diff --git a/src/automation/tasks/standupMissingReminderSend.test.ts b/src/automation/tasks/standupMissingReminderSend.test.ts
new file mode 100644
index 0000000..f1715b7
--- /dev/null
+++ b/src/automation/tasks/standupMissingReminderSend.test.ts
@@ -0,0 +1,351 @@
+/**
+ * Offline unit tests for the missing-standup reminder task. No live DB or Slack/SMTP
+ * access — the prisma, sendSlackProjectMessage, and sendTemplateEmail collaborators are
+ * injected via runStandupMissingReminderSend's dependency seam, mirroring the
+ * projectParticipations(prisma) seam used by syncAlumniInteractions.test.ts.
+ *
+ * These tests pin the idempotency-flag invariants that are easy to silently regress:
+ * the Slack flag must be flipped only after a successful send, a failed send must leave
+ * it false so the next cron tick retries, and every "nothing to send" branch must still
+ * close the flag out so a row is never re-fetched forever.
+ *
+ * Run with:
+ *   npx tsx src/automation/tasks/standupMissingReminderSend.test.ts
+ *   (or: npx ts-node --transpile-only src/automation/tasks/standupMissingReminderSend.test.ts)
+ */
+/* eslint-disable @typescript-eslint/no-var-requires, import/no-dynamic-require */
+import 'reflect-metadata';
+import type { StandupMissingReminderDeps } from './standupMissingReminderSend';
+
+// The production module transitively imports src/config (via ../../email), which throws at
+// load time if a long list of env vars is missing. Stub every required var so the module can
+// be imported in a test environment with no real database/SMTP/Slack wired up. The module is
+// loaded with `require` (below) AFTER this block so the stubs are in place first: top-level
+// `import` declarations are hoisted above this statement block and would run too early.
+const REQUIRED_ENV_VARS = [
+  'DATABASE_URL', 'ELASTIC_URL', 'ELASTIC_INDEX', 'AUTH_SECRET', 'AUTH_AUDIENCE',
+  'EMAIL_HOST', 'EMAIL_PORT', 'EMAIL_USER', 'EMAIL_PASS', 'EMAIL_INBOUND_DOMAIN',
+  'GEOCODIO_API_KEY', 'OPENAI_API_KEY', 'OPENAI_ORGANIZATION', 'WEBHOOK_KEY',
+  'BADGR_USERNAME', 'BADGR_PASSWORD', 'BADGR_ISSUER', 'SHOPIFY_API_TOKEN',
+  'SHOPIFY_API_KEY', 'SHOPIFY_API_SECRET_KEY', 'SHOPIFY_STORE_DOMAIN',
+  'LINEAR_API_KEY', 'LINEAR_TEAM_ID', 'LINEAR_PROBLEM_LABEL_ID', 'METRICS_KEY',
+  'PLACID_API_TOKEN', 'ATTIO_API_TOKEN', 'ATTIO_ALUMNI_LIST',
+];
+for (const k of REQUIRED_ENV_VARS) if (!process.env[k]) process.env[k] = `test-stub:${k}`;
+
+const { runStandupMissingReminderSend } = require('./standupMissingReminderSend') as typeof import('./standupMissingReminderSend');
+
+let failures = 0;
+
+function assert(condition: boolean, message: string): void {
+  if (!condition) {
+    failures += 1;
+    console.error(`FAILED: ${message}`);
+  } else {
+    console.log(`PASSED: ${message}`);
+  }
+}
+
+// --- Fakes ----------------------------------------------------------------------------------
+
+type TestStudent = {
+  id: string; slackId: string | null; email: string; givenName: string;
+  weeks: number; minHours: number;
+  event: { id: string; name: string; emailSignature: string | null; title: string; defaultWeeks: number; startsAt: Date };
+};
+type TestProject = {
+  slackChannelId: string | null;
+  event: { slackWorkspaceAccessToken: string | null; slackWorkspaceId: string | null; name: string; startsAt: Date } | null;
+  students: TestStudent[];
+};
+type TestRow = {
+  id: string; projectId: string; dueAt: Date;
+  sentMissingReminderSlack: boolean; sentMissingReminderEmail: boolean;
+  results: { student: { id: string } }[];
+  project: TestProject;
+};
+
+type UpdateCall = { id: string; data: Record<string, unknown> };
+
+interface FakePrisma {
+  standupThread: {
+    findMany: (args?: unknown) => Promise<TestRow[]>;
