diff --git a/fn_operations/db.go b/fn_operations/db.go
new file mode 100644
index 00000000..0ce57f2a
--- /dev/null
+++ b/fn_operations/db.go
@@ -0,0 +1,140 @@
+package fn_operations
+
+import (
+	"database/sql"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+const schemaSQL = `
+CREATE TABLE IF NOT EXISTS types_snapshot (
+	id          TEXT PRIMARY KEY,
+	version     TEXT NOT NULL DEFAULT '1.0.0',
+	lang        TEXT NOT NULL,
+	algebraic   TEXT NOT NULL CHECK(algebraic IN ('product','sum')),
+	definition  TEXT NOT NULL DEFAULT '',
+	description TEXT NOT NULL DEFAULT '',
+	snapped_at  TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS entities (
+	id          TEXT PRIMARY KEY,
+	name        TEXT NOT NULL,
+	type_ref    TEXT NOT NULL,
+	status      TEXT NOT NULL DEFAULT 'active' CHECK(status IN ('active','stale','corrupted','archived')),
+	description TEXT NOT NULL DEFAULT '',
+	domain      TEXT NOT NULL DEFAULT '',
+	tags        TEXT NOT NULL DEFAULT '[]',
+	source      TEXT NOT NULL,
+	metadata    TEXT NOT NULL DEFAULT '{}',
+	notes       TEXT NOT NULL DEFAULT '',
+	created_at  TEXT NOT NULL,
+	updated_at  TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS relations (
+	id          TEXT PRIMARY KEY,
+	name        TEXT NOT NULL,
+	from_entity TEXT NOT NULL DEFAULT '',
+	to_entity   TEXT NOT NULL,
+	via         TEXT NOT NULL DEFAULT '',
+	description TEXT NOT NULL DEFAULT '',
+	purity      TEXT NOT NULL DEFAULT '' CHECK(purity IN ('','pure','impure')),
+	direction   TEXT NOT NULL DEFAULT 'unidirectional' CHECK(direction IN ('unidirectional','bidirectional','inverse')),
+	weight      REAL,
+	status      TEXT NOT NULL DEFAULT 'designed' CHECK(status IN ('designed','implemented','tested','running','deprecated')),
+	started_at  TEXT,
+	ended_at    TEXT,
+	"order"     INTEGER,
+	tags        TEXT NOT NULL DEFAULT '[]',
+	notes       TEXT NOT NULL DEFAULT '',
+	created_at  TEXT NOT NULL,
+	updated_at  TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS relation_inputs (
+	id          TEXT PRIMARY KEY,
+	relation_id TEXT NOT NULL REFERENCES relations(id) ON DELETE CASCADE,
+	entity_id   TEXT NOT NULL REFERENCES entities(id),
+	role        TEXT NOT NULL,
+	"order"     INTEGER
+);
+
+CREATE VIRTUAL TABLE IF NOT EXISTS entities_fts USING fts5(
+	id,
+	name,
+	description,
+	tags,
+	domain,
+	content='entities',
+	content_rowid='rowid'
+);
+
+-- Triggers to keep entities FTS in sync
+CREATE TRIGGER IF NOT EXISTS entities_ai AFTER INSERT ON entities BEGIN
+	INSERT INTO entities_fts(rowid, id, name, description, tags, domain)
+	VALUES (new.rowid, new.id, new.name, new.description, new.tags, new.domain);
+END;
+
+CREATE TRIGGER IF NOT EXISTS entities_ad AFTER DELETE ON entities BEGIN
+	INSERT INTO entities_fts(entities_fts, rowid, id, name, description, tags, domain)
+	VALUES ('delete', old.rowid, old.id, old.name, old.description, old.tags, old.domain);
+END;
+
+CREATE TRIGGER IF NOT EXISTS entities_au AFTER UPDATE ON entities BEGIN
+	INSERT INTO entities_fts(entities_fts, rowid, id, name, description, tags, domain)
+	VALUES ('delete', old.rowid, old.id, old.name, old.description, old.tags, old.domain);
+	INSERT INTO entities_fts(rowid, id, name, description, tags, domain)
+	VALUES (new.rowid, new.id, new.name, new.description, new.tags, new.domain);
+END;
+`
+
+// DB wraps a SQLite connection for an operations database.
+type DB struct {
+	conn *sql.DB
+	path string
+}
+
+// Open opens or creates an operations database at the given path.
+func Open(path string) (*DB, error) {
+	dir := filepath.Dir(path)
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return nil, fmt.Errorf("creating db directory: %w", err)
+	}
+
+	conn, err := sql.Open("sqlite3", path+"?_foreign_keys=on")
+	if err != nil {
+		return nil, fmt.Errorf("opening database: %w", err)
+	}
+
+	if _, err := conn.Exec("PRAGMA journal_mode=WAL"); err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("setting WAL mode: %w", err)
+	}
+
+	if _, err := conn.Exec(schemaSQL); err != nil {
+		conn.Close()
+		return nil, fmt.Errorf("applying schema: %w", err)
+	}
+
+	return &DB{conn: conn, path: path}, nil
+}
+
+// Conn returns the underlying sql.DB for transaction use.
+func (db *DB) Conn() *sql.DB {
+	return db.conn
+}
+
+// Close closes the database connection.
+func (db *DB) Close() error {
+	return db.conn.Close()
+}
+
+// Drop removes the database file.
+func (db *DB) Drop() error {
+	db.Close()
+	return os.Remove(db.path)
+}
diff --git a/fn_operations/models.go b/fn_operations/models.go
new file mode 100644
index 00000000..2eefeab3
--- /dev/null
+++ b/fn_operations/models.go
@@ -0,0 +1,90 @@
+package fn_operations
+
+import "time"
+
+// EntityStatus represents the lifecycle state of an entity.
+type EntityStatus string
+
+const (
+	StatusActive    EntityStatus = "active"
+	StatusStale     EntityStatus = "stale"
+	StatusCorrupted EntityStatus = "corrupted"
+	StatusArchived  EntityStatus = "archived"
+)
+
+// RelationStatus represents the lifecycle state of a relation.
+type RelationStatus string
+
+const (
+	RelDesigned    RelationStatus = "designed"
+	RelImplemented RelationStatus = "implemented"
+	RelTested      RelationStatus = "tested"
+	RelRunning     RelationStatus = "running"
+	RelDeprecated  RelationStatus = "deprecated"
+)
+
+// Direction represents the directionality of a relation.
+type Direction string
+
+const (
+	DirUnidirectional Direction = "unidirectional"
+	DirBidirectional  Direction = "bidirectional"
+	DirInverse        Direction = "inverse"
+)
+
+// Entity is a concrete instance of a registry type within a project context.
