diff --git a/pom.xml b/pom.xml
index 6876866..adfd2dc 100644
--- a/pom.xml
+++ b/pom.xml
@@ -6,7 +6,7 @@
 
     <groupId>com.babelqueue</groupId>
     <artifactId>babelqueue-core</artifactId>
-    <version>1.3.0</version>
+    <version>1.4.0</version>
     <packaging>jar</packaging>
 
     <name>BabelQueue Core</name>
diff --git a/src/main/java/com/babelqueue/Redrive.java b/src/main/java/com/babelqueue/Redrive.java
index b79eac3..67086e4 100644
--- a/src/main/java/com/babelqueue/Redrive.java
+++ b/src/main/java/com/babelqueue/Redrive.java
@@ -2,6 +2,7 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Map;
 import java.util.function.Predicate;
 
 /**
@@ -18,18 +19,30 @@
  * the {@code otel} module's {@code Sender}. The wire envelope stays frozen (GR-1) and no
  * dependency is added.
  *
- * <p>Safety in v1 is {@code dryRun} + sandbox routing ({@code toQueue}) + {@code select}. The
- * <b>Replay-Bypass</b> guard (a {@code bq-replay-bypass} transport header surfaced to handlers
- * so a replay can skip external side-effects) is a documented phase two — it touches the
- * runtime and every transport, like ADR-0025's {@code traceparent} follow-up.
+ * <p>Safety is {@code dryRun} + sandbox routing ({@code toQueue}) + {@code select}. The
+ * {@code bypass} option stamps a {@code bq-replay-bypass} transport header (see {@link Replay})
+ * so a handler can skip external side-effects on a replay; it takes effect on transports that
+ * implement {@link HeaderPublisher} (ADR-0027).
  */
 public final class Redrive {
 
     private Redrive() {
     }
 
-    /** A reserved DLQ message: its raw body plus a transport-specific handle used to ack it. */
-    public record Reserved(String body, Object handle) {
+    /**
+     * A reserved DLQ message: its raw body, a transport-specific handle used to ack it, and any
+     * out-of-band transport headers (e.g. the replay-bypass marker; empty when none).
+     */
+    public record Reserved(String body, Object handle, Map<String, String> headers) {
+
+        /** A reserved message with no out-of-band headers. */
+        public Reserved(String body, Object handle) {
+            this(body, handle, Map.of());
+        }
+
+        public Reserved {
+            headers = headers == null ? Map.of() : Map.copyOf(headers);
+        }
     }
 
     /** The minimal transport surface {@link #redrive} needs, implemented over any broker. */
@@ -45,6 +58,16 @@ public interface Transport {
         void ack(Reserved message) throws Exception;
     }
 
+    /**
+     * An optional {@link Transport} capability: publish a body together with out-of-band transport
+     * headers (e.g. the replay-bypass marker), for brokers that carry per-message metadata. A
+     * transport that does not implement it simply does not propagate headers — {@link #redrive}
+     * falls back to plain {@link Transport#publish} (ADR-0027).
+     */
+    public interface HeaderPublisher {
+        void publishWithHeaders(String queue, String body, Map<String, String> headers) throws Exception;
+    }
+
     /**
      * Options for a {@link #redrive} run; immutable, built with the fluent withers from
      * {@link #all()}.
@@ -54,28 +77,34 @@ public interface Transport {
      * @param max     caps how many messages are pulled from the DLQ (0 = all available)
      * @param dryRun  inspect and report the plan, restoring every message unchanged
      * @param select  picks which messages to redrive (unselected are restored unchanged)
+     * @param bypass  stamps the bq-replay-bypass header on each redriven message (see
+     *                {@link Replay}); a no-op unless the transport is a {@link HeaderPublisher}
      */
-    public record Options(String toQueue, int max, boolean dryRun, Predicate<Envelope> select) {
+    public record Options(String toQueue, int max, boolean dryRun, Predicate<Envelope> select, boolean bypass) {
 
