diff --git a/tasks/select-runner-test-files.test.ts b/tasks/select-runner-test-files.test.ts
index c2fcdd9e7d..5375ec7b47 100644
--- a/tasks/select-runner-test-files.test.ts
+++ b/tasks/select-runner-test-files.test.ts
@@ -1,0 +2,3 @@ import { assertEquals } from "@std/assert";
+import { expect } from "@std/expect";
+import { toFileUrl } from "@std/path";
+import { describe, it } from "@std/testing/bdd";
@@ -9,0 +13,41 @@ const TOTAL_SHARDS = 8;
+describe("listRunnerTests", () => {
+  it("includes nested tests with distinct relative paths in exactly one shard", async () => {
+    const directory = await Deno.makeTempDir({ prefix: "runner-discovery-" });
+    try {
+      await Deno.mkdir(`${directory}/executor/nested`, { recursive: true });
+      for (
+        const name of [
+          "event.test.ts",
+          "executor/event.test.ts",
+          "executor/nested/event.test.ts",
+          "executor/helper.ts",
+        ]
+      ) {
+        await Deno.writeTextFile(`${directory}/${name}`, "");
+      }
+      await Deno.symlink(
+        `${directory}/event.test.ts`,
+        `${directory}/alias.test.ts`,
+      );
+      const files = await listRunnerTests(toFileUrl(`${directory}/`));
+      const expected = [
+        "event.test.ts",
+        "executor/event.test.ts",
+        "executor/nested/event.test.ts",
+      ];
+      expect(files.map((file) => file.name)).toEqual(expected);
+      const selected = Array.from(
+        { length: TOTAL_SHARDS },
+        (_, offset) =>
+          selectRunnerTestFiles(files, {
+            index: offset + 1,
+            total: TOTAL_SHARDS,
+          }),
+      ).flat().sort();
+      expect(selected).toEqual(expected);
+    } finally {
+      await Deno.remove(directory, { recursive: true });
+    }
+  });
+});
+
diff --git a/tasks/select-runner-test-files.ts b/tasks/select-runner-test-files.ts
index 299e4e8a63..897a6e1c9f 100644
--- a/tasks/select-runner-test-files.ts
+++ b/tasks/select-runner-test-files.ts
@@ -2,0 +3,3 @@
+import { walk } from "@std/fs/walk";
+import { fromFileUrl, relative, SEPARATOR } from "@std/path";
+
@@ -25,2 +28,4 @@ export function selectRunnerTestFiles(
-export async function listRunnerTests(): Promise<{ name: string }[]> {
-  const testDir = new URL("../packages/runner/test/", import.meta.url);
+/** Lists every `.test.ts` file by its path relative to the test directory. */
+export async function listRunnerTests(
+  testDir = new URL("../packages/runner/test/", import.meta.url),
+): Promise<{ name: string }[]> {
@@ -29,4 +34,10 @@ export async function listRunnerTests(): Promise<{ name: string }[]> {
-  for await (const entry of Deno.readDir(testDir)) {
-    if (entry.isFile && entry.name.endsWith(".test.ts")) {
-      files.push({ name: entry.name });
-    }
+  for await (
+    const entry of walk(testDir, {
+      includeDirs: false,
+      includeSymlinks: false,
+      match: [/\.test\.ts$/],
+    })
+  ) {
+    const name = relative(fromFileUrl(testDir), entry.path)
+      .split(SEPARATOR).join("/");
+    files.push({ name });