feat(quests): add issuer quest cancellation

README.md

@@ -6,7 +6,7 @@ The approved behavior is specified in the [OKF knowledge bundle](knowledge/index
 
 ## Status
 
-Administrators can register persistent shared quest boards by targeting a block within five blocks and running `/questadmin createboard`. Right-clicking any registered board opens a native dialog where a player can request a block and quantity while escrowing the exact reward stack held in their main hand. Every board and `/quests list` show the same active quests with requested blocks, rewards, issuers, and time remaining. Players can complete quests at any board or with `/quests complete <quest>` by delivering the required blocks. Exact escrowed rewards are granted immediately, and delivered blocks are held for the issuer. Claim collection remains under development.
+Administrators can register persistent shared quest boards by targeting a block within five blocks and running `/questadmin createboard`. Right-clicking any registered board opens a native dialog where a player can request a block and quantity while escrowing the exact reward stack held in their main hand. Every board and `/quests list` show the same active quests with requested blocks, rewards, issuers, and time remaining. Players can complete quests at any board or with `/quests complete <quest>` by delivering the required blocks. Exact escrowed rewards are granted immediately, and delivered blocks are held for the issuer. Issuers can cancel their own active quests at any board or with `/quests cancel <quest>`; exact rewards are held for later claim collection. Claim collection remains under development.
 
 ## Requirements
 

knowledge/log.md

@@ -54,3 +54,10 @@ description: Chronological record of material decisions affecting Spigot Quest B
 - Completion serializes state transitions, holds delivered blocks in durable issuer claims, and grants exact escrowed rewards with owner-protected overflow drops.
 - Added durable online and next-login issuer notifications and rollback before persistence succeeds.
 - Verified 49 tests and the plugin JAR with `./gradlew clean check jar`.
+
+## 2026-09-05 — Issuer quest cancellation
+
+- Added issuer-only board actions and `/quests cancel <quest>` with ownership-filtered autocomplete.
+- Cancellation atomically moves exact reward escrow into an issuer claim without directly changing inventory.
+- Serialized cancellation against completion and retained active state when persistence fails.
+- Verified 58 tests and the plugin JAR with `./gradlew clean check jar`.

knowledge/user-stories/us-006-cancel-an-owned-quest.md

@@ -2,7 +2,7 @@
 type: User Story
 title: "US-006: Cancel an owned quest"
 description: Let an issuer cancel an active quest and reclaim its escrowed reward safely.
-status: backlog
+status: done
 ---
 
 # US-006: Cancel an owned quest
@@ -11,15 +11,15 @@ As a **quest issuer**, I want to cancel my active quest so that I can reclaim th
 
 ## Acceptance criteria
 
-- [ ] An issuer can cancel their own active quest through any registered board.
+- [x] An issuer can cancel their own active quest through any registered board.
-- [ ] A player cannot cancel a quest issued by another player.
+- [x] A player cannot cancel a quest issued by another player.
-- [ ] Completed, cancelled, and expired quests cannot be cancelled.
+- [x] Completed, cancelled, and expired quests cannot be cancelled.
-- [ ] Cancellation removes the quest from active listings and converts its exact escrowed reward into a claim for the issuer.
+- [x] Cancellation removes the quest from active listings and converts its exact escrowed reward into a claim for the issuer.
-- [ ] The reward is not inserted directly into the issuer's inventory during cancellation.
+- [x] The reward is not inserted directly into the issuer's inventory during cancellation.
-- [ ] When player commands are enabled, `/quests cancel <quest>` provides equivalent behavior and only autocompletes the player's cancellable quest identifiers.
+- [x] When player commands are enabled, `/quests cancel <quest>` provides equivalent behavior and only autocompletes the player's cancellable quest identifiers.
-- [ ] Cancellation and simultaneous completion are serialized so items cannot be duplicated or lost.
+- [x] Cancellation and simultaneous completion are serialized so items cannot be duplicated or lost.
-- [ ] Persistence failure leaves the quest active and its reward escrowed.
+- [x] Persistence failure leaves the quest active and its reward escrowed.
-- [ ] Automated tests verify ownership, state validation, claim creation, autocomplete, rollback, and completion races.
+- [x] Automated tests verify ownership, state validation, claim creation, autocomplete, rollback, and completion races.
 
