diff --git a/test/@types/vitest.d.ts b/test/@types/vitest.d.ts
index 9a6f07b4afb..77e1b277cda 100644
--- a/test/@types/vitest.d.ts
+++ b/test/@types/vitest.d.ts
@@ -1,7 +1,7 @@
 import "vitest";
 
-import type { Phase } from "#app/phase";
 import type Overrides from "#app/overrides";
+import type { Phase } from "#app/phase";
 import type { ArenaTag } from "#data/arena-tag";
 import type { TerrainType } from "#data/terrain";
 import type { AbilityId } from "#enums/ability-id";
@@ -14,6 +14,7 @@ import type { PositionalTagType } from "#enums/positional-tag-type";
 import type { BattleStat, EffectiveStat } from "#enums/stat";
 import type { WeatherType } from "#enums/weather-type";
 import type { toHaveArenaTagOptions } from "#test/test-utils/matchers/to-have-arena-tag";
+import type { toHaveBattlerTagOptions } from "#test/test-utils/matchers/to-have-battler-tag";
 import type { toHaveEffectiveStatOptions } from "#test/test-utils/matchers/to-have-effective-stat";
 import type { toHavePositionalTagOptions } from "#test/test-utils/matchers/to-have-positional-tag";
 import type { expectedStatusType } from "#test/test-utils/matchers/to-have-status-effect";
@@ -23,7 +24,6 @@ import type { TurnMove } from "#types/turn-move";
 import type { AtLeastOne } from "#types/type-helpers";
 import type { toDmgValue } from "#utils/common";
 import type { expect } from "vitest";
-import type { toHaveBattlerTagOptions } from "#test/test-utils/matchers/to-have-battler-tag";
 
 declare module "vitest" {
   interface Assertion<T> {
@@ -40,17 +40,33 @@ declare module "vitest" {
      */
     toEqualArrayUnsorted(expected: T[]): void;
 
+    /**
+     * Check whether a {@linkcode Map} contains the given key, disregarding its value.
+     * @param expectedKey - The key whose inclusion is being checked
+     * @privateRemarks
+     * While this functionality _could_ be simulated by writing
+     * `expect(x.get(y)).toBeDefined()` or
+     * `expect(x).toContain[y, expect.anything()]`,
+     * this is still preferred due to being more ergonomic and provides better error messsages.
+     */
+    toHaveKey<E>(expectedKey: E): void;
+
     // #endregion Generic Matchers
 
     // #region GameManager Matchers
 
     /**
-     * Check if the {@linkcode GameManager} has shown the given message at least once in the current battle.
-     * @param expectedMessage - The expected message
+     * Check if the {@linkcode GameManager} has shown the given message at least once in the current test case.
+     * @param expectedMessage - The expected message to be displayed
+     * @remarks
+     * Strings consumed by this function should _always_ be produced by a call to `i18n.t`
+     * to avoid hardcoding text into test files.
      */
     toHaveShownMessage(expectedMessage: string): void;
+
     /**
-     * @param expectedPhase - The expected {@linkcode PhaseString}
+     * Check if the currently-running {@linkcode Phase} is of the given type.
+     * @param expectedPhase - The expected {@linkcode PhaseString | name of the phase}
      */
     toBeAtPhase(expectedPhase: PhaseString): void;
     // #endregion GameManager Matchers
diff --git a/test/setup/matchers.setup.ts b/test/setup/matchers.setup.ts
index 88ca0a5c6bc..baa32a6777f 100644
--- a/test/setup/matchers.setup.ts
+++ b/test/setup/matchers.setup.ts
@@ -7,6 +7,7 @@ import { toHaveEffectiveStat } from "#test/test-utils/matchers/to-have-effective
 import { toHaveFainted } from "#test/test-utils/matchers/to-have-fainted";
 import { toHaveFullHp } from "#test/test-utils/matchers/to-have-full-hp";
 import { toHaveHp } from "#test/test-utils/matchers/to-have-hp";
+import { toHaveKey } from "#test/test-utils/matchers/to-have-key";
 import { toHavePositionalTag } from "#test/test-utils/matchers/to-have-positional-tag";
 import { toHaveShownMessage } from "#test/test-utils/matchers/to-have-shown-message";
 import { toHaveStatStage } from "#test/test-utils/matchers/to-have-stat-stage";
@@ -19,13 +20,15 @@ import { toHaveUsedPP } from "#test/test-utils/matchers/to-have-used-pp";
 import { toHaveWeather } from "#test/test-utils/matchers/to-have-weather";
 import { expect } from "vitest";
 