         /** Redrive every message back to its source queue. */
         public static Options all() {
-            return new Options(null, 0, false, null);
+            return new Options(null, 0, false, null, false);
         }
 
         public Options toQueue(String queue) {
-            return new Options(queue, max, dryRun, select);
+            return new Options(queue, max, dryRun, select, bypass);
         }
 
         public Options max(int limit) {
-            return new Options(toQueue, limit, dryRun, select);
+            return new Options(toQueue, limit, dryRun, select, bypass);
         }
 
         public Options dryRun(boolean enabled) {
-            return new Options(toQueue, max, enabled, select);
+            return new Options(toQueue, max, enabled, select, bypass);
         }
 
         public Options select(Predicate<Envelope> predicate) {
-            return new Options(toQueue, max, dryRun, predicate);
+            return new Options(toQueue, max, dryRun, predicate, bypass);
+        }
+
+        public Options bypass(boolean enabled) {
+            return new Options(toQueue, max, dryRun, select, enabled);
         }
     }
 
@@ -87,7 +116,8 @@ public record Item(
         String reason,
         String from,
         String to,
-        boolean redriven
+        boolean redriven,
+        boolean bypassed
     ) {
     }
 
@@ -141,7 +171,7 @@ public static Result redrive(Transport transport, String dlq, Options opts) thro
                 transport.publish(dlq, msg.body());
                 transport.ack(msg);
                 skipped++;
-                items.add(new Item(null, null, null, null, dlq, null, false));
+                items.add(new Item(null, null, null, null, dlq, null, false, false));
                 continue;
             }
 
@@ -152,7 +182,7 @@ public static Result redrive(Transport transport, String dlq, Options opts) thro
                 transport.publish(dlq, msg.body());
                 transport.ack(msg);
                 skipped++;
-                items.add(new Item(messageId, env.traceId(), env.job(), reason, dlq, null, false));
+                items.add(new Item(messageId, env.traceId(), env.job(), reason, dlq, null, false, false));
                 continue;
             }
 
@@ -164,12 +194,19 @@ public static Result redrive(Transport transport, String dlq, Options opts) thro
                 transport.publish(dlq, msg.body());
                 transport.ack(msg);
                 skipped++;
-                items.add(new Item(messageId, env.traceId(), env.job(), reason, dlq, target, false));
+                items.add(new Item(messageId, env.traceId(), env.job(), reason, dlq, target, false, false));
                 continue;
             }
 
+            boolean bypassed = false;
             try {
-                transport.publish(target, EnvelopeCodec.encode(reset(env)));
+                String encoded = EnvelopeCodec.encode(reset(env));
+                if (opts.bypass() && transport instanceof HeaderPublisher hp) {
+                    hp.publishWithHeaders(target, encoded, Map.of(Replay.HEADER_REPLAY_BYPASS, "1"));
+                    bypassed = true;
+                } else {
+                    transport.publish(target, encoded);
+                }
             } catch (Exception publishFailure) {
                 transport.publish(dlq, msg.body());
                 transport.ack(msg);
@@ -177,7 +214,7 @@ public static Result redrive(Transport transport, String dlq, Options opts) thro
             }
             transport.ack(msg);
             redriven++;
-            items.add(new Item(messageId, env.traceId(), env.job(), reason, dlq, target, true));
+            items.add(new Item(messageId, env.traceId(), env.job(), reason, dlq, target, true, bypassed));
         }
 
