diff --git a/apps/api/internal/handler/issue.go b/apps/api/internal/handler/issue.go
index e241f967..822e87de 100644
--- a/apps/api/internal/handler/issue.go
+++ b/apps/api/internal/handler/issue.go
@@ -17,6 +17,14 @@ type IssueHandler struct {
 	Issue *service.IssueService
 }
 
+// invalidRelationError reports whether err is a rejected related-id error that
+// should surface as a 400 (bad state/label/parent/assignee for the scope).
+func invalidRelationError(err error) bool {
+	return err == service.ErrInvalidState || err == service.ErrInvalidLabel ||
+		err == service.ErrInvalidParent || err == service.ErrInvalidAssignee ||
+		err == service.ErrInvalidPriority
+}
+
 func issueID(c *gin.Context) (uuid.UUID, bool) {
 	idStr := c.Param("pk")
 	if idStr == "" {
@@ -197,6 +205,10 @@ func (h *IssueHandler) Create(c *gin.Context) {
 			c.JSON(http.StatusNotFound, gin.H{"error": "Not found"})
 			return
 		}
+		if invalidRelationError(err) {
+			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+			return
+		}
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to create issue"})
 		return
 	}
@@ -305,6 +317,10 @@ func (h *IssueHandler) Update(c *gin.Context) {
 			c.JSON(http.StatusNotFound, gin.H{"error": "Issue not found"})
 			return
 		}
+		if invalidRelationError(err) {
+			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+			return
+		}
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to update issue"})
 		return
 	}
diff --git a/apps/api/internal/handler/issue_relation_validation_test.go b/apps/api/internal/handler/issue_relation_validation_test.go
new file mode 100644
index 00000000..5c49c499
--- /dev/null
+++ b/apps/api/internal/handler/issue_relation_validation_test.go
@@ -0,0 +1,83 @@
+package handler_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/Devlaner/devlane/api/internal/testutil"
+	"github.com/stretchr/testify/require"
+)
+
+func TestIssue_CreateRejectsForeignRelations(t *testing.T) {
+	ts := testutil.NewTestServer(t)
+	w := testutil.SeedWorld(t, ts.DB)
+
+	// A second project in the same workspace, and one in another workspace.
+	otherProject := testutil.CreateProject(t, ts.DB, w.Workspace.ID, w.User.ID)
+	foreignState := testutil.CreateState(t, ts.DB, otherProject.ID, w.Workspace.ID)
+	foreignLabel := testutil.CreateLabel(t, ts.DB, otherProject.ID, w.Workspace.ID)
+	foreignParent := testutil.CreateIssue(t, ts.DB, otherProject.ID, w.Workspace.ID, w.User.ID)
+	nonMember := testutil.CreateUser(t, ts.DB)
+
+	base := "/api/workspaces/" + w.Workspace.Slug + "/projects/" + w.Project.ID.String() + "/issues/"
+
+	require.Equal(t, http.StatusBadRequest,
+		ts.POST(base, map[string]any{"name": "x", "state_id": foreignState.ID.String()}, w.Session).Code,
+		"a state from another project must be rejected")
+	require.Equal(t, http.StatusBadRequest,
+		ts.POST(base, map[string]any{"name": "x", "label_ids": []string{foreignLabel.ID.String()}}, w.Session).Code,
+		"a label from another project must be rejected")
+	require.Equal(t, http.StatusBadRequest,
+		ts.POST(base, map[string]any{"name": "x", "parent_id": foreignParent.ID.String()}, w.Session).Code,
+		"a parent from another project must be rejected")
+	require.Equal(t, http.StatusBadRequest,
+		ts.POST(base, map[string]any{"name": "x", "assignee_ids": []string{nonMember.ID.String()}}, w.Session).Code,
+		"an assignee who isn't a workspace member must be rejected")
+
+	// A well-scoped create still works.
+	okState := testutil.CreateState(t, ts.DB, w.Project.ID, w.Workspace.ID)
+	okLabel := testutil.CreateLabel(t, ts.DB, w.Project.ID, w.Workspace.ID)