 ## Related
 

src/main/java/games/dmg/spigotquestboard/QuestBoardDialogUi.java

@@ -21,10 +21,11 @@ final class QuestBoardDialogUi implements QuestBoardUi {
     private final QuestCreationGateway creator;
     private final QuestBrowser browser;
     private final QuestCompletionGateway completer;
+    private final QuestCancellationGateway canceller;
     private final Clock clock;
 
     QuestBoardDialogUi(QuestCreationGateway creator, QuestBrowser browser, Clock clock) {
-        this(creator, browser, null, clock);
+        this(creator, browser, null, null, clock);
     }
 
     QuestBoardDialogUi(
@@ -32,10 +33,21 @@ final class QuestBoardDialogUi implements QuestBoardUi {
         QuestBrowser browser,
         QuestCompletionGateway completer,
         Clock clock
+    ) {
+        this(creator, browser, completer, null, clock);
+    }
+
+    QuestBoardDialogUi(
+        QuestCreationGateway creator,
+        QuestBrowser browser,
+        QuestCompletionGateway completer,
+        QuestCancellationGateway canceller,
+        Clock clock
     ) {
         this.creator = Objects.requireNonNull(creator, "creator");
         this.browser = Objects.requireNonNull(browser, "browser");
         this.completer = completer;
+        this.canceller = canceller;
         this.clock = Objects.requireNonNull(clock, "clock");
     }
 
@@ -62,10 +74,13 @@ final class QuestBoardDialogUi implements QuestBoardUi {
         String listing = QuestListingFormatter.formatAll(activeQuests, now);
         List<ActionButton> actions = new ArrayList<>();
         actions.add(create);
-        if (completer != null) {
         for (Quest quest : activeQuests) {
+            if (completer != null) {
                 actions.add(completionButton(quest));
             }
+            if (canceller != null && quest.issuerId().equals(player.getUniqueId())) {
+                actions.add(cancellationButton(quest));
+            }
         }
         DialogBase base = DialogBase.builder(Component.text("Quest Board"))
             .externalTitle(Component.text("Quest Board — Active quests and create"))
@@ -116,6 +131,44 @@ final class QuestBoardDialogUi implements QuestBoardUi {
             .build();
     }
 
+    private ActionButton cancellationButton(Quest quest) {
+        String id = quest.id().toString();
+        return ActionButton.builder(Component.text(
+                "Cancel " + quest.requestedAmount() + " " + quest.requestedMaterial()
+            ))
+            .tooltip(Component.text("Return the escrowed reward to claims for quest " + id))
+            .width(250)
+            .action(DialogAction.customClick((response, audience) -> {
+                if (audience instanceof Player respondingPlayer) {
+                    submitCancellation(respondingPlayer, id);
+                }
+            }, ClickCallback.Options.builder()
+                .uses(1)
+                .lifetime(Duration.ofMinutes(10))
+                .build()))
+            .build();
+    }
+
+    List<String> cancellableQuestIds(Player player) {
+        return browser.cancellableQuestIds(player.getUniqueId(), clock.instant());
+    }
+
+    void submitCancellation(Player player, String questId) {
+        if (canceller == null) {
+            player.sendMessage("Quest cancellation is unavailable.");
+            return;
+        }
+        try {
+            canceller.cancel(player, questId, clock.instant());
+        } catch (IllegalArgumentException | IllegalStateException exception) {
+            player.sendMessage(exception.getMessage());
+        } catch (IOException exception) {
+            player.sendMessage(
+                "The quest could not be saved. It remains active and its reward remains escrowed."
+            );
+        }
+    }
+
     void submitCompletion(Player player, String questId) {
         if (completer == null) {
             player.sendMessage("Quest completion is unavailable.");

