imu_sample_count;
+    SwitchImuSample imu_samples[3];
+} SwitchInputState;
diff --git a/tests/firmware/test_switch_input.cpp b/tests/firmware/test_switch_input.cpp
new file mode 100644
index 0000000..11781e0
--- /dev/null
+++ b/tests/firmware/test_switch_input.cpp
@@ -0,0 +1,52 @@
+#include "test_support.h"
+
+#include <cstddef>
+
+#include "../../switch_input.h"
+
+namespace {
+
+bool switch_input_constants_match_legacy_values() {
+    // Given/When: the shared input constants are compiled after extraction.
+    // Then: every UART-visible legacy value remains unchanged.
+    CHECK(SWITCH_PRO_HAT_UP == 0x00 && SWITCH_PRO_HAT_UPRIGHT == 0x01);
+    CHECK(SWITCH_PRO_HAT_RIGHT == 0x02 && SWITCH_PRO_HAT_DOWNRIGHT == 0x03);
+    CHECK(SWITCH_PRO_HAT_DOWN == 0x04 && SWITCH_PRO_HAT_DOWNLEFT == 0x05);
+    CHECK(SWITCH_PRO_HAT_LEFT == 0x06 && SWITCH_PRO_HAT_UPLEFT == 0x07);
+    CHECK(SWITCH_PRO_HAT_NOTHING == 0x08);
+    CHECK(SWITCH_PRO_MASK_Y == (1U << 0) && SWITCH_PRO_MASK_B == (1U << 1));
+    CHECK(SWITCH_PRO_MASK_A == (1U << 2) && SWITCH_PRO_MASK_X == (1U << 3));
+    CHECK(SWITCH_PRO_MASK_L == (1U << 4) && SWITCH_PRO_MASK_R == (1U << 5));
+    CHECK(SWITCH_PRO_MASK_ZL == (1U << 6) && SWITCH_PRO_MASK_ZR == (1U << 7));
+    CHECK(SWITCH_PRO_MASK_MINUS == (1U << 8) && SWITCH_PRO_MASK_PLUS == (1U << 9));
+    CHECK(SWITCH_PRO_MASK_L3 == (1U << 10) && SWITCH_PRO_MASK_R3 == (1U << 11));
+    CHECK(SWITCH_PRO_MASK_HOME == (1U << 12) && SWITCH_PRO_MASK_CAPTURE == (1U << 13));
+    CHECK(SWITCH_PRO_JOYSTICK_MIN == 0x0000);
+    CHECK(SWITCH_PRO_JOYSTICK_MID == 0x7FFF);
+    CHECK(SWITCH_PRO_JOYSTICK_MAX == 0xFFFF);
+    return true;
+}
+
+bool switch_input_layout_matches_legacy_layout() {
+    // Given/When: the shared input types are compiled after extraction.
+    // Then: field offsets and aggregate sizes match the former driver-owned layout.
+    CHECK(sizeof(SwitchImuSample) == 12);
+    CHECK(offsetof(SwitchImuSample, accel_x) == 0);
+    CHECK(offsetof(SwitchImuSample, gyro_z) == 10);
+    CHECK(offsetof(SwitchInputState, dpad_up) == 0);
+    CHECK(offsetof(SwitchInputState, button_a) == 4);
+    CHECK(offsetof(SwitchInputState, button_r3) == 17);
+    CHECK(offsetof(SwitchInputState, lx) == 18);
+    CHECK(offsetof(SwitchInputState, ry) == 24);
+    CHECK(offsetof(SwitchInputState, imu_sample_count) == 26);
+    CHECK(offsetof(SwitchInputState, imu_samples) == 28);
+    CHECK(sizeof(SwitchInputState) == 64);
+    return true;
+}
+
+}  // namespace
+
+void run_switch_input_tests(TestRunner& runner) {
+    runner.run("shared input constants preserve legacy values", switch_input_constants_match_legacy_values);
+    runner.run("shared input types preserve legacy layout", switch_input_layout_matches_legacy_layout);
+}