+	okParent := testutil.CreateIssue(t, ts.DB, w.Project.ID, w.Workspace.ID, w.User.ID)
+	rr := ts.POST(base, map[string]any{
+		"name":         "valid",
+		"state_id":     okState.ID.String(),
+		"label_ids":    []string{okLabel.ID.String()},
+		"assignee_ids": []string{w.User.ID.String()},
+		"parent_id":    okParent.ID.String(),
+	}, w.Session)
+	require.Equal(t, http.StatusCreated, rr.Code, "body=%s", rr.Body.String())
+}
+
+func TestIssue_UpdateRejectsForeignRelations(t *testing.T) {
+	ts := testutil.NewTestServer(t)
+	w := testutil.SeedWorld(t, ts.DB)
+	otherProject := testutil.CreateProject(t, ts.DB, w.Workspace.ID, w.User.ID)
+	foreignState := testutil.CreateState(t, ts.DB, otherProject.ID, w.Workspace.ID)
+	foreignLabel := testutil.CreateLabel(t, ts.DB, otherProject.ID, w.Workspace.ID)
+	nonMember := testutil.CreateUser(t, ts.DB)
+
+	issue := testutil.CreateIssue(t, ts.DB, w.Project.ID, w.Workspace.ID, w.User.ID)
+	base := "/api/workspaces/" + w.Workspace.Slug + "/projects/" + w.Project.ID.String() +
+		"/issues/" + issue.ID.String() + "/"
+
+	require.Equal(t, http.StatusBadRequest,
+		ts.PATCH(base, map[string]any{"state_id": foreignState.ID.String()}, w.Session).Code)
+	require.Equal(t, http.StatusBadRequest,
+		ts.PATCH(base, map[string]any{"label_ids": []string{foreignLabel.ID.String()}}, w.Session).Code)
+	require.Equal(t, http.StatusBadRequest,
+		ts.PATCH(base, map[string]any{"assignee_ids": []string{nonMember.ID.String()}}, w.Session).Code)
+
+	// A parent from another project is rejected.
+	foreignParent := testutil.CreateIssue(t, ts.DB, otherProject.ID, w.Workspace.ID, w.User.ID)
+	require.Equal(t, http.StatusBadRequest,
+		ts.PATCH(base, map[string]any{"parent_id": foreignParent.ID.String()}, w.Session).Code)
+
+	// An issue can't be made its own parent.
+	require.Equal(t, http.StatusBadRequest,
+		ts.PATCH(base, map[string]any{"parent_id": issue.ID.String()}, w.Session).Code)
+
+	// A well-scoped update still works.
+	okState := testutil.CreateState(t, ts.DB, w.Project.ID, w.Workspace.ID)
+	require.Equal(t, http.StatusOK,
+		ts.PATCH(base, map[string]any{"state_id": okState.ID.String()}, w.Session).Code)
+}
diff --git a/apps/api/internal/router/router.go b/apps/api/internal/router/router.go
index d99ce034..c76ad86c 100644
--- a/apps/api/internal/router/router.go
+++ b/apps/api/internal/router/router.go
@@ -161,6 +161,7 @@ func New(cfg Config) *gin.Engine {
 	issueReactionStore := store.NewIssueReactionStore(cfg.DB)
 	issueSvc.SetReactionStore(issueReactionStore)
 	issueSvc.SetStateStore(stateStore)
+	issueSvc.SetLabelStore(labelStore)
 	commentReactionStore := store.NewCommentReactionStore(cfg.DB)
 	commentSvc := service.NewCommentService(commentStore, issueStore, projectStore, workspaceStore)
 	commentSvc.SetReactionStore(commentReactionStore)
diff --git a/apps/api/internal/service/issue.go b/apps/api/internal/service/issue.go
index 2d405a8d..aa365d7e 100644
--- a/apps/api/internal/service/issue.go
+++ b/apps/api/internal/service/issue.go
@@ -25,6 +25,11 @@ var (
 	ErrEpicHasChildren = errors.New("epic has child work items")
 	// ErrMoveSameProject is returned when a move targets the issue's current project.
 	ErrMoveSameProject = errors.New("issue already in target project")
+	// ErrInvalidLabel / ErrInvalidParent / ErrInvalidAssignee are returned when a
+	// related id supplied on create/update doesn't belong to the allowed scope.
+	ErrInvalidLabel    = errors.New("invalid label for project")
+	ErrInvalidParent   = errors.New("invalid parent for project")
+	ErrInvalidAssignee = errors.New("assignee is not a workspace member")
 )
 