src/main/java/games/dmg/spigotquestboard/QuestCancellationController.java

@@ -0,0 +1,31 @@
+package games.dmg.spigotquestboard;
+
+import java.io.IOException;
+import java.time.Instant;
+import java.util.Objects;
+import java.util.UUID;
+import org.bukkit.entity.Player;
+
+final class QuestCancellationController implements QuestCancellationGateway {
+    private final QuestService quests;
+
+    QuestCancellationController(QuestService quests) {
+        this.quests = Objects.requireNonNull(quests, "quests");
+    }
+
+    @Override
+    public QuestClaim cancel(Player player, String questId, Instant cancelledAt) throws IOException {
+        Objects.requireNonNull(player, "player");
+        final UUID id;
+        try {
+            id = UUID.fromString(Objects.requireNonNull(questId, "questId"));
+        } catch (IllegalArgumentException exception) {
+            throw new IllegalArgumentException("Quest identifier must be a valid UUID.", exception);
+        }
+        QuestClaim claim = quests.cancel(id, player.getUniqueId(), cancelledAt);
+        player.sendMessage(
+            "Quest " + id + " cancelled. Your exact escrowed reward is ready to claim at a quest board."
+        );
+        return claim;
+    }
+}

src/main/java/games/dmg/spigotquestboard/QuestCancellationGateway.java

@@ -0,0 +1,9 @@
+package games.dmg.spigotquestboard;
+
+import java.io.IOException;
+import java.time.Instant;
+import org.bukkit.entity.Player;
+
+interface QuestCancellationGateway {
+    QuestClaim cancel(Player player, String questId, Instant cancelledAt) throws IOException;
+}

src/main/java/games/dmg/spigotquestboard/QuestCommand.java

@@ -17,10 +17,11 @@ final class QuestCommand implements CommandExecutor, TabCompleter {
     private final QuestCreationGateway creator;
     private final QuestBrowser browser;
     private final QuestCompletionGateway completer;
+    private final QuestCancellationGateway canceller;
     private final Clock clock;
 
     QuestCommand(QuestCreationGateway creator, QuestBrowser browser, Clock clock) {
-        this(creator, browser, null, clock);
+        this(creator, browser, null, null, clock);
     }
 
     QuestCommand(
@@ -28,10 +29,21 @@ final class QuestCommand implements CommandExecutor, TabCompleter {
         QuestBrowser browser,
         QuestCompletionGateway completer,
         Clock clock
+    ) {
+        this(creator, browser, completer, null, clock);
+    }
+
+    QuestCommand(
+        QuestCreationGateway creator,
+        QuestBrowser browser,
+        QuestCompletionGateway completer,
+        QuestCancellationGateway canceller,
+        Clock clock
     ) {
         this.creator = Objects.requireNonNull(creator, "creator");
         this.browser = Objects.requireNonNull(browser, "browser");
         this.completer = completer;
+        this.canceller = canceller;
         this.clock = Objects.requireNonNull(clock, "clock");
     }
 
@@ -65,6 +77,26 @@ final class QuestCommand implements CommandExecutor, TabCompleter {
             }
             return true;
         }
+        if (arguments.length == 2 && "cancel".equalsIgnoreCase(arguments[0])) {
+            if (!(sender instanceof Player player)) {
+                sender.sendMessage("Only players can cancel quests.");
+                return true;
+            }
+            if (canceller == null) {
+                sender.sendMessage("Quest cancellation is unavailable.");
+                return true;
+            }
+            try {
+                canceller.cancel(player, arguments[1], clock.instant());
+            } catch (IllegalArgumentException | IllegalStateException exception) {
+                sender.sendMessage(exception.getMessage());
+            } catch (IOException exception) {
+                sender.sendMessage(
+                    "The quest could not be saved. It remains active and its reward remains escrowed."
+                );
+            }
+            return true;
+        }
         if (arguments.length != 3 || !"create".equalsIgnoreCase(arguments[0])) {
             usage(sender);
             return true;
@@ -99,7 +131,7 @@ final class QuestCommand implements CommandExecutor, TabCompleter {
             return List.of();
         }
         if (arguments.length == 1) {
-            return startsWith(List.of("create", "list", "complete"), arguments[0]);
+            return startsWith(List.of("create", "list", "complete", "cancel"), arguments[0]);
         }
         if (arguments.length == 2 && "create".equalsIgnoreCase(arguments[0])) {
             return creator.suggestBlockMaterials(arguments[1]);
@@ -129,7 +161,7 @@ final class QuestCommand implements CommandExecutor, TabCompleter {
     private static void usage(CommandSender sender) {
         sender.sendMessage(
             "Usage: /quests [list] | /quests create <block> <quantity> | "
-                + "/quests complete <quest>"
+                + "/quests complete <quest> | /quests cancel <quest>"
         );
         sender.sendMessage("Hold the entire reward stack in your main hand; its exact metadata will be escrowed.");
     }

src/main/java/games/dmg/spigotquestboard/QuestService.java

@@ -107,6 +107,42 @@ final class QuestService implements QuestBrowser {
         return new QuestCompletion(completed, claim, completed.reward());
     }
 
