diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 706afae..cb11382 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -49,6 +49,25 @@ jobs:
           echo "coverage=$pct" >> "$GITHUB_OUTPUT"
           echo "tests passed: $passed | sim line coverage: ${pct}%"
 
+  mcdc:
+    # 100% MC/DC (condition coverage) enforced on every Tier-1 driver harness.
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: install gcc-14
+        run: sudo apt-get update -qq && sudo apt-get install -y -qq gcc-14
+      - name: enforce 100% MC/DC on drivers/*/test
+        run: |
+          fail=0; ran=0
+          for t in drivers/*/test; do
+            [ -f "$t/Makefile" ] || continue
+            ran=1
+            echo "== MC/DC: $t =="
+            make -C "$t" check CC=gcc-14 GCOV=gcov-14 || fail=1
+          done
+          [ "$ran" = 1 ] || { echo "no driver MC/DC harnesses found"; exit 1; }
+          exit $fail
+
   badges:
     needs: [test]
     if: github.event_name == 'push' && github.ref == 'refs/heads/bringup'
diff --git a/.gitignore b/.gitignore
index ffe69c5..dd0a7de 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,3 +11,6 @@ target/
 *.swp
 *~
 .DS_Store
+
+# driver MC/DC harness build dirs
+**/test/build/
diff --git a/drivers/relays/relays.c b/drivers/relays/relays.c
new file mode 100644
index 0000000..924b7be
--- /dev/null
+++ b/drivers/relays/relays.c
@@ -0,0 +1,132 @@
+#include "relays.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+
+/* GPIO1_A0 and GPIO1_A1; gpiochip1 base is 32. See relays.h. */
+static const int  s_gpio[WARDEN_RELAY_COUNT] = { 32, 33 };
+static const char * const s_name[WARDEN_RELAY_COUNT] = { "Relay 1", "Relay 2" };
+
+/* ---- default sysfs backend (Tier-2 plumbing; covered by integration tests) ---- */
+
+static const char * gpio_root(void)
+{
+    const char * r = getenv("WARDEN_GPIO_ROOT");
+    return (r && *r) ? r : "/sys/class/gpio";
+}
+
+static bool sysfs_exists(const char * path)
+{
+    struct stat st;
+    return stat(path, &st) == 0;
+}
+
+static void sysfs_write(const char * path, const char * value)
+{
+    FILE * f = fopen(path, "w");
+    if(!f) return;
+    fputs(value, f);
+    fclose(f);
+}
+
+static bool sysfs_read_line(const char * path, char * buf, size_t n)
+{
+    FILE * f = fopen(path, "r");
+    if(!f) return false;
+    bool ok = fgets(buf, (int)n, f) != NULL;
+    fclose(f);
+    return ok;
+}
+
+static bool sysfs_read_int(const char * path, int * out)
+{
+    FILE * f = fopen(path, "r");
+    if(!f) return false;
+    bool ok = fscanf(f, "%d", out) == 1;
+    fclose(f);
+    return ok;
+}
+
+static const struct relay_io s_sysfs_io = {
+    sysfs_exists, sysfs_write, sysfs_read_line, sysfs_read_int
+};
+
+static const struct relay_io * s_io = &s_sysfs_io;
+
+void warden_relay__set_io(const struct relay_io * io) { s_io = io ? io : &s_sysfs_io; }
+
+/* ---- decision logic (Tier-1; measured to 100% MC/DC via the fake io) ---- */
+
+const char * warden_relay_name(uint32_t idx)
+{
+    return idx < WARDEN_RELAY_COUNT ? s_name[idx] : "-";
+}
+
+static bool exported(int gpio)
+{
+    char p[80];
+    snprintf(p, sizeof(p), "%s/gpio%d", gpio_root(), gpio);
+    return s_io->exists(p);
+}
+
+/**
+ * Export and set the direction, without disturbing the level. "out" on an
+ * unexported pin latches the kernel's default low, which would drop a closed
+ * relay the first time this page is opened; reading the current level first and
+ * writing it straight back makes the export transparent.
