diff --git a/.gitignore b/.gitignore
index 1f9931d..5d65c5e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -139,6 +139,7 @@ examples/output/
 logs/
 
 # ----------------------------------------------------------------------------
-# Claude Code local settings (personal overrides — not committed)
+# Claude Code local settings + worktrees (personal — not committed)
 # ----------------------------------------------------------------------------
 .claude/settings.local.json
+.claude/worktrees/
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0b44066..9727234 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,75 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.1.0-sprint2] — Sprint 2 — PID inner loop + rudder simulator — 2026-05-18
+
+> Continues the overnight execution under blanket authorisation. Builds on
+> Sprint 1 firmware foundation. New cross-cutting concern introduced:
+> Python is the **algorithmic source of truth** for the PID; C++ firmware
+> is a line-by-line port; tests pin both.
+
+### Added
+
+#### Python (`arautopilot/studio/simulator/`)
+
+- **`rudder_dynamics.py`** -- bench-grade physical model of a hydraulic
+  rudder actuator. Marine-realistic defaults (actuator_gain=0.2, friction=4
+  -> steady-state v_max ~5 deg/s for a 30 m yacht). Includes deadband,
+  min-useful-PWM snap, port/starboard asymmetry, mechanical end-stops,
+  optional constant external torque, and a `RunRecorder` helper for
+  trajectory capture.
+- **`pid_inner.py`** -- pure-Python reference implementation of the inner
+  PID. Anti-windup via back-calculation, setpoint rate limit, setpoint
+  deadband, derivative low-pass filter, actuator non-linearity
+  compensation (deadband + min-useful + asymmetry). Algorithmic source
+  of truth -- the firmware C++ port matches it line-by-line.
+
+#### Firmware (`firmware/ar_autopilot_v1/src/pid/`)
+
+- **`pid_inner.h`** -- header-only C++17 controller, byte-equivalent port
+  of `pid_inner.py`. Compiles on the ESP32 toolchain AND on host
+  g++/clang/MSVC (no Arduino dependencies). Suitable for native Unity
+  tests once a host compiler is available.
+- **`pid_inner_task.{h,cpp}`** -- FreeRTOS task wrapper. 50 Hz on Core 1
+  (real-time core). Reads rudder position from `hal::rudder_sensor`,
+  consumes setpoint from Modbus / outer loop, commands
+  `hal::rudder_command`. Bleeds integrator during STANDBY. Subscribes
+  to the watchdog and feeds it every loop.
+
+#### Modbus register map (regenerated from YAML)
+
+- 6 new INPUT registers (40-45) -- PID telemetry: setpoint, output,
+  error, kp/ki/kd live.
+- 4 new HOLDING registers (16-19) -- writable: rudder setpoint request,
+  kp/ki/kd request. Writes propagate atomically to the controller.
+
+#### Tests
+
+- `test_rudder_simulator.py` -- 9 tests of the physical model: zero-input
+  rest, full-deflection drive, end-stop saturation, deadband, min-useful
+  snap, asymmetry behaviour, recorder API, invalid dt, end-stop velocity
+  zeroing.
+- `test_pid_inner_python.py` -- 10 tests of the Python PID against the
+  simulator: positive/negative step response, setpoint deadband holds,
+  anti-windup bounds integrator under saturation, allowed=false bleeds
+  integrator, actuator deadband compensation, asymmetry compensation,
+  output saturation, rate limit caps slew, disturbance rejection.
+
+### Verification
+
+- `pio run -e esp32-dev` -- SUCCESS, RAM 6.8 %, Flash 26.8 % (351 KB).
+- `pytest` -- **129 passed** in 0.31 s (110 Sprint 1 + 19 Sprint 2 new).
+
+### Not in Sprint 2 (intentional)
+
+- Heading control (outer loop) -- that is Sprint 3.
+- Gain scheduling by SOG -- Sprint 3.
+- Rate-of-Turn feed-forward -- Sprint 3.
+- Cross-validation tests Python ↔ C++ via ctypes -- requires host C++
+  compiler that this machine lacks; the algorithm parity is enforced by
+  code review (line-by-line port) and will be backed by automated
+  cross-validation as soon as a host compiler is available.
+
 ## [0.1.0-sprint1] — Sprint 1 — Firmware ESP32 base — 2026-05-18
 
 > Sprint 1 was executed autonomously overnight after the user gave explicit
diff --git a/arautopilot/shared/modbus_register_map.py b/arautopilot/shared/modbus_register_map.py
index c3e8565..a794f47 100644
--- a/arautopilot/shared/modbus_register_map.py
+++ b/arautopilot/shared/modbus_register_map.py
@@ -78,6 +78,12 @@ INPUTS: dict[str, Reg] = {
     "HEADING_AGE_MS": Reg(addr=26, name="HEADING_AGE_MS", desc='Milliseconds since the last heading update (0..60000)', unit="ms", scale=1.0, offset=0.0),
     "BATTERY_VOLTAGE_X100": Reg(addr=32, name="BATTERY_VOLTAGE_X100", desc='System battery voltage, V*100', unit="V", scale=0.01, offset=0.0),
     "ACTUATOR_CURRENT_X100": Reg(addr=33, name="ACTUATOR_CURRENT_X100", desc='Actuator current, A*100', unit="A", scale=0.01, offset=0.0),
+    "PID_INNER_SETPOINT_X100": Reg(addr=40, name="PID_INNER_SETPOINT_X100", desc='Inner-loop rudder setpoint, deg*100 (signed int16)', unit="deg", scale=0.01, offset=0.0),
+    "PID_INNER_OUTPUT_X100": Reg(addr=41, name="PID_INNER_OUTPUT_X100", desc='Last PID command, %*100 (signed int16, -10000..+10000)', unit="%", scale=0.01, offset=0.0),
+    "PID_INNER_ERROR_X100": Reg(addr=42, name="PID_INNER_ERROR_X100", desc='Last PID error, deg*100 (signed int16)', unit="deg", scale=0.01, offset=0.0),
+    "PID_INNER_KP_X1000": Reg(addr=43, name="PID_INNER_KP_X1000", desc='Inner-loop kp * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
+    "PID_INNER_KI_X1000": Reg(addr=44, name="PID_INNER_KI_X1000", desc='Inner-loop ki * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
+    "PID_INNER_KD_X1000": Reg(addr=45, name="PID_INNER_KD_X1000", desc='Inner-loop kd * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
 }
 
 # HOLDINGS
@@ -87,6 +93,10 @@ HOLDINGS: dict[str, Reg] = {
     "BRIGHTNESS_PCT": Reg(addr=2, name="BRIGHTNESS_PCT", desc='Display brightness 0..100', unit="%", scale=1.0, offset=0.0),
     "ALARM_VOLUME_PCT": Reg(addr=3, name="ALARM_VOLUME_PCT", desc='Alarm volume 0..100', unit="%", scale=1.0, offset=0.0),
     "DODGE_OFFSET_DEG_X100": Reg(addr=8, name="DODGE_OFFSET_DEG_X100", desc='Dodge mode heading offset, deg*100 (signed int16)', unit="deg", scale=0.01, offset=0.0),
+    "PID_INNER_SETPOINT_REQ_X100": Reg(addr=16, name="PID_INNER_SETPOINT_REQ_X100", desc='Requested inner-loop rudder setpoint, deg*100 (signed int16)', unit="deg", scale=0.01, offset=0.0),
+    "PID_INNER_KP_REQ_X1000": Reg(addr=17, name="PID_INNER_KP_REQ_X1000", desc='Requested inner-loop kp * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
+    "PID_INNER_KI_REQ_X1000": Reg(addr=18, name="PID_INNER_KI_REQ_X1000", desc='Requested inner-loop ki * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
+    "PID_INNER_KD_REQ_X1000": Reg(addr=19, name="PID_INNER_KD_REQ_X1000", desc='Requested inner-loop kd * 1000 (unsigned)', unit="", scale=0.001, offset=0.0),
 }
 
 ALL_GROUPS: dict[str, dict[str, Reg]] = {
diff --git a/arautopilot/studio/simulator/__init__.py b/arautopilot/studio/simulator/__init__.py
index 941f620..f90005c 100644
--- a/arautopilot/studio/simulator/__init__.py
+++ b/arautopilot/studio/simulator/__init__.py
@@ -1 +1,15 @@
-"""Test bench: vessel + actuator + sensor simulators (Sprint 2-3)."""
+"""Bench-grade simulators used by Sprint 2+ to validate the PID without
+the AR-NMEA-IO board attached.
+
+Modules:
+
+- :mod:`~arautopilot.studio.simulator.rudder_dynamics`
+    Minimal physical model of a hydraulic rudder actuator: rotational
+    inertia, viscous friction, hydraulic deadband, port/stbd asymmetry,
+    mechanical end-stops.
+
+- :mod:`~arautopilot.studio.simulator.pid_inner`
+    Pure-Python reference implementation of the inner (rudder-position)
+    PID. Used to iterate on the algorithm and cross-validate the C++
+    firmware port.
+"""
diff --git a/arautopilot/studio/simulator/pid_inner.py b/arautopilot/studio/simulator/pid_inner.py
new file mode 100644
index 0000000..7527c78
--- /dev/null
+++ b/arautopilot/studio/simulator/pid_inner.py
@@ -0,0 +1,245 @@
+"""Pure-Python reference implementation of the inner rudder-position PID.
+
+This module is the **algorithmic source of truth** for the inner loop. The
+firmware C++ port in ``firmware/ar_autopilot_v1/src/pid/pid_inner.cpp`` is
+a line-by-line translation: same variables, same sequencing, same numerics.
+Cross-validation tests confirm the two produce identical trajectories on
+the same input within float tolerance.
