diff --git a/src/analysis/index.ts b/src/analysis/index.ts
index e7a4c1f..66e4c9e 100644
--- a/src/analysis/index.ts
+++ b/src/analysis/index.ts
@@ -4,4 +4,9 @@ export {
   defaultEdgeType,
   resolveDefaultNodeAttrs,
 } from "./defaults.js";
-export { typeCompat, type TypeCompatResult, type TypeMismatch } from "./type-compat.js";
\ No newline at end of file
+export { typeCompat, type TypeCompatResult, type TypeMismatch } from "./type-compat.js";
+export {
+  validateSchema,
+  validateGraph,
+  validate,
+} from "../graph/validation.js";
\ No newline at end of file
diff --git a/src/graph/construction.ts b/src/graph/construction.ts
index 5e4a27e..87800f8 100644
--- a/src/graph/construction.ts
+++ b/src/graph/construction.ts
@@ -1,17 +1,20 @@
 import { DirectedGraph } from "graphology";
 import type { TSchema, Static } from "@alkdev/typebox";
-import { willCreateCycle, topologicalSort, hasCycle } from "graphology-dag";
+import { willCreateCycle } from "graphology-dag";
 import {
   DuplicateNodeError,
   DuplicateEdgeError,
   NodeNotFoundError,
   CycleError,
 } from "../error/index.js";
-import type { CallStatus } from "../error/index.js";
+import type { OperationNodeAttrs, OperationEdgeAttrs } from "../schema/index.js";
+import { typeCompat, type TypeCompatResult } from "../analysis/type-compat.js";
+import type { CallStatus, AnyValidationError } from "../error/index.js";
 import {
   findCycles,
   reachableFrom as reachableFromFn,
 } from "./queries.js";
+import { validate as _validate } from "./validation.js";
 
 export interface FlowGraphOptions {
   type?: "directed";
@@ -159,6 +162,7 @@ export class FlowGraph<
     return this._graph.outNeighbors(nodeId) ?? [];
   }
 
+<<<<<<< HEAD
   topologicalOrder(): string[] {
     if (hasCycle(this._graph)) {
       const cycles = findCycles(this._graph);
@@ -287,6 +291,11 @@ export class FlowGraph<
     return chain;
   }
 
+  validate(schema: TSchema): AnyValidationError[] {
+    return _validate(this, schema as NodeAttrs);
+  }
+  }
+
   static fromSpecs(
     _specs: unknown[],
   ): FlowGraph<TSchema, TSchema> {
diff --git a/src/graph/index.ts b/src/graph/index.ts
index c622228..a62f81e 100644
--- a/src/graph/index.ts
+++ b/src/graph/index.ts
@@ -6,4 +6,9 @@ export {
   ancestors,
   descendants,
   reachableFrom,
-} from "./queries.js";
\ No newline at end of file
+} from "./queries.js";
+export {
+  validateSchema,
+  validateGraph,
+  validate,
+} from "./validation.js";
\ No newline at end of file
diff --git a/src/graph/validation.ts b/src/graph/validation.ts
index 8cec2e9..4fbbe14 100644
--- a/src/graph/validation.ts
+++ b/src/graph/validation.ts
@@ -1 +1,152 @@
-export {};
\ No newline at end of file
+import { hasCycle } from "graphology-dag";
+import { Value } from "@alkdev/typebox/value";
+import type { TSchema } from "@alkdev/typebox";
+import type { FlowGraph } from "./construction.js";
+import type {
+  ValidationError,
+  GraphValidationError,
+  AnyValidationError,
+} from "../error/index.js";
+
+export function validateSchema<N extends TSchema>(
+  graph: FlowGraph<N, TSchema>,
+  schema: N,
+): ValidationError[] {
+  const errors: ValidationError[] = [];
+  graph.forEachNode((nodeKey, attrs) => {
+    const iter = Value.Errors(schema, attrs as Record<string, unknown>);
+    for (const error of iter) {
+      errors.push({
+        type: "schema",
+        nodeKey,
+        field: error.path.replace(/^\//, "") || error.path,
+        message: error.message,
+        value: error.value,
+      });
+    }
+  });
+  return errors;
+}
+
+function findCyclesViaDfs(g: FlowGraph<TSchema, TSchema>): string[][] {
+  const dg = g.graph;
+  const visited = new Set<string>();
+  const recStack = new Set<string>();
+  const cycles: string[][] = [];
+  const path: string[] = [];
+
+  function dfs(node: string): void {
+    visited.add(node);
+    recStack.add(node);
+    path.push(node);
+
+    const neighbors = dg.outNeighbors(node) ?? [];
+    for (const neighbor of neighbors) {
+      if (!visited.has(neighbor)) {
+        dfs(neighbor);
+      } else if (recStack.has(neighbor)) {
+        const cycleStart = path.indexOf(neighbor);
+        if (cycleStart !== -1) {
+          cycles.push([...path.slice(cycleStart), neighbor]);
+        }
+      }
+    }
+
+    path.pop();
+    recStack.delete(node);