+ */
+static bool ensure_ready(int gpio)
+{
+    if(!exported(gpio)) {
+        char ex[80], n[16];
+        snprintf(ex, sizeof(ex), "%s/export", gpio_root());
+        snprintf(n, sizeof(n), "%d", gpio);
+        s_io->write(ex, n);
+        if(!exported(gpio)) return false;
+    }
+
+    char p[96];
+    snprintf(p, sizeof(p), "%s/gpio%d/direction", gpio_root(), gpio);
+    char dir[16] = "";
+    if(!s_io->read_line(p, dir, sizeof(dir))) return false;
+
+    if(strncmp(dir, "out", 3) != 0) {
+        /* Preserve the level across the switch to output. */
+        char vp[96];
+        int cur;
+        snprintf(vp, sizeof(vp), "%s/gpio%d/value", gpio_root(), gpio);
+        if(!s_io->read_int(vp, &cur)) cur = 0;
+        s_io->write(p, cur ? "high" : "low");
+    }
+    return true;
+}
+
+bool warden_relay_available(uint32_t idx)
+{
+    if(idx >= WARDEN_RELAY_COUNT) return false;
+    return ensure_ready(s_gpio[idx]);
+}
+
+bool warden_relay_get(uint32_t idx)
+{
+    if(idx >= WARDEN_RELAY_COUNT) return false;
+    if(!exported(s_gpio[idx])) return false;
+
+    char p[96];
+    int v;
+    snprintf(p, sizeof(p), "%s/gpio%d/value", gpio_root(), s_gpio[idx]);
+    if(!s_io->read_int(p, &v)) v = 0;
+    return v != 0;
+}
+
+void warden_relay_set(uint32_t idx, bool on)
+{
+    if(idx >= WARDEN_RELAY_COUNT) return;
+    if(!ensure_ready(s_gpio[idx])) return;
+
+    char p[96];
+    snprintf(p, sizeof(p), "%s/gpio%d/value", gpio_root(), s_gpio[idx]);
+    s_io->write(p, on ? "1" : "0");
+}
diff --git a/drivers/relays/relays.h b/drivers/relays/relays.h
new file mode 100644
index 0000000..0ea97a0
--- /dev/null
+++ b/drivers/relays/relays.h
@@ -0,0 +1,55 @@
+/**
+ * The two onboard relays (86-Panel bottom board).
+ *
+ *     RELAY1  <-  GPIO1_A0   (sysfs gpio 32)
+ *     RELAY2  <-  GPIO1_A1   (sysfs gpio 33)
+ *
+ * gpiochip1 base is 32; A0..A7 are offsets 0..7. Nothing in the DT claims these
+ * (there is no relay driver), so we drive them through /sys/class/gpio directly.
+ * An off-by-one here would toggle the neighbouring RS485 pair (GPIO1_B0/B1)
+ * under a running Modbus master, so the numbers come from the schematic, not
+ * inference. State is never assumed at startup: a relay may be holding a
+ * contactor closed, and deciding it should be off because we just booted is not
+ * this module's call.
+ *
+ * Hardened for warden-sdk: the sysfs plumbing sits behind a `relay_io` seam so
+ * the decision logic runs and is measured to 100% MC/DC on the host, and the
+ * gpio root is `$WARDEN_GPIO_ROOT`-overridable so the real backend can run
+ * against a scratch tree in an integration test.