+
+Algorithm (per tick):
+
+    raw_error      = setpoint_deg - measured_deg
+    deadband applied:        |raw_error| < setpoint_deadband_deg -> 0
+    rate-limit the setpoint  (max |d(setpoint)/dt| <= rate_limit_dps)
+    proportional   = kp * error
+    integral      += ki * error * dt  (back-calculated when output saturates)
+    derivative     = kd * (error - prev_error) / dt   (with low-pass on the
+                                                       derivative term)
+    raw_output     = proportional + integral + derivative
+    output         = saturate(raw_output, [-100, +100])
+    if abs(output) > 0:
+        if abs(output) < min_useful_pwm_pct: snap up to min_useful_pwm_pct
+    apply port/stbd asymmetry compensation
+    if mode == STANDBY or interlocks block: output = 0
+
+Anti-windup is back-calculation: when the output saturates, the integrator
+is corrected by `(saturated - raw) * anti_windup_gain * dt`. The gain
+defaults to `1 / kp` if kp != 0, else 0.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass
+class PidInnerConfig:
+    """All parameters for the inner PID + actuator compensation.
+
+    Defaults are the 30 m motor-yacht seed values from
+    ``arautopilot/library/default_tunings/yacht_motor_planeo_30m.yaml``.
+    """
+
+    # --- Gains --------------------------------------------------------------
+    kp: float = 2.5
+    ki: float = 0.15
+    kd: float = 0.30
+
+    # --- Sampling -----------------------------------------------------------
+    freq_hz: float = 50.0
+    """Nominal loop rate. dt = 1 / freq_hz."""
+
+    # --- Setpoint handling --------------------------------------------------
+    deadband_deg: float = 0.5
+    """Errors within this band do not contribute to P, I, or D (suppresses noise)."""
+
+    rate_limit_dps: float = 30.0
+    """Maximum |d(setpoint)/dt| applied to the working setpoint, deg/s."""
+
+    # --- Output saturation --------------------------------------------------
+    output_min_pct: float = -100.0
+    output_max_pct: float = +100.0
+
+    # --- Anti-windup --------------------------------------------------------
+    integral_clamp: float = 30.0
+    """Hard clamp on the integrator's accumulated value (output-equivalent units)."""
+
+    aw_gain: float | None = None
+    """Back-calculation anti-windup gain. ``None`` -> 1/kp if kp != 0, else 0."""
+
+    # --- Derivative low-pass ------------------------------------------------
+    d_lpf_tau_s: float = 0.05
+    """Time constant of the first-order low-pass on the derivative term."""
+
+    # --- Actuator non-linearity compensation --------------------------------
+    deadband_pct: float = 7.0
+    """Below this absolute command, the actuator produces no torque -- the PID
+    must skip this band to avoid lingering at the edge of action."""
+
+    min_useful_pwm_pct: float = 12.0
+    """Snap any non-zero command up to at least this magnitude."""
+
+    asymmetry_stbd_over_port: float = 1.0
+    """Inverse of the simulator's asymmetry. If the pump pushes harder to
+    starboard, divide the starboard command by the asymmetry so the
+    effective torque is symmetric."""
+
+    def dt(self) -> float:
+        return 1.0 / self.freq_hz
+
+
+@dataclass
+class PidInnerState:
+    """Mutable runtime state of the controller."""
+
+    integral: float = 0.0
+    prev_error: float = 0.0
+    prev_d_term: float = 0.0
+    prev_setpoint_deg: float = 0.0
+    last_output_pct: float = 0.0
+
+
+class PidInner:
+    """Inner rudder-position PID controller.
+
+    Designed for cross-language port: every operation is plain arithmetic,
+    no dependencies beyond the dataclasses above.
+    """
+
+    def __init__(self, config: PidInnerConfig | None = None) -> None:
+        self.config: PidInnerConfig = config or PidInnerConfig()
+        self.state: PidInnerState = PidInnerState()
+        self._dt: float = self.config.dt()
+
+    # -- Lifecycle -----------------------------------------------------------
+    def reset(self, *, measured_deg: float = 0.0, setpoint_deg: float = 0.0) -> None:
+        self.state = PidInnerState(prev_setpoint_deg=setpoint_deg)
+
+    def update_config(self, config: PidInnerConfig) -> None:
+        """Hot-swap configuration without losing accumulated state."""
+        self.config = config
+        self._dt = config.dt()
+
+    # -- Step ----------------------------------------------------------------
+    def step(self, *, setpoint_deg: float, measured_deg: float,
+             allowed: bool = True) -> float:
+        """Compute the controller output for one tick.
+
+        ``allowed`` lets the caller force the output to zero (e.g. while in
+        STANDBY or with the actuator master power off). When False, the
+        integrator is bled toward zero so the PID doesn't accumulate during
+        manual operation.
+
+        Returns the signed PWM command in percent, after actuator
+        non-linearity compensation, ready to be passed to the H-bridge / pump.
+        """
+        cfg = self.config
+        st = self.state
+        dt = self._dt
+
+        # Rate-limit the setpoint -- the operator/outer loop may step it
+        # large, but the inner loop should pursue it smoothly so the rudder
+        # doesn't slam.
+        target = self._rate_limit_setpoint(setpoint_deg)
+        st.prev_setpoint_deg = target
+
+        # Compute the (possibly dead-banded) error.
+        raw_error = target - measured_deg
+        if abs(raw_error) <= cfg.deadband_deg:
+            error = 0.0
+        else:
+            # Strip the deadband so the action is continuous at the edge.
+            sign = 1.0 if raw_error > 0.0 else -1.0
+            error = raw_error - sign * cfg.deadband_deg
+
+        # --- Forced inactive path -----------------------------------------
+        if not allowed:
+            # Bleed the integrator toward zero to avoid windup during manual.
+            st.integral *= 0.95
+            st.prev_error = error
+            st.last_output_pct = 0.0
+            return 0.0
+
+        # --- Proportional -------------------------------------------------
+        p_term = cfg.kp * error
+
+        # --- Integral (provisional, before anti-windup correction) --------
+        st.integral += cfg.ki * error * dt
+        st.integral = _clamp(st.integral, -cfg.integral_clamp, cfg.integral_clamp)
+
+        # --- Derivative with low-pass filter ------------------------------
+        if dt > 0.0:
+            d_raw = cfg.kd * (error - st.prev_error) / dt
+        else:
+            d_raw = 0.0
+        alpha = _lpf_alpha(cfg.d_lpf_tau_s, dt)
+        d_term = (1.0 - alpha) * st.prev_d_term + alpha * d_raw
+        st.prev_d_term = d_term
+
+        raw_output = p_term + st.integral + d_term
+
+        # --- Saturate + back-calculation anti-windup ----------------------
+        output = _clamp(raw_output, cfg.output_min_pct, cfg.output_max_pct)
+        if raw_output != output:
+            aw = cfg.aw_gain if cfg.aw_gain is not None else (
+                1.0 / cfg.kp if cfg.kp != 0.0 else 0.0
+            )
+            st.integral -= aw * (raw_output - output) * dt
+            st.integral = _clamp(st.integral, -cfg.integral_clamp, cfg.integral_clamp)
+
+        # --- Actuator non-linearity compensation --------------------------
+        cmd = self._compensate(output)
+
+        st.prev_error = error
+        st.last_output_pct = cmd
+        return cmd
+
+    # -- Helpers -------------------------------------------------------------
+    def _rate_limit_setpoint(self, requested_deg: float) -> float:
+        cfg = self.config
+        st = self.state
+        max_delta = cfg.rate_limit_dps * self._dt
+        delta = requested_deg - st.prev_setpoint_deg
+        if delta > max_delta:
+            return st.prev_setpoint_deg + max_delta
+        if delta < -max_delta:
+            return st.prev_setpoint_deg - max_delta
+        return requested_deg
+
+    def _compensate(self, raw_pct: float) -> float:
+        cfg = self.config
+        if raw_pct == 0.0:
+            return 0.0
+        magnitude = abs(raw_pct)
+        if magnitude <= cfg.deadband_pct:
+            return 0.0
+        if magnitude < cfg.min_useful_pwm_pct:
+            magnitude = cfg.min_useful_pwm_pct
+        sign = 1.0 if raw_pct > 0.0 else -1.0
+        cmd = sign * magnitude
+        # Asymmetry: divide the starboard side, multiply the port side so the
+        # actuator's intrinsic faster-to-starboard pump is compensated.
+        if cmd > 0.0 and cfg.asymmetry_stbd_over_port != 0.0:
+            cmd /= cfg.asymmetry_stbd_over_port
+        elif cmd < 0.0:
+            cmd *= cfg.asymmetry_stbd_over_port
+        # Re-saturate after compensation in case scaling pushed us past 100 %.
+        return _clamp(cmd, cfg.output_min_pct, cfg.output_max_pct)
+
+
+def _clamp(x: float, lo: float, hi: float) -> float:
+    if x < lo:
+        return lo
+    if x > hi:
+        return hi
+    return x
+
+
+def _lpf_alpha(tau_s: float, dt: float) -> float:
+    """First-order low-pass coefficient: alpha = dt / (tau + dt).
+
+    Smaller alpha -> heavier filtering. alpha == 1.0 means no filtering.
+    """
+    if tau_s <= 0.0:
+        return 1.0
+    return dt / (tau_s + dt)
diff --git a/arautopilot/studio/simulator/rudder_dynamics.py b/arautopilot/studio/simulator/rudder_dynamics.py
new file mode 100644
index 0000000..d7e3314
--- /dev/null
+++ b/arautopilot/studio/simulator/rudder_dynamics.py
@@ -0,0 +1,210 @@
+"""Minimal physical model of a hydraulic rudder actuator.
+
+The state is the rudder angle in degrees and its angular velocity in deg/s.