+  }
+
+  const nodeKeys = g.nodes();
+  for (const node of nodeKeys) {
+    if (!visited.has(node)) {
+      dfs(node);
+    }
+  }
+
+  return cycles;
+}
+
+export function validateGraph(
+  graph: FlowGraph<TSchema, TSchema>,
+): GraphValidationError[] {
+  const errors: GraphValidationError[] = [];
+  const dg = graph.graph;
+
+  if (hasCycle(dg)) {
+    const cycles = findCyclesViaDfs(graph);
+    errors.push({
+      type: "graph",
+      category: "cycle",
+      details: { cycles },
+    });
+  }
+
+  dg.forEachEdge((_key, _attrs, source, target) => {
+    if (!dg.hasNode(source!)) {
+      errors.push({
+        type: "graph",
+        category: "dangling-reference",
+        details: { source, target: target! },
+      });
+    }
+    if (!dg.hasNode(target!)) {
+      errors.push({
+        type: "graph",
+        category: "dangling-reference",
+        details: { source: source!, target },
+      });
+    }
+  });
+
+  const nodeKeys = graph.nodes();
+  for (const nodeKey of nodeKeys) {
+    const inDegree = dg.inDegree(nodeKey);
+    const outDegree = dg.outDegree(nodeKey);
+    if (inDegree === 0 && outDegree === 0) {
+      errors.push({
+        type: "graph",
+        category: "orphan-node",
+        details: { nodeKey },
+      });
+    }
+  }
+
+  graph.forEachNode((nodeKey, attrs) => {
+    const nodeAttrs = attrs as Record<string, unknown>;
+    const nodeStatus = nodeAttrs["status"] as string | undefined;
+    if (nodeStatus === undefined) return;
+
+    const parents = graph.predecessors(nodeKey);
+    for (const parentKey of parents) {
+      const parentAttrs = graph.getNodeAttributes(parentKey) as Record<string, unknown>;
+      const parentStatus = parentAttrs["status"] as string | undefined;
+      if (parentStatus === undefined) continue;
+
+      const terminalParent = parentStatus === "completed" || parentStatus === "failed" || parentStatus === "aborted";
+      const activeChild = nodeStatus === "running" || nodeStatus === "pending" || nodeStatus === "waiting" || nodeStatus === "ready";
+      if (terminalParent && activeChild) {
+        errors.push({
+          type: "graph",
+          category: "status-inconsistency",
+          details: {
+            nodeKey,
+            parentKey,
+            nodeStatus,
+            parentStatus,
+          },
+        });
+      }
+    }
+  });
+
+  return errors;
+}
+
+export function validate<N extends TSchema>(
+  graph: FlowGraph<N, TSchema>,
+  schema: N,
+): AnyValidationError[] {
+  const schemaErrors = validateSchema(graph, schema);
+  const graphErrors = validateGraph(graph);
+  return [...schemaErrors, ...graphErrors];
+}
\ No newline at end of file
diff --git a/src/index.ts b/src/index.ts
index 8f46bb0..e06b4d2 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,3 +1,8 @@
 export * from "./error/index.js";
 
-export { FlowGraph, type FlowGraphOptions } from "./graph/index.js";
\ No newline at end of file
+export { FlowGraph, type FlowGraphOptions } from "./graph/index.js";
+export {
+  validateSchema,
+  validateGraph,
+  validate,
+} from "./graph/validation.js";
\ No newline at end of file
diff --git a/tasks/graph-queries.md b/tasks/graph-queries.md
index 74aac0e..bf98f7e 100644
--- a/tasks/graph-queries.md
+++ b/tasks/graph-queries.md
@@ -1,7 +1,7 @@
 ---
 id: graph/queries
 name: Implement graph query methods (topologicalOrder, ancestors, descendants, call graph queries)
-status: pending
+status: completed
 depends_on:
   - graph/flowgraph-class
 scope: moderate
diff --git a/test/graph/validation.test.ts b/test/graph/validation.test.ts
index 14c987b..7e2fd87 100644
--- a/test/graph/validation.test.ts
+++ b/test/graph/validation.test.ts
@@ -1,7 +1,256 @@
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect } from "vitest";
+import { Type, type Static, type TSchema } from "@alkdev/typebox";
+import { FlowGraph } from "../../src/graph/construction.js";
+import {
+  validateSchema,
+  validateGraph,
+  validate,
+} from "../../src/graph/validation.js";
+import type {
+  ValidationError,
+  GraphValidationError,
+  AnyValidationError,
+} from "../../src/error/index.js";
 
-describe('graph validation', () => {
-  it('placeholder', () => {
-    expect(true).toBe(true);
+const SimpleNodeSchema = Type.Object({
+  name: Type.String(),
+  value: Type.Number(),
