diff --git a/functions/src/__tests__/meetup-join-contention.test.ts b/functions/src/__tests__/meetup-join-contention.test.ts
new file mode 100644
index 0000000..a31353a
--- /dev/null
+++ b/functions/src/__tests__/meetup-join-contention.test.ts
@@ -0,0 +1,113 @@
+import "./setup";
+import { afterAll, afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import type { Transaction } from "firebase-admin/firestore";
+import { admin, db } from "../platform";
+import { joinMeetupCallable } from "../meetups";
+import { createPetCallable } from "../pets";
+import { CONTENTION_RETRY } from "../shared";
+import { callAs, clearRateLimits, errorCodeOf, purge } from "./helpers";
+
+// Everyone joining one meetup reads and writes the same document. Two hundred
+// at once on the test project had two joins aborted for contention, past the
+// client library's own retries, and those two were told "internal". A
+// contended join now waits and tries again. Running it again is safe: someone
+// already on the roster is answered as joined.
+
+const ORGANIZER = "jc-organizer";
+const JOINER = "jc-joiner";
+const MEETUP_ID = "jc-meetup";
+const MEETUP = `meetups/${MEETUP_ID}`;
+
+const aborted = () =>
+  Object.assign(new Error("10 ABORTED: Aborted due to cross-transaction contention."), { code: 10 });
+
+/** Thrown inside the transaction to roll it back; the library does not retry it. */
+class GiveUp extends Error {}
+
+/**
+ * The first `failures` transactions that read the meetup end as though the
+ * client library had given up on them for contention. Others, such as the
+ * rate limit's, run as they are.
+ */
+function joinsContended(failures: number) {
+  const real = db.runTransaction.bind(db);
+  let joins = 0;
+  vi.spyOn(db, "runTransaction").mockImplementation((async (
+    update: (t: Transaction) => Promise<unknown>,
+    options?: Parameters<typeof db.runTransaction>[1]
+  ) => {
+    try {
+      return await real(async (t) => {
+        let readsTheMeetup = false;
+        const get = t.get.bind(t) as (ref: unknown) => unknown;
+        (t as unknown as { get: (ref: { path?: string }) => unknown }).get = (ref) => {
+          if (ref?.path === MEETUP) readsTheMeetup = true;
+          return get(ref);
+        };
+        const result = await update(t);
+        if (readsTheMeetup && joins < failures) {
+          joins += 1;
+          throw new GiveUp();
+        }
+        return result;
+      }, options);
+    } catch (error) {
+      if (error instanceof GiveUp) throw aborted();
+      throw error;
+    }
+  }) as typeof db.runTransaction);
+  return { aborted: () => joins };
+}
+
+let petId = "";
+const saved = { ...CONTENTION_RETRY };
+
+beforeEach(async () => {
+  Object.assign(CONTENTION_RETRY, { baseMs: 1, capMs: 2 });
+  await purge(MEETUP, `users/${ORGANIZER}`, `users/${JOINER}`);
+  await clearRateLimits();
+  await db.doc(`users/${ORGANIZER}`).set({ displayName: ORGANIZER });
+  await db.doc(`users/${JOINER}`).set({ displayName: JOINER });
+  await db.doc(MEETUP).set({
+    organizerId: ORGANIZER,
+    title: "A crowded walk",
+    status: "upcoming",
+    participantCount: 1,
+    requirements: { maxPets: 20 },
+    date: admin.firestore.Timestamp.fromMillis(Date.now() + 86_400_000),
+  });
+  const pet = await callAs<{ id?: string; petId?: string }>(createPetCallable, JOINER, { name: "Crowd", species: "dog", gender: "male" });
+  petId = (pet.id ?? pet.petId) as string;
+});
+afterEach(() => {
+  vi.restoreAllMocks();
+  Object.assign(CONTENTION_RETRY, saved);
+});
+afterAll(async () => {
+  await purge(MEETUP, `users/${ORGANIZER}`, `users/${JOINER}`, `pets/${petId}`);
+  await clearRateLimits();
+});
+
+describe("joining a meetup a crowd is joining", () => {
+  it("waits and tries again when the join is aborted for contention, and joins once", async () => {
+    const contention = joinsContended(3);
+
+    const result = await callAs<{ success: boolean }>(joinMeetupCallable, JOINER, { meetupId: MEETUP_ID, petId });
+
+    expect(result.success).toBe(true);
+    expect(contention.aborted()).toBe(3);
+    expect((await db.doc(`${MEETUP}/participants/${JOINER}`).get()).exists).toBe(true);
+    expect((await db.doc(MEETUP).get()).get("participantCount")).toBe(2);
+  });
+
+  it("still fails when every try is contended", async () => {
+    const contention = joinsContended(Number.POSITIVE_INFINITY);
+
+    const code = await errorCodeOf(() => callAs(joinMeetupCallable, JOINER, { meetupId: MEETUP_ID, petId }));
+
+    expect(code).toContain("ABORTED");
+    expect(contention.aborted()).toBe(CONTENTION_RETRY.retries + 1);
+    expect((await db.doc(`${MEETUP}/participants/${JOINER}`).get()).exists).toBe(false);
+    expect((await db.doc(MEETUP).get()).get("participantCount")).toBe(1);
+  });
+});
diff --git a/functions/src/meetups.ts b/functions/src/meetups.ts
index 39980f9..cc8ac2a 100644
--- a/functions/src/meetups.ts
+++ b/functions/src/meetups.ts
@@ -14,6 +14,7 @@ import {
   requestData,
   requiredDocId,
   requiredTrimmedString,
+  retryOnContention,
   runEventOnce,
   stripUndefined,
   TRUSTED_MEDIA_URL_HOSTS,
@@ -743,7 +744,12 @@ export const joinMeetupCallable = onCall(async (request) => {
     await assertNoBlockBetween(callerUid, admissionOrganizerId, "Joining this meetup");
   }
 