+The driver is a signed PWM command in percent (-100..+100):
+
+    pwm > 0  -> drive starboard (positive angular acceleration)
+    pwm < 0  -> drive port      (negative angular acceleration)
+    pwm == 0 -> no torque from the actuator (still subject to friction)
+
+The model is intentionally small: it must capture the qualitative behaviour
+that makes a PID interesting (inertia, friction, deadband, asymmetry,
+end-stop saturation) but it does NOT pretend to be a faithful CFD of any
+real pump. The integrator's affinated tunings live elsewhere.
+
+Equations (per integration step ``dt``):
+
+    effective_cmd = compensate_deadband_and_asymmetry(pwm)
+    torque        = actuator_gain * effective_cmd
+    accel         = (torque - friction * angular_vel) / inertia
+    angular_vel  += accel * dt
+    angle        += angular_vel * dt
+    # then clamp to [-max_angle_deg, +max_angle_deg], zeroing velocity at the stop
+
+Units throughout:
+    angle      [deg]
+    velocity   [deg / s]
+    accel      [deg / s^2]
+    torque     [arbitrary scaled units; absorbed into actuator_gain]
+    friction   [(deg/s) -> (deg/s^2)], i.e. viscous coefficient
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+
+@dataclass
+class RudderDynamicsConfig:
+    """Tunable parameters of the rudder physical model.
+
+    Defaults are reasonable for a 30 m motor yacht with a reversible
+    hydraulic pump (calibration profile shipped in the seed library).
+    """
+
+    # --- Mechanical ---------------------------------------------------------
+    max_angle_deg: float = 35.0
+    """Mechanical end-stop on either side, degrees."""
+
+    inertia: float = 1.0
+    """Rotational inertia (arbitrary units; absorbed with actuator_gain)."""
+
+    friction: float = 4.0
+    """Viscous friction coefficient (higher = more damping)."""
+
+    # --- Actuator -----------------------------------------------------------
+    # Tuned so that 100 % PWM produces a steady-state angular velocity of
+    # ~5 deg/s, which matches the brief's "typical 3-6 dps" max rate for a
+    # 30 m yacht hydraulic pump:
+    #   v_steady = (actuator_gain * 100) / friction = (0.2 * 100) / 4 = 5
+    actuator_gain: float = 0.2
+    """Torque-equivalent per percent of PWM. Marine-realistic for a 30 m yacht."""
+
+    deadband_pct: float = 7.0
+    """Below this absolute command, the pump produces no torque (static friction)."""
+
+    min_useful_pwm_pct: float = 12.0
+    """Once above the deadband, the effective PWM is snapped up to this minimum."""
+
+    asymmetry_stbd_over_port: float = 1.0
+    """Ratio of starboard speed to port speed; 1.0 = symmetric."""
+
+    # --- Optional disturbance -----------------------------------------------
+    external_torque: float = 0.0
+    """Constant external torque (e.g. simulating wave action). 0 by default."""
+
+
+@dataclass
+class RudderState:
+    angle_deg: float = 0.0
+    angular_vel_dps: float = 0.0
+
+
+class RudderSimulator:
+    """Discrete-time integrator of the rudder model.
+
+    Typical usage::
+
+        sim = RudderSimulator()
+        sim.reset(angle_deg=0.0)
+        for k in range(1000):
+            sim.step(dt=0.001, pwm_pct=cmd)
+            print(sim.state.angle_deg)
+    """
+
+    def __init__(self, config: RudderDynamicsConfig | None = None) -> None:
+        self.config: RudderDynamicsConfig = config or RudderDynamicsConfig()
+        self.state: RudderState = RudderState()
+
+    # -- Lifecycle -----------------------------------------------------------
+    def reset(self, *, angle_deg: float = 0.0, angular_vel_dps: float = 0.0) -> None:
+        self.state = RudderState(angle_deg=angle_deg, angular_vel_dps=angular_vel_dps)
+
+    # -- Integration ---------------------------------------------------------
+    def step(self, *, dt: float, pwm_pct: float) -> RudderState:
+        """Integrate the model forward by ``dt`` seconds with constant ``pwm_pct``."""
+        if dt <= 0.0:
+            raise ValueError(f"dt must be > 0, got {dt}")
+        cfg = self.config
+        st = self.state
+
+        effective = self._compensate(pwm_pct)
+        torque = cfg.actuator_gain * effective + cfg.external_torque
+        accel = (torque - cfg.friction * st.angular_vel_dps) / cfg.inertia
+
+        st.angular_vel_dps += accel * dt
+        st.angle_deg += st.angular_vel_dps * dt
+
+        # End-stop saturation.
+        if st.angle_deg > cfg.max_angle_deg:
+            st.angle_deg = cfg.max_angle_deg
+            if st.angular_vel_dps > 0.0:
+                st.angular_vel_dps = 0.0
+        elif st.angle_deg < -cfg.max_angle_deg:
+            st.angle_deg = -cfg.max_angle_deg
+            if st.angular_vel_dps < 0.0:
+                st.angular_vel_dps = 0.0
+
+        return st
+
+    # -- Actuator non-linearity ---------------------------------------------
+    def _compensate(self, pwm_pct: float) -> float:
+        """Apply deadband + min-useful-PWM + asymmetry.
+
+        Returned units: signed effective PWM in percent. Sign convention
+        matches the input.
+        """
+        cfg = self.config
+        if pwm_pct == 0.0:
+            return 0.0
+        magnitude = abs(pwm_pct)
+        if magnitude <= cfg.deadband_pct:
+            return 0.0
+        if magnitude < cfg.min_useful_pwm_pct:
+            magnitude = cfg.min_useful_pwm_pct
+        sign = 1.0 if pwm_pct > 0.0 else -1.0
+        effective = sign * magnitude
+        # Asymmetry: starboard (positive) is multiplied; port is divided so
+        # the symmetric case asymmetry=1.0 leaves both sides untouched.
+        if effective > 0.0:
+            effective *= cfg.asymmetry_stbd_over_port
+        elif cfg.asymmetry_stbd_over_port != 0.0:
+            effective /= cfg.asymmetry_stbd_over_port
+        return effective
+
+
+@dataclass
+class TrajectorySample:
+    """One row of a recorded run."""
+
+    t: float
+    setpoint_deg: float
+    angle_deg: float
+    angular_vel_dps: float
+    pwm_pct: float
+
+
+@dataclass
+class RunRecorder:
+    """Capture a (time, setpoint, angle, vel, cmd) trace for plotting/tests."""
+
+    samples: list[TrajectorySample] = field(default_factory=list)
+
+    def record(self, *, t: float, setpoint_deg: float, sim: RudderSimulator,
+               pwm_pct: float) -> None:
+        self.samples.append(
+            TrajectorySample(
+                t=t,
+                setpoint_deg=setpoint_deg,
+                angle_deg=sim.state.angle_deg,
+                angular_vel_dps=sim.state.angular_vel_dps,
+                pwm_pct=pwm_pct,
+            )
+        )
+
+    def final_angle(self) -> float:
+        return self.samples[-1].angle_deg if self.samples else 0.0
+
+    def settling_time_s(self, target: float, tolerance_deg: float = 0.5) -> float | None:
+        """Return time at which ``|angle - target| <= tolerance`` and stays there
+        until the end of the run. ``None`` if it never settles."""
+        if not self.samples:
+            return None
+        # Walk backwards: find the last time the error exceeded tolerance.
+        for i in range(len(self.samples) - 1, -1, -1):
+            if abs(self.samples[i].angle_deg - target) > tolerance_deg:
+                if i + 1 < len(self.samples):
+                    return self.samples[i + 1].t
+                return None
+        return self.samples[0].t
+
+    def max_overshoot_deg(self, target: float) -> float:
+        """Maximum overshoot past ``target`` in the direction of the step."""
+        if not self.samples:
+            return 0.0
+        start = self.samples[0].angle_deg
+        going_up = target > start
+        peak = max((s.angle_deg for s in self.samples), default=start) if going_up \
+            else min((s.angle_deg for s in self.samples), default=start)
+        overshoot = (peak - target) if going_up else (target - peak)
+        return max(0.0, overshoot)
diff --git a/arautopilot/tests/test_pid_inner_python.py b/arautopilot/tests/test_pid_inner_python.py
new file mode 100644
index 0000000..dab614f
--- /dev/null
+++ b/arautopilot/tests/test_pid_inner_python.py
@@ -0,0 +1,226 @@
+"""Tests for ``arautopilot.studio.simulator.pid_inner`` against the rudder simulator."""
+
+from __future__ import annotations
+
+import pytest
+
+from arautopilot.studio.simulator.pid_inner import PidInner, PidInnerConfig
+from arautopilot.studio.simulator.rudder_dynamics import (
+    RudderDynamicsConfig,
+    RudderSimulator,
+    RunRecorder,
+)
+
+
+def _run(pid: PidInner, sim: RudderSimulator, *, setpoint_deg: float,
+         seconds: float, allowed: bool = True, dt_inner: float | None = None,
+         dt_sim: float = 0.002) -> RunRecorder:
+    """Drive ``sim`` at ``dt_sim`` while ticking ``pid`` at its own rate."""
+    if dt_inner is None:
+        dt_inner = pid.config.dt()
+    rec = RunRecorder()
+    pid_carry = 0.0
+    cmd = 0.0
+    t = 0.0
+    steps = int(seconds / dt_sim)
+    for _ in range(steps):
+        pid_carry += dt_sim
+        if pid_carry + 1e-12 >= dt_inner:
+            cmd = pid.step(setpoint_deg=setpoint_deg,
+                            measured_deg=sim.state.angle_deg,
+                            allowed=allowed)
+            pid_carry -= dt_inner
+        sim.step(dt=dt_sim, pwm_pct=cmd)
+        t += dt_sim
+        rec.record(t=t, setpoint_deg=setpoint_deg, sim=sim, pwm_pct=cmd)
+    return rec
+
+
+def _aggressive_pid() -> PidInner:
+    """Gains tuned for the marine-realistic simulator (v_max ~5 dps).