+type Entity struct {
+	ID          string            `json:"id"`
+	Name        string            `json:"name"`
+	TypeRef     string            `json:"type_ref"`
+	Status      EntityStatus      `json:"status"`
+	Description string            `json:"description"`
+	Domain      string            `json:"domain"`
+	Tags        []string          `json:"tags"`
+	Source      string            `json:"source"`
+	Metadata    map[string]any    `json:"metadata"`
+	Notes       string            `json:"notes"`
+	CreatedAt   time.Time         `json:"created_at"`
+	UpdatedAt   time.Time         `json:"updated_at"`
+}
+
+// Relation describes how one entity connects to or transforms into another.
+type Relation struct {
+	ID          string         `json:"id"`
+	Name        string         `json:"name"`
+	FromEntity  string         `json:"from_entity"`
+	ToEntity    string         `json:"to_entity"`
+	Via         string         `json:"via"`
+	Description string         `json:"description"`
+	Purity      string         `json:"purity"`
+	Direction   Direction      `json:"direction"`
+	Weight      *float64       `json:"weight"`
+	Status      RelationStatus `json:"status"`
+	StartedAt   *time.Time     `json:"started_at"`
+	EndedAt     *time.Time     `json:"ended_at"`
+	Order       *int           `json:"order"`
+	Tags        []string       `json:"tags"`
+	Notes       string         `json:"notes"`
+	CreatedAt   time.Time      `json:"created_at"`
+	UpdatedAt   time.Time      `json:"updated_at"`
+}
+
+// RelationInput represents one input entity in a multi-input relation.
+type RelationInput struct {
+	ID         string `json:"id"`
+	RelationID string `json:"relation_id"`
+	EntityID   string `json:"entity_id"`
+	Role       string `json:"role"`
+	Order      *int   `json:"order"`
+}
+
+// TypeSnapshot is an immutable copy of a registry type at point of use.
+type TypeSnapshot struct {
+	ID          string    `json:"id"`
+	Version     string    `json:"version"`
+	Lang        string    `json:"lang"`
+	Algebraic   string    `json:"algebraic"`
+	Definition  string    `json:"definition"`
+	Description string    `json:"description"`
+	SnappedAt   time.Time `json:"snapped_at"`
+}
diff --git a/fn_operations/operations.go b/fn_operations/operations.go
new file mode 100644
index 00000000..d52fc9a5
--- /dev/null
+++ b/fn_operations/operations.go
@@ -0,0 +1,202 @@
+package fn_operations
+
+import (
+	"fmt"
+	"time"
+
+	"fn-registry/registry"
+)
+
+// InsertEntityWithSnapshot inserts an entity, snapshotting its type from the registry if needed.
+// registryDB can be nil if the type is already snapshotted.
+func InsertEntityWithSnapshot(opsDB *DB, registryDB *registry.DB, e *Entity) error {
+	if err := ValidateEntity(e); err != nil {
+		return err
+	}
+
+	// Check if type is already snapshotted
+	snap, err := opsDB.GetTypeSnapshot(e.TypeRef)
+	if err != nil {
+		return fmt.Errorf("checking type snapshot: %w", err)
+	}
+
+	if snap == nil {
+		// Need to fetch from registry
+		if registryDB == nil {
+			return fmt.Errorf("type %q not found in local snapshots and no registry provided", e.TypeRef)
+		}
+		if err := SnapshotType(opsDB, registryDB, e.TypeRef); err != nil {
+			return err
+		}
+	}
+
+	return opsDB.InsertEntity(e)
+}
+
+// SnapshotType fetches a type from the registry and copies it to types_snapshot.
+func SnapshotType(opsDB *DB, registryDB *registry.DB, typeID string) error {
+	t, err := registryDB.GetType(typeID)
+	if err != nil {
+		return fmt.Errorf("fetching type %q from registry: %w", typeID, err)
+	}
+
+	snap := &TypeSnapshot{
+		ID:          t.ID,
+		Version:     t.Version,
+		Lang:        t.Lang,
+		Algebraic:   string(t.Algebraic),
+		Definition:  t.Definition,
+		Description: t.Description,
+		SnappedAt:   time.Now().UTC(),
+	}
+
+	return opsDB.InsertTypeSnapshot(snap)
+}
+
+// InsertRelationSafe validates, checks for cycles, and inserts a relation.
+func InsertRelationSafe(db *DB, r *Relation) error {
+	entities, err := buildEntitySet(db)
+	if err != nil {
+		return err
+	}
+
+	if err := ValidateRelation(r, entities); err != nil {
+		return err
+	}
+
+	// from_entity is required when not using relation_inputs
+	if r.FromEntity == "" {
+		return fmt.Errorf("from_entity is required (use InsertRelationWithInputs for multi-input relations)")
+	}
+
+	// Cycle detection only for causal relations
+	if r.Via != "" {
+		if err := DetectCycle(db, r.FromEntity, r.ToEntity); err != nil {
+			return err
+		}
+	}
+
+	return db.InsertRelation(r)
+}
+
+// InsertRelationWithInputs validates and inserts a relation with multiple inputs in a transaction.
+func InsertRelationWithInputs(db *DB, r *Relation, inputs []RelationInput) error {
+	entities, err := buildEntitySet(db)
+	if err != nil {
+		return err
+	}
+
+	if err := ValidateRelation(r, entities); err != nil {
+		return err
+	}
+	if err := ValidateRelationInputs(inputs, entities); err != nil {
+		return err
+	}
+
+	// Cycle detection for each input if causal
+	if r.Via != "" {
+		for _, input := range inputs {
+			if err := DetectCycle(db, input.EntityID, r.ToEntity); err != nil {
+				return err
+			}
+		}
+	}
+
+	tx, err := db.Conn().Begin()
+	if err != nil {
+		return fmt.Errorf("beginning transaction: %w", err)
+	}
+	defer tx.Rollback()
+
+	// Insert relation
+	now := time.Now().UTC()
+	if r.CreatedAt.IsZero() {
+		r.CreatedAt = now
+	}
+	r.UpdatedAt = now
+
+	var startedAt, endedAt *string
+	if r.StartedAt != nil {
+		s := r.StartedAt.Format(time.RFC3339)
+		startedAt = &s
+	}
+	if r.EndedAt != nil {
+		s := r.EndedAt.Format(time.RFC3339)
+		endedAt = &s
+	}
+
+	_, err = tx.Exec(`
+		INSERT OR REPLACE INTO relations (id, name, from_entity, to_entity, via, description, purity, direction, weight, status, started_at, ended_at, "order", tags, notes, created_at, updated_at)
+		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+		r.ID, r.Name, r.FromEntity, r.ToEntity, r.Via, r.Description,
+		r.Purity, string(r.Direction), r.Weight, string(r.Status),
+		startedAt, endedAt, r.Order, marshalStrings(r.Tags), r.Notes,
+		r.CreatedAt.Format(time.RFC3339), r.UpdatedAt.Format(time.RFC3339),
+	)
+	if err != nil {
+		return fmt.Errorf("inserting relation: %w", err)
+	}
+
+	// Insert inputs
+	for _, ri := range inputs {
+		_, err = tx.Exec(`
+			INSERT INTO relation_inputs (id, relation_id, entity_id, role, "order")
+			VALUES (?, ?, ?, ?, ?)`,
+			ri.ID, ri.RelationID, ri.EntityID, ri.Role, ri.Order,
+		)
+		if err != nil {
+			return fmt.Errorf("inserting relation_input: %w", err)
+		}
+	}
+
+	return tx.Commit()
+}
+
+// Graph holds the full entity-relation graph for a project.
