diff --git a/docs/development/task-backlog.md b/docs/development/task-backlog.md
index e5d8a93..a04e16e 100644
--- a/docs/development/task-backlog.md
+++ b/docs/development/task-backlog.md
@@ -378,10 +378,10 @@ Status markers:
 
 **Tasks:**
 
-- [ ] Define node types: `start`, `agent`, `tool`, `condition`, `humanApproval`, and `end`.
-- [ ] Define edges, variables, permissions, status, version, and timestamps.
-- [ ] Validate exactly one start node and at least one end node.
-- [ ] Commit with message `feat: define workflow dag schema`.
+- [x] Define node types: `start`, `agent`, `tool`, `condition`, `humanApproval`, and `end`.
+- [x] Define edges, variables, permissions, status, version, and timestamps.
+- [x] Validate exactly one start node and at least one end node.
+- [x] Commit with message `feat: define workflow dag schema`.
 
 **Acceptance Criteria:**
 
diff --git a/packages/workflow-core/src/index.ts b/packages/workflow-core/src/index.ts
index 6db5932..9c3244e 100644
--- a/packages/workflow-core/src/index.ts
+++ b/packages/workflow-core/src/index.ts
@@ -1 +1,25 @@
 export const workflowCorePackageName = "@agentdeck/workflow-core";
+
+export {
+  WORKFLOW_NODE_TYPES,
+  WORKFLOW_STATUSES,
+  createWorkflowDefinition,
+  validateWorkflowDefinition,
+} from "./workflow-schema.js";
+export type {
+  AgentNode,
+  ConditionNode,
+  EndNode,
+  HumanApprovalNode,
+  StartNode,
+  ToolNode,
+  WorkflowDefinition,
+  WorkflowEdge,
+  WorkflowNode,
+  WorkflowNodeBase,
+  WorkflowNodeType,
+  WorkflowPermissions,
+  WorkflowSchemaResult,
+  WorkflowStatus,
+  WorkflowValidationResult,
+} from "./workflow-schema.js";
diff --git a/packages/workflow-core/src/workflow-schema.test.ts b/packages/workflow-core/src/workflow-schema.test.ts
new file mode 100644
index 0000000..35bd907
--- /dev/null
+++ b/packages/workflow-core/src/workflow-schema.test.ts
@@ -0,0 +1,117 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  WORKFLOW_NODE_TYPES,
+  WORKFLOW_STATUSES,
+  createWorkflowDefinition,
+  validateWorkflowDefinition,
+  type WorkflowDefinition,
+} from "./workflow-schema.js";
+
+const validWorkflowInput = {
+  id: "workflow-review",
+  name: "Review workflow",
+  version: 1,
+  status: "draft",
+  variables: {
+    branch: "main",
+  },
+  permissions: {
+    read: true,
+    write: false,
+    install: false,
+    arbitraryCommands: false,
+    commit: false,
+    push: false,
+  },
+  nodes: [
+    { id: "start", type: "start", label: "Start" },
+    { id: "agent-review", type: "agent", label: "Review", agentId: "reviewer" },
+    { id: "tool-diff", type: "tool", label: "Diff", toolId: "git.diff" },
+    { id: "condition-risk", type: "condition", label: "Risk?", expression: "risk > 0" },
+    { id: "approval", type: "humanApproval", label: "Approve", approverRole: "maintainer" },
+    { id: "end", type: "end", label: "Done" },
+  ],
+  edges: [
+    { id: "edge-1", from: "start", to: "agent-review" },
+    { id: "edge-2", from: "agent-review", to: "tool-diff" },
+    { id: "edge-3", from: "tool-diff", to: "condition-risk" },
+    { id: "edge-4", from: "condition-risk", to: "approval", condition: "needsApproval" },
+    { id: "edge-5", from: "approval", to: "end" },
+  ],
+  createdAt: "2026-05-29T00:00:00.000Z",
+  updatedAt: "2026-05-29T00:00:00.000Z",
+} as const;
+
+describe("workflow schema constants", () => {
+  it("defines MVP workflow node types and statuses", () => {
+    expect(WORKFLOW_NODE_TYPES).toEqual([
+      "start",
+      "agent",
+      "tool",
+      "condition",