+    synchronized QuestClaim cancel(UUID questId, UUID issuerId, Instant cancelledAt)
+        throws IOException {
+        Objects.requireNonNull(questId, "questId");
+        Objects.requireNonNull(issuerId, "issuerId");
+        Objects.requireNonNull(cancelledAt, "cancelledAt");
+        Quest active = state.quests().get(questId);
+        if (active == null) {
+            throw new IllegalArgumentException("Quest not found: " + questId);
+        }
+        if (!active.issuerId().equals(issuerId)) {
+            throw new IllegalArgumentException("Only the quest issuer can cancel this quest.");
+        }
+        if (active.status() != QuestStatus.ACTIVE || !cancelledAt.isBefore(active.expiresAt())) {
+            throw new IllegalStateException("That quest is no longer active or has expired.");
+        }
+
+        Quest cancelled = new Quest(
+            active.id(), active.issuerId(), active.issuerName(), active.requestedMaterial(),
+            active.requestedAmount(), active.reward(), active.createdAt(), active.expiresAt(),
+            QuestStatus.CANCELLED
+        );
+        QuestClaim claim = new QuestClaim(
+            UUID.randomUUID(), active.id(), active.issuerId(), active.reward(), cancelledAt
+        );
+        Map<UUID, Quest> quests = new LinkedHashMap<>(state.quests());
+        quests.put(active.id(), cancelled);
+        Map<UUID, List<QuestClaim>> claims = new LinkedHashMap<>(state.claims());
+        List<QuestClaim> ownerClaims = new ArrayList<>(
+            claims.getOrDefault(active.issuerId(), List.of())
+        );
+        ownerClaims.add(claim);
+        claims.put(active.issuerId(), ownerClaims);
+        save(new QuestState(quests, claims, state.notifications()));
+        return claim;
+    }
+
     synchronized List<IssuerNotification> pendingNotifications(UUID recipientId) {
         Objects.requireNonNull(recipientId, "recipientId");
         return state.notifications().values().stream()

src/main/java/games/dmg/spigotquestboard/SpigotQuestBoardPlugin.java

@@ -38,13 +38,16 @@ public final class SpigotQuestBoardPlugin extends JavaPlugin {
         QuestCompletionGateway completer = new QuestCompletionController(
             quests, new BukkitQuestCompletionInventory(), notifier
         );
-        QuestCommand questCommand = new QuestCommand(creator, quests, completer, clock);
+        QuestCancellationGateway canceller = new QuestCancellationController(quests);
+        QuestCommand questCommand = new QuestCommand(
+            creator, quests, completer, canceller, clock
+        );
         command("quests").setExecutor(questCommand);
         command("quests").setTabCompleter(questCommand);
         getServer().getPluginManager().registerEvents(notifier, this);
         getServer().getPluginManager().registerEvents(
             new QuestBoardInteractionListener(
-                boards, new QuestBoardDialogUi(creator, quests, completer, clock)
+                boards, new QuestBoardDialogUi(creator, quests, completer, canceller, clock)
             ),
             this
         );

src/test/java/games/dmg/spigotquestboard/QuestBoardDialogUiTest.java

@@ -52,6 +52,41 @@ final class QuestBoardDialogUiTest {
         assertEquals(NOW, completer.completedAt);
     }
 