+type Graph struct {
+	Entities  []Entity
+	Relations []Relation
+	Inputs    map[string][]RelationInput
+}
+
+// GetEntityGraph returns all entities and relations for visualization.
+func GetEntityGraph(db *DB) (*Graph, error) {
+	entities, err := db.ListEntities("", "")
+	if err != nil {
+		return nil, fmt.Errorf("listing entities: %w", err)
+	}
+
+	relations, err := db.ListRelations("")
+	if err != nil {
+		return nil, fmt.Errorf("listing relations: %w", err)
+	}
+
+	inputs := map[string][]RelationInput{}
+	for _, r := range relations {
+		ri, err := db.GetRelationInputs(r.ID)
+		if err != nil {
+			return nil, fmt.Errorf("getting inputs for relation %s: %w", r.ID, err)
+		}
+		if len(ri) > 0 {
+			inputs[r.ID] = ri
+		}
+	}
+
+	return &Graph{
+		Entities:  entities,
+		Relations: relations,
+		Inputs:    inputs,
+	}, nil
+}
+
+func buildEntitySet(db *DB) (map[string]bool, error) {
+	all, err := db.ListEntities("", "")
+	if err != nil {
+		return nil, fmt.Errorf("building entity set: %w", err)
+	}
+	set := make(map[string]bool, len(all))
+	for _, e := range all {
+		set[e.ID] = true
+	}
+	return set, nil
+}
diff --git a/fn_operations/operations_test.go b/fn_operations/operations_test.go
new file mode 100644
index 00000000..23b81ec7
--- /dev/null
+++ b/fn_operations/operations_test.go
@@ -0,0 +1,375 @@
+package fn_operations
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func tempDB(t *testing.T) *DB {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "test.db")
+	db, err := Open(path)
+	if err != nil {
+		t.Fatalf("opening test db: %v", err)
+	}
+	t.Cleanup(func() { db.Close() })
+	return db
+}
+
+func TestOpenAndClose(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "test.db")
+	db, err := Open(path)
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	if err := db.Close(); err != nil {
+		t.Fatalf("close: %v", err)
+	}
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		t.Fatal("db file should exist")
+	}
+}
+
+func TestTypeSnapshotCRUD(t *testing.T) {
+	db := tempDB(t)
+
+	ts := &TypeSnapshot{
+		ID:          "ohlcv_go_finance",
+		Version:     "1.0.0",
+		Lang:        "go",
+		Algebraic:   "product",
+		Definition:  "type OHLCV struct { ... }",
+		Description: "Vela de mercado",
+	}
+
+	if err := db.InsertTypeSnapshot(ts); err != nil {
+		t.Fatalf("insert: %v", err)
+	}
+
+	got, err := db.GetTypeSnapshot("ohlcv_go_finance")
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if got == nil {
+		t.Fatal("expected snapshot, got nil")
+	}
+	if got.Definition != ts.Definition {
+		t.Errorf("definition = %q, want %q", got.Definition, ts.Definition)
+	}
+
+	// INSERT OR IGNORE: second insert should not overwrite
+	ts2 := &TypeSnapshot{
+		ID:          "ohlcv_go_finance",
+		Version:     "2.0.0",
+		Lang:        "go",
+		Algebraic:   "product",
+		Definition:  "type OHLCV struct { CHANGED }",
+		Description: "Changed",
+	}
+	if err := db.InsertTypeSnapshot(ts2); err != nil {
+		t.Fatalf("insert duplicate: %v", err)
+	}
+	got2, _ := db.GetTypeSnapshot("ohlcv_go_finance")
+	if got2.Version != "1.0.0" {
+		t.Errorf("snapshot should be immutable, got version %q", got2.Version)
+	}
+
+	// Not found
+	missing, err := db.GetTypeSnapshot("nonexistent")
+	if err != nil {
+		t.Fatalf("get missing: %v", err)
+	}
+	if missing != nil {
+		t.Error("expected nil for missing snapshot")
+	}
+
+	all, err := db.ListTypeSnapshots()
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if len(all) != 1 {
+		t.Errorf("expected 1 snapshot, got %d", len(all))
+	}
+}
+
+func TestEntityCRUD(t *testing.T) {
+	db := tempDB(t)
+
+	// Insert snapshot first (type_ref)
+	db.InsertTypeSnapshot(&TypeSnapshot{
+		ID: "tick_go_finance", Version: "1.0.0", Lang: "go", Algebraic: "product",
+	})
+
+	e := &Entity{
+		ID:      "ticks_btcusdt_2024",
+		Name:    "ticks_btcusdt_2024",
+		TypeRef: "tick_go_finance",
+		Status:  StatusActive,
+		Source:  "binance_api",
+		Domain:  "market_data",
+		Tags:    []string{"btc", "binance"},
+		Metadata: map[string]any{
+			"pair":     "BTCUSDT",
+			"exchange": "binance",
+		},
+	}
+
+	if err := db.InsertEntity(e); err != nil {
+		t.Fatalf("insert: %v", err)
+	}
+
+	got, err := db.GetEntity("ticks_btcusdt_2024")
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if got == nil {
+		t.Fatal("expected entity, got nil")
+	}
+	if got.Source != "binance_api" {
+		t.Errorf("source = %q, want binance_api", got.Source)
+	}
+	if len(got.Tags) != 2 {
+		t.Errorf("tags len = %d, want 2", len(got.Tags))
+	}
+	if got.Metadata["pair"] != "BTCUSDT" {
+		t.Errorf("metadata pair = %v, want BTCUSDT", got.Metadata["pair"])
+	}
+
+	// Update
+	got.Status = StatusStale
+	if err := db.UpdateEntity(got); err != nil {
+		t.Fatalf("update: %v", err)
+	}
+	updated, _ := db.GetEntity("ticks_btcusdt_2024")
+	if updated.Status != StatusStale {