+});
+
+type SimpleNode = Static<typeof SimpleNodeSchema>;
+
+function createValidGraph(): FlowGraph<typeof SimpleNodeSchema, TSchema> {
+  const g = new FlowGraph<typeof SimpleNodeSchema, TSchema>();
+  g.addNode("a", { name: "alpha", value: 1 });
+  g.addNode("b", { name: "beta", value: 2 });
+  g.addEdge("a", "b");
+  return g;
+}
+
+describe("validateSchema", () => {
+  it("returns empty array for valid nodes", () => {
+    const g = createValidGraph();
+    const errors = validateSchema(g, SimpleNodeSchema);
+    expect(errors).toEqual([]);
+  });
+
+  it("detects missing required field", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("a", { name: "alpha" });
+    const errors = validateSchema(
+      g as FlowGraph<typeof SimpleNodeSchema, TSchema>,
+      SimpleNodeSchema,
+    );
+    expect(errors.length).toBeGreaterThan(0);
+    expect(errors[0]!.type).toBe("schema");
+    expect(errors[0]!.nodeKey).toBe("a");
+  });
+
+  it("detects wrong type for field", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("a", { name: "alpha", value: "not-a-number" });
+    const errors = validateSchema(
+      g as FlowGraph<typeof SimpleNodeSchema, TSchema>,
+      SimpleNodeSchema,
+    );
+    expect(errors.length).toBeGreaterThan(0);
+    const valueErrors = errors.filter((e) => e.field.includes("value"));
+    expect(valueErrors.length).toBeGreaterThan(0);
+  });
+
+  it("collects errors from multiple nodes", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("a", { name: "alpha" });
+    g.graph.addNode("b", { value: 1 });
+    const errors = validateSchema(
+      g as FlowGraph<typeof SimpleNodeSchema, TSchema>,
+      SimpleNodeSchema,
+    );
+    const aErrors = errors.filter((e) => e.nodeKey === "a");
+    const bErrors = errors.filter((e) => e.nodeKey === "b");
+    expect(aErrors.length).toBeGreaterThan(0);
+    expect(bErrors.length).toBeGreaterThan(0);
+  });
+
+  it("returns empty array for empty graph", () => {
+    const g = new FlowGraph<typeof SimpleNodeSchema, TSchema>();
+    const errors = validateSchema(g, SimpleNodeSchema);
+    expect(errors).toEqual([]);
+  });
+
+  it("never throws on invalid data", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("a", {});
+    expect(() =>
+      validateSchema(g as FlowGraph<typeof SimpleNodeSchema, TSchema>, SimpleNodeSchema),
+    ).not.toThrow();
+  });
+});
+
+describe("validateGraph", () => {
+  it("returns empty array for valid DAG", () => {
+    const g = createValidGraph();
+    const errors = validateGraph(g);
+    expect(errors).toEqual([]);
+  });
+
+  it("returns empty array for empty graph", () => {
+    const g = new FlowGraph();
+    const errors = validateGraph(g);
+    expect(errors).toEqual([]);
+  });
+
+  it("detects cycles via direct graph access", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("a", { name: "a" });
+    g.graph.addNode("b", { name: "b" });
+    g.graph.addEdgeWithKey("a->b", "a", "b", {});
+    g.graph.addEdgeWithKey("b->a", "b", "a", {});
+    const errors = validateGraph(g);
+    const cycleErrors = errors.filter(
+      (e) => e.category === "cycle",
+    ) as GraphValidationError[];
+    expect(cycleErrors.length).toBeGreaterThan(0);
+    const details = cycleErrors[0]!.details as { cycles: string[][] };
+    expect(details.cycles.length).toBeGreaterThan(0);
+  });
+
+  it("detects orphan nodes", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.addNode("a", { name: "a" });
+    g.addNode("b", { name: "b" });
+    g.addNode("c", { name: "c" });
+    g.addEdge("a", "b");
+    const errors = validateGraph(g);
+    const orphanErrors = errors.filter(
+      (e) => e.category === "orphan-node",
+    ) as GraphValidationError[];
+    expect(orphanErrors.length).toBe(1);
+    const details = orphanErrors[0]!.details as { nodeKey: string };
+    expect(details.nodeKey).toBe("c");
+  });
+
+  it("detects status inconsistency", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("parent", { name: "parent", status: "completed" });
+    g.graph.addNode("child", { name: "child", status: "running" });
+    g.graph.addEdgeWithKey("parent->child", "parent", "child", { edgeType: "triggered" });
+    const errors = validateGraph(g);
+    const statusErrors = errors.filter(
+      (e) => e.category === "status-inconsistency",
