diff --git a/apps/backend/internal/mcp/handlers/config_handlers_test.go b/apps/backend/internal/mcp/handlers/config_handlers_test.go
index a00f4e3091..089cd6adf4 100644
--- a/apps/backend/internal/mcp/handlers/config_handlers_test.go
+++ b/apps/backend/internal/mcp/handlers/config_handlers_test.go
@@ -515,6 +515,45 @@ func TestHandleArchiveTask_InvalidPayload(t *testing.T) {
 	assertWSError(t, resp, ws.ErrorCodeBadRequest)
 }
 
+// TestHandleArchiveTask_AlreadyArchived_IsIdempotent pins that re-archiving a
+// task the caller already archived reports success rather than surfacing the
+// ErrTaskAlreadyArchived sentinel as an opaque INTERNAL ERROR. Archiving is a
+// goal-state operation, so the requested state already holding is not a
+// failure — but the response flags it so the caller can tell the two apart.
+func TestHandleArchiveTask_AlreadyArchived_IsIdempotent(t *testing.T) {
+	svc, repo := newTestTaskService(t)
+	ctx := context.Background()
+	require.NoError(t, repo.CreateWorkspace(ctx, &models.Workspace{ID: "ws-1", Name: "Test"}))
+	require.NoError(t, repo.CreateWorkflow(ctx, &models.Workflow{ID: "wf-1", WorkspaceID: "ws-1", Name: "Board"}))
+	task, err := svc.CreateTask(ctx, &service.CreateTaskRequest{
+		WorkspaceID: "ws-1",
+		WorkflowID:  "wf-1",
+		Title:       "Archive me",
+	})
+	require.NoError(t, err)
+
+	h := &Handlers{taskSvc: svc, logger: testLogger(t).WithFields()}
+	msg := makeWSMessage(t, ws.ActionMCPArchiveTask, map[string]string{"task_id": task.ID})
+
+	resp, err := h.handleArchiveTask(ctx, msg)
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+	require.Equal(t, ws.MessageTypeResponse, resp.Type)
+	var first map[string]interface{}
+	require.NoError(t, json.Unmarshal(resp.Payload, &first))
+	assert.Equal(t, true, first["success"])
+	assert.NotContains(t, first, "already_archived", "first archive is a real state change")
+
+	resp2, err := h.handleArchiveTask(ctx, msg)
+	require.NoError(t, err)
+	require.NotNil(t, resp2)
+	require.Equal(t, ws.MessageTypeResponse, resp2.Type, "re-archiving must not surface as an error")
+	var second map[string]interface{}
+	require.NoError(t, json.Unmarshal(resp2.Payload, &second))
+	assert.Equal(t, true, second["success"])
+	assert.Equal(t, true, second["already_archived"])
+}
+
 func TestHandleUpdateTaskState_MissingTaskID(t *testing.T) {
 	h := &Handlers{}
 	msg := makeWSMessage(t, ws.ActionMCPUpdateTaskState, map[string]interface{}{
diff --git a/apps/backend/internal/mcp/handlers/config_task_handlers.go b/apps/backend/internal/mcp/handlers/config_task_handlers.go
index d8bc6ee499..22dd7288e5 100644
--- a/apps/backend/internal/mcp/handlers/config_task_handlers.go
+++ b/apps/backend/internal/mcp/handlers/config_task_handlers.go
@@ -11,6 +11,7 @@ import (
 	"github.com/kandev/kandev/internal/task/dto"
 	"github.com/kandev/kandev/internal/task/models"
 	taskrepo "github.com/kandev/kandev/internal/task/repository/sqlite"
+	"github.com/kandev/kandev/internal/task/service"
 	v1 "github.com/kandev/kandev/pkg/api/v1"
 	ws "github.com/kandev/kandev/pkg/websocket"
 	"go.uber.org/zap"
@@ -335,6 +336,16 @@ func (h *Handlers) handleArchiveTask(ctx context.Context, msg *ws.Message) (*ws.
 	}
 
 	if err := h.taskSvc.ArchiveTask(ctx, taskID); err != nil {
+		// Archiving is a goal-state operation: a task that is already archived
+		// is in the requested state, so report success instead of an opaque
+		// internal error. The flag lets the caller tell a no-op from a real
+		// state change.
+		if errors.Is(err, service.ErrTaskAlreadyArchived) {
+			return ws.NewResponse(msg.ID, msg.Action, map[string]interface{}{
+				"success":          true,
+				"already_archived": true,
+			})
+		}
 		h.logger.Error("failed to archive task", zap.Error(err))
 		return ws.NewError(msg.ID, msg.Action, ws.ErrorCodeInternalError, "Failed to archive task", nil)
 	}
diff --git a/apps/backend/internal/mcp/server/server.go b/apps/backend/internal/mcp/server/server.go
index b39440f4df..0366676425 100644
--- a/apps/backend/internal/mcp/server/server.go
+++ b/apps/backend/internal/mcp/server/server.go
@@ -694,7 +694,7 @@ func (s *Server) registerKanbanTools() {
 	)
 	s.mcpServer.AddTool(
 		mcp.NewTool("archive_task_kandev",
-			mcp.WithDescription("Archive a task. The task is hidden from active board views but kept in the database. Use to tidy up finished or abandoned tasks. Unarchiving is a user action done from the UI, not via MCP."),
+			mcp.WithDescription("Archive a task. The task is hidden from active board views but kept in the database. Use to tidy up finished or abandoned tasks. Archiving an already-archived task is a no-op that succeeds with already_archived: true. Unarchiving is a user action done from the UI, not via MCP."),
 			mcp.WithString("task_id", mcp.Required(), mcp.Description("The task ID to archive")),
 		),
 		s.wrapHandler("archive_task_kandev", s.archiveTaskHandler()),