+		t.Errorf("status = %q, want stale", updated.Status)
+	}
+
+	// List
+	all, err := db.ListEntities("market_data", "")
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if len(all) != 1 {
+		t.Errorf("expected 1, got %d", len(all))
+	}
+
+	// Search
+	found, err := db.SearchEntities("btcusdt", "")
+	if err != nil {
+		t.Fatalf("search: %v", err)
+	}
+	if len(found) != 1 {
+		t.Errorf("search expected 1, got %d", len(found))
+	}
+
+	// Delete
+	if err := db.DeleteEntity("ticks_btcusdt_2024"); err != nil {
+		t.Fatalf("delete: %v", err)
+	}
+	deleted, _ := db.GetEntity("ticks_btcusdt_2024")
+	if deleted != nil {
+		t.Error("expected nil after delete")
+	}
+}
+
+func TestRelationCRUD(t *testing.T) {
+	db := tempDB(t)
+
+	// Setup entities
+	db.InsertTypeSnapshot(&TypeSnapshot{ID: "t1", Version: "1.0.0", Lang: "go", Algebraic: "product"})
+	db.InsertEntity(&Entity{ID: "a", Name: "a", TypeRef: "t1", Status: StatusActive, Source: "test"})
+	db.InsertEntity(&Entity{ID: "b", Name: "b", TypeRef: "t1", Status: StatusActive, Source: "test"})
+
+	r := &Relation{
+		ID:        "a__to__b__via__transform",
+		Name:      "TRANSFORMA",
+		FromEntity: "a",
+		ToEntity:  "b",
+		Direction: DirUnidirectional,
+		Status:    RelDesigned,
+	}
+
+	if err := InsertRelationSafe(db, r); err != nil {
+		t.Fatalf("insert relation: %v", err)
+	}
+
+	got, err := db.GetRelation("a__to__b__via__transform")
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if got == nil {
+		t.Fatal("expected relation, got nil")
+	}
+	if got.Name != "TRANSFORMA" {
+		t.Errorf("name = %q, want TRANSFORMA", got.Name)
+	}
+
+	// List by entity
+	rels, err := db.ListRelations("a")
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if len(rels) != 1 {
+		t.Errorf("expected 1, got %d", len(rels))
+	}
+
+	// Delete
+	if err := db.DeleteRelation("a__to__b__via__transform"); err != nil {
+		t.Fatalf("delete: %v", err)
+	}
+	deleted, _ := db.GetRelation("a__to__b__via__transform")
+	if deleted != nil {
+		t.Error("expected nil after delete")
+	}
+}
+
+func TestRelationInputs(t *testing.T) {
+	db := tempDB(t)
+
+	db.InsertTypeSnapshot(&TypeSnapshot{ID: "t1", Version: "1.0.0", Lang: "go", Algebraic: "product"})
+	db.InsertEntity(&Entity{ID: "a", Name: "a", TypeRef: "t1", Status: StatusActive, Source: "test"})
+	db.InsertEntity(&Entity{ID: "b", Name: "b", TypeRef: "t1", Status: StatusActive, Source: "test"})
+	db.InsertEntity(&Entity{ID: "c", Name: "c", TypeRef: "t1", Status: StatusActive, Source: "test"})
+
+	r := &Relation{
+		ID:       "multi__to__c",
+		Name:     "ENRIQUECE",
+		ToEntity: "c",
+		Direction: DirUnidirectional,
+		Status:   RelDesigned,
+	}
+
+	inputs := []RelationInput{
+		{ID: "i1", RelationID: "multi__to__c", EntityID: "a", Role: "base"},
+		{ID: "i2", RelationID: "multi__to__c", EntityID: "b", Role: "lookup"},
+	}
+
+	if err := InsertRelationWithInputs(db, r, inputs); err != nil {
+		t.Fatalf("insert with inputs: %v", err)
+	}
+
+	got, err := db.GetRelationInputs("multi__to__c")
+	if err != nil {
+		t.Fatalf("get inputs: %v", err)
+	}
+	if len(got) != 2 {
+		t.Errorf("expected 2 inputs, got %d", len(got))
+	}
+}
+
+func TestCycleDetectionCausal(t *testing.T) {
+	db := tempDB(t)
+
+	db.InsertTypeSnapshot(&TypeSnapshot{ID: "t1", Version: "1.0.0", Lang: "go", Algebraic: "product"})
+	db.InsertEntity(&Entity{ID: "a", Name: "a", TypeRef: "t1", Status: StatusActive, Source: "test"})
+	db.InsertEntity(&Entity{ID: "b", Name: "b", TypeRef: "t1", Status: StatusActive, Source: "test"})
+	db.InsertEntity(&Entity{ID: "c", Name: "c", TypeRef: "t1", Status: StatusActive, Source: "test"})
+
+	// a -> b (causal)
+	InsertRelationSafe(db, &Relation{
+		ID: "ab", Name: "T1", FromEntity: "a", ToEntity: "b", Via: "fn1",
+		Purity: "impure", Direction: DirUnidirectional, Status: RelDesigned,
+	})
+
+	// b -> c (causal)
+	InsertRelationSafe(db, &Relation{
+		ID: "bc", Name: "T2", FromEntity: "b", ToEntity: "c", Via: "fn2",
+		Purity: "impure", Direction: DirUnidirectional, Status: RelDesigned,
+	})
+
+	// c -> a (causal) should fail — creates cycle
+	err := InsertRelationSafe(db, &Relation{
+		ID: "ca", Name: "T3", FromEntity: "c", ToEntity: "a", Via: "fn3",
+		Purity: "impure", Direction: DirUnidirectional, Status: RelDesigned,
+	})
+	if err == nil {
+		t.Fatal("expected cycle error, got nil")
+	}
+}
+
+func TestCycleDetectionSemanticAllowed(t *testing.T) {
+	db := tempDB(t)
+
+	db.InsertTypeSnapshot(&TypeSnapshot{ID: "t1", Version: "1.0.0", Lang: "go", Algebraic: "product"})
+	db.InsertEntity(&Entity{ID: "juan", Name: "juan", TypeRef: "t1", Status: StatusActive, Source: "test"})
+	db.InsertEntity(&Entity{ID: "paula", Name: "paula", TypeRef: "t1", Status: StatusActive, Source: "test"})
+
+	// juan -> paula (semantic, no via)
+	if err := InsertRelationSafe(db, &Relation{
+		ID: "jp", Name: "CONOCE A", FromEntity: "juan", ToEntity: "paula",