+ */
+#ifndef WARDEN_RELAYS_H
+#define WARDEN_RELAYS_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#define WARDEN_RELAY_COUNT 2
+
+/** Display name for @p idx, e.g. "Relay 1"; "-" if out of range. */
+const char * warden_relay_name(uint32_t idx);
+
+/** Current output state, false if it cannot be read. */
+bool warden_relay_get(uint32_t idx);
+
+/** Drive the output. Exports and sets the direction on first use. */
+void warden_relay_set(uint32_t idx, bool on);
+
+/** True if the GPIO is exported and usable — the page says so if it is not. */
+bool warden_relay_available(uint32_t idx);
+
+/* --- Hardware-abstraction seam ---------------------------------------------
+ * The four filesystem primitives the relay logic needs. Production binds the
+ * sysfs backend (the default); a unit test binds an in-memory fake that can
+ * model export -> node-appears, which a passive scratch tree cannot. */
+struct relay_io {
+    bool (*exists)(const char *path);                      /* stat(path)==0 */
+    void (*write)(const char *path, const char *value);    /* best-effort */
+    bool (*read_line)(const char *path, char *buf, size_t n); /* a line was read */
+    bool (*read_int)(const char *path, int *out);          /* one int parsed */
+};
+
+/** Swap the io backend (test hook). Pass NULL to restore the sysfs default. */
+void warden_relay__set_io(const struct relay_io *io);
+
+#endif /* WARDEN_RELAYS_H */
diff --git a/drivers/relays/test/Makefile b/drivers/relays/test/Makefile
new file mode 100644
index 0000000..58ed59e
--- /dev/null
+++ b/drivers/relays/test/Makefile
@@ -0,0 +1,41 @@
+# MC/DC unit harness for drivers/relays/relays.c.
+#
+#   make check   — build, run, and FAIL unless relays.c hits 100% MC/DC
+#                  (condition) coverage and every unit check passes.
+#   make report  — per-condition gcov annotation (build/relays.c.gcov).
+#   make clean
+#
+# Requires gcc >= 14 (for -fcondition-coverage) and its matching gcov.
+CC      ?= gcc
+GCOV    ?= gcov
+CFLAGS  := -O0 -g -Wall -Wextra -I..
+COVFLAGS := --coverage -fcondition-coverage
+BUILD   := build
+
+.PHONY: check report clean
+.DEFAULT_GOAL := check
+
+$(BUILD):
+	@mkdir -p $(BUILD)
+
+# relays.c carries the coverage flags; the test driver is plain.
+$(BUILD)/test: test_relays.c ../relays.c ../relays.h | $(BUILD)
+	@$(CC) $(CFLAGS) $(COVFLAGS) -c ../relays.c -o $(BUILD)/relays.o
+	@$(CC) $(CFLAGS) -c test_relays.c -o $(BUILD)/test_relays.o
+	@$(CC) $(COVFLAGS) $(BUILD)/relays.o $(BUILD)/test_relays.o -o $(BUILD)/test
+
+check: $(BUILD)/test
+	@echo "== running relays MC/DC harness =="
+	@rm -f $(BUILD)/relays.gcda
+	@$(BUILD)/test; echo $$? > $(BUILD)/test.rc
+	@echo
+	@echo "== MC/DC (condition) coverage of relays.c =="
+	@$(GCOV) --conditions --branch-probabilities -o $(BUILD) ../relays.c >$(BUILD)/gcov.log 2>&1 || true
+	@mv -f *.gcov $(BUILD)/ 2>/dev/null || true
+	@bash enforce-mcdc.sh $(BUILD)/gcov.log $(BUILD)/relays.c.gcov $(BUILD)/test.rc
+
+report: check
+	@grep -nE "condition.*not covered|conditions covered" $(BUILD)/relays.c.gcov || true
+
+clean:
+	@rm -rf $(BUILD)
diff --git a/drivers/relays/test/enforce-mcdc.sh b/drivers/relays/test/enforce-mcdc.sh
new file mode 100755
index 0000000..4fdb4ed
--- /dev/null
+++ b/drivers/relays/test/enforce-mcdc.sh
@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# Gate: fail unless every unit check passed AND relays.c reached 100% MC/DC
+# (condition) coverage with no uncovered executable lines.
+#   $1 = gcov stdout log   $2 = relays.c.gcov   $3 = test exit-code file
+set -uo pipefail
+LOG="$1"; GCOV="$2"; RCFILE="$3"
+rc=0
+
+testrc="$(cat "$RCFILE" 2>/dev/null || echo 1)"
+if [ "$testrc" != "0" ]; then
+	echo "RESULT: unit checks FAILED (test exit $testrc)"; rc=1
+else
+	echo "RESULT: all unit checks passed"
+fi
+
+if [ ! -f "$GCOV" ]; then
+	echo "RESULT: no coverage file ($GCOV) produced"; exit 1
+fi
+
+notcov="$(grep -nE "condition[s]? .*not covered" "$GCOV" || true)"
+uncov_lines="$(grep -nE "^ +#####:" "$GCOV" || true)"
+
+# Summary lines for relays.c from gcov stdout. Match the exact file so
+# 'test_relays.c' (which also contains "relays.c") is NOT picked up.