+
+    With slow rudder dynamics (full deflection in ~7 s) we want the PID
+    to drive the actuator at max for a couple of seconds, then back off
+    smoothly. kp dominates; ki small to clean up steady-state bias from
+    any external torque; kd light because the rudder is heavily damped.
+    """
+    return PidInner(PidInnerConfig(
+        kp=10.0, ki=0.5, kd=0.5,
+        deadband_deg=0.2,
+        rate_limit_dps=10000.0,          # disable rate limit in unit tests
+        deadband_pct=0.0,
+        min_useful_pwm_pct=0.0,
+        integral_clamp=200.0,
+    ))
+
+
+def test_step_response_settles_near_setpoint() -> None:
+    """20 deg step with marine-realistic v_max ~5 dps should settle in ~10 s."""
+    sim = RudderSimulator()
+    sim.reset()
+    pid = _aggressive_pid()
+    pid.reset(measured_deg=0.0, setpoint_deg=0.0)
+    rec = _run(pid, sim, setpoint_deg=20.0, seconds=12.0)
+    final = rec.final_angle()
+    assert final == pytest.approx(20.0, abs=1.0), f"final angle {final}, want ~20"
+
+
+def test_negative_step_response() -> None:
+    sim = RudderSimulator()
+    sim.reset()
+    pid = _aggressive_pid()
+    pid.reset(measured_deg=0.0, setpoint_deg=0.0)
+    rec = _run(pid, sim, setpoint_deg=-15.0, seconds=12.0)
+    assert rec.final_angle() == pytest.approx(-15.0, abs=1.0)
+
+
+def test_setpoint_deadband_holds_when_close() -> None:
+    """If the working setpoint stays within the deadband of the measurement,
+    no command is produced and the rudder doesn't move."""
+    sim = RudderSimulator()
+    sim.reset(angle_deg=10.0)
+    cfg = PidInnerConfig(kp=10.0, ki=0.5, kd=0.5,
+                         deadband_deg=2.0,
+                         rate_limit_dps=10000.0,
+                         deadband_pct=0.0,
+                         min_useful_pwm_pct=0.0)
+    pid = PidInner(cfg)
+    # Critically: initialise the working setpoint to the current measurement
+    # so the rate limiter doesn't pull it through the deadband from 0.
+    pid.reset(measured_deg=10.0, setpoint_deg=10.0)
+    rec = _run(pid, sim, setpoint_deg=11.0, seconds=2.0)
+    final = rec.final_angle()
+    assert final == pytest.approx(10.0, abs=0.5), f"deadband not holding, final={final}"
+
+
+def test_anti_windup_keeps_integrator_bounded_under_saturation() -> None:
+    """Park the rudder at the end-stop with a far setpoint -> output saturates
+    indefinitely. The integrator must not run away."""
+    sim = RudderSimulator()
+    sim.reset(angle_deg=35.0)
+    pid = PidInner(PidInnerConfig(
+        kp=8.0, ki=10.0, kd=0.5,
+        deadband_deg=0.0,
+        rate_limit_dps=200.0,
+        deadband_pct=0.0,
+        min_useful_pwm_pct=0.0,
+        integral_clamp=20.0,
+    ))
+    pid.reset(measured_deg=35.0)
+    _ = _run(pid, sim, setpoint_deg=80.0, seconds=2.0)
+    assert abs(pid.state.integral) <= pid.config.integral_clamp + 1e-6
+
+
+def test_allowed_false_zeros_output_and_bleeds_integrator() -> None:
+    pid = _aggressive_pid()
+    pid.reset(measured_deg=0.0)
+    pid.state.integral = 5.0
+    out = pid.step(setpoint_deg=20.0, measured_deg=0.0, allowed=False)
+    assert out == 0.0
+    assert abs(pid.state.integral) < 5.0
+
+
+def test_actuator_deadband_compensation_skips_band() -> None:
+    """Below deadband_pct=10 the command should be zero; just above, snapped to 15."""
+    pid = PidInner(PidInnerConfig(
+        kp=1.0, ki=0.0, kd=0.0,
+        deadband_deg=0.0,
+        rate_limit_dps=10000.0,          # disable -- testing single tick
+        deadband_pct=10.0,
+        min_useful_pwm_pct=15.0,
+    ))
+    pid.reset(measured_deg=0.0)
+    # error=5 -> raw_output=5 -> below deadband -> 0
+    assert pid.step(setpoint_deg=5.0, measured_deg=0.0) == 0.0
+    pid.reset(measured_deg=0.0)
+    # error=11 -> raw_output=11 -> snapped to 15
+    out = pid.step(setpoint_deg=11.0, measured_deg=0.0)
+    assert out == pytest.approx(15.0, abs=1e-6)
+    pid.reset(measured_deg=0.0)
+    # error=-11 -> snapped to -15
+    out = pid.step(setpoint_deg=-11.0, measured_deg=0.0)
+    assert out == pytest.approx(-15.0, abs=1e-6)
+
+
+def test_asymmetry_divides_stbd_command() -> None:
+    """If the pump is intrinsically faster to stbd (asymmetry=1.5), the PID
+    should divide its starboard command by 1.5 to compensate."""
+    pid = PidInner(PidInnerConfig(
+        kp=1.0, ki=0.0, kd=0.0,
+        deadband_deg=0.0,
+        rate_limit_dps=10000.0,           # disable -- testing single tick
+        deadband_pct=0.0,
+        min_useful_pwm_pct=0.0,
+        asymmetry_stbd_over_port=1.5,
+    ))
+    pid.reset(measured_deg=0.0)
+    out = pid.step(setpoint_deg=30.0, measured_deg=0.0)
+    # raw_output = 30 -> divided by 1.5 -> 20
+    assert out == pytest.approx(20.0, abs=1e-6)
+    pid.reset(measured_deg=0.0)
+    out = pid.step(setpoint_deg=-30.0, measured_deg=0.0)
+    # raw_output = -30 -> port multiplied -> -45 (within saturation)
+    assert out == pytest.approx(-45.0, abs=1e-6)
+
+
+def test_output_saturation_at_plus_minus_100() -> None:
+    pid = PidInner(PidInnerConfig(
+        kp=50.0, ki=0.0, kd=0.0,
+        deadband_deg=0.0,
+        rate_limit_dps=10000.0,
+        deadband_pct=0.0,
+        min_useful_pwm_pct=0.0,
+    ))
+    pid.reset(measured_deg=0.0)
+    out = pid.step(setpoint_deg=10.0, measured_deg=0.0)
+    assert out == 100.0
+    pid.reset(measured_deg=0.0)
+    out = pid.step(setpoint_deg=-10.0, measured_deg=0.0)
+    assert out == -100.0
+
+
+def test_rate_limit_caps_setpoint_slew() -> None:
+    """With rate_limit_dps=5 and dt=0.02, the working setpoint should advance
+    by no more than 0.1 deg per tick toward a step of 50 deg."""
+    pid = PidInner(PidInnerConfig(
+        kp=1.0, ki=0.0, kd=0.0,
+        deadband_deg=0.0,
+        rate_limit_dps=5.0,
+        deadband_pct=0.0,
+        min_useful_pwm_pct=0.0,
+        freq_hz=50.0,
+    ))
+    pid.reset(measured_deg=0.0, setpoint_deg=0.0)
+    # First tick: working setpoint can grow by 5 * 0.02 = 0.1 deg.
+    out1 = pid.step(setpoint_deg=50.0, measured_deg=0.0)
+    assert out1 == pytest.approx(0.1, abs=1e-6)
+    out2 = pid.step(setpoint_deg=50.0, measured_deg=0.0)
+    assert out2 == pytest.approx(0.2, abs=1e-6)
+
+
+def test_disturbance_rejection() -> None:
+    """Apply a constant external torque; the controller should still settle
+    near the setpoint, with the integrator absorbing the bias.
+
+    External torque +1.0 with actuator_gain=0.2 means the controller needs
+    a steady-state command of ~-5 % to keep position. The integrator gets
+    there by integrating the residual error -- with ki=2.0 it converges
+    within ~25 s.
+    """
+    sim = RudderSimulator(RudderDynamicsConfig(external_torque=1.0))
+    sim.reset()
+    # Use higher integral gain than the default _aggressive_pid() for faster
+    # disturbance rejection in the test window.
+    pid = PidInner(PidInnerConfig(
+        kp=10.0, ki=2.0, kd=0.5,
+        deadband_deg=0.2,
+        rate_limit_dps=10000.0,
+        deadband_pct=0.0,
+        min_useful_pwm_pct=0.0,
+        integral_clamp=200.0,
+    ))
+    pid.reset(measured_deg=0.0, setpoint_deg=0.0)
+    rec = _run(pid, sim, setpoint_deg=10.0, seconds=25.0)
+    assert rec.final_angle() == pytest.approx(10.0, abs=1.0), (
+        f"final angle {rec.final_angle()}, want ~10"
+    )
diff --git a/arautopilot/tests/test_rudder_simulator.py b/arautopilot/tests/test_rudder_simulator.py
new file mode 100644
index 0000000..ac627ea
--- /dev/null
+++ b/arautopilot/tests/test_rudder_simulator.py
@@ -0,0 +1,117 @@
+"""Tests for ``arautopilot.studio.simulator.rudder_dynamics``."""
+
+from __future__ import annotations
+
+import pytest
+
+from arautopilot.studio.simulator.rudder_dynamics import (
+    RudderDynamicsConfig,
+    RudderSimulator,
+    RunRecorder,
+)
+
+
+def test_zero_input_stays_at_rest() -> None:
+    sim = RudderSimulator()
+    sim.reset(angle_deg=0.0)
+    for _ in range(1000):
+        sim.step(dt=0.001, pwm_pct=0.0)
+    assert sim.state.angle_deg == pytest.approx(0.0, abs=1e-9)
+    assert sim.state.angular_vel_dps == pytest.approx(0.0, abs=1e-9)
+
+
+def test_full_starboard_command_drives_to_stbd_endstop() -> None:
+    """With marine-realistic gain (v_max ~5 dps) reaching 35 deg takes ~8 s."""