+    update: (args: { where: { id: string }; data: Record<string, unknown> }) => Promise<TestRow>;
+  };
+}
+
+function makeFakePrisma(rows: TestRow[], updates: UpdateCall[]): FakePrisma {
+  return {
+    standupThread: {
+      findMany: async () => rows.filter(
+        (r) => r.sentMissingReminderSlack === false || r.sentMissingReminderEmail === false
+      ),
+      update: async (args) => {
+        updates.push({ id: args.where.id, data: { ...args.data } });
+        const row = rows.find((r) => r.id === args.where.id)!;
+        Object.assign(row, args.data);
+        return row;
+      },
+    },
+  };
+}
+
+type SlackCall = { channelId: string | null; message: string };
+
+function makeSendSlack(behaviors: ('resolve' | 'reject')[], calls: SlackCall[]) {
+  let i = 0;
+  return async (project: { slackChannelId: string | null }, message: string) => {
+    calls.push({ channelId: project.slackChannelId, message });
+    const behavior = behaviors[i] ?? 'resolve';
+    i += 1;
+    if (behavior === 'reject') throw new Error('transient Slack chat.postMessage 5xx');
+  };
+}
+
+function makeSendEmail(behaviors: ('resolve' | 'reject')[]) {
+  let i = 0;
+  return async (
+    _template: string,
+    _context: unknown,
+    _subject: string,
+    _to: TestStudent[],
+  ) => {
+    const behavior = behaviors[i] ?? 'resolve';
+    i += 1;
+    if (behavior === 'reject') throw new Error('transient SMTP failure');
+  };
+}
+
+// Row factories -----------------------------------------------------------------------------
+
+const EVENT_START = new Date('2025-09-01T00:00:00Z');
+const DUE_AT = new Date('2025-09-01T00:00:00Z'); // == startsAt => weeksSinceStart === 0
+
+function student(over: Partial<TestStudent> = {}): TestStudent {
+  return {
+    id: 'student-1',
+    slackId: 'U123',
+    email: 'student@example.com',
+    givenName: 'Student',
+    weeks: 5,
+    minHours: 10,
+    event: { id: 'evt-1', name: 'CodeDay Labs', emailSignature: null, title: 'Labs', defaultWeeks: 10, startsAt: EVENT_START },
+    ...over,
+  };
+}
+
+function row(over: Partial<TestRow> = {}): TestRow {
+  return {
+    id: 'standup-1',
+    projectId: 'project-1',
+    dueAt: DUE_AT,
+    sentMissingReminderSlack: false,
+    sentMissingReminderEmail: false,
+    results: [],
+    project: {
+      slackChannelId: 'C123',
+      event: { slackWorkspaceAccessToken: 'xoxb-token', slackWorkspaceId: 'T123', name: 'CodeDay Labs', startsAt: EVENT_START },
+      students: [student()],
+    },
+    ...over,
+  };
+}
+
+function depsWith(
+  rows: TestRow[],
+  updates: UpdateCall[],
+  slackBehaviors: ('resolve' | 'reject')[],
+  slackCalls: SlackCall[],
+  emailBehaviors: ('resolve' | 'reject')[],
+): StandupMissingReminderDeps {
+  return {
+    prisma: makeFakePrisma(rows, updates) as unknown as StandupMissingReminderDeps['prisma'],
+    sendSlackProjectMessage: makeSendSlack(slackBehaviors, slackCalls) as StandupMissingReminderDeps['sendSlackProjectMessage'],
+    sendTemplateEmail: makeSendEmail(emailBehaviors) as StandupMissingReminderDeps['sendTemplateEmail'],
+  };
+}
+
+function slackFlagSetFor(updates: UpdateCall[], id: string): boolean {
+  return updates.some((u) => u.id === id && u.data.sentMissingReminderSlack === true);
+}
+function combinedFlagUpdateFor(updates: UpdateCall[], id: string): boolean {
+  return updates.some((u) => u.id === id && u.data.sentMissingReminderSlack === true && u.data.sentMissingReminderEmail === true);
+}
+
+// --- Tests ---------------------------------------------------------------------------------
+
+// Core regression: a transient Slack send failure must NOT flip sentMissingReminderSlack to
+// true. The flag must stay false so the next cron tick retries.