+		Direction: DirBidirectional, Status: RelRunning,
+	}); err != nil {
+		t.Fatalf("insert semantic: %v", err)
+	}
+
+	// paula -> juan (semantic, no via) — should succeed, no cycle check
+	if err := InsertRelationSafe(db, &Relation{
+		ID: "pj", Name: "CONOCE A", FromEntity: "paula", ToEntity: "juan",
+		Direction: DirBidirectional, Status: RelRunning,
+	}); err != nil {
+		t.Fatalf("semantic cycle should be allowed: %v", err)
+	}
+}
+
+func TestValidateEntity(t *testing.T) {
+	tests := []struct {
+		name    string
+		entity  Entity
+		wantErr bool
+	}{
+		{
+			name:    "valid",
+			entity:  Entity{ID: "x", Name: "x", TypeRef: "t1", Status: StatusActive, Source: "test"},
+			wantErr: false,
+		},
+		{
+			name:    "missing name",
+			entity:  Entity{ID: "x", TypeRef: "t1", Status: StatusActive, Source: "test"},
+			wantErr: true,
+		},
+		{
+			name:    "missing source",
+			entity:  Entity{ID: "x", Name: "x", TypeRef: "t1", Status: StatusActive},
+			wantErr: true,
+		},
+		{
+			name:    "missing type_ref",
+			entity:  Entity{ID: "x", Name: "x", Status: StatusActive, Source: "test"},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateEntity(&tt.entity)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ValidateEntity() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestGetEntityGraph(t *testing.T) {
+	db := tempDB(t)
+
+	db.InsertTypeSnapshot(&TypeSnapshot{ID: "t1", Version: "1.0.0", Lang: "go", Algebraic: "product"})
+	db.InsertEntity(&Entity{ID: "a", Name: "a", TypeRef: "t1", Status: StatusActive, Source: "test"})
+	db.InsertEntity(&Entity{ID: "b", Name: "b", TypeRef: "t1", Status: StatusActive, Source: "test"})
+	InsertRelationSafe(db, &Relation{
+		ID: "ab", Name: "FLUYE", FromEntity: "a", ToEntity: "b",
+		Direction: DirUnidirectional, Status: RelDesigned,
+	})
+
+	g, err := GetEntityGraph(db)
+	if err != nil {
+		t.Fatalf("graph: %v", err)
+	}
+	if len(g.Entities) != 2 {
+		t.Errorf("entities = %d, want 2", len(g.Entities))
+	}
+	if len(g.Relations) != 1 {
+		t.Errorf("relations = %d, want 1", len(g.Relations))
+	}
+}
diff --git a/fn_operations/store.go b/fn_operations/store.go
new file mode 100644
index 00000000..52f2389a
--- /dev/null
+++ b/fn_operations/store.go
@@ -0,0 +1,459 @@
+package fn_operations
+
+import (
+	"database/sql"
+	"encoding/json"
+	"fmt"
+	"strings"
+	"time"
+)
+
+func marshalStrings(ss []string) string {
+	if ss == nil {
+		ss = []string{}
+	}
+	b, _ := json.Marshal(ss)
+	return string(b)
+}
+
+func unmarshalStrings(s string) []string {
+	var out []string
+	json.Unmarshal([]byte(s), &out)
+	if out == nil {
+		out = []string{}
+	}
+	return out
+}
+
+func marshalJSON(v map[string]any) string {
+	if v == nil {
+		v = map[string]any{}
+	}
+	b, _ := json.Marshal(v)
+	return string(b)
+}
+
+func unmarshalJSON(s string) map[string]any {
+	var out map[string]any
+	json.Unmarshal([]byte(s), &out)
+	if out == nil {
+		out = map[string]any{}
+	}
+	return out
+}
+
+// --- TypeSnapshot CRUD ---
+
+// InsertTypeSnapshot inserts a type snapshot.
+func (db *DB) InsertTypeSnapshot(ts *TypeSnapshot) error {
+	if ts.SnappedAt.IsZero() {
+		ts.SnappedAt = time.Now().UTC()
+	}
+	_, err := db.conn.Exec(`
+		INSERT OR IGNORE INTO types_snapshot (id, version, lang, algebraic, definition, description, snapped_at)
+		VALUES (?, ?, ?, ?, ?, ?, ?)`,
+		ts.ID, ts.Version, ts.Lang, ts.Algebraic, ts.Definition, ts.Description,
+		ts.SnappedAt.Format(time.RFC3339),
+	)
+	return err
+}
+
+// GetTypeSnapshot returns a type snapshot by ID.
+func (db *DB) GetTypeSnapshot(id string) (*TypeSnapshot, error) {
+	row := db.conn.QueryRow("SELECT id, version, lang, algebraic, definition, description, snapped_at FROM types_snapshot WHERE id = ?", id)
+	var ts TypeSnapshot
+	var snappedAt string
+	err := row.Scan(&ts.ID, &ts.Version, &ts.Lang, &ts.Algebraic, &ts.Definition, &ts.Description, &snappedAt)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("scanning type_snapshot: %w", err)
+	}
+	ts.SnappedAt, _ = time.Parse(time.RFC3339, snappedAt)
+	return &ts, nil
+}
+
+// ListTypeSnapshots returns all type snapshots.
+func (db *DB) ListTypeSnapshots() ([]TypeSnapshot, error) {
+	rows, err := db.conn.Query("SELECT id, version, lang, algebraic, definition, description, snapped_at FROM types_snapshot ORDER BY id")
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var result []TypeSnapshot
+	for rows.Next() {
+		var ts TypeSnapshot
+		var snappedAt string
+		if err := rows.Scan(&ts.ID, &ts.Version, &ts.Lang, &ts.Algebraic, &ts.Definition, &ts.Description, &snappedAt); err != nil {
+			return nil, fmt.Errorf("scanning type_snapshot: %w", err)
+		}
+		ts.SnappedAt, _ = time.Parse(time.RFC3339, snappedAt)
+		result = append(result, ts)
+	}
+	return result, nil
+}
+
+// --- Entity CRUD ---
+
+// InsertEntity inserts or replaces an entity.