+FMATCH="File '([^']*/)?relays[.]c'"
+cond_line="$(awk -v patt="$FMATCH" '$0 ~ patt {f=1} f&&/Condition outcomes covered:/{print; f=0}' "$LOG")"
+line_line="$(awk -v patt="$FMATCH" '$0 ~ patt {f=1} f&&/Lines executed:/{print; f=0}' "$LOG")"
+echo "  ${line_line:-Lines executed: (n/a)}"
+echo "  ${cond_line:-Condition outcomes covered: (n/a)}"
+
+if [ -n "$notcov" ]; then
+	echo "RESULT: MC/DC gaps (conditions not covered):"
+	echo "$notcov" | sed 's/^/    /'
+	rc=1
+fi
+if [ -n "$uncov_lines" ]; then
+	echo "RESULT: uncovered executable lines in relays.c:"
+	echo "$uncov_lines" | sed 's/^/    /'
+	rc=1
+fi
+
+if ! echo "$cond_line" | grep -q "100.00%"; then
+	echo "RESULT: condition coverage is below 100%"
+	rc=1
+fi
+
+[ "$rc" = "0" ] && echo "RESULT: 100% MC/DC + all checks green ✓"
+exit "$rc"
diff --git a/drivers/relays/test/test_relays.c b/drivers/relays/test/test_relays.c
new file mode 100644
index 0000000..1ef90dd
--- /dev/null
+++ b/drivers/relays/test/test_relays.c
@@ -0,0 +1,218 @@
+/* MC/DC harness for drivers/relays/relays.c.
+ *
+ * Two layers, one binary, so the combined run covers every decision in relays.c:
+ *   1. unit tests through a FAKE relay_io — exercise the decision logic, incl.
+ *      the export->node-appears path a passive tree cannot model.
+ *   2. integration tests through the real sysfs backend + $WARDEN_GPIO_ROOT
+ *      pointed at a scratch tree — exercise the backend's fopen/stat branches.
+ */
+#include "../relays.h"
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+static int g_fail = 0, g_checks = 0;
+#define EXPECT(cond) do { \
+    g_checks++; \
+    if(!(cond)) { g_fail++; fprintf(stderr, "FAIL %s:%d: %s\n", __FILE__, __LINE__, #cond); } \
+} while(0)
+
+/* ---------------- layer 1: fake io ---------------- */
+
+static struct {
+    bool exists_ret;
+    bool export_makes_exist;   /* write(.../export) flips exists_ret true */
+    bool read_line_ok;
+    const char *read_line_val;
+    bool read_int_ok;
+    int read_int_val;
+    char last_write_path[160];
+    char last_write_val[32];
+} fk;
+
+static bool fk_exists(const char *p) { (void)p; return fk.exists_ret; }
+static void fk_write(const char *p, const char *v) {
+    snprintf(fk.last_write_path, sizeof fk.last_write_path, "%s", p);
+    snprintf(fk.last_write_val, sizeof fk.last_write_val, "%s", v);
+    if(fk.export_makes_exist && strstr(p, "/export")) fk.exists_ret = true;
+}
+static bool fk_read_line(const char *p, char *b, size_t n) {
+    (void)p;
+    if(!fk.read_line_ok) return false;
+    snprintf(b, n, "%s", fk.read_line_val);
+    return true;
+}
+static bool fk_read_int(const char *p, int *o) {
+    (void)p;
+    if(!fk.read_int_ok) return false;
+    *o = fk.read_int_val;
+    return true;
+}
+static const struct relay_io FAKE = { fk_exists, fk_write, fk_read_line, fk_read_int };
+
+static void fk_reset(void) { memset(&fk, 0, sizeof fk); }
+
+static void unit_tests(void)
+{
+    warden_relay__set_io(&FAKE);
+
+    /* warden_relay_name: both sides of idx < COUNT */
+    EXPECT(strcmp(warden_relay_name(0), "Relay 1") == 0);
+    EXPECT(strcmp(warden_relay_name(WARDEN_RELAY_COUNT), "-") == 0);
+
+    /* available: idx >= COUNT (true) */