+    @Test
+    void cancellationActionUsesEquivalentCancellationFlow() {
+        RecordingCreator creator = new RecordingCreator(false);
+        RecordingCanceller canceller = new RecordingCanceller();
+        QuestBoardDialogUi ui = new QuestBoardDialogUi(
+            creator, now -> List.of(), null, canceller, Clock.fixed(NOW, ZoneOffset.UTC)
+        );
+        Player player = mock(Player.class);
+        String id = UUID.randomUUID().toString();
+
+        ui.submitCancellation(player, id);
+
+        assertEquals(player, canceller.player);
+        assertEquals(id, canceller.questId);
+        assertEquals(NOW, canceller.cancelledAt);
+    }
+
+    @Test
+    void boardOnlyOffersCancellationActionsForPlayersOwnedActiveQuests() {
+        RecordingCreator creator = new RecordingCreator(false);
+        Quest owned = creator.quest(NOW);
+        Quest other = new Quest(
+            UUID.randomUUID(), UUID.randomUUID(), "Other", "DIRT", 1,
+            List.of(new EscrowItem("COAL", 1, null)), NOW, NOW.plusSeconds(604800)
+        );
+        QuestBoardDialogUi ui = new QuestBoardDialogUi(
+            creator, now -> List.of(owned, other), null, new RecordingCanceller(),
+            Clock.fixed(NOW, ZoneOffset.UTC)
+        );
+        Player issuer = mock(Player.class);
+        org.mockito.Mockito.when(issuer.getUniqueId()).thenReturn(owned.issuerId());
+
+        assertEquals(List.of(owned.id().toString()), ui.cancellableQuestIds(issuer));
+    }
+
     @Test
     void everyOpeningBuildsAListingFromTheCurrentActiveQuests() {
         RecordingCreator creator = new RecordingCreator(false);
@@ -77,6 +112,20 @@ final class QuestBoardDialogUiTest {
         );
     }
 
+    private static final class RecordingCanceller implements QuestCancellationGateway {
+        private Player player;
+        private String questId;
+        private Instant cancelledAt;
+
+        @Override
+        public QuestClaim cancel(Player cancellingPlayer, String id, Instant instant) {
+            player = cancellingPlayer;
+            questId = id;
+            cancelledAt = instant;
+            return null;
+        }
+    }
+
     private static final class RecordingCompleter implements QuestCompletionGateway {
         private Player player;
         private String questId;

src/test/java/games/dmg/spigotquestboard/QuestCancellationControllerTest.java

@@ -0,0 +1,58 @@
+package games.dmg.spigotquestboard;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.mockito.ArgumentMatchers.contains;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import java.io.IOException;
+import java.time.Instant;
+import java.util.List;
+import java.util.UUID;
+import org.bukkit.entity.Player;
+import org.junit.jupiter.api.Test;
+
+final class QuestCancellationControllerTest {
+    @Test
+    void cancelsAsPlayerIdentityAndCreatesClaimWithoutInventorySettlement() throws Exception {
+        Repository repository = new Repository();
+        QuestService service = new QuestService(repository);
+        UUID issuer = UUID.randomUUID();
+        Quest quest = service.create(
+            issuer, "Issuer", "STONE", 1,
+            List.of(new EscrowItem("DIAMOND", 2, "exact-data")), Instant.EPOCH
+        );
+        Player player = mock(Player.class);
+        when(player.getUniqueId()).thenReturn(issuer);
+
+        QuestClaim claim = new QuestCancellationController(service).cancel(
+            player, quest.id().toString(), Instant.EPOCH.plusSeconds(1)
+        );
+
+        assertEquals(quest.reward(), claim.items());
+        assertEquals(List.of(claim), service.state().claims().get(issuer));
+        verify(player).sendMessage(contains("claim"));
+    }
+
+    @Test
+    void rejectsMalformedQuestIdentifier() throws Exception {
+        QuestCancellationController controller = new QuestCancellationController(
+            new QuestService(new Repository())
+        );
+
+        IllegalArgumentException exception = assertThrows(
+            IllegalArgumentException.class,
+            () -> controller.cancel(mock(Player.class), "not-a-uuid", Instant.EPOCH)
+        );
+
+        assertEquals("Quest identifier must be a valid UUID.", exception.getMessage());
+    }
+
+    private static final class Repository implements QuestRepository {
+        private QuestState state = QuestState.empty();
+        @Override public QuestState load() { return state; }
+        @Override public void save(QuestState candidate) throws IOException { state = candidate; }
+    }
+}