+async function testSlackFailureLeavesFlagFalse(): Promise<void> {
+  const rows = [row()];
+  const updates: UpdateCall[] = [];
+  const slackCalls: SlackCall[] = [];
+  const deps = depsWith(rows, updates, ['reject'], slackCalls, ['resolve']);
+
+  await runStandupMissingReminderSend(deps);
+
+  assert(slackCalls.length === 1, 'Slack send was attempted exactly once');
+  assert(!slackFlagSetFor(updates, 'standup-1'), 'Slack flag is NOT set when sendSlackProjectMessage throws (retry stays possible)');
+  assert(rows[0].sentMissingReminderSlack === false, 'Row retains sentMissingReminderSlack=false after a failed Slack send');
+}
+
+// Happy path: successful Slack send marks sentMissingReminderSlack true (no duplicate sends
+// on subsequent ticks since the OR filter excludes a fully-true row).
+async function testSlackSuccessMarksFlagTrueOnce(): Promise<void> {
+  const rows = [row()];
+  const updates: UpdateCall[] = [];
+  const slackCalls: SlackCall[] = [];
+  const deps = depsWith(rows, updates, ['resolve'], slackCalls, ['resolve']);
+
+  await runStandupMissingReminderSend(deps);
+
+  assert(slackCalls.length === 1, 'Slack send called exactly once');
+  assert(slackFlagSetFor(updates, 'standup-1'), 'Slack flag is set true after a successful send');
+  assert(rows[0].sentMissingReminderSlack === true, 'Row reflects sentMissingReminderSlack=true');
+}
+
+// Ordering guarantee: the Slack flag update happens AFTER the send resolves, not before.
+// This is the exact invariant the bug violated — a regression that moves the update earlier
+// into "both flags up front" must turn this red.
+async function testSlackFlagSetAfterSendNotBefore(): Promise<void> {
+  const rows = [row()];
+  const updates: UpdateCall[] = [];
+  const slackCalls: SlackCall[] = [];
+  const events: string[] = [];
+  const slack = async (p: { slackChannelId: string | null }, m: string) => {
+    events.push('slack-send-start');
+    slackCalls.push({ channelId: p.slackChannelId, message: m });
+    events.push('slack-send-end');
+  };
+  const prisma = makeFakePrisma(rows, updates);
+  const origUpdate = prisma.standupThread.update.bind(prisma.standupThread);
+  prisma.standupThread.update = async (args) => {
+    events.push('update:' + Object.keys(args.data).join(','));
+    return origUpdate(args);
+  };
+  const deps: StandupMissingReminderDeps = {
+    prisma: prisma as unknown as StandupMissingReminderDeps['prisma'],
+    sendSlackProjectMessage: slack as unknown as StandupMissingReminderDeps['sendSlackProjectMessage'],
+    sendTemplateEmail: makeSendEmail(['resolve']) as StandupMissingReminderDeps['sendTemplateEmail'],
+  };
+
+  await runStandupMissingReminderSend(deps);
+
+  const slackUpdateIdx = events.findIndex((e) => e === 'update:sentMissingReminderSlack');
+  const slackSendEndIdx = events.findIndex((e) => e === 'slack-send-end');
+  assert(slackUpdateIdx > slackSendEndIdx, 'Slack flag update is written AFTER sendSlackProjectMessage resolves (not before)');
+}
+
+// Batch isolation: a transient Slack failure on row N must not abort row N+1 in the same tick.