+func (db *DB) InsertEntity(e *Entity) error {
+	now := time.Now().UTC()
+	if e.CreatedAt.IsZero() {
+		e.CreatedAt = now
+	}
+	e.UpdatedAt = now
+
+	_, err := db.conn.Exec(`
+		INSERT OR REPLACE INTO entities (id, name, type_ref, status, description, domain, tags, source, metadata, notes, created_at, updated_at)
+		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+		e.ID, e.Name, e.TypeRef, string(e.Status), e.Description, e.Domain,
+		marshalStrings(e.Tags), e.Source, marshalJSON(e.Metadata), e.Notes,
+		e.CreatedAt.Format(time.RFC3339), e.UpdatedAt.Format(time.RFC3339),
+	)
+	return err
+}
+
+// GetEntity returns an entity by ID.
+func (db *DB) GetEntity(id string) (*Entity, error) {
+	row := db.conn.QueryRow(`
+		SELECT id, name, type_ref, status, description, domain, tags, source, metadata, notes, created_at, updated_at
+		FROM entities WHERE id = ?`, id)
+
+	var e Entity
+	var tagsJSON, metadataJSON, createdAt, updatedAt string
+	err := row.Scan(&e.ID, &e.Name, &e.TypeRef, &e.Status, &e.Description, &e.Domain,
+		&tagsJSON, &e.Source, &metadataJSON, &e.Notes, &createdAt, &updatedAt)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("scanning entity: %w", err)
+	}
+	e.Tags = unmarshalStrings(tagsJSON)
+	e.Metadata = unmarshalJSON(metadataJSON)
+	e.CreatedAt, _ = time.Parse(time.RFC3339, createdAt)
+	e.UpdatedAt, _ = time.Parse(time.RFC3339, updatedAt)
+	return &e, nil
+}
+
+// UpdateEntity updates an existing entity.
+func (db *DB) UpdateEntity(e *Entity) error {
+	e.UpdatedAt = time.Now().UTC()
+	_, err := db.conn.Exec(`
+		UPDATE entities SET name=?, type_ref=?, status=?, description=?, domain=?, tags=?, source=?, metadata=?, notes=?, updated_at=?
+		WHERE id=?`,
+		e.Name, e.TypeRef, string(e.Status), e.Description, e.Domain,
+		marshalStrings(e.Tags), e.Source, marshalJSON(e.Metadata), e.Notes,
+		e.UpdatedAt.Format(time.RFC3339), e.ID,
+	)
+	return err
+}
+
+// DeleteEntity removes an entity by ID.
+func (db *DB) DeleteEntity(id string) error {
+	_, err := db.conn.Exec("DELETE FROM entities WHERE id = ?", id)
+	return err
+}
+
+// ListEntities returns entities filtered by domain and/or status.
+func (db *DB) ListEntities(domain string, status EntityStatus) ([]Entity, error) {
+	where := []string{}
+	args := []any{}
+	if domain != "" {
+		where = append(where, "domain = ?")
+		args = append(args, domain)
+	}
+	if status != "" {
+		where = append(where, "status = ?")
+		args = append(args, string(status))
+	}
+
+	q := "SELECT id, name, type_ref, status, description, domain, tags, source, metadata, notes, created_at, updated_at FROM entities"
+	if len(where) > 0 {
+		q += " WHERE " + strings.Join(where, " AND ")
+	}
+	q += " ORDER BY name"
+
+	rows, err := db.conn.Query(q, args...)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	return scanEntities(rows)
+}
+
+// SearchEntities performs FTS search on entities.
+func (db *DB) SearchEntities(query, domain string) ([]Entity, error) {
+	where := []string{}
+	args := []any{}
+	if query != "" {
+		where = append(where, "e.id IN (SELECT id FROM entities_fts WHERE entities_fts MATCH ?)")
+		args = append(args, query)
+	}
+	if domain != "" {
+		where = append(where, "e.domain = ?")
+		args = append(args, domain)
+	}
+
+	q := "SELECT e.id, e.name, e.type_ref, e.status, e.description, e.domain, e.tags, e.source, e.metadata, e.notes, e.created_at, e.updated_at FROM entities e"
+	if len(where) > 0 {
+		q += " WHERE " + strings.Join(where, " AND ")
+	}
+	q += " ORDER BY e.name"
+
+	rows, err := db.conn.Query(q, args...)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	return scanEntities(rows)
+}
+
+func scanEntities(rows *sql.Rows) ([]Entity, error) {
+	var result []Entity
+	for rows.Next() {
+		var e Entity
+		var tagsJSON, metadataJSON, createdAt, updatedAt string
+		if err := rows.Scan(&e.ID, &e.Name, &e.TypeRef, &e.Status, &e.Description, &e.Domain,
+			&tagsJSON, &e.Source, &metadataJSON, &e.Notes, &createdAt, &updatedAt); err != nil {
+			return nil, fmt.Errorf("scanning entity: %w", err)
+		}
+		e.Tags = unmarshalStrings(tagsJSON)
+		e.Metadata = unmarshalJSON(metadataJSON)
+		e.CreatedAt, _ = time.Parse(time.RFC3339, createdAt)
+		e.UpdatedAt, _ = time.Parse(time.RFC3339, updatedAt)
+		result = append(result, e)
+	}
+	return result, nil
+}
+
+// --- Relation CRUD ---
+
+// InsertRelation inserts or replaces a relation.
+func (db *DB) InsertRelation(r *Relation) error {
+	now := time.Now().UTC()
+	if r.CreatedAt.IsZero() {
+		r.CreatedAt = now
+	}
+	r.UpdatedAt = now
+
+	var startedAt, endedAt *string
+	if r.StartedAt != nil {
+		s := r.StartedAt.Format(time.RFC3339)
+		startedAt = &s
+	}
+	if r.EndedAt != nil {
+		s := r.EndedAt.Format(time.RFC3339)
+		endedAt = &s
+	}
+
+	_, err := db.conn.Exec(`
+		INSERT OR REPLACE INTO relations (id, name, from_entity, to_entity, via, description, purity, direction, weight, status, started_at, ended_at, "order", tags, notes, created_at, updated_at)
+		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+		r.ID, r.Name, r.FromEntity, r.ToEntity, r.Via, r.Description,
+		r.Purity, string(r.Direction), r.Weight, string(r.Status),
+		startedAt, endedAt, r.Order, marshalStrings(r.Tags), r.Notes,
+		r.CreatedAt.Format(time.RFC3339), r.UpdatedAt.Format(time.RFC3339),
+	)
+	return err
+}
+
+// GetRelation returns a relation by ID.
+func (db *DB) GetRelation(id string) (*Relation, error) {
+	row := db.conn.QueryRow(`
+		SELECT id, name, from_entity, to_entity, via, description, purity, direction, weight, status, started_at, ended_at, "order", tags, notes, created_at, updated_at
+		FROM relations WHERE id = ?`, id)
+	return scanRelation(row)
+}
+
+// UpdateRelation updates an existing relation.