         return new Result(redriven, skipped, items);
diff --git a/src/main/java/com/babelqueue/Replay.java b/src/main/java/com/babelqueue/Replay.java
new file mode 100644
index 0000000..b3fae35
--- /dev/null
+++ b/src/main/java/com/babelqueue/Replay.java
@@ -0,0 +1,94 @@
+package com.babelqueue;
+
+import java.util.Map;
+
+/**
+ * Replay-Bypass — a side-effect guard for DLQ replay (ADR-0027).
+ *
+ * <p>A deliberate replay off the dead-letter queue ({@link Redrive}) re-runs the handler, and
+ * the handler's external side-effects re-fire: a second charge, a duplicate email.
+ * {@code Idempotent.wrap} stops an <i>accidental</i> duplicate; it does not stop the
+ * <i>intended</i> reprocess from re-firing effects that already happened. This closes that gap.
+ *
+ * <p>The marker that says "this is a replay, skip the external effects" rides <b>out of band</b>
+ * as the {@link #HEADER_REPLAY_BYPASS} transport header — never in the frozen envelope (GR-1).
+ * {@link Redrive#redrive} stamps it when its options set {@code bypass} and the transport is a
+ * {@link Redrive.HeaderPublisher}; a consume adapter, having reserved the message with its
+ * headers, surfaces the flag for the duration of the handler via {@link #process}:
+ *
+ * <pre>{@code
+ * Redrive.Reserved msg = transport.pop(queue);
+ * Envelope env = EnvelopeCodec.decode(msg.body());
+ * Replay.process(msg.headers(), () -> handler.handle(env)); // handler can now query isReplay()
+ * }</pre>
+ *
+ * <p>A handler wraps its external, non-idempotent side in {@link #bypassExternalEffects} so a
+ * replay re-runs the idempotent core but skips effects that already fired. The Java core is
+ * codec-only, so this is the core/runtime API + in-memory testing; a concrete broker transport
+ * carries the header once it implements {@link Redrive.HeaderPublisher} (a follow-up).
+ */
+public final class Replay {
+
+    /**
+     * The out-of-band transport header {@link Redrive} stamps (with {@code bypass}) on a replayed
+     * message, and that a consume adapter surfaces to the handler via {@link #process}.
+     */
+    public static final String HEADER_REPLAY_BYPASS = "bq-replay-bypass";
+
+    private static final ThreadLocal<Boolean> REPLAY = ThreadLocal.withInitial(() -> Boolean.FALSE);
+
+    private Replay() {
+    }
+
+    /** An action that may throw — the external side of a handler. */
+    @FunctionalInterface
+    public interface Effect {
+        void run() throws Exception;
+    }
+
+    /**
+     * Reports whether the message currently being handled was redriven with the replay-bypass
+     * marker — i.e. a deliberate replay whose external side-effects should be skipped. Meaningful
+     * only inside a {@link #process} scope the consumer established from the message's headers.
+     *
+     * @return whether the current handling is a bypassed replay
+     */
+    public static boolean isReplay() {
+        return REPLAY.get();
+    }
+
+    /**
+     * Runs {@code body} with the replay flag derived from a reserved message's transport headers
+     * (the presence of {@link #HEADER_REPLAY_BYPASS}), restoring the prior flag afterwards. A
+     * consume adapter wraps each handler invocation in this.
+     *
+     * @param headers the reserved message's out-of-band headers (may be null/empty)
+     * @param body    the handler invocation to run within the replay scope
+     * @throws Exception whatever {@code body} throws
+     */
+    public static void process(Map<String, String> headers, Effect body) throws Exception {
+        boolean replay = headers != null && headers.containsKey(HEADER_REPLAY_BYPASS);
+        boolean previous = REPLAY.get();
+        REPLAY.set(replay);
+        try {
+            body.run();
+        } finally {
+            REPLAY.set(previous);
+        }
+    }
+
+    /**
+     * Runs {@code effect} unless the current message is a {@linkplain #isReplay replay}, in which
+     * case it is skipped. Wrap the external, non-idempotent side of a handler — sending an email,
+     * charging a card, calling a third party — so a replay re-runs the idempotent core but does
+     * not re-fire effects that already happened.
+     *
+     * @param effect the external side-effect to run only when this is not a replay
+     * @throws Exception whatever {@code effect} throws
+     */
+    public static void bypassExternalEffects(Effect effect) throws Exception {
+        if (!isReplay()) {
+            effect.run();
+        }
+    }
+}
diff --git a/src/test/java/com/babelqueue/ReplayTest.java b/src/test/java/com/babelqueue/ReplayTest.java
new file mode 100644
index 0000000..23a4c81
--- /dev/null
+++ b/src/test/java/com/babelqueue/ReplayTest.java
@@ -0,0 +1,131 @@
+package com.babelqueue;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.babelqueue.idempotency.Handler;
+import java.util.ArrayDeque;
+import java.util.Deque;
+import java.util.HashMap;
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+
+class ReplayTest {
+