+// A future refactor that removes the per-row try/catch would re-introduce this footgun.
+async function testSlackFailureOnOneRowDoesNotAbortRestOfBatch(): Promise<void> {
+  const rows = [
+    row({ id: 'bad', project: { slackChannelId: 'C-bad', event: { slackWorkspaceAccessToken: 'xoxb', slackWorkspaceId: 'T', name: 'CodeDay Labs', startsAt: EVENT_START }, students: [student({ id: 's-bad', slackId: 'U-bad', email: 'bad@example.com' })] } }),
+    row({ id: 'good', project: { slackChannelId: 'C-good', event: { slackWorkspaceAccessToken: 'xoxb', slackWorkspaceId: 'T', name: 'CodeDay Labs', startsAt: EVENT_START }, students: [student({ id: 's-good', slackId: 'U-good', email: 'good@example.com' })] } }),
+  ];
+  const updates: UpdateCall[] = [];
+  const slackCalls: SlackCall[] = [];
+  const deps = depsWith(rows, updates, ['reject', 'resolve'], slackCalls, ['resolve', 'resolve']);
+
+  await runStandupMissingReminderSend(deps);
+
+  assert(slackCalls.length === 2, 'Both rows attempt a Slack send in the same tick despite the first failing');
+  assert(!slackFlagSetFor(updates, 'bad'), 'The failing row retains sentMissingReminderSlack=false (will be retried)');
+  assert(slackFlagSetFor(updates, 'good'), 'The subsequent row still gets sentMissingReminderSlack=true in the same tick');
+}
+
+// End-to-end retry via the findMany OR filter: tick 1 fails (flag false), tick 2 re-selects
+// the row and succeeds (flag true), tick 3 no longer selects it.
+async function testRetrySucceedsOnNextTickAfterTransientFailure(): Promise<void> {
+  const rows = [row()];
+  const updates: UpdateCall[] = [];
+
+  const slack1: SlackCall[] = [];
+  await runStandupMissingReminderSend(depsWith(rows, updates, ['reject'], slack1, ['resolve']));
+  assert(!slackFlagSetFor(updates, 'standup-1'), 'After tick 1 (failed send) sentMissingReminderSlack is still false');
+  assert(rows[0].sentMissingReminderSlack === false, 'Row remains eligible for re-selection after a failed Slack send');
+
+  const selectedAfterTick1 = rows.filter((r) => r.sentMissingReminderSlack === false || r.sentMissingReminderEmail === false);
+  assert(selectedAfterTick1.length === 1, 'Row is re-selected by the OR filter after a failed Slack send');
+
+  await runStandupMissingReminderSend(depsWith(rows, updates, ['resolve'], [], ['resolve']));
+  assert(slackFlagSetFor(updates, 'standup-1'), 'After tick 2 (successful retry) sentMissingReminderSlack is set true');
+
+  const selectedAfterTick2 = rows.filter((r) => r.sentMissingReminderSlack === false || r.sentMissingReminderEmail === false);
+  assert(selectedAfterTick2.length === 0, 'Row is no longer re-selected once both idempotency flags are true');
+}
+
+// "Nothing to send" branches must still mark the flag true so a row is never re-fetched
+// forever. Covers both no-Slack-integration-configured and no-connected-Slack-students.