+func (db *DB) UpdateRelation(r *Relation) error {
+	r.UpdatedAt = time.Now().UTC()
+
+	var startedAt, endedAt *string
+	if r.StartedAt != nil {
+		s := r.StartedAt.Format(time.RFC3339)
+		startedAt = &s
+	}
+	if r.EndedAt != nil {
+		s := r.EndedAt.Format(time.RFC3339)
+		endedAt = &s
+	}
+
+	_, err := db.conn.Exec(`
+		UPDATE relations SET name=?, from_entity=?, to_entity=?, via=?, description=?, purity=?, direction=?, weight=?, status=?, started_at=?, ended_at=?, "order"=?, tags=?, notes=?, updated_at=?
+		WHERE id=?`,
+		r.Name, r.FromEntity, r.ToEntity, r.Via, r.Description,
+		r.Purity, string(r.Direction), r.Weight, string(r.Status),
+		startedAt, endedAt, r.Order, marshalStrings(r.Tags), r.Notes,
+		r.UpdatedAt.Format(time.RFC3339), r.ID,
+	)
+	return err
+}
+
+// DeleteRelation removes a relation by ID (cascades to relation_inputs).
+func (db *DB) DeleteRelation(id string) error {
+	_, err := db.conn.Exec("DELETE FROM relations WHERE id = ?", id)
+	return err
+}
+
+// ListRelations returns all relations, optionally filtered by entity involvement.
+func (db *DB) ListRelations(entityID string) ([]Relation, error) {
+	q := `SELECT id, name, from_entity, to_entity, via, description, purity, direction, weight, status, started_at, ended_at, "order", tags, notes, created_at, updated_at FROM relations`
+	var args []any
+	if entityID != "" {
+		q += " WHERE from_entity = ? OR to_entity = ?"
+		args = append(args, entityID, entityID)
+	}
+	q += " ORDER BY name"
+
+	rows, err := db.conn.Query(q, args...)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var result []Relation
+	for rows.Next() {
+		r, err := scanRelationFromRows(rows)
+		if err != nil {
+			return nil, err
+		}
+		result = append(result, *r)
+	}
+	return result, nil
+}
+
+// GetRelationsFrom returns all relations where from_entity matches.
+func (db *DB) GetRelationsFrom(entityID string) ([]Relation, error) {
+	rows, err := db.conn.Query(`
+		SELECT id, name, from_entity, to_entity, via, description, purity, direction, weight, status, started_at, ended_at, "order", tags, notes, created_at, updated_at
+		FROM relations WHERE from_entity = ? ORDER BY name`, entityID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var result []Relation
+	for rows.Next() {
+		r, err := scanRelationFromRows(rows)
+		if err != nil {
+			return nil, err
+		}
+		result = append(result, *r)
+	}
+	return result, nil
+}
+
+// GetRelationsTo returns all relations where to_entity matches.
+func (db *DB) GetRelationsTo(entityID string) ([]Relation, error) {
+	rows, err := db.conn.Query(`
+		SELECT id, name, from_entity, to_entity, via, description, purity, direction, weight, status, started_at, ended_at, "order", tags, notes, created_at, updated_at
+		FROM relations WHERE to_entity = ? ORDER BY name`, entityID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var result []Relation
+	for rows.Next() {
+		r, err := scanRelationFromRows(rows)
+		if err != nil {
+			return nil, err
+		}
+		result = append(result, *r)
+	}
+	return result, nil
+}
+
+func scanRelation(row *sql.Row) (*Relation, error) {
+	var r Relation
+	var tagsJSON, createdAt, updatedAt string
+	var startedAt, endedAt *string
+	err := row.Scan(&r.ID, &r.Name, &r.FromEntity, &r.ToEntity, &r.Via, &r.Description,
+		&r.Purity, &r.Direction, &r.Weight, &r.Status,
+		&startedAt, &endedAt, &r.Order, &tagsJSON, &r.Notes, &createdAt, &updatedAt)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("scanning relation: %w", err)
+	}
+	r.Tags = unmarshalStrings(tagsJSON)
+	r.CreatedAt, _ = time.Parse(time.RFC3339, createdAt)
+	r.UpdatedAt, _ = time.Parse(time.RFC3339, updatedAt)
+	if startedAt != nil {
+		t, _ := time.Parse(time.RFC3339, *startedAt)
+		r.StartedAt = &t
+	}
+	if endedAt != nil {
+		t, _ := time.Parse(time.RFC3339, *endedAt)
+		r.EndedAt = &t
+	}
+	return &r, nil
+}
+
+func scanRelationFromRows(rows *sql.Rows) (*Relation, error) {
+	var r Relation
+	var tagsJSON, createdAt, updatedAt string
+	var startedAt, endedAt *string
+	err := rows.Scan(&r.ID, &r.Name, &r.FromEntity, &r.ToEntity, &r.Via, &r.Description,
+		&r.Purity, &r.Direction, &r.Weight, &r.Status,
+		&startedAt, &endedAt, &r.Order, &tagsJSON, &r.Notes, &createdAt, &updatedAt)
+	if err != nil {
+		return nil, fmt.Errorf("scanning relation: %w", err)
+	}
+	r.Tags = unmarshalStrings(tagsJSON)
+	r.CreatedAt, _ = time.Parse(time.RFC3339, createdAt)
+	r.UpdatedAt, _ = time.Parse(time.RFC3339, updatedAt)
+	if startedAt != nil {
+		t, _ := time.Parse(time.RFC3339, *startedAt)
+		r.StartedAt = &t
+	}
+	if endedAt != nil {
+		t, _ := time.Parse(time.RFC3339, *endedAt)
+		r.EndedAt = &t
+	}
+	return &r, nil
+}
+
+// --- RelationInput CRUD ---
+
+// InsertRelationInput inserts a relation input.
+func (db *DB) InsertRelationInput(ri *RelationInput) error {
+	_, err := db.conn.Exec(`
+		INSERT INTO relation_inputs (id, relation_id, entity_id, role, "order")
+		VALUES (?, ?, ?, ?, ?)`,
+		ri.ID, ri.RelationID, ri.EntityID, ri.Role, ri.Order,
+	)
+	return err
+}
+
+// GetRelationInputs returns all inputs for a relation.