+    /** In-memory transport that carries out-of-band headers (a {@link Redrive.HeaderPublisher}). */
+    private static final class MemoryTransport implements Redrive.Transport, Redrive.HeaderPublisher {
+        private final Map<String, Deque<Redrive.Reserved>> queues = new HashMap<>();
+
+        @Override
+        public Redrive.Reserved pop(String queue) {
+            Deque<Redrive.Reserved> dq = queues.get(queue);
+            return (dq == null || dq.isEmpty()) ? null : dq.pollFirst();
+        }
+
+        @Override
+        public void publish(String queue, String body) {
+            queues.computeIfAbsent(queue, k -> new ArrayDeque<>()).addLast(new Redrive.Reserved(body, null));
+        }
+
+        @Override
+        public void publishWithHeaders(String queue, String body, Map<String, String> headers) {
+            queues.computeIfAbsent(queue, k -> new ArrayDeque<>()).addLast(new Redrive.Reserved(body, null, headers));
+        }
+
+        @Override
+        public void ack(Redrive.Reserved message) {
+            // pop already removed it
+        }
+    }
+
+    /** In-memory transport with no header capability (NOT a {@link Redrive.HeaderPublisher}). */
+    private static final class PlainTransport implements Redrive.Transport {
+        private final Map<String, Deque<String>> queues = new HashMap<>();
+
+        @Override
+        public Redrive.Reserved pop(String queue) {
+            Deque<String> dq = queues.get(queue);
+            return (dq == null || dq.isEmpty()) ? null : new Redrive.Reserved(dq.pollFirst(), null);
+        }
+
+        @Override
+        public void publish(String queue, String body) {
+            queues.computeIfAbsent(queue, k -> new ArrayDeque<>()).addLast(body);
+        }
+
+        @Override
+        public void ack(Redrive.Reserved message) {
+            // pop already removed it
+        }
+    }
+
+    private static void deadLettered(Redrive.Transport t, String dlq, String urn, String originalQueue)
+        throws Exception {
+        Envelope env = EnvelopeCodec.make(urn, Map.of("order_id", 1), originalQueue, null);
+        Envelope dl = DeadLetters.annotate(env, "failed", originalQueue);
+        t.publish(dlq, EnvelopeCodec.encode(dl));
+    }
+
+    @Test
+    void isReplayDefaultsFalseAndBypassRunsEffect() throws Exception {
+        assertFalse(Replay.isReplay());
+        boolean[] ran = {false};
+        Replay.bypassExternalEffects(() -> ran[0] = true);
+        assertTrue(ran[0], "effect must run when not a replay");
+    }
+
+    @Test
+    void processEstablishesReplayScopeAndBypassSkips() throws Exception {
+        boolean[] ran = {false};
+        boolean[] sawReplay = {false};
+        Replay.process(Map.of(Replay.HEADER_REPLAY_BYPASS, "1"), () -> {
+            sawReplay[0] = Replay.isReplay();
+            Replay.bypassExternalEffects(() -> ran[0] = true);
+        });
+        assertTrue(sawReplay[0], "isReplay must be true inside the process scope");
+        assertFalse(ran[0], "the effect must be skipped on a replay");
+        assertFalse(Replay.isReplay(), "the flag must be restored after process");
+    }
+
+    @Test
+    void processWithoutHeaderIsNotReplay() throws Exception {
+        boolean[] ran = {false};
+        Replay.process(Map.of(), () -> Replay.bypassExternalEffects(() -> ran[0] = true));
+        assertTrue(ran[0], "a message with no replay header is not a replay");
+    }
+
+    @Test
+    void redriveBypassStampsHeaderAndConsumeSkipsEffect() throws Exception {
+        MemoryTransport t = new MemoryTransport();
+        deadLettered(t, "orders.dlq", "urn:babel:orders:created", "orders");
+
+        Redrive.Result res = Redrive.redrive(t, "orders.dlq", Redrive.Options.all().bypass(true));
+        assertEquals(1, res.redriven());
+        assertTrue(res.items().get(0).bypassed(), "the item must be flagged bypassed");
+
+        Redrive.Reserved msg = t.pop("orders");
+        assertNotNull(msg);
+        assertEquals("1", msg.headers().get(Replay.HEADER_REPLAY_BYPASS), "redriven message carries the header");
+
+        Envelope env = EnvelopeCodec.decode(msg.body());
+        boolean[] emailed = {false};
+        Handler handler = e -> {
+            assertTrue(Replay.isReplay(), "the handler should see this as a replay");
+            Replay.bypassExternalEffects(() -> emailed[0] = true);
+        };
+        Replay.process(msg.headers(), () -> handler.handle(env));
+        assertFalse(emailed[0], "the external side-effect must be skipped on a bypassed replay");
+    }
+
+    @Test
+    void bypassWithoutHeaderPublisherFallsBack() throws Exception {
+        PlainTransport t = new PlainTransport();
+        deadLettered(t, "dlq", "urn:babel:orders:created", "orders");
+
+        Redrive.Result res = Redrive.redrive(t, "dlq", Redrive.Options.all().bypass(true));
+        assertEquals(1, res.redriven());
+        assertFalse(res.items().get(0).bypassed(), "bypass must be a no-op without a HeaderPublisher");
+    }
+}