+async function testNothingToSendMarksFlagTrueToAvoidInfiniteRefetch(): Promise<void> {
+  const noConfigRows = [row({ project: { slackChannelId: 'C123', event: { slackWorkspaceAccessToken: null, slackWorkspaceId: 'T123', name: 'CodeDay Labs', startsAt: EVENT_START }, students: [student({ slackId: 'U123' })] } })];
+  const noConfigUpdates: UpdateCall[] = [];
+  const noConfigSlack: SlackCall[] = [];
+  await runStandupMissingReminderSend(depsWith(noConfigRows, noConfigUpdates, [], noConfigSlack, ['resolve']));
+  assert(noConfigSlack.length === 0, 'No Slack send attempted when Slack is not configured');
+  assert(slackFlagSetFor(noConfigUpdates, 'standup-1'), 'Flag marked true when Slack is not configured (avoids infinite re-fetch)');
+
+  const noSlackIdRows = [row({ project: { slackChannelId: 'C123', event: { slackWorkspaceAccessToken: 'xoxb', slackWorkspaceId: 'T', name: 'CodeDay Labs', startsAt: EVENT_START }, students: [student({ slackId: null, email: 'noslack@example.com' })] } })];
+  const noSlackIdUpdates: UpdateCall[] = [];
+  const noSlackIdSlack: SlackCall[] = [];
+  await runStandupMissingReminderSend(depsWith(noSlackIdRows, noSlackIdUpdates, [], noSlackIdSlack, ['resolve']));
+  assert(noSlackIdSlack.length === 0, 'No Slack send attempted when no missing student has a slackId');
+  assert(slackFlagSetFor(noSlackIdUpdates, 'standup-1'), 'Flag marked true when there is no connected-Slack recipient (avoids infinite re-fetch)');
+}
+
+// Zero-missing-students short-circuit: nothing to send at all. Both flags set true in one
+// update; no sends; row excluded on the next tick.
+async function testNoMissingStudentsSetsBothFlagsAndContinues(): Promise<void> {
+  const s = student({ id: 'present-1' });
+  const rows = [row({ results: [{ student: { id: 'present-1' } }], project: { slackChannelId: 'C123', event: { slackWorkspaceAccessToken: 'xoxb', slackWorkspaceId: 'T', name: 'CodeDay Labs', startsAt: EVENT_START }, students: [s] } })];
+  const updates: UpdateCall[] = [];
+  const slackCalls: SlackCall[] = [];
+  const deps = depsWith(rows, updates, [], slackCalls, []);
+
+  await runStandupMissingReminderSend(deps);
+
+  assert(slackCalls.length === 0, 'No Slack send when no students missed');
+  assert(combinedFlagUpdateFor(updates, 'standup-1'), 'Both flags set true in one update when no students missed');
+  assert(rows[0].sentMissingReminderSlack === true && rows[0].sentMissingReminderEmail === true, 'Row excluded from future findMany once both flags are true');
+  assert(rows.filter((r) => r.sentMissingReminderSlack === false || r.sentMissingReminderEmail === false).length === 0, 'Zero-missing row is not re-selected on the next tick');
+}
+
+// Email channel behavior is intentionally unchanged: the email flag is set regardless of
+// email send success/failure (errors swallowed, no retry) — matching sendScheduledAnnouncements.
+// Guards against an accidental "make email retry too" change that would diverge from the
+// sibling task's accepted convention.