src/test/java/games/dmg/spigotquestboard/QuestCancellationServiceTest.java

@@ -0,0 +1,159 @@
+package games.dmg.spigotquestboard;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.IOException;
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.UUID;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import org.junit.jupiter.api.Test;
+
+final class QuestCancellationServiceTest {
+    @Test
+    void onlyIssuerCanCancelAndExactRewardBecomesClaim() throws Exception {
+        Repository repository = new Repository();
+        QuestService service = new QuestService(repository);
+        UUID issuer = UUID.randomUUID();
+        EscrowItem reward = new EscrowItem("DIAMOND", 3, "exact-item-data");
+        Quest quest = service.create(
+            issuer, "Issuer", "STONE", 64, List.of(reward), Instant.EPOCH
+        );
+
+        assertThrows(IllegalArgumentException.class, () -> service.cancel(
+            quest.id(), UUID.randomUUID(), Instant.EPOCH.plusSeconds(1)
+        ));
+        QuestClaim claim = service.cancel(
+            quest.id(), issuer, Instant.EPOCH.plusSeconds(1)
+        );
+
+        assertEquals(QuestStatus.CANCELLED, service.state().quests().get(quest.id()).status());
+        assertEquals(List.of(reward), claim.items());
+        assertEquals(List.of(claim), service.state().claims().get(issuer));
+        assertTrue(service.activeQuests(Instant.EPOCH.plusSeconds(1)).isEmpty());
+    }
+
+    @Test
+    void completedCancelledAndExpiredQuestsCannotBeCancelled() throws Exception {
+        Repository repository = new Repository();
+        QuestService service = new QuestService(repository);
+        UUID issuer = UUID.randomUUID();
+        Quest completed = quest(service, issuer);
+        service.complete(
+            completed.id(), new EscrowItem("STONE", 1, null), Instant.EPOCH.plusSeconds(1)
+        );
+        Quest cancelled = quest(service, issuer);
+        service.cancel(cancelled.id(), issuer, Instant.EPOCH.plusSeconds(1));
+        Quest expired = quest(service, issuer);
+
+        assertThrows(IllegalStateException.class, () -> service.cancel(
+            completed.id(), issuer, Instant.EPOCH.plusSeconds(2)
+        ));
+        assertThrows(IllegalStateException.class, () -> service.cancel(
+            cancelled.id(), issuer, Instant.EPOCH.plusSeconds(2)
+        ));
+        assertThrows(IllegalStateException.class, () -> service.cancel(
+            expired.id(), issuer, expired.expiresAt()
+        ));
+    }
+
+    @Test
+    void persistenceFailureLeavesQuestActiveAndRewardEscrowed() throws Exception {
+        Repository repository = new Repository();
+        QuestService service = new QuestService(repository);
+        UUID issuer = UUID.randomUUID();
+        Quest quest = quest(service, issuer);
+        QuestState before = service.state();
+        repository.fail = true;
+
+        assertThrows(IOException.class, () -> service.cancel(
+            quest.id(), issuer, Instant.EPOCH.plusSeconds(1)
+        ));
+
+        assertEquals(before, service.state());
+        assertEquals(before, repository.state);
+        assertEquals(QuestStatus.ACTIVE, service.state().quests().get(quest.id()).status());
+        assertTrue(service.state().claims().isEmpty());
+    }
+
+    @Test
+    void simultaneousCancellationAndCompletionSettleEscrowExactlyOnce() throws Exception {
+        Repository repository = new Repository();
+        QuestService service = new QuestService(repository);
+        UUID issuer = UUID.randomUUID();
+        Quest quest = quest(service, issuer);
+        CountDownLatch start = new CountDownLatch(1);
+        try (ExecutorService executor = Executors.newFixedThreadPool(2)) {
+            List<Future<Boolean>> attempts = new ArrayList<>();
+            attempts.add(executor.submit(() -> {
+                start.await();
+                try {
+                    service.cancel(quest.id(), issuer, Instant.EPOCH.plusSeconds(1));
+                    return true;
+                } catch (IllegalStateException exception) {
+                    return false;
+                }
+            }));
+            attempts.add(executor.submit(() -> {
+                start.await();
+                try {
+                    service.complete(
+                        quest.id(), new EscrowItem("STONE", 1, null),
+                        Instant.EPOCH.plusSeconds(1)
+                    );
+                    return true;
+                } catch (IllegalStateException exception) {
+                    return false;
+                }
+            }));
+            start.countDown();
+            assertEquals(1, attempts.stream().filter(attempt -> {
+                try {
+                    return attempt.get();
+                } catch (Exception exception) {
+                    throw new AssertionError(exception);
+                }
+            }).count());
+        }
+
+        QuestStatus status = service.state().quests().get(quest.id()).status();
+        assertTrue(status == QuestStatus.CANCELLED || status == QuestStatus.COMPLETED);
+        assertEquals(1, service.state().claims().get(issuer).size());
+        QuestClaim settlement = service.state().claims().get(issuer).getFirst();
+        if (status == QuestStatus.CANCELLED) {
+            assertEquals(quest.reward(), settlement.items());
+            assertTrue(service.state().notifications().isEmpty());
+        } else {
+            assertEquals(List.of(new EscrowItem("STONE", 1, null)), settlement.items());
+            assertEquals(1, service.state().notifications().size());
+        }
+    }
+
+    private static Quest quest(QuestService service, UUID issuer) throws IOException {
+        return service.create(
+            issuer, "Issuer", "STONE", 1,
+            List.of(new EscrowItem("DIAMOND", 2, null)), Instant.EPOCH
+        );
+    }
+
+    private static final class Repository implements QuestRepository {
+        private QuestState state = QuestState.empty();
+        private boolean fail;
+
+        @Override public QuestState load() { return state; }
+
+        @Override
+        public void save(QuestState candidate) throws IOException {
+            if (fail) {
+                throw new IOException("disk full");
+            }
+            state = candidate;
+        }
+    }
+}