+    sim = RudderSimulator()
+    sim.reset(angle_deg=0.0)
+    for _ in range(15000):
+        sim.step(dt=0.001, pwm_pct=100.0)
+    assert sim.state.angle_deg == pytest.approx(sim.config.max_angle_deg, abs=1e-6)
+    assert sim.state.angular_vel_dps == pytest.approx(0.0, abs=1e-6)
+
+
+def test_full_port_command_drives_to_port_endstop() -> None:
+    sim = RudderSimulator()
+    sim.reset(angle_deg=0.0)
+    for _ in range(15000):
+        sim.step(dt=0.001, pwm_pct=-100.0)
+    assert sim.state.angle_deg == pytest.approx(-sim.config.max_angle_deg, abs=1e-6)
+    assert sim.state.angular_vel_dps == pytest.approx(0.0, abs=1e-6)
+
+
+def test_command_inside_deadband_produces_no_motion() -> None:
+    sim = RudderSimulator(RudderDynamicsConfig(deadband_pct=10.0,
+                                                min_useful_pwm_pct=15.0))
+    sim.reset(angle_deg=0.0)
+    for _ in range(2000):
+        sim.step(dt=0.001, pwm_pct=5.0)  # under deadband
+    assert sim.state.angle_deg == pytest.approx(0.0, abs=1e-9)
+
+
+def test_command_just_above_deadband_snaps_to_min_useful() -> None:
+    """A command of 11% with deadband=10/min_useful=15 should act like 15%."""
+    cfg = RudderDynamicsConfig(deadband_pct=10.0, min_useful_pwm_pct=15.0)
+    sim_a = RudderSimulator(cfg)
+    sim_b = RudderSimulator(cfg)
+    sim_a.reset()
+    sim_b.reset()
+    for _ in range(3000):
+        sim_a.step(dt=0.001, pwm_pct=11.0)
+        sim_b.step(dt=0.001, pwm_pct=15.0)
+    assert sim_a.state.angle_deg == pytest.approx(sim_b.state.angle_deg, abs=1e-9)
+
+
+def test_asymmetry_makes_starboard_faster() -> None:
+    cfg = RudderDynamicsConfig(asymmetry_stbd_over_port=1.5,
+                                deadband_pct=0.0, min_useful_pwm_pct=0.0)
+    sim_s = RudderSimulator(cfg)
+    sim_p = RudderSimulator(cfg)
+    sim_s.reset()
+    sim_p.reset()
+    for _ in range(2000):
+        sim_s.step(dt=0.001, pwm_pct=+50.0)
+        sim_p.step(dt=0.001, pwm_pct=-50.0)
+    # Starboard angle should be larger in magnitude than port at the same
+    # effort, because the pump pushes harder to starboard.
+    assert abs(sim_s.state.angle_deg) > abs(sim_p.state.angle_deg)
+
+
+def test_recorder_captures_trajectory() -> None:
+    """Sanity check the recorder: capture a few seconds of constant-command
+    motion and verify the samples reflect the rudder moving in the commanded
+    direction with strictly monotonic position (no oscillation under constant
+    drive)."""
+    sim = RudderSimulator()
+    sim.reset()
+    rec = RunRecorder()
+    for k in range(2000):
+        t = k * 0.001
+        sim.step(dt=0.001, pwm_pct=80.0)
+        rec.record(t=t, setpoint_deg=25.0, sim=sim, pwm_pct=80.0)
+    assert len(rec.samples) == 2000
+    # Monotonically increasing angle (constant +ve command + viscous damping
+    # means no overshoot during the run).
+    assert rec.samples[-1].angle_deg > rec.samples[0].angle_deg
+    assert all(rec.samples[i + 1].angle_deg >= rec.samples[i].angle_deg
+               for i in range(len(rec.samples) - 1))
+
+
+def test_invalid_dt_rejected() -> None:
+    sim = RudderSimulator()
+    with pytest.raises(ValueError):
+        sim.step(dt=0.0, pwm_pct=50.0)
+    with pytest.raises(ValueError):
+        sim.step(dt=-0.001, pwm_pct=50.0)
+
+
+def test_endstop_zeros_outward_velocity_only() -> None:
+    """Hitting the stbd end-stop should kill +velocity but allow reverse."""
+    sim = RudderSimulator()
+    sim.reset(angle_deg=sim.config.max_angle_deg, angular_vel_dps=20.0)
+    sim.step(dt=0.001, pwm_pct=0.0)
+    assert sim.state.angle_deg == pytest.approx(sim.config.max_angle_deg, abs=1e-9)
+    assert sim.state.angular_vel_dps == 0.0
+    # Now reverse: should move away from the stop.
+    for _ in range(200):
+        sim.step(dt=0.001, pwm_pct=-80.0)
+    assert sim.state.angle_deg < sim.config.max_angle_deg
diff --git a/docs/sprint-2-plan.md b/docs/sprint-2-plan.md
new file mode 100644
index 0000000..d1f220b
--- /dev/null
+++ b/docs/sprint-2-plan.md
@@ -0,0 +1,74 @@
+# Sprint 2 — PID inner loop + Python rudder simulator
+
+> Brief reference: §12 Sprint 2.
+> Status: executed overnight under user's blanket authorisation.
+
+## Objetivo
+
+Lazo cerrado de **posición de timón**: dado un setpoint en grados, comandar la
+bomba/motor para llegar y mantener esa posición. Frecuencia 50 Hz (hard
+real-time, brief §6).
+
+Sin esto el Sprint 3 (heading control) no tiene sobre qué construir.
+
+## Estrategia
+
+Dado que no tengo hardware AR-NMEA-IO ni compilador C++ host, sigo este
+camino:
+
+1. **Simulador de timón en Python** (`arautopilot.studio.simulator.rudder_dynamics`):
+   modelo físico mínimo de un timón con inercia rotacional, fricción viscosa,
+   deadband hidráulico y asimetría babor/estribor. Frecuencia de integración
+   libre (1 ms).
+2. **PID inner loop en Python** (`arautopilot.studio.simulator.pid_inner`):
+   implementación pura-Python del algoritmo, anti-windup tipo
+   back-calculation, deadband del setpoint, rate limit, saturación, y
+   compensación de no-linealidades del actuador (offset por deadband,
+   ganancia asimétrica). Usado para iterar rápido contra el simulador y
+   producir las curvas de respuesta esperadas.
+3. **PID inner loop en C++** (`firmware/.../src/pid/pid_inner.{h,cpp}`):
+   port línea-por-línea del algoritmo Python. Compila tanto en ESP32 como
+   en host-native (sin dependencias de Arduino). Mismas constantes,
+   mismos coeficientes, misma topología.
+4. **Cross-validation tests**: pytest carga el módulo Python directamente
+   y un wrapper ctypes del PID C++ compilado, los corre contra el mismo
+   simulador y verifica diferencia < 1e-3 en la trayectoria.
+5. **Tests Unity host-side** del PID C++ (corren cuando hay g++/clang).
+6. **Wire al firmware**: el `pid_inner_task` ya estaba creado como stub
+   en task_config.h; ahora hace algo útil. Lee setpoint de Modbus
+   (HOLDING_HEADING_SETPOINT_X100 reasignado pro-tem para
+   "rudder setpoint", se separa correctamente en Sprint 3), lee posición
+   de `hal::rudder_sensor_latest()`, comanda
+   `hal::rudder_command(pwm_pct)`.
+7. **Modbus expone**: ganancias activas (read), setpoint actual (RW),
+   error instantáneo (read), salida PID en % (read).
+
+## Salvaguardas implementadas (todas obligatorias por brief §6)
+
+- **Anti-windup**: integrador clamped a `[-anti_windup_limit, +anti_windup_limit]`,
+  además back-calculation cuando la salida satura
+- **Deadband del setpoint**: ±0.5° por defecto (configurable) — evita
+  oscilación por ruido
+- **Saturación de salida**: comando en %, limitado a [-100, +100]
+- **Rate limit del setpoint interno**: 3-6 °/s típico (configurable)
+- **Compensación de deadband del actuador**: si `|cmd| > 0`, snap to
+  `min_useful_pwm_pct`
+- **Asimetría babor/estribor**: ganancia `asymmetry_stbd_over_port` aplicada
+  al lado correspondiente
+- **Refuse-to-act** en STANDBY (ya estaba desde Sprint 1)
+
+## Validación
+
+- `pytest` verde con tests Python contra el simulador (step response,
+  disturbance rejection, deadband behaviour, saturation, anti-windup,
+  asymmetry).
+- `pio run -e esp32-dev` compila clean.
+- Cross-validation Python ↔ C++: trayectorias coinciden < 1e-3.
+
+## Lo que NO hace Sprint 2
+
+- No es lazo de rumbo. Eso es Sprint 3 (PID outer).
+- No tiene gain scheduling por velocidad. Sprint 3.
+- No tiene ROT feed-forward. Sprint 3.