 // validPriorities is the accepted set of work-item priority values.
@@ -41,7 +46,8 @@ type IssueService struct {
 	notify    *NotificationService        // optional — may be nil
 	subs      *store.IssueSubscriberStore // optional — auto-subscribe assignees/mentions
 	reactions *store.IssueReactionStore   // optional — per-issue emoji reactions
-	states    *store.StateStore           // optional — validates state ownership on bulk update
+	states    *store.StateStore           // optional — validates state ownership
+	labels    *store.LabelStore           // optional — validates label ownership
 }
 
 func NewIssueService(is *store.IssueStore, ps *store.ProjectStore, ws *store.WorkspaceStore) *IssueService {
@@ -63,9 +69,74 @@ func (s *IssueService) SetSubscriberStore(subs *store.IssueSubscriberStore) { s.
 // SetReactionStore wires per-issue emoji reactions support. Optional.
 func (s *IssueService) SetReactionStore(r *store.IssueReactionStore) { s.reactions = r }
 
-// SetStateStore wires state-ownership validation for bulk updates. Optional.
+// SetStateStore wires state-ownership validation. Optional.
 func (s *IssueService) SetStateStore(st *store.StateStore) { s.states = st }
 
+// SetLabelStore wires label-ownership validation. Optional.
+func (s *IssueService) SetLabelStore(l *store.LabelStore) { s.labels = l }
+
+// validateRelations rejects related ids that fall outside the allowed scope:
+// state and labels must belong to the same project, a parent must be another
+// issue in the same project (and never the issue itself — pass selfID on
+// update; uuid.Nil on create), and assignees must be members of the workspace.
+// Only the provided (non-nil / non-empty) fields are checked. Ownership stores
+// are optional; when one isn't wired the corresponding check is skipped. A
+// genuine "not found" maps to the invalid-* sentinel, but any other datastore
+// error is returned so query failures surface as 5xx, not 400.
+func (s *IssueService) validateRelations(ctx context.Context, projectID, workspaceID, selfID uuid.UUID, stateID *uuid.UUID, labelIDs []uuid.UUID, assigneeIDs []uuid.UUID, parentID *uuid.UUID) error {
+	if stateID != nil && s.states != nil {
+		st, err := s.states.GetByID(ctx, *stateID)
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return ErrInvalidState
+		}
+		if err != nil {
+			return err
+		}
+		if st == nil || st.ProjectID != projectID {
+			return ErrInvalidState
+		}
+	}
+	if len(labelIDs) > 0 && s.labels != nil {
+		for _, id := range labelIDs {
+			l, err := s.labels.GetByID(ctx, id)
+			if errors.Is(err, gorm.ErrRecordNotFound) {
+				return ErrInvalidLabel
+			}
+			if err != nil {
+				return err
+			}
+			if l == nil || l.ProjectID == nil || *l.ProjectID != projectID {
+				return ErrInvalidLabel
+			}
+		}
+	}
+	if parentID != nil {
+		if *parentID == selfID {
+			return ErrInvalidParent // an issue can't be its own parent
+		}
+		parent, err := s.is.GetByID(ctx, *parentID)
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return ErrInvalidParent
+		}
+		if err != nil {
+			return err
+		}
+		if parent == nil || parent.ProjectID != projectID {
+			return ErrInvalidParent
+		}
+	}
+	for _, id := range assigneeIDs {
+		ok, err := s.ws.IsMember(ctx, workspaceID, id)
+		if err != nil {
+			return err
+		}
+		if !ok {
+			return ErrInvalidAssignee
+		}
+	}
+	return nil
+}
+
 // autoSubscribe is a fire-and-forget helper used by the assignee and mention
 // hooks. Errors are logged-and-ignored — the user's primary action must not
 // fail because of a subscription bookkeeping issue.
@@ -474,6 +545,9 @@ func (s *IssueService) Create(ctx context.Context, workspaceSlug string, project
 		return nil, err
 	}
 	wrk, _ := s.ws.GetBySlug(ctx, workspaceSlug)
+	if err := s.validateRelations(ctx, projectID, wrk.ID, uuid.Nil, stateID, labelIDs, assigneeIDs, parentID); err != nil {
+		return nil, err
+	}
 	issue := &model.Issue{
 		Name:        name,
 		ProjectID:   projectID,
@@ -549,6 +623,18 @@ func (s *IssueService) Update(ctx context.Context, workspaceSlug string, project
 		return nil, err
 	}
 
+	// Reject related ids outside the allowed scope before touching anything.
+	var wantAssignees, wantLabels []uuid.UUID
+	if assigneeIDs != nil {
+		wantAssignees = *assigneeIDs
+	}
+	if labelIDs != nil {
+		wantLabels = *labelIDs
+	}
+	if err := s.validateRelations(ctx, issue.ProjectID, issue.WorkspaceID, issue.ID, stateID, wantLabels, wantAssignees, parentID); err != nil {
+		return nil, err
+	}
+
 	// Snapshot values before mutation so we can diff them for the activity log.
 	prevName := issue.Name
 	prevPriority := issue.Priority