src/test/java/games/dmg/spigotquestboard/QuestCommandTest.java

@@ -105,6 +105,25 @@ final class QuestCommandTest {
         assertEquals(NOW, completer.completedAt);
     }
 
+    @Test
+    void cancelCommandRoutesPlayerAndCurrentTimeToEquivalentGateway() {
+        RecordingCreator creator = new RecordingCreator();
+        RecordingCanceller canceller = new RecordingCanceller();
+        QuestCommand executor = new QuestCommand(
+            creator, now -> List.of(), null, canceller, Clock.fixed(NOW, ZoneOffset.UTC)
+        );
+        Player player = mock(Player.class);
+        String id = UUID.randomUUID().toString();
+
+        assertTrue(executor.onCommand(
+            player, mock(Command.class), "quests", new String[] {"cancel", id}
+        ));
+
+        assertEquals(player, canceller.player);
+        assertEquals(id, canceller.questId);
+        assertEquals(NOW, canceller.cancelledAt);
+    }
+
     @Test
     void questIdentifierAutocompleteOnlyUsesActiveAndOwnedQuests() {
         RecordingCreator creator = new RecordingCreator();
@@ -139,6 +158,8 @@ final class QuestCommandTest {
             executor.onTabComplete(player, command, "quests", new String[] {"cr"}));
         assertEquals(List.of("list"),
             executor.onTabComplete(player, command, "quests", new String[] {"li"}));
+        assertEquals(List.of("cancel"),
+            executor.onTabComplete(player, command, "quests", new String[] {"ca"}));
         assertEquals(List.of("STONE", "STONE_BRICKS"), executor.onTabComplete(
             player, command, "quests", new String[] {"create", "sto"}
         ));
@@ -159,6 +180,20 @@ final class QuestCommandTest {
         return new QuestCommand(creator, browser, Clock.fixed(NOW, ZoneOffset.UTC));
     }
 
+    private static final class RecordingCanceller implements QuestCancellationGateway {
+        private Player player;
+        private String questId;
+        private Instant cancelledAt;
+
+        @Override
+        public QuestClaim cancel(Player cancellingPlayer, String id, Instant instant) {
+            player = cancellingPlayer;
+            questId = id;
+            cancelledAt = instant;
+            return null;
+        }
+    }
+
     private static final class RecordingCompleter implements QuestCompletionGateway {
         private Player player;
         private String questId;