```

src/discord/bot.ts

@@ -1,6 +1,7 @@
 import type { Client } from "discord.js";
 import type { AppConfig } from "../config";
 import { createDb } from "../db";
+import { logger } from "../logger";
 import { BoosterRoleService } from "../services/boosterRoleService";
 import { DrizzleBoosterRoleStore } from "../services/drizzleBoosterRoleStore";
 import { DiscordRoleRepository } from "../services/discordRoleRepository";
@@ -33,10 +34,13 @@ export function attachBotHandlers(client: Client, config: AppConfig): void {
     const hadBooster = oldMember.roles.cache.has(eligibilityRoleId);
     const hasBooster = newMember.roles.cache.has(eligibilityRoleId);
     if (!hadBooster && hasBooster && config.boosterGreetingChannelId) {
-      await sendBoostGreeting(newMember, config.boosterGreetingChannelId).catch((err) => {
+      const channel = newMember.guild.channels.cache.get(config.boosterGreetingChannelId);
+      if (channel && "send" in channel) {
+        await sendBoostGreeting(channel as { send(input: { content: string }): Promise<unknown> }, newMember.id, config.boosterGreetingChannelId).catch((err) => {
           logger.error("Failed to send boost greeting", { error: String(err), userId: newMember.id });
         });
       }
+    }
 
     // Handle boost loss (existing behaviour)
     const service = new BoosterRoleService(

src/discord/events/boostGreeting.test.ts

@@ -0,0 +1,35 @@
+import { describe, expect, test } from "bun:test";
+import { sendBoostGreeting } from "./boostGreeting";
+
+describe("sendBoostGreeting", () => {
+  test("sends boost greeting to the given channel", async () => {
+    const sent: string[] = [];
+    const channel = {
+      send: async (input: { content: string }): Promise<void> => {
+        sent.push(input.content);
+      },
+    };
+
+    await sendBoostGreeting(channel, "user-123", "channel-456");
+
+    expect(sent.length).toBe(1);
+    expect(sent[0]).toContain("Thank you for boosting");
+    expect(sent[0]).toContain("<@user-123>");
+    expect(sent[0]).toContain("/booster-role claim");
+    expect(sent[0]).toContain("<#channel-456>");
+  });
+
+  test("handles different user and channel IDs", async () => {
+    const sent: string[] = [];
+    const channel = {
+      send: async (input: { content: string }): Promise<void> => {
+        sent.push(input.content);
+      },
+    };
+
+    await sendBoostGreeting(channel, "other-user", "other-channel");
+
+    expect(sent[0]).toContain("<@other-user>");
+    expect(sent[0]).toContain("<#other-channel>");
+  });
+});

src/discord/events/boostGreeting.ts

@@ -1,50 +1,19 @@
 import { logger } from "../../logger";
 
-type GuildTextChannel = {
-  send(input: { content: string }): Promise<unknown>;
-};
-
-type GuildMemberLike = {
-  id: string;
-  user?: { id: string };
-  guild: {
-    id: string;
-    channels: {
-      cache: {
-        get(channelId: string): GuildTextChannel | undefined;
-      };
-    };
-  };
-};
-
 /**
- * Sends a welcome message to the configured greeting channel.
- * Call this after confirming the member newly gained the booster eligibility role.
+ * Sends a welcome message via the given channel when a member newly
+ * acquires the booster eligibility role (i.e. just boosted the server).
  */
 export async function sendBoostGreeting(
-  member: GuildMemberLike,
+  channel: { send(input: { content: string }): Promise<unknown> },
+  userId: string,
   greetingChannelId: string,
 ): Promise<void> {
-  const channel = member.guild.channels.cache.get(greetingChannelId);
-
-  if (!channel) {
-    logger.warn("Boost greeting channel not found", {
-      channelId: greetingChannelId,
-      guildId: member.guild.id,
-    });
-    return;
-  }
-
-  const mention = member.user?.id ?? member.id;
   const message =
-    `🎉 Thank you for boosting <@${mention}>! ` +
+    `🎉 Thank you for boosting <@${userId}>! ` +
     `You can now claim a custom role using \`/booster-role claim\` in <#${greetingChannelId}>.`;
 
   await channel.send({ content: message });
 
-  logger.info("Boost greeting sent", {
-    userId: member.id,
-    guildId: member.guild.id,
-    channelId: greetingChannelId,
-  });
+  logger.info("Boost greeting sent", { userId, channelId: greetingChannelId });
 }