+async function testEmailFailureStillMarksFlagTrue(): Promise<void> {
+  const rows = [row()];
+  const updates: UpdateCall[] = [];
+  const slackCalls: SlackCall[] = [];
+  const deps = depsWith(rows, updates, ['resolve'], slackCalls, ['reject']);
+
+  await runStandupMissingReminderSend(deps);
+
+  assert(emailFlagTrueByEmailState(rows[0]), 'Email flag set true even when the email send fails (unchanged email convention: no retry)');
+}
+
+function emailFlagTrueByEmailState(r: TestRow): boolean {
+  return r.sentMissingReminderEmail === true;
+}
+
+// --- Runner --------------------------------------------------------------------------------
+
+async function main(): Promise<void> {
+  await testSlackFailureLeavesFlagFalse();
+  await testSlackSuccessMarksFlagTrueOnce();
+  await testSlackFlagSetAfterSendNotBefore();
+  await testSlackFailureOnOneRowDoesNotAbortRestOfBatch();
+  await testRetrySucceedsOnNextTickAfterTransientFailure();
+  await testNothingToSendMarksFlagTrueToAvoidInfiniteRefetch();
+  await testNoMissingStudentsSetsBothFlagsAndContinues();
+  await testEmailFailureStillMarksFlagTrue();
+
+  if (failures > 0) {
+    console.error(`\n${failures} test(s) failed.`);
+    process.exit(1);
+  }
+  console.log('\nAll tests passed.');
+}
+
+main();
diff --git a/src/automation/tasks/standupMissingReminderSend.ts b/src/automation/tasks/standupMissingReminderSend.ts
index 951b8d9..eea93cf 100644
--- a/src/automation/tasks/standupMissingReminderSend.ts
+++ b/src/automation/tasks/standupMissingReminderSend.ts
@@ -19,8 +19,22 @@ const MISSING_STANDUP_SLACK_MESSAGES = [
   'As a reminder, standups are required.',
 ];
 
+export type StandupMissingReminderDeps = {
+  prisma: PrismaClient;
+  sendSlackProjectMessage: typeof sendSlackProjectMessage;
+  sendTemplateEmail: typeof sendTemplateEmail;
+};
+
 export default async function standupMissingReminderSend() {
-  const prisma = Container.get(PrismaClient);
+  return runStandupMissingReminderSend({
+    prisma: Container.get(PrismaClient),
+    sendSlackProjectMessage,
+    sendTemplateEmail,
+  });
+}
+
+export async function runStandupMissingReminderSend(deps: StandupMissingReminderDeps) {
+  const { prisma, sendSlackProjectMessage, sendTemplateEmail } = deps;
   const standups = await prisma.standupThread.findMany({
     where: {
       event: {
@@ -75,15 +89,21 @@ export default async function standupMissingReminderSend() {
     ));
 
     DEBUG(`Sending missing standup reminders to ${missingStudents.length} for ${standup.id} (project: ${standup.projectId})`)
-    await prisma.standupThread.update({
-      where: { id: standup.id },
-      data: { sentMissingReminderSlack: true, sentMissingReminderEmail: true },
-    });
 
-    if (missingStudents.length === 0) continue;
+    if (missingStudents.length === 0) {
+      await prisma.standupThread.update({
+        where: { id: standup.id },
+        data: { sentMissingReminderSlack: true, sentMissingReminderEmail: true },
+      });
+      continue;
+    }
 
     // Send email reminders
     if (!standup.sentMissingReminderEmail) {
+      await prisma.standupThread.update({
+        where: { id: standup.id },
+        data: { sentMissingReminderEmail: true },
+      });
       try {
         await sendTemplateEmail(
           'standupMissingReminder',
@@ -101,12 +121,24 @@ export default async function standupMissingReminderSend() {
       const missingStudentsWithConnectedSlack = missingStudents.filter(s => !!s.slackId);
       DEBUG(`Sending Slack reminders to ${missingStudentsWithConnectedSlack.length} students for ${standup.id}`);
       if (canSendSlackProjectMessage(standup.project) && missingStudentsWithConnectedSlack.length > 0) {
-        await sendSlackProjectMessage(
-          standup.project,
-          `${missingStudentsWithConnectedSlack.map(s => `<@${s.slackId}>`).join(' ')} ${randomSlackMessage}`
-        );
+        try {
+          await sendSlackProjectMessage(
+            standup.project,
+            `${missingStudentsWithConnectedSlack.map(s => `<@${s.slackId}>`).join(' ')} ${randomSlackMessage}`
+          );
+          await prisma.standupThread.update({
+            where: { id: standup.id },
+            data: { sentMissingReminderSlack: true },
+          });
+        } catch (ex) {
+          DEBUG(ex);
+        }
+      } else {
+        await prisma.standupThread.update({
+          where: { id: standup.id },
+          data: { sentMissingReminderSlack: true },
+        });
       }
-
     }
   }
 }