+    EXPECT(warden_relay_available(WARDEN_RELAY_COUNT) == false);
+
+    /* ensure_ready: not exported -> export FAILS (inner if true) */
+    fk_reset();
+    fk.exists_ret = false; fk.export_makes_exist = false;
+    EXPECT(warden_relay_available(0) == false);
+    EXPECT(strstr(fk.last_write_path, "/export") != NULL); /* export was attempted */
+
+    /* ensure_ready: not exported -> export SUCCEEDS -> direction read FAILS */
+    fk_reset();
+    fk.exists_ret = false; fk.export_makes_exist = true; fk.read_line_ok = false;
+    EXPECT(warden_relay_available(0) == false);
+
+    /* already exported -> direction == "out" -> no level-preserve write */
+    fk_reset();
+    fk.exists_ret = true; fk.read_line_ok = true; fk.read_line_val = "out\n";
+    EXPECT(warden_relay_available(0) == true);
+
+    /* exported -> direction "in" -> read_int FAILS -> cur=0 -> write "low" */
+    fk_reset();
+    fk.exists_ret = true; fk.read_line_ok = true; fk.read_line_val = "in\n";
+    fk.read_int_ok = false;
+    EXPECT(warden_relay_available(0) == true);
+    EXPECT(strcmp(fk.last_write_val, "low") == 0);
+
+    /* exported -> direction "in" -> read_int OK, cur=0 -> ternary false -> "low" */
+    fk_reset();
+    fk.exists_ret = true; fk.read_line_ok = true; fk.read_line_val = "in\n";
+    fk.read_int_ok = true; fk.read_int_val = 0;
+    EXPECT(warden_relay_available(0) == true);
+    EXPECT(strcmp(fk.last_write_val, "low") == 0);
+
+    /* exported -> direction "in" -> read_int OK, cur=1 -> ternary true -> "high" */
+    fk_reset();
+    fk.exists_ret = true; fk.read_line_ok = true; fk.read_line_val = "in\n";
+    fk.read_int_ok = true; fk.read_int_val = 1;
+    EXPECT(warden_relay_available(0) == true);
+    EXPECT(strcmp(fk.last_write_val, "high") == 0);
+
+    /* get: idx >= COUNT (true) */
+    EXPECT(warden_relay_get(WARDEN_RELAY_COUNT) == false);
+
+    /* get: not exported -> false */
+    fk_reset(); fk.exists_ret = false;
+    EXPECT(warden_relay_get(0) == false);
+
+    /* get: exported, read_int FAILS -> v=0 -> v!=0 false */
+    fk_reset(); fk.exists_ret = true; fk.read_int_ok = false;
+    EXPECT(warden_relay_get(0) == false);
+
+    /* get: exported, read_int OK v=1 -> v!=0 true */
+    fk_reset(); fk.exists_ret = true; fk.read_int_ok = true; fk.read_int_val = 1;
+    EXPECT(warden_relay_get(0) == true);
+
+    /* get: exported, read_int OK v=0 -> v!=0 false */
+    fk_reset(); fk.exists_ret = true; fk.read_int_ok = true; fk.read_int_val = 0;
+    EXPECT(warden_relay_get(0) == false);
+
+    /* set: idx >= COUNT (true) -> no-op */
+    fk_reset();
+    warden_relay_set(WARDEN_RELAY_COUNT, true);
+    EXPECT(fk.last_write_path[0] == '\0');
+
+    /* set: ensure_ready FAILS -> return before writing value */
+    fk_reset(); fk.exists_ret = false; fk.export_makes_exist = false;
+    warden_relay_set(0, true);
+    EXPECT(strstr(fk.last_write_path, "/value") == NULL);
+
+    /* set: ensure_ready OK (exported, "out"), on=true -> write "1" */
+    fk_reset(); fk.exists_ret = true; fk.read_line_ok = true; fk.read_line_val = "out\n";
+    warden_relay_set(0, true);
+    EXPECT(strcmp(fk.last_write_val, "1") == 0);
+