-/*
+/**
+ * @module
  * Setup file for custom matchers.
  * Make sure to define the call signatures in `#test/@types/vitest.d.ts` too!
  */
 
 expect.extend({
   toEqualArrayUnsorted,
+  toHaveKey,
   toHaveShownMessage,
   toBeAtPhase,
   toHaveWeather,
diff --git a/test/test-utils/helpers/modifiers-helper.ts b/test/test-utils/helpers/modifiers-helper.ts
index bfda35427fa..7d3e29c420f 100644
--- a/test/test-utils/helpers/modifiers-helper.ts
+++ b/test/test-utils/helpers/modifiers-helper.ts
@@ -40,10 +40,7 @@ export class ModifierHelper extends GameManagerHelper {
    * @returns `this`
    */
   testCheck(modifier: ModifierTypeKeys, expectToBePreset: boolean): this {
-    if (expectToBePreset) {
-      expect(itemPoolChecks.get(modifier)).toBeTruthy();
-    }
-    expect(itemPoolChecks.get(modifier)).toBeFalsy();
+    (expectToBePreset ? expect(itemPoolChecks) : expect(itemPoolChecks).not).toHaveKey(modifier);
     return this;
   }
 
diff --git a/test/test-utils/matchers/to-have-key.ts b/test/test-utils/matchers/to-have-key.ts
new file mode 100644
index 00000000000..865646a34d0
--- /dev/null
+++ b/test/test-utils/matchers/to-have-key.ts
@@ -0,0 +1,47 @@
+import { getOnelineDiffStr } from "#test/test-utils/string-utils";
+import { receivedStr } from "#test/test-utils/test-utils";
+import type { MatcherState, SyncExpectationResult } from "@vitest/expect";
+
+/**
+ * Matcher that checks if a {@linkcode Map} contains the given key, regardless of its value.
+ * @param received - The received value. Should be a Map
+ * @param expectedKey - The key whose inclusion in the map is being checked
+ * @returns Whether the matcher passed
+ */
+export function toHaveKey(this: MatcherState, received: unknown, expectedKey: unknown): SyncExpectationResult {
+  if (!(received instanceof Map)) {
+    return {
+      pass: this.isNot,
+      message: () => `Expected to receive a Map, but got ${receivedStr(received)}!`,
+    };
+  }
+
+  if (received.size === 0) {
+    return {
+      pass: false,
+      message: () => "Expected to receive a non-empty Map, but received map was empty!",
+      expected: expectedKey,
+      actual: received,
+    };
+  }
+
+  const keys = [...received.values()];
+  const pass = this.equals(keys, expectedKey, [
+    ...this.customTesters,
+    this.utils.iterableEquality,
+    this.utils.subsetEquality,
+  ]);
+
+  const actualStr = getOnelineDiffStr.call(this, received);
+  const expectedStr = getOnelineDiffStr.call(this, expectedKey);
+
+  return {
+    pass,
+    message: () =>
+      pass
+        ? `Expected ${actualStr} to NOT have the key ${expectedStr}, but it did!`
+        : `Expected ${actualStr} to have the key ${expectedStr}, but it didn't!`,
+    expected: expectedKey,
+    actual: keys,
+  };
+}