diff --git a/apps/api/src/messaging-inbound.test.ts b/apps/api/src/messaging-inbound.test.ts
index b20083e..5f6991d 100644
--- a/apps/api/src/messaging-inbound.test.ts
+++ b/apps/api/src/messaging-inbound.test.ts
@@ -939,7 +939,7 @@ describe("createMessagingInboundHandler channel routing", () => {
     };
     const deps = createDeps({ members: [alice, carol] });
     const handle = createMessagingInboundHandler(deps);
-    await handle(groupEvent);
+    await handle({ ...groupEvent, participantsAuthoritative: true });
 
     expect(deps.prisma.messagingChannelMember.updateMany).toHaveBeenCalledWith(
       expect.objectContaining({
@@ -1004,6 +1004,35 @@ describe("createMessagingInboundHandler channel routing", () => {
     expect(sparse.prisma.messagingChannelMember.updateMany).not.toHaveBeenCalled();
   });
 
+  it("does not sweep approved members when the webhook roster is partial", async () => {
+    const alice = {
+      id: "cm-1",
+      channelId: "ch-1",
+      address: "+15551111111",
+      identityId: "mi-1",
+      status: "approved",
+    };
+    const bob = {
+      id: "cm-2",
+      channelId: "ch-1",
+      address: "+15552222222",
+      identityId: "mi-2",
+      status: "approved",
+    };
+    const deps = createDeps({ members: [alice, bob] });
+    const handle = createMessagingInboundHandler(deps);
+    // Sendblue often reports only the sender — not a complete roster.
+    await handle({
+      ...groupEvent,
+      participants: ["+15551111111"],
+    });
+
+    expect(deps.prisma.messagingChannelMember.updateMany).not.toHaveBeenCalledWith(
+      expect.objectContaining({ data: { status: "left" } }),
+    );
+    expect(bob.status).toBe("approved");
+  });
+
   it("sanitizes attacker-controlled channel names before storing them", async () => {
     const deps = createDeps();
     const handle = createMessagingInboundHandler(deps);
diff --git a/apps/api/src/messaging-inbound.ts b/apps/api/src/messaging-inbound.ts
index f536e64..0a00b7a 100644
--- a/apps/api/src/messaging-inbound.ts
+++ b/apps/api/src/messaging-inbound.ts
@@ -537,7 +537,7 @@ async function handleChannelEvent(
   // Someone removed from the group must stop receiving its content.
   // A webhook without a participants roster says nothing about membership —
   // never sweep on partial data.
-  if (event.participants.length > 0) {
+  if (event.participantsAuthoritative) {
     await deps.prisma.messagingChannelMember.updateMany({
       where: {
         channelId: channel.id,
diff --git a/packages/adapter-kit/src/types.ts b/packages/adapter-kit/src/types.ts
index cfb3a73..4e060be 100644
--- a/packages/adapter-kit/src/types.ts
+++ b/packages/adapter-kit/src/types.ts
@@ -587,6 +587,11 @@ export interface MessagingInboundMessage {
   channelName: string | null;
   /** Group roster addresses when the platform reports them; often empty. */
   participants: string[];
+  /**
+   * True only when {@link participants} is a complete group roster from the
+   * provider. Partial rosters must not trigger membership sweeps.
+   */
+  participantsAuthoritative?: boolean;
   content: string;
   mediaUrl: string | null;
   /** Team-room workspace/team id when the platform reports one (Slack team_id, …). */