+      "humanApproval",
+      "end",
+    ]);
+    expect(WORKFLOW_STATUSES).toEqual(["draft", "active", "paused", "archived"]);
+  });
+});
+
+describe("createWorkflowDefinition", () => {
+  it("normalizes workflow fields, nodes, edges, variables, permissions, status, version, and timestamps", () => {
+    const result = createWorkflowDefinition(validWorkflowInput);
+
+    expect(result.success).toBe(true);
+    if (!result.success) {
+      throw new Error(result.errors.join("\n"));
+    }
+
+    expect(result.value).toEqual(validWorkflowInput);
+  });
+
+  it("rejects invalid node types, missing start node, and missing end node", () => {
+    const result = createWorkflowDefinition({
+      ...validWorkflowInput,
+      nodes: [{ id: "agent-review", type: "agent", label: "Review", agentId: "reviewer" }],
+      edges: [],
+    });
+
+    expect(result).toEqual({
+      success: false,
+      errors: [
+        "workflow must contain exactly one start node.",
+        "workflow must contain at least one end node.",
+      ],
+    });
+
+    expect(
+      validateWorkflowDefinition({
+        ...validWorkflowInput,
+        nodes: [{ id: "bad", type: "timer", label: "Timer" }],
+      } as unknown as WorkflowDefinition),
+    ).toEqual({
+      success: false,
+      errors: [
+        'node "bad" has invalid type "timer".',
+        "workflow must contain exactly one start node.",
+        "workflow must contain at least one end node.",
+      ],
+    });
+  });
+
+  it("rejects workflows with duplicate start nodes", () => {
+    const result = createWorkflowDefinition({
+      ...validWorkflowInput,
+      nodes: [
+        { id: "start-1", type: "start", label: "Start 1" },
+        { id: "start-2", type: "start", label: "Start 2" },
+        { id: "end", type: "end", label: "Done" },
+      ],
+    });
+
+    expect(result).toEqual({
+      success: false,
+      errors: ["workflow must contain exactly one start node."],
+    });
+  });
+});
diff --git a/packages/workflow-core/src/workflow-schema.ts b/packages/workflow-core/src/workflow-schema.ts
new file mode 100644
index 0000000..c9f862c
--- /dev/null
+++ b/packages/workflow-core/src/workflow-schema.ts
@@ -0,0 +1,187 @@
+export const WORKFLOW_NODE_TYPES = [
+  "start",
+  "agent",
+  "tool",
+  "condition",
+  "humanApproval",
+  "end",
+] as const;
+
+export const WORKFLOW_STATUSES = ["draft", "active", "paused", "archived"] as const;
+
+export type WorkflowNodeType = (typeof WORKFLOW_NODE_TYPES)[number];
+export type WorkflowStatus = (typeof WORKFLOW_STATUSES)[number];
+
+export interface WorkflowPermissions {
+  readonly read: boolean;
+  readonly write: boolean;
+  readonly install: boolean;
+  readonly arbitraryCommands: boolean;
+  readonly commit: boolean;
+  readonly push: boolean;
+}
+
+export interface WorkflowNodeBase<TType extends WorkflowNodeType = WorkflowNodeType> {
+  readonly id: string;
+  readonly type: TType;
+  readonly label: string;
+}
+
+export type StartNode = WorkflowNodeBase<"start">;
+export type EndNode = WorkflowNodeBase<"end">;
+
+export interface AgentNode extends WorkflowNodeBase<"agent"> {
+  readonly agentId: string;
+}
+
+export interface ToolNode extends WorkflowNodeBase<"tool"> {
+  readonly toolId: string;
+}
+
+export interface ConditionNode extends WorkflowNodeBase<"condition"> {
+  readonly expression: string;
+}
+
+export interface HumanApprovalNode extends WorkflowNodeBase<"humanApproval"> {
+  readonly approverRole: string;
+}
+
+export type WorkflowNode =
+  | StartNode
+  | AgentNode