+func (db *DB) GetRelationInputs(relationID string) ([]RelationInput, error) {
+	rows, err := db.conn.Query(`
+		SELECT id, relation_id, entity_id, role, "order"
+		FROM relation_inputs WHERE relation_id = ? ORDER BY "order"`, relationID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var result []RelationInput
+	for rows.Next() {
+		var ri RelationInput
+		if err := rows.Scan(&ri.ID, &ri.RelationID, &ri.EntityID, &ri.Role, &ri.Order); err != nil {
+			return nil, fmt.Errorf("scanning relation_input: %w", err)
+		}
+		result = append(result, ri)
+	}
+	return result, nil
+}
+
+// DeleteRelationInputs removes all inputs for a relation.
+func (db *DB) DeleteRelationInputs(relationID string) error {
+	_, err := db.conn.Exec("DELETE FROM relation_inputs WHERE relation_id = ?", relationID)
+	return err
+}
diff --git a/fn_operations/validate.go b/fn_operations/validate.go
new file mode 100644
index 00000000..5eb759a0
--- /dev/null
+++ b/fn_operations/validate.go
@@ -0,0 +1,181 @@
+package fn_operations
+
+import (
+	"fmt"
+	"strings"
+)
+
+// ValidationError represents one or more integrity violations.
+type ValidationError struct {
+	ID     string
+	Errors []string
+}
+
+func (v *ValidationError) Error() string {
+	return fmt.Sprintf("%s: %s", v.ID, strings.Join(v.Errors, "; "))
+}
+
+// ValidateEntity checks entity integrity rules.
+func ValidateEntity(e *Entity) *ValidationError {
+	var errs []string
+
+	if e.ID == "" {
+		errs = append(errs, "id is required")
+	}
+	if e.Name == "" {
+		errs = append(errs, "name is required")
+	}
+	if e.TypeRef == "" {
+		errs = append(errs, "type_ref is required")
+	}
+	if e.Source == "" {
+		errs = append(errs, "source is required")
+	}
+
+	switch e.Status {
+	case StatusActive, StatusStale, StatusCorrupted, StatusArchived:
+	case "":
+		errs = append(errs, "status is required")
+	default:
+		errs = append(errs, fmt.Sprintf("invalid status: %s", e.Status))
+	}
+
+	if len(errs) > 0 {
+		return &ValidationError{ID: e.ID, Errors: errs}
+	}
+	return nil
+}
+
+// ValidateRelation checks relation integrity rules.
+// knownEntities is a set of entity IDs that exist.
+func ValidateRelation(r *Relation, knownEntities map[string]bool) *ValidationError {
+	var errs []string
+
+	if r.ID == "" {
+		errs = append(errs, "id is required")
+	}
+	if r.Name == "" {
+		errs = append(errs, "name is required")
+	}
+	if r.ToEntity == "" {
+		errs = append(errs, "to_entity is required")
+	}
+
+	// from_entity or relation_inputs — validated at operation level
+	if r.FromEntity != "" && r.ToEntity != "" && r.FromEntity == r.ToEntity {
+		errs = append(errs, "from_entity and to_entity cannot be the same")
+	}
+
+	if r.FromEntity != "" && !knownEntities[r.FromEntity] {
+		errs = append(errs, fmt.Sprintf("from_entity references unknown entity: %s", r.FromEntity))
+	}
+	if r.ToEntity != "" && !knownEntities[r.ToEntity] {
+		errs = append(errs, fmt.Sprintf("to_entity references unknown entity: %s", r.ToEntity))
+	}
+
+	if r.Weight != nil {
+		if *r.Weight < 0.0 || *r.Weight > 1.0 {
+			errs = append(errs, "weight must be between 0.0 and 1.0")
+		}
+	}
+
+	if r.StartedAt != nil && r.EndedAt != nil {
+		if r.StartedAt.After(*r.EndedAt) {
+			errs = append(errs, "started_at must be before ended_at")
+		}
+	}
+
+	switch r.Direction {
+	case DirUnidirectional, DirBidirectional, DirInverse, "":
+	default:
+		errs = append(errs, fmt.Sprintf("invalid direction: %s", r.Direction))
+	}
+
+	if len(errs) > 0 {
+		return &ValidationError{ID: r.ID, Errors: errs}
+	}
+	return nil
+}
+
+// ValidateRelationInputs checks relation_inputs integrity.
+func ValidateRelationInputs(inputs []RelationInput, knownEntities map[string]bool) *ValidationError {
+	var errs []string
+
+	if len(inputs) < 2 {
+		errs = append(errs, "relation_inputs must have at least 2 entries")
+	}
+
+	for i, ri := range inputs {
+		if ri.RelationID == "" {
+			errs = append(errs, fmt.Sprintf("input[%d]: relation_id is required", i))
+		}
+		if ri.EntityID == "" {
+			errs = append(errs, fmt.Sprintf("input[%d]: entity_id is required", i))
+		}
+		if ri.Role == "" {
+			errs = append(errs, fmt.Sprintf("input[%d]: role is required", i))
+		}
+		if ri.EntityID != "" && !knownEntities[ri.EntityID] {
+			errs = append(errs, fmt.Sprintf("input[%d]: entity_id references unknown entity: %s", i, ri.EntityID))
+		}
+	}
+
+	if len(errs) > 0 {
+		id := "relation_inputs"
+		if len(inputs) > 0 {
+			id = inputs[0].RelationID
+		}
+		return &ValidationError{ID: id, Errors: errs}
+	}
+	return nil
+}
+
+// DetectCycle checks if adding a causal relation (from -> to) creates a cycle.
+// Only considers relations where via != "" (causal/transformational).
+// Semantic relations (via == "") are exempt from cycle detection.
+func DetectCycle(db *DB, fromEntity, toEntity string) error {
+	if fromEntity == "" || toEntity == "" {
+		return nil
+	}
+
+	// BFS from toEntity following only causal relations.
+	// If we reach fromEntity, there's a cycle.
+	visited := map[string]bool{}
+	queue := []string{toEntity}
+
+	for len(queue) > 0 {
+		current := queue[0]
+		queue = queue[1:]
+
+		if visited[current] {
+			continue
+		}
+		visited[current] = true
+
+		if current == fromEntity {
+			return fmt.Errorf("cycle detected: adding relation %s -> %s would create a causal cycle", fromEntity, toEntity)
+		}
+
+		// Follow causal relations from current entity
+		rows, err := db.conn.Query(`
+			SELECT to_entity FROM relations
+			WHERE from_entity = ? AND via != ''`, current)
+		if err != nil {
+			return fmt.Errorf("querying relations for cycle detection: %w", err)
+		}
+
+		for rows.Next() {
+			var next string
+			if err := rows.Scan(&next); err != nil {
+				rows.Close()
+				return err
+			}
+			if !visited[next] {
+				queue = append(queue, next)
+			}
+		}
+		rows.Close()
+	}
+
+	return nil
+}