diff --git a/coderd/x/chatd/chattool/goal.go b/coderd/x/chatd/chattool/goal.go
index 7d46e66dd4..6e360afec6 100644
--- a/coderd/x/chatd/chattool/goal.go
+++ b/coderd/x/chatd/chattool/goal.go
@@ -32,8 +32,8 @@ type GoalToolOptions struct {
 type getGoalArgs struct{}
 
 type completeGoalArgs struct {
-	GoalID  uuid.UUID `json:"goal_id" description:"The expected current goal ID. The tool fails if the current goal changed."`
-	Summary string    `json:"summary" description:"A concise non-empty summary of how the goal was completed."`
+	GoalID  string `json:"goal_id" description:"The expected current goal ID. The tool fails if the current goal changed."`
+	Summary string `json:"summary" description:"A concise non-empty summary of how the goal was completed."`
 }
 
 type goalResult struct {
@@ -74,9 +74,16 @@ func CompleteGoal(db database.Store, options GoalToolOptions) fantasy.AgentTool
 			if !options.IsRootChat {
 				return fantasy.NewTextErrorResponse("complete_goal can only be used from the root chat"), nil
 			}
-			if args.GoalID == uuid.Nil {
+			goalIDStr := strings.TrimSpace(args.GoalID)
+			if goalIDStr == "" {
 				return fantasy.NewTextErrorResponse("goal_id is required"), nil
 			}
+			goalID, err := uuid.Parse(goalIDStr)
+			if err != nil {
+				return fantasy.NewTextErrorResponse(
+					xerrors.Errorf("invalid goal_id: %w", err).Error(),
+				), nil
+			}
 			summary := strings.TrimSpace(args.Summary)
 			if summary == "" {
 				return fantasy.NewTextErrorResponse("summary is required"), nil
@@ -98,7 +105,7 @@ func CompleteGoal(db database.Store, options GoalToolOptions) fantasy.AgentTool
 					}
 					return err
 				}
-				if current.ID != args.GoalID {
+				if current.ID != goalID {
 					return sql.ErrNoRows
 				}
 				if current.Status != database.ChatGoalStatusActive {
@@ -109,7 +116,7 @@ func CompleteGoal(db database.Store, options GoalToolOptions) fantasy.AgentTool
 				}
 				completed, err = tx.CompleteChatGoalByID(ctx, database.CompleteChatGoalByIDParams{
 					RootChatID: options.RootChatID,
-					ID:         args.GoalID,
+					ID:         goalID,
 					CompletionSummary: sql.NullString{
 						String: summary,
 						Valid:  true,
diff --git a/coderd/x/chatd/chattool/goal_test.go b/coderd/x/chatd/chattool/goal_test.go
index 3cc6dd95e3..10518149d4 100644
--- a/coderd/x/chatd/chattool/goal_test.go
+++ b/coderd/x/chatd/chattool/goal_test.go
@@ -81,6 +81,17 @@ func TestGoalTools(t *testing.T) {
 	require.Equal(t, database.ChatGoalStatusComplete, completedGoal.Status)
 }
 
+func TestCompleteGoalSchemaUsesStringGoalID(t *testing.T) {
+	t.Parallel()
+
+	tool := chattool.CompleteGoal(nil, chattool.GoalToolOptions{})
+	info := tool.Info()
+	goalIDParam, ok := info.Parameters["goal_id"].(map[string]any)
+	require.True(t, ok)
+	require.Equal(t, "string", goalIDParam["type"])
+	require.NotEqual(t, "array", goalIDParam["type"])
+}
+
 func TestGetGoalReturnsNullWithoutCurrentGoal(t *testing.T) {
 	t.Parallel()
 
@@ -134,6 +145,16 @@ func TestCompleteGoalValidatesInput(t *testing.T) {
 			input:   `{"summary":"done"}`,
 			message: "goal_id is required",
 		},
+		{
+			name:    "empty goal id",
+			input:   `{"goal_id":"   ","summary":"done"}`,
+			message: "goal_id is required",
+		},
+		{
+			name:    "invalid goal id",
+			input:   `{"goal_id":"not-a-uuid","summary":"done"}`,
+			message: "invalid goal_id",
+		},
 		{
 			name:    "empty summary",
 			input:   `{"goal_id":"00000000-0000-4000-8000-000000000001","summary":"  "}`,