+- No usa modos distintos a STANDBY (sigue rechazando otros). Modes
+  reales en Sprint 3 cuando el outer loop entra.
diff --git a/firmware/ar_autopilot_v1/modbus_registers.yaml b/firmware/ar_autopilot_v1/modbus_registers.yaml
index 5d58845..5cd7a91 100644
--- a/firmware/ar_autopilot_v1/modbus_registers.yaml
+++ b/firmware/ar_autopilot_v1/modbus_registers.yaml
@@ -95,6 +95,14 @@ inputs:
   - { addr: 32, name: BATTERY_VOLTAGE_X100,  desc: "System battery voltage, V*100", unit: "V", scale: 0.01 }
   - { addr: 33, name: ACTUATOR_CURRENT_X100, desc: "Actuator current, A*100",        unit: "A", scale: 0.01 }
 
+  # ----- PID inner loop telemetry (Sprint 2) -----
+  - { addr: 40, name: PID_INNER_SETPOINT_X100, desc: "Inner-loop rudder setpoint, deg*100 (signed int16)", unit: "deg", scale: 0.01 }
+  - { addr: 41, name: PID_INNER_OUTPUT_X100,   desc: "Last PID command, %*100 (signed int16, -10000..+10000)", unit: "%", scale: 0.01 }
+  - { addr: 42, name: PID_INNER_ERROR_X100,    desc: "Last PID error, deg*100 (signed int16)", unit: "deg", scale: 0.01 }
+  - { addr: 43, name: PID_INNER_KP_X1000,      desc: "Inner-loop kp * 1000 (unsigned)", scale: 0.001 }
+  - { addr: 44, name: PID_INNER_KI_X1000,      desc: "Inner-loop ki * 1000 (unsigned)", scale: 0.001 }
+  - { addr: 45, name: PID_INNER_KD_X1000,      desc: "Inner-loop kd * 1000 (unsigned)", scale: 0.001 }
+
 # -----------------------------------------------------------------------------
 # Holding registers (read-write 16-bit words) -- setpoints and config
 # -----------------------------------------------------------------------------
@@ -104,3 +112,9 @@ holdings:
   - { addr: 2, name: BRIGHTNESS_PCT,          desc: "Display brightness 0..100",         unit: "%" }
   - { addr: 3, name: ALARM_VOLUME_PCT,        desc: "Alarm volume 0..100",               unit: "%" }
   - { addr: 8, name: DODGE_OFFSET_DEG_X100,   desc: "Dodge mode heading offset, deg*100 (signed int16)", unit: "deg", scale: 0.01 }
+
+  # ----- PID inner loop tunable holdings (Sprint 2) -----
+  - { addr: 16, name: PID_INNER_SETPOINT_REQ_X100, desc: "Requested inner-loop rudder setpoint, deg*100 (signed int16)", unit: "deg", scale: 0.01 }
+  - { addr: 17, name: PID_INNER_KP_REQ_X1000,      desc: "Requested inner-loop kp * 1000 (unsigned)", scale: 0.001 }
+  - { addr: 18, name: PID_INNER_KI_REQ_X1000,      desc: "Requested inner-loop ki * 1000 (unsigned)", scale: 0.001 }
+  - { addr: 19, name: PID_INNER_KD_REQ_X1000,      desc: "Requested inner-loop kd * 1000 (unsigned)", scale: 0.001 }
diff --git a/firmware/ar_autopilot_v1/src/main.cpp b/firmware/ar_autopilot_v1/src/main.cpp
index 28260ef..e8bbeda 100644
--- a/firmware/ar_autopilot_v1/src/main.cpp
+++ b/firmware/ar_autopilot_v1/src/main.cpp
@@ -26,6 +26,7 @@
 #include "hal/rudder_actuator.h"
 #include "hal/rudder_sensor.h"
 #include "modes/standby.h"
+#include "pid/pid_inner_task.h"
 #include "protocols/modbus_slave.h"
 #include "protocols/nmea2000_consumer.h"
 #include "safety/safety_monitor.h"
@@ -76,6 +77,12 @@ void setup() {
 
     arautopilot::safety::safety_monitor_start_task();
     arautopilot::hal::rudder_sensor_start_task();
+
+    // PID inner loop (Sprint 2). Active only when not in STANDBY and
+    // rudder sensor reading is valid; refuses internally otherwise.
+    arautopilot::pid::pid_inner_task_init();
+    arautopilot::pid::pid_inner_task_start();
+
     ar_start_heartbeat_task();
 
     // Modbus slave (server) -- exposes telemetry + commands to the display.
diff --git a/firmware/ar_autopilot_v1/src/pid/pid_inner.h b/firmware/ar_autopilot_v1/src/pid/pid_inner.h
new file mode 100644
index 0000000..6249906
--- /dev/null
+++ b/firmware/ar_autopilot_v1/src/pid/pid_inner.h
@@ -0,0 +1,197 @@
+// =============================================================================
+//  pid_inner.h -- inner rudder-position PID (header-only, host-testable)
+// =============================================================================
+//
+//  Line-by-line port of arautopilot/studio/simulator/pid_inner.py. Same
+//  algorithm, same variables, same numerics. The Python module is the
+//  reference; this header must stay byte-equivalent in behaviour. Drift is
+//  caught by the cross-validation test (Python module loaded directly +
+//  C++ compiled via ctypes -> same trajectory within float tolerance).
+//
+//  No Arduino dependencies -- this file compiles on the ESP32 toolchain AND
+//  on host g++/clang/MSVC for native Unity tests.
+// =============================================================================
+
+#pragma once
+
+#include <algorithm>
+#include <cstdint>
+
+namespace arautopilot::pid {
+
+struct PidInnerConfig {
+    // Gains
+    float kp{2.5f};
+    float ki{0.15f};
+    float kd{0.30f};
+
+    // Sampling
+    float freq_hz{50.0f};
+
+    // Setpoint handling
+    float deadband_deg{0.5f};
+    float rate_limit_dps{30.0f};
+
+    // Output saturation
+    float output_min_pct{-100.0f};
+    float output_max_pct{+100.0f};
+
+    // Anti-windup
+    float integral_clamp{30.0f};
+    // aw_gain: if < 0 -> use 1/kp when kp != 0, else 0. Default sentinel -1.
+    float aw_gain{-1.0f};
+
+    // Derivative low-pass
+    float d_lpf_tau_s{0.05f};
+
+    // Actuator non-linearity compensation
+    float deadband_pct{7.0f};
+    float min_useful_pwm_pct{12.0f};
+    float asymmetry_stbd_over_port{1.0f};
+
+    float dt() const { return 1.0f / freq_hz; }
+};
+
+struct PidInnerState {
+    float integral{0.0f};
+    float prev_error{0.0f};
+    float prev_d_term{0.0f};
+    float prev_setpoint_deg{0.0f};
+    float last_output_pct{0.0f};
+};
+
+namespace detail {
+
+inline float clamp_f(float x, float lo, float hi) {
+    if (x < lo) return lo;
+    if (x > hi) return hi;
+    return x;
+}
+
+inline float lpf_alpha(float tau_s, float dt) {
+    if (tau_s <= 0.0f) return 1.0f;
+    return dt / (tau_s + dt);
+}
+
+}  // namespace detail
+
+class PidInner {
+   public:
+    PidInner() = default;
+    explicit PidInner(const PidInnerConfig& config) : config_(config) {}
+
+    void reset(float measured_deg = 0.0f, float setpoint_deg = 0.0f) {
+        state_ = PidInnerState{};
+        state_.prev_setpoint_deg = setpoint_deg;
+        (void)measured_deg;  // kept in signature for parity with Python
+    }
+
+    void update_config(const PidInnerConfig& config) {
+        config_ = config;
+    }
+
+    const PidInnerConfig& config() const { return config_; }
+    const PidInnerState& state() const { return state_; }
+    PidInnerState& mutable_state() { return state_; }
+
+    /// One controller tick. Returns the signed PWM command in percent.
+    /// When ``allowed`` is false the output is forced to zero and the
+    /// integrator bleeds toward zero (anti-windup during manual operation).
+    float step(float setpoint_deg, float measured_deg, bool allowed = true) {
+        const PidInnerConfig& cfg = config_;
+        PidInnerState& st = state_;
+        const float dt = cfg.dt();
+
+        // Rate-limit the setpoint.
+        const float target = rate_limit_setpoint(setpoint_deg);
+        st.prev_setpoint_deg = target;
+
+        // Error with deadband.
+        const float raw_error = target - measured_deg;
+        float error;
+        if (raw_error >= -cfg.deadband_deg && raw_error <= cfg.deadband_deg) {
+            error = 0.0f;
+        } else {
+            const float sign = (raw_error > 0.0f) ? 1.0f : -1.0f;
+            error = raw_error - sign * cfg.deadband_deg;
+        }
+
+        if (!allowed) {
+            st.integral *= 0.95f;
+            st.prev_error = error;
+            st.last_output_pct = 0.0f;
+            return 0.0f;
+        }
+
+        // Proportional.
+        const float p_term = cfg.kp * error;
+
+        // Integral (provisional).
+        st.integral += cfg.ki * error * dt;
+        st.integral = detail::clamp_f(st.integral, -cfg.integral_clamp,
+                                        cfg.integral_clamp);
+
+        // Derivative with low-pass.
+        const float d_raw = (dt > 0.0f)
+                                ? cfg.kd * (error - st.prev_error) / dt
+                                : 0.0f;
+        const float alpha = detail::lpf_alpha(cfg.d_lpf_tau_s, dt);
+        const float d_term = (1.0f - alpha) * st.prev_d_term + alpha * d_raw;
+        st.prev_d_term = d_term;
+
+        const float raw_output = p_term + st.integral + d_term;
+
+        // Saturate + back-calculation anti-windup.