-  return await db.runTransaction(async (t) => {
+  // Everyone joining one meetup reads and writes the same document, and two
+  // hundred at once had two joins aborted for contention on the test project,
+  // past the client library's own retries: told "internal" rather than that
+  // the meetup was full. Running the join again is safe — someone already on
+  // the roster is answered as joined.
+  return await retryOnContention(() => db.runTransaction(async (t) => {
     const meetupSnap = await t.get(meetupRef);
     if (!meetupSnap.exists) throw new HttpsError("not-found", "Meetup not found.");
     const meetup = meetupSnap.data() as Record<string, unknown>;
@@ -884,7 +890,7 @@ export const joinMeetupCallable = onCall(async (request) => {
     });
 
     return { success: true };
-  });
+  }));
 });
 
 // Flip expired meetups to "completed" even when no user visits the detail
diff --git a/functions/src/shared.ts b/functions/src/shared.ts
index b926fa4..3359891 100644
--- a/functions/src/shared.ts
+++ b/functions/src/shared.ts
@@ -383,9 +383,21 @@ export async function runEventOnce(
   // their counts aborted for contention on the test project, and these
   // triggers are not redelivered, so those likes were never counted. The
   // ledger makes another run safe, so a contended one waits and tries again.
+  return retryOnContention(once);
+}
+
+/**
+ * Runs `attempt` again, after a growing wait, while it fails for contention.
+ *
+ * The client library retries a contended transaction too, but back to back
+ * and five times at most, and a crowd on one document outlasts that. Only for
+ * work that is safe to run again: runEventOnce has its ledger, and a meetup
+ * join finds the person already on the roster.
+ */
+export async function retryOnContention<T>(attempt: () => Promise<T>): Promise<T> {
   for (let retry = 0; ; retry += 1) {
     try {
-      return await once();
+      return await attempt();
     } catch (error) {
       if (retry >= CONTENTION_RETRY.retries || !isContention(error)) throw error;
       const wait = Math.min(CONTENTION_RETRY.capMs, CONTENTION_RETRY.baseMs * 2 ** retry);