diff --git a/workspace-server/internal/handlers/chat_files.go b/workspace-server/internal/handlers/chat_files.go
index aa2b8c05..ccfa0d4c 100644
--- a/workspace-server/internal/handlers/chat_files.go
+++ b/workspace-server/internal/handlers/chat_files.go
@@ -651,18 +651,19 @@ func (h *ChatFilesHandler) uploadPollMode(c *gin.Context, ctx context.Context, w
 		}
 
 		// Activity row so the workspace's inbox poller picks this up
-		// on its next cycle. type=chat_upload_receive is a new
-		// activity_type the workspace's message_from_activity adapter
-		// (Phase 2) will handle by fetching content via the GET
-		// endpoint. The request_body carries everything the
-		// workspace needs to dispatch the fetch — including the
-		// synthetic URI canvas embeds in the chat message.
+		// on its next cycle. activity_type=a2a_receive (NOT a new
+		// type) so the existing poll filter
+		// `?type=a2a_receive` catches it without poll-side changes;
+		// method=chat_upload_receive is the discriminator the
+		// workspace's adapter (Phase 2) uses to route to the upload
+		// fetcher instead of the agent's message handler. Same
+		// shape as A2A's tasks/send vs message/send method split.
 		uri := fmt.Sprintf("platform-pending:%s/%s", workspaceID, fileID)
 		summary := "chat_upload_receive: " + sanitized
 		method := "chat_upload_receive"
 		LogActivity(ctx, h.broadcaster, ActivityParams{
 			WorkspaceID:  workspaceID,
-			ActivityType: "chat_upload_receive",
+			ActivityType: "a2a_receive",
 			TargetID:     &workspaceID,
 			Method:       &method,
 			Summary:      &summary,
diff --git a/workspace-server/internal/handlers/chat_files_poll_test.go b/workspace-server/internal/handlers/chat_files_poll_test.go
index 40b289ff..c064bd6a 100644
--- a/workspace-server/internal/handlers/chat_files_poll_test.go
+++ b/workspace-server/internal/handlers/chat_files_poll_test.go
@@ -96,6 +96,37 @@ func expectActivityInsert(mock sqlmock.Sqlmock) {
 		WillReturnResult(sqlmock.NewResult(1, 1))
 }
 
+// expectActivityInsertWithTypeAndMethod is a strict variant that pins
+// the activity_type and method positional args. Used in the discriminator
+// regression test below — the workspace inbox poller filters
+// `?type=a2a_receive`, so writing any other activity_type silently breaks
+// poll-mode delivery without a build/test error. Pin the two discriminator
+// fields so a refactor that flips activity_type back to a custom value is
+// caught here instead of at runtime by a confused poller.
+//
+// Positional args (LogActivity uses ExecContext with 12 positional params):
+//   $1 workspace_id, $2 activity_type, $3 source_id, $4 target_id,
+//   $5 method, $6 summary, $7 request_body, $8 response_body,
+//   $9 tool_trace, $10 duration_ms, $11 status, $12 error_detail.
+func expectActivityInsertWithTypeAndMethod(mock sqlmock.Sqlmock, workspaceID, activityType, method string) {
+	mock.ExpectExec(`INSERT INTO activity_logs`).
+		WithArgs(
+			workspaceID,             // $1 workspace_id
+			activityType,            // $2 activity_type ← pinned
+			sqlmock.AnyArg(),        // $3 source_id
+			sqlmock.AnyArg(),        // $4 target_id (workspaceID, but already covered)
+			method,                  // $5 method ← pinned
+			sqlmock.AnyArg(),        // $6 summary
+			sqlmock.AnyArg(),        // $7 request_body
+			sqlmock.AnyArg(),        // $8 response_body
+			sqlmock.AnyArg(),        // $9 tool_trace
+			sqlmock.AnyArg(),        // $10 duration_ms
+			sqlmock.AnyArg(),        // $11 status
+			sqlmock.AnyArg(),        // $12 error_detail
+		).
+		WillReturnResult(sqlmock.NewResult(1, 1))
+}
+
 // pollUploadFixture builds a multipart body with N named files.
 func pollUploadFixture(t *testing.T, files map[string][]byte) (*bytes.Buffer, string) {
 	t.Helper()
@@ -518,3 +549,41 @@ func TestPollUpload_SanitizesFilenameInResponse(t *testing.T) {
 		t.Errorf("storage Put didn't receive sanitized filename: %+v", store.puts)
 	}
 }
+
+// TestPollUpload_ActivityRowDiscriminator pins the
+// activity_type / method shape that the workspace inbox poller depends
+// on. The poller filters `GET /workspaces/:id/activity?type=a2a_receive`
+// so the handler MUST write activity_type=a2a_receive (NOT a custom
+// type), and use method=chat_upload_receive as the
+// upload-vs-message-vs-task discriminator.
+//
+// Why pinned: a previous iteration of this handler used
+// activity_type="chat_upload_receive" — silently invisible to the
+// existing poller. The branch passed every push-mode test, every
+// storage test, and every per-file content test; the bug only
+// surfaced at runtime when the workspace polled and got nothing.
+// Encode the contract in a unit test so the next refactor can't
+// re-break it without a red CI.
+func TestPollUpload_ActivityRowDiscriminator(t *testing.T) {
+	mock := setupTestDB(t)
+	setupTestRedis(t)
+
+	wsID := "abc12345-6789-4abc-8def-000000000999"
+	expectPollDeliveryMode(mock, wsID, "poll")
+	expectActivityInsertWithTypeAndMethod(mock, wsID, "a2a_receive", "chat_upload_receive")
+
+	store := newInMemStorage()
+	h := NewChatFilesHandler(NewTemplatesHandler(t.TempDir(), nil)).
+		WithPendingUploads(store, nil)
+
+	body, ct := pollUploadFixture(t, map[string][]byte{"x.pdf": []byte("xx")})
+	c, w := makeUploadRequest(t, wsID, body, ct)
+	h.Upload(c)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("status=%d body=%s", w.Code, w.Body.String())
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("expectations: %v", err)
+	}
+}