+  | ToolNode
+  | ConditionNode
+  | HumanApprovalNode
+  | EndNode;
+
+export interface WorkflowEdge {
+  readonly id: string;
+  readonly from: string;
+  readonly to: string;
+  readonly condition?: string;
+}
+
+export interface WorkflowDefinition {
+  readonly id: string;
+  readonly name: string;
+  readonly version: number;
+  readonly status: WorkflowStatus;
+  readonly variables: Readonly<Record<string, string>>;
+  readonly permissions: WorkflowPermissions;
+  readonly nodes: readonly WorkflowNode[];
+  readonly edges: readonly WorkflowEdge[];
+  readonly createdAt: string;
+  readonly updatedAt: string;
+}
+
+export type WorkflowSchemaResult<T> =
+  | {
+      readonly success: true;
+      readonly value: T;
+    }
+  | {
+      readonly success: false;
+      readonly errors: readonly string[];
+    };
+
+export interface WorkflowValidationResult {
+  readonly success: boolean;
+  readonly errors: readonly string[];
+}
+
+export function createWorkflowDefinition(
+  input: WorkflowDefinition,
+): WorkflowSchemaResult<WorkflowDefinition> {
+  const workflow: WorkflowDefinition = {
+    id: input.id.trim(),
+    name: input.name.trim(),
+    version: input.version,
+    status: input.status,
+    variables: { ...input.variables },
+    permissions: { ...input.permissions },
+    nodes: input.nodes.map(normalizeNode),
+    edges: input.edges.map(normalizeEdge),
+    createdAt: input.createdAt,
+    updatedAt: input.updatedAt,
+  };
+  const validation = validateWorkflowDefinition(workflow);
+
+  if (!validation.success) {
+    return {
+      success: false,
+      errors: validation.errors,
+    };
+  }
+
+  return {
+    success: true,
+    value: workflow,
+  };
+}
+
+export function validateWorkflowDefinition(workflow: WorkflowDefinition): WorkflowValidationResult {
+  const errors: string[] = [];
+
+  if (!workflow.id.trim()) {
+    errors.push("id is required.");
+  }
+
+  if (!workflow.name.trim()) {
+    errors.push("name is required.");
+  }
+
+  if (!Number.isInteger(workflow.version) || workflow.version < 1) {
+    errors.push("version must be a positive integer.");
+  }
+
+  if (!isWorkflowStatus(workflow.status)) {
+    errors.push(`workflow status "${workflow.status}" is invalid.`);
+  }
+
+  for (const node of workflow.nodes) {
+    if (!isWorkflowNodeType(node.type)) {
+      errors.push(`node "${node.id}" has invalid type "${node.type}".`);
+    }
+  }
+
+  const startNodeCount = workflow.nodes.filter((node) => node.type === "start").length;
+  const endNodeCount = workflow.nodes.filter((node) => node.type === "end").length;
+
+  if (startNodeCount !== 1) {
+    errors.push("workflow must contain exactly one start node.");
+  }
+
+  if (endNodeCount < 1) {
+    errors.push("workflow must contain at least one end node.");
+  }
+
+  return {
+    success: errors.length === 0,
+    errors,
+  };
+}
+
+function normalizeNode(node: WorkflowNode): WorkflowNode {
+  return {
+    ...node,
+    id: node.id.trim(),
+    label: node.label.trim(),
+  };
+}
+
+function normalizeEdge(edge: WorkflowEdge): WorkflowEdge {
+  return {
+    id: edge.id.trim(),
+    from: edge.from.trim(),
+    to: edge.to.trim(),
+    ...(edge.condition ? { condition: edge.condition.trim() } : {}),
+  };
+}
+
+function isWorkflowNodeType(value: string): value is WorkflowNodeType {
+  return WORKFLOW_NODE_TYPES.includes(value as WorkflowNodeType);
+}
+
+function isWorkflowStatus(value: string): value is WorkflowStatus {
+  return WORKFLOW_STATUSES.includes(value as WorkflowStatus);
+}