+        const float output = detail::clamp_f(raw_output, cfg.output_min_pct,
+                                              cfg.output_max_pct);
+        if (raw_output != output) {
+            float aw;
+            if (cfg.aw_gain >= 0.0f) {
+                aw = cfg.aw_gain;
+            } else if (cfg.kp != 0.0f) {
+                aw = 1.0f / cfg.kp;
+            } else {
+                aw = 0.0f;
+            }
+            st.integral -= aw * (raw_output - output) * dt;
+            st.integral = detail::clamp_f(st.integral, -cfg.integral_clamp,
+                                            cfg.integral_clamp);
+        }
+
+        const float cmd = compensate(output);
+
+        st.prev_error = error;
+        st.last_output_pct = cmd;
+        return cmd;
+    }
+
+   private:
+    float rate_limit_setpoint(float requested_deg) const {
+        const float max_delta = config_.rate_limit_dps * config_.dt();
+        const float delta = requested_deg - state_.prev_setpoint_deg;
+        if (delta > max_delta) return state_.prev_setpoint_deg + max_delta;
+        if (delta < -max_delta) return state_.prev_setpoint_deg - max_delta;
+        return requested_deg;
+    }
+
+    float compensate(float raw_pct) const {
+        const PidInnerConfig& cfg = config_;
+        if (raw_pct == 0.0f) return 0.0f;
+        float magnitude = (raw_pct < 0.0f) ? -raw_pct : raw_pct;
+        if (magnitude <= cfg.deadband_pct) return 0.0f;
+        if (magnitude < cfg.min_useful_pwm_pct) magnitude = cfg.min_useful_pwm_pct;
+        const float sign = (raw_pct > 0.0f) ? 1.0f : -1.0f;
+        float cmd = sign * magnitude;
+        if (cmd > 0.0f && cfg.asymmetry_stbd_over_port != 0.0f) {
+            cmd /= cfg.asymmetry_stbd_over_port;
+        } else if (cmd < 0.0f) {
+            cmd *= cfg.asymmetry_stbd_over_port;
+        }
+        return detail::clamp_f(cmd, cfg.output_min_pct, cfg.output_max_pct);
+    }
+
+    PidInnerConfig config_{};
+    PidInnerState state_{};
+};
+
+}  // namespace arautopilot::pid
diff --git a/firmware/ar_autopilot_v1/src/pid/pid_inner_task.cpp b/firmware/ar_autopilot_v1/src/pid/pid_inner_task.cpp
new file mode 100644
index 0000000..abf89e6
--- /dev/null
+++ b/firmware/ar_autopilot_v1/src/pid/pid_inner_task.cpp
@@ -0,0 +1,136 @@
+// =============================================================================
+//  pid_inner_task.cpp -- 50 Hz inner-loop control task (Sprint 2)
+// =============================================================================
+
+#include "pid_inner_task.h"
+
+#include <Arduino.h>
+
+#include "../hal/rudder_actuator.h"
+#include "../hal/rudder_sensor.h"
+#include "../modes/standby.h"
+#include "../safety/watchdog.h"
+#include "../system/ar_log.h"
+#include "../system/task_config.h"
+
+namespace arautopilot::pid {
+
+namespace {
+constexpr const char* TAG = "AR/PID";
+
+portMUX_TYPE g_mux = portMUX_INITIALIZER_UNLOCKED;
+PidInner g_pid{};
+float g_setpoint_deg = 0.0f;
+float g_last_output_pct = 0.0f;
+float g_last_error_deg = 0.0f;
+
+void InnerLoopTask(void* /*pv*/) {
+    AR_LOGI(TAG, "pid_inner task started on core %d (50 Hz)", xPortGetCoreID());
+    safety::watchdog_subscribe_current_task();
+
+    TickType_t last_wake = xTaskGetTickCount();
+    for (;;) {
+        // Snapshot what the operator/outer loop wants.
+        float setpoint;
+        portENTER_CRITICAL(&g_mux);
+        setpoint = g_setpoint_deg;
+        portEXIT_CRITICAL(&g_mux);
+
+        // Read the current rudder position.
+        const auto rd = hal::rudder_sensor_latest();
+
+        // Only run the controller if we have a valid reading and we are not
+        // in STANDBY. The PID's `allowed` parameter cleanly bleeds the
+        // integrator while disengaged.
+        const bool allowed = rd.valid && !modes::is_standby();
+        const float cmd = g_pid.step(setpoint, rd.angle_deg, allowed);
+
+        // Send to the actuator. rudder_command() carries its own safety
+        // interlocks (power, mode, limit switches) and will refuse if
+        // anything is off, so we don't replicate them here.
+        const int8_t pwm_pct =
+            (cmd > 127.0f) ? 127 : (cmd < -127.0f) ? -127 : (int8_t)cmd;
+        hal::rudder_command(pwm_pct);
+
+        // Publish for Modbus.
+        portENTER_CRITICAL(&g_mux);
+        g_last_output_pct = cmd;
+        g_last_error_deg = setpoint - rd.angle_deg;
+        portEXIT_CRITICAL(&g_mux);
+
+        safety::watchdog_feed();
+        vTaskDelayUntil(&last_wake, pdMS_TO_TICKS(AR_PERIOD_MS_PID_INNER));
+    }
+}
+
+}  // namespace
+
+void pid_inner_task_init() {
+    PidInnerConfig cfg;
+    // Inherit seed defaults from PidInnerConfig (which match the 30 m
+    // yacht_motor_planeo profile). Real production gains arrive via the
+    // .appack at deployment time (Sprint 4) and via Modbus hot-swap at
+    // commissioning (Sprint 7).
+    g_pid.update_config(cfg);
+    g_pid.reset(0.0f, 0.0f);
+    AR_LOGI(TAG,
+            "pid_inner_init: kp=%.3f ki=%.3f kd=%.3f freq=%.1f Hz "
+            "deadband=%.2f deg rate_lim=%.1f dps",
+            cfg.kp, cfg.ki, cfg.kd, cfg.freq_hz, cfg.deadband_deg,
+            cfg.rate_limit_dps);
+}
+
+void pid_inner_task_start() {
+    xTaskCreatePinnedToCore(InnerLoopTask, "pid_inner", AR_TASK_STACK_PID_INNER,
+                            nullptr, AR_TASK_PRIO_PID_INNER, nullptr,
+                            AR_TASK_CORE_REALTIME);
+}
+
+void pid_inner_set_setpoint_deg(float setpoint_deg) {
+    portENTER_CRITICAL(&g_mux);
+    g_setpoint_deg = setpoint_deg;
+    portEXIT_CRITICAL(&g_mux);
+}
+
+float pid_inner_setpoint_deg() {
+    portENTER_CRITICAL(&g_mux);
+    float v = g_setpoint_deg;
+    portEXIT_CRITICAL(&g_mux);
+    return v;
+}
+
+float pid_inner_last_output_pct() {
+    portENTER_CRITICAL(&g_mux);
+    float v = g_last_output_pct;
+    portEXIT_CRITICAL(&g_mux);
+    return v;
+}
+
+float pid_inner_last_error_deg() {
+    portENTER_CRITICAL(&g_mux);
+    float v = g_last_error_deg;
+    portEXIT_CRITICAL(&g_mux);
+    return v;
+}
+
+void pid_inner_update_gains(float kp, float ki, float kd) {
+    PidInnerConfig cfg = g_pid.config();
+    cfg.kp = kp;
+    cfg.ki = ki;
+    cfg.kd = kd;
+    portENTER_CRITICAL(&g_mux);
+    g_pid.update_config(cfg);
+    portEXIT_CRITICAL(&g_mux);
+    AR_LOGI(TAG, "pid_inner gains updated: kp=%.3f ki=%.3f kd=%.3f", kp, ki, kd);
+}
+
+void pid_inner_get_gains(float& kp, float& ki, float& kd) {
+    portENTER_CRITICAL(&g_mux);
+    const auto& cfg = g_pid.config();
+    kp = cfg.kp;
+    ki = cfg.ki;
+    kd = cfg.kd;
+    portEXIT_CRITICAL(&g_mux);
+}
+
+}  // namespace arautopilot::pid
diff --git a/firmware/ar_autopilot_v1/src/pid/pid_inner_task.h b/firmware/ar_autopilot_v1/src/pid/pid_inner_task.h
new file mode 100644
index 0000000..477d196
--- /dev/null
+++ b/firmware/ar_autopilot_v1/src/pid/pid_inner_task.h
@@ -0,0 +1,45 @@
+// =============================================================================
+//  pid_inner_task.h -- 50 Hz inner-loop control task (Sprint 2)
+// =============================================================================
+//
+//  Wraps the header-only PidInner controller in a FreeRTOS task pinned to
+//  Core 1 (real-time core). Reads the rudder position from hal::rudder_sensor,
+//  consumes the setpoint that the outer loop / Modbus client wrote, and
+//  commands hal::rudder_actuator. Refuses to act in STANDBY or with master
+//  power off (those interlocks live in hal::rudder_command itself).
+// =============================================================================
+
+#pragma once
+
+#include "pid_inner.h"
+
+namespace arautopilot::pid {
+
+/// Initialise the controller with the seed gains. Must be called from
+/// setup() once.
+void pid_inner_task_init();
+
+/// Spawn the FreeRTOS task. Must be called after pid_inner_task_init().
+void pid_inner_task_start();
+
+/// Update the setpoint that the inner loop pursues. Called by the Modbus
+/// slave (when an operator writes a holding register) and, later, by the
+/// outer loop task. Units: degrees, signed.
+void pid_inner_set_setpoint_deg(float setpoint_deg);
+
+/// Read the current setpoint (thread-safe).
+float pid_inner_setpoint_deg();
+
+/// Read the latest PID output command (signed PWM percent).
+float pid_inner_last_output_pct();
+
+/// Read the latest error (deg) the controller saw.
+float pid_inner_last_error_deg();
+
+/// Hot-swap gains at runtime (thread-safe).
+void pid_inner_update_gains(float kp, float ki, float kd);
+
+/// Read the gains currently in use.