+    /* set: on=false -> write "0" */
+    fk_reset(); fk.exists_ret = true; fk.read_line_ok = true; fk.read_line_val = "out\n";
+    warden_relay_set(0, false);
+    EXPECT(strcmp(fk.last_write_val, "0") == 0);
+}
+
+/* ---------------- layer 2: real sysfs backend over a scratch tree ---------------- */
+
+static char g_root[128];
+
+static void wr(const char *rel, const char *content) {
+    char p[256]; snprintf(p, sizeof p, "%s/%s", g_root, rel);
+    FILE *f = fopen(p, "w"); if(f) { fputs(content, f); fclose(f); }
+}
+static void mkgpio(int n) {
+    char p[256]; snprintf(p, sizeof p, "%s/gpio%d", g_root, n); mkdir(p, 0777);
+}
+
+static void integration_tests(void)
+{
+    char tmpl[] = "/tmp/warden-relays-XXXXXX";
+    char *d = mkdtemp(tmpl);
+    assert(d);
+    snprintf(g_root, sizeof g_root, "%s", d);
+    setenv("WARDEN_GPIO_ROOT", g_root, 1);
+    warden_relay__set_io(NULL);   /* restore the real sysfs backend */
+
+    /* empty $WARDEN_GPIO_ROOT -> gpio_root() falls back to the default (covers the
+       `*r` false arm of `r && *r`). The default /sys path is absent on the host,
+       so the call simply reports unavailable. */
+    setenv("WARDEN_GPIO_ROOT", "", 1);
+    (void)warden_relay_available(0);
+    setenv("WARDEN_GPIO_ROOT", g_root, 1);
+
+    /* sysfs_exists false + sysfs_write fopen FAIL: root has no /export parent issue;
+       use a nonexistent root so export write and stat both fail cleanly. */
+    setenv("WARDEN_GPIO_ROOT", "/nonexistent-warden-root/xyz", 1);
+    EXPECT(warden_relay_available(0) == false);   /* exists=false, write(export) fopen fails */
+    setenv("WARDEN_GPIO_ROOT", g_root, 1);
+
+    /* gpio32 exists, direction file ABSENT -> sysfs_read_line fopen fail */
+    mkgpio(32);
+    EXPECT(warden_relay_available(0) == false);
+
+    /* direction EMPTY -> fgets returns NULL -> read_line false */
+    wr("gpio32/direction", "");
+    EXPECT(warden_relay_available(0) == false);
+
+    /* direction "out" -> read_line ok, skip block -> available true (sysfs_write success path
+       is exercised by set below); also value ABSENT -> get: read_int fopen fail -> v=0 */
+    wr("gpio32/direction", "out\n");
+    EXPECT(warden_relay_available(0) == true);
+    EXPECT(warden_relay_get(0) == false);          /* value file absent: read_int fopen fail */
+
+    /* value non-numeric -> fscanf != 1 -> read_int false */
+    wr("gpio32/value", "xyz\n");
+    EXPECT(warden_relay_get(0) == false);
+
+    /* value "1" -> read_int ok -> get true; and set(0,true) -> sysfs_write success */
+    wr("gpio32/value", "1\n");
+    EXPECT(warden_relay_get(0) == true);
+    warden_relay_set(0, false);
+    EXPECT(warden_relay_get(0) == false);          /* wrote "0" over the value file */
+
+    /* direction "in" with a real value -> exercises the level-preserve write via sysfs */
+    mkgpio(33);
+    wr("gpio33/direction", "in\n");
+    wr("gpio33/value", "1\n");
+    EXPECT(warden_relay_available(1) == true);      /* reads value=1, writes direction "high" */
+}
+
+int main(void)
+{
+    unit_tests();
+    integration_tests();
+    fprintf(stderr, "%d checks, %d failures\n", g_checks, g_fail);
+    return g_fail ? 1 : 0;
+}