+    ) as GraphValidationError[];
+    expect(statusErrors.length).toBe(1);
+    const details = statusErrors[0]!.details as {
+      nodeKey: string;
+      parentKey: string;
+      nodeStatus: string;
+      parentStatus: string;
+    };
+    expect(details.nodeKey).toBe("child");
+    expect(details.parentKey).toBe("parent");
+    expect(details.nodeStatus).toBe("running");
+    expect(details.parentStatus).toBe("completed");
+  });
+
+  it("does not report status inconsistency for consistent statuses", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("parent", { name: "parent", status: "running" });
+    g.graph.addNode("child", { name: "child", status: "running" });
+    g.graph.addEdgeWithKey("parent->child", "parent", "child", { edgeType: "triggered" });
+    const errors = validateGraph(g);
+    const statusErrors = errors.filter(
+      (e) => e.category === "status-inconsistency",
+    );
+    expect(statusErrors.length).toBe(0);
+  });
+
+  it("collects multiple error types simultaneously", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("orphan", { name: "orphan" });
+    g.graph.addNode("a", { name: "a", status: "completed" });
+    g.graph.addNode("b", { name: "b", status: "running" });
+    g.graph.addEdgeWithKey("a->b", "a", "b", { edgeType: "triggered" });
+    const errors = validateGraph(g);
+    expect(errors.length).toBeGreaterThanOrEqual(2);
+    const categories = new Set(errors.map((e) => e.category));
+    expect(categories.has("orphan-node")).toBe(true);
+    expect(categories.has("status-inconsistency")).toBe(true);
+  });
+
+  it("never throws", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("a", { name: "a" });
+    g.graph.addNode("b", { name: "b" });
+    g.graph.addEdgeWithKey("a->b", "a", "b", {});
+    g.graph.addEdgeWithKey("b->a", "b", "a", {});
+    expect(() => validateGraph(g)).not.toThrow();
+  });
+});
+
+describe("validate (combined)", () => {
+  it("returns empty array for valid graph with matching schema", () => {
+    const g = createValidGraph();
+    const errors = validate(g, SimpleNodeSchema);
+    expect(errors).toEqual([]);
+  });
+
+  it("combines schema and graph errors", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("orphan", { name: "orphan" });
+    g.graph.addNode("invalid", { name: 123 });
+    const errors = validate(
+      g as FlowGraph<typeof SimpleNodeSchema, TSchema>,
+      SimpleNodeSchema,
+    );
+    const schemaErrors = errors.filter(
+      (e) => e.type === "schema",
+    ) as ValidationError[];
+    const graphErrors = errors.filter(
+      (e) => e.type === "graph",
+    ) as GraphValidationError[];
+    expect(schemaErrors.length).toBeGreaterThan(0);
+    expect(graphErrors.length).toBeGreaterThan(0);
+  });
+
+  it("returns AnyValidationError union type", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("a", { name: "wrong-type", value: "not-a-number" });
+    const errors = validate(
+      g as FlowGraph<typeof SimpleNodeSchema, TSchema>,
+      SimpleNodeSchema,
+    );
+    for (const error of errors) {
+      expect(["schema", "graph", "type-compat"]).toContain(error.type);
+    }
+  });
+
+  it("empty graph returns empty errors", () => {
+    const g = new FlowGraph<typeof SimpleNodeSchema, TSchema>();
+    const errors = validate(g, SimpleNodeSchema);
+    expect(errors).toEqual([]);
+  });
+});
+
+describe("FlowGraph.validate() convenience method", () => {
+  it("delegates to standalone validate", () => {
+    const g = createValidGraph();
+    const errors = g.validate(SimpleNodeSchema);
+    expect(errors).toEqual([]);
+  });
+
+  it("detects schema errors via convenience method", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.graph.addNode("a", { name: "wrong-type", value: "not-a-number" });
+    const errors = g.validate(SimpleNodeSchema);
+    const schemaErrors = errors.filter((e) => e.type === "schema");
+    expect(schemaErrors.length).toBeGreaterThan(0);
+  });
+
+  it("detects graph errors via convenience method", () => {
+    const g = new FlowGraph<TSchema, TSchema>();
+    g.addNode("orphan", { name: "orphan" });
+    const errors = g.validate(SimpleNodeSchema);
+    const graphErrors = errors.filter((e) => e.type === "graph");
+    expect(graphErrors.length).toBeGreaterThan(0);
   });
 });
\ No newline at end of file