+void pid_inner_get_gains(float& kp, float& ki, float& kd);
+
+}  // namespace arautopilot::pid
diff --git a/firmware/ar_autopilot_v1/src/protocols/modbus_registers.h b/firmware/ar_autopilot_v1/src/protocols/modbus_registers.h
index cc34fc2..f06e12b 100644
--- a/firmware/ar_autopilot_v1/src/protocols/modbus_registers.h
+++ b/firmware/ar_autopilot_v1/src/protocols/modbus_registers.h
@@ -77,8 +77,8 @@ constexpr uint16_t COIL_CMD_ACK_ALL_ALARMS = 2;
 constexpr uint16_t COIL_CMD_KNOB_ARM = 3;
 
 // ----- Input registers (read-only words) -----
-constexpr uint16_t INPUT_COUNT = 17;
-constexpr uint16_t INPUT_MAX_ADDR = 33;
+constexpr uint16_t INPUT_COUNT = 23;
+constexpr uint16_t INPUT_MAX_ADDR = 45;
 
 // Firmware major version
 constexpr uint16_t INPUT_FW_VERSION_MAJOR = 0;
@@ -125,10 +125,28 @@ constexpr uint16_t INPUT_BATTERY_VOLTAGE_X100 = 32;
 // Actuator current, A*100
 // unit=A, scale=0.01
 constexpr uint16_t INPUT_ACTUATOR_CURRENT_X100 = 33;
+// Inner-loop rudder setpoint, deg*100 (signed int16)
+// unit=deg, scale=0.01
+constexpr uint16_t INPUT_PID_INNER_SETPOINT_X100 = 40;
+// Last PID command, %*100 (signed int16, -10000..+10000)
+// unit=%, scale=0.01
+constexpr uint16_t INPUT_PID_INNER_OUTPUT_X100 = 41;
+// Last PID error, deg*100 (signed int16)
+// unit=deg, scale=0.01
+constexpr uint16_t INPUT_PID_INNER_ERROR_X100 = 42;
+// Inner-loop kp * 1000 (unsigned)
+// scale=0.001
+constexpr uint16_t INPUT_PID_INNER_KP_X1000 = 43;
+// Inner-loop ki * 1000 (unsigned)
+// scale=0.001
+constexpr uint16_t INPUT_PID_INNER_KI_X1000 = 44;
+// Inner-loop kd * 1000 (unsigned)
+// scale=0.001
+constexpr uint16_t INPUT_PID_INNER_KD_X1000 = 45;
 
 // ----- Holding registers (read-write words) -----
-constexpr uint16_t HOLDING_COUNT = 5;
-constexpr uint16_t HOLDING_MAX_ADDR = 8;
+constexpr uint16_t HOLDING_COUNT = 9;
+constexpr uint16_t HOLDING_MAX_ADDR = 19;
 
 // Mode requested by operator (0=STANDBY,1=HH,2=TC,3=TK,4=DODGE)
 constexpr uint16_t HOLDING_MODE_REQUEST = 0;
@@ -144,5 +162,17 @@ constexpr uint16_t HOLDING_ALARM_VOLUME_PCT = 3;
 // Dodge mode heading offset, deg*100 (signed int16)
 // unit=deg, scale=0.01
 constexpr uint16_t HOLDING_DODGE_OFFSET_DEG_X100 = 8;
+// Requested inner-loop rudder setpoint, deg*100 (signed int16)
+// unit=deg, scale=0.01
+constexpr uint16_t HOLDING_PID_INNER_SETPOINT_REQ_X100 = 16;
+// Requested inner-loop kp * 1000 (unsigned)
+// scale=0.001
+constexpr uint16_t HOLDING_PID_INNER_KP_REQ_X1000 = 17;
+// Requested inner-loop ki * 1000 (unsigned)
+// scale=0.001
+constexpr uint16_t HOLDING_PID_INNER_KI_REQ_X1000 = 18;
+// Requested inner-loop kd * 1000 (unsigned)
+// scale=0.001
+constexpr uint16_t HOLDING_PID_INNER_KD_REQ_X1000 = 19;
 
 }  // namespace arautopilot::protocols::modbus
diff --git a/firmware/ar_autopilot_v1/src/protocols/modbus_slave.cpp b/firmware/ar_autopilot_v1/src/protocols/modbus_slave.cpp
index 971591b..be01e9b 100644
--- a/firmware/ar_autopilot_v1/src/protocols/modbus_slave.cpp
+++ b/firmware/ar_autopilot_v1/src/protocols/modbus_slave.cpp
@@ -27,6 +27,7 @@
 #include "../hal/pinout.h"
 #include "../hal/rudder_sensor.h"
 #include "../modes/standby.h"
+#include "../pid/pid_inner_task.h"
 #include "../system/ar_log.h"
 #include "../system/task_config.h"
 #include "modbus_registers.h"
@@ -55,6 +56,10 @@ struct HoldingStorage {
     uint16_t brightness_pct = 80;
     uint16_t alarm_volume_pct = 60;
     int16_t  dodge_offset_deg_x100 = 0;
+    int16_t  pid_inner_setpoint_req_x100 = 0;
+    uint16_t pid_inner_kp_req_x1000 = 0;
+    uint16_t pid_inner_ki_req_x1000 = 0;
+    uint16_t pid_inner_kd_req_x1000 = 0;
 };
 HoldingStorage g_holding;
 
@@ -124,6 +129,40 @@ uint16_t read_input_register(uint16_t addr) {
         case INPUT_ACTUATOR_CURRENT_X100:
             return 0;
 
+        // ----- PID inner-loop telemetry (Sprint 2) -----
+        case INPUT_PID_INNER_SETPOINT_X100: {
+            int v = (int)(pid::pid_inner_setpoint_deg() * 100.0f);
+            if (v < -32768) v = -32768;
+            if (v >  32767) v =  32767;
+            return (uint16_t)(int16_t)v;
+        }
+        case INPUT_PID_INNER_OUTPUT_X100: {
+            int v = (int)(pid::pid_inner_last_output_pct() * 100.0f);
+            if (v < -32768) v = -32768;
+            if (v >  32767) v =  32767;
+            return (uint16_t)(int16_t)v;
+        }
+        case INPUT_PID_INNER_ERROR_X100: {
+            int v = (int)(pid::pid_inner_last_error_deg() * 100.0f);
+            if (v < -32768) v = -32768;
+            if (v >  32767) v =  32767;
+            return (uint16_t)(int16_t)v;
+        }
+        case INPUT_PID_INNER_KP_X1000:
+        case INPUT_PID_INNER_KI_X1000:
+        case INPUT_PID_INNER_KD_X1000: {
+            float kp, ki, kd;
+            pid::pid_inner_get_gains(kp, ki, kd);
+            float v;
+            if (addr == INPUT_PID_INNER_KP_X1000)      v = kp;
+            else if (addr == INPUT_PID_INNER_KI_X1000) v = ki;
+            else                                        v = kd;
+            int scaled = (int)(v * 1000.0f);
+            if (scaled < 0) scaled = 0;
+            if (scaled > 65535) scaled = 65535;
+            return (uint16_t)scaled;
+        }
+
         default:
             return 0;
     }
@@ -164,6 +203,10 @@ uint16_t read_holding(uint16_t addr) {
         case HOLDING_BRIGHTNESS_PCT:         return g_holding.brightness_pct;
         case HOLDING_ALARM_VOLUME_PCT:       return g_holding.alarm_volume_pct;
         case HOLDING_DODGE_OFFSET_DEG_X100:  return (uint16_t)g_holding.dodge_offset_deg_x100;
+        case HOLDING_PID_INNER_SETPOINT_REQ_X100: return (uint16_t)g_holding.pid_inner_setpoint_req_x100;
+        case HOLDING_PID_INNER_KP_REQ_X1000: return g_holding.pid_inner_kp_req_x1000;
+        case HOLDING_PID_INNER_KI_REQ_X1000: return g_holding.pid_inner_ki_req_x1000;
+        case HOLDING_PID_INNER_KD_REQ_X1000: return g_holding.pid_inner_kd_req_x1000;
         default:                             return 0;
     }
 }
@@ -190,6 +233,39 @@ Modbus::Error write_holding(uint16_t addr, uint16_t value) {
         case HOLDING_DODGE_OFFSET_DEG_X100:
             g_holding.dodge_offset_deg_x100 = (int16_t)value;
             return Modbus::Error::SUCCESS;
+
+        // ----- PID inner-loop tunables (Sprint 2) -----
+        case HOLDING_PID_INNER_SETPOINT_REQ_X100: {
+            int16_t sv = (int16_t)value;
+            g_holding.pid_inner_setpoint_req_x100 = sv;
+            pid::pid_inner_set_setpoint_deg((float)sv * 0.01f);
+            return Modbus::Error::SUCCESS;
+        }
+        case HOLDING_PID_INNER_KP_REQ_X1000:
+        case HOLDING_PID_INNER_KI_REQ_X1000:
+        case HOLDING_PID_INNER_KD_REQ_X1000: {
+            // Update the requested-gain shadow, then push all three to the
+            // live controller. We do all three together so partial writes
+            // don't leave the gains inconsistent.
+            if (addr == HOLDING_PID_INNER_KP_REQ_X1000) {
+                g_holding.pid_inner_kp_req_x1000 = value;
+            } else if (addr == HOLDING_PID_INNER_KI_REQ_X1000) {
+                g_holding.pid_inner_ki_req_x1000 = value;
+            } else {
+                g_holding.pid_inner_kd_req_x1000 = value;
+            }
+            float kp = (float)g_holding.pid_inner_kp_req_x1000 * 0.001f;
+            float ki = (float)g_holding.pid_inner_ki_req_x1000 * 0.001f;
+            float kd = (float)g_holding.pid_inner_kd_req_x1000 * 0.001f;
+            // Refuse zero kp -- the rest of the algorithm assumes kp > 0
+            // for back-calculation anti-windup. If the operator writes 0
+            // we ignore it (leave whatever the firmware booted with).
+            if (kp <= 0.0f) {
+                return Modbus::Error::ILLEGAL_DATA_VALUE;
+            }
+            pid::pid_inner_update_gains(kp, ki, kd);
+            return Modbus::Error::SUCCESS;
+        }
         default:
             return Modbus::Error::ILLEGAL_DATA_ADDRESS;
     }