diff --git a/src/features/team-mode/team-mailbox/ack.test.ts b/src/features/team-mode/team-mailbox/ack.test.ts
new file mode 100644
index 000000000..4c337c1dd
--- /dev/null
+++ b/src/features/team-mode/team-mailbox/ack.test.ts
@@ -0,0 +1,45 @@
+/// <reference types="bun-types" />
+
+import { describe, expect, test } from "bun:test"
+import { mkdtemp, readdir } from "node:fs/promises"
+import { randomUUID } from "node:crypto"
+import { tmpdir } from "node:os"
+import path from "node:path"
+
+import { TeamModeConfigSchema } from "../../../config/schema/team-mode"
+import { getInboxDir, resolveBaseDir } from "../team-registry/paths"
+import { ackMessages } from "./ack"
+import { sendMessage } from "./send"
+
+async function createBaseDirectory(): Promise<string> {
+  return await mkdtemp(path.join(tmpdir(), "team-mailbox-ack-"))
+}
+
+describe("ackMessages", () => {
+  test("moves inbox files into processed and stays idempotent", async () => {
+    // given
+    const config = TeamModeConfigSchema.parse({ base_dir: await createBaseDirectory() })
+    const teamRunId = randomUUID()
+    const messageId = randomUUID()
+    await sendMessage({
+      version: 1,
+      messageId,
+      from: "lead",
+      to: "m1",
+      kind: "message",
+      body: "hello",
+      timestamp: 100,
+    }, teamRunId, config, { isLead: true, activeMembers: ["m1"] })
+
+    // when
+    await ackMessages(teamRunId, "m1", [messageId], config)
+    await ackMessages(teamRunId, "m1", [messageId], config)
+
+    // then
+    const inboxDir = getInboxDir(resolveBaseDir(config), teamRunId, "m1")
+    const inboxEntries = await readdir(inboxDir)
+    const processedEntries = await readdir(path.join(inboxDir, "processed"))
+    expect(inboxEntries).not.toContain(`${messageId}.json`)
+    expect(processedEntries).toContain(`${messageId}.json`)
+  })
+})
diff --git a/src/features/team-mode/team-mailbox/ack.ts b/src/features/team-mode/team-mailbox/ack.ts
new file mode 100644
index 000000000..9428f7ae5
--- /dev/null
+++ b/src/features/team-mode/team-mailbox/ack.ts
@@ -0,0 +1,34 @@
+import { mkdir, rename } from "node:fs/promises"
+import path from "node:path"
+
+import type { TeamModeConfig } from "../../../config/schema/team-mode"
+import { getInboxDir, resolveBaseDir } from "../team-registry/paths"
+
+export async function ackMessages(
+  teamRunId: string,
+  memberName: string,
+  messageIds: string[],
+  config: TeamModeConfig,
+): Promise<void> {
+  const baseDir = resolveBaseDir(config)
+  const inboxDir = getInboxDir(baseDir, teamRunId, memberName)
+  const processedDir = path.join(inboxDir, "processed")
+  await mkdir(processedDir, { recursive: true, mode: 0o700 })
+
+  for (const messageId of messageIds) {
+    const messageFileName = `${messageId}.json`
+    const sourcePath = path.join(inboxDir, messageFileName)
+    const targetPath = path.join(processedDir, messageFileName)
+
+    try {
+      await rename(sourcePath, targetPath)
+    } catch (error) {
+      const err = error as NodeJS.ErrnoException
+      if (err.code === "ENOENT") {
+        continue
+      }
+
+      throw error
+    }
+  }
+}