diff --git a/.changeset/FN-7113-workflow-graph-integrity.md b/.changeset/FN-7113-workflow-graph-integrity.md
new file mode 100644
index 0000000000..97f29a21d1
--- /dev/null
+++ b/.changeset/FN-7113-workflow-graph-integrity.md
@@ -0,0 +1,7 @@
+---
+"@runfusion/fusion": minor
+---
+
+summary: Reject malformed workflow graphs before they can be saved or launched.
+category: feature
+dev: Hardens the central parseWorkflowIr/validateV2 gate (duplicate-node-id and required top-level reachability rejection) and fail-closed re-validation at the WorkflowGraphTaskRunner run boundary before any side effects (FN-7113).
diff --git a/docs/custom-workflow-reliability-acceptance-map.md b/docs/custom-workflow-reliability-acceptance-map.md
index 72ae74c28e..195126c1a7 100644
--- a/docs/custom-workflow-reliability-acceptance-map.md
+++ b/docs/custom-workflow-reliability-acceptance-map.md
@@ -9,6 +9,9 @@ This artifact distinguishes MVP/blocking requirements from nice-to-have enhancem
 
 FNXC:WorkflowRouting 2026-06-22-12:00:
 Workflow selection acceptance must distinguish operator intent and task creator ownership from executor opportunism. Agents can assign workflows when the user asked or when creating the task; executors cannot reroute the task under execution unless instructed.
+
+FNXC:WorkflowValidation 2026-06-27-00:00:
+FN-7113 upgrades graph integrity from an authoring-only expectation to a save-and-run acceptance criterion: malformed workflow DAGs must be rejected before persistence and revalidated before graph execution side effects.
 -->
 
 ## Purpose
@@ -37,7 +40,7 @@ Use this document to write engineering tasks, QA plans, and release checks. It i
 - **Actor / need:** A workflow author needs to create or copy a workflow that can be reviewed, saved, and selected without corrupting built-in definitions.
 - **Trigger:** Open the [Workflow Editor](./workflow-editor.md) from the dashboard, duplicate a built-in with **Duplicate to customize**, start from Blank, import a JSON envelope, or use workflow tools such as `fn_workflow_create` / `fn_workflow_update`.
 - **Expected happy path + lifecycle transitions + feedback:** The editor serializes graph nodes/edges, columns, fields, and setting declarations into Workflow IR, saves the custom definition, and keeps built-ins read-only. The saved workflow appears in the editor picker and `fn_workflow_list`; no task lifecycle transition occurs until a task selects the workflow. The editor reports whether the workflow can run on the linear engine or must run on the graph interpreter.
-- **Failure / recovery expectation:** Invalid JSON, dangling edges, illegal cycles, unplaced nodes, blocking column-trait violations, invalid setting/field declarations, and attempts to mutate built-ins are rejected before partial persistence. Import errors and server validation errors render in a persistent inline error region; built-ins show read-only hints and disable mutation controls.
+- **Failure / recovery expectation:** Invalid JSON, duplicate top-level node ids, missing/multiple start or end nodes, dangling edges, illegal cycles, invalid step/template references, unknown plugin workflow-extension keys, unplaced nodes, blocking column-trait violations, invalid setting/field declarations, and attempts to mutate built-ins are rejected before partial persistence. Import errors and server validation errors render in a persistent inline error region; built-ins show read-only hints and disable mutation controls.
 - **Measurable success signal:** A stable workflow ID is returned/listed by `fn_workflow_list`; `fn_workflow_get` or the editor reload shows the saved IR; invalid saves return a typed validation failure without changing the prior persisted definition.
 - **Priority:** MVP/blocking for save/validation/discovery; enhancement for AI-assisted design quality and richer telemetry around definition registration.
 
@@ -64,7 +67,7 @@ Use this document to write engineering tasks, QA plans, and release checks. It i
 - **Actor / need:** The scheduler/executor needs to run the selected workflow deterministically while preserving Fusion's observable task lifecycle.
 - **Trigger:** A schedulable task with a selected or default workflow is picked up for execution.
 - **Expected happy path + lifecycle transitions + feedback:** `TaskExecutor.execute()` resolves the workflow, pins graph execution for the run, and `WorkflowGraphExecutor` traverses nodes through workflow runtime primitives such as planning, execute, workflow-step, review, merge, schedule, and step-execute. Standard coding work continues to show `todo → in-progress → in-review → done` (or equivalent workflow-defined columns/holds where enabled), workflow checks appear on task cards/list/detail, and task documents/artifacts are persisted as produced.
-- **Failure / recovery expectation:** Unsupported edge conditions throw `WorkflowIrError`; explicit custom workflow resolution failures fail closed; interpreter failures park as workflow failures rather than re-running a legacy imperative path. File-scope guards (`FileScopeViolationError`), squash overlap enforcement, `autoMerge:false` terminal-until-human behavior, and `moveTask(in-progress → todo)` hard-cancel semantics remain non-bypassable.
+- **Failure / recovery expectation:** Unsupported edge conditions throw `WorkflowIrError`; explicit custom workflow resolution failures fail closed; resolved IR is revalidated before any graph side effects and malformed graphs fail closed with `invalid-ir: <message>` rather than partially running or falling back into the wrong legacy workflow; interpreter failures after side effects park as workflow failures rather than re-running a legacy imperative path. File-scope guards (`FileScopeViolationError`), squash overlap enforcement, `autoMerge:false` terminal-until-human behavior, and `moveTask(in-progress → todo)` hard-cancel semantics remain non-bypassable.
 - **Measurable success signal:** Workflow results are visible in task card/list/detail surfaces; node outcomes route according to `success`, `failure`, or `outcome:<value>` edges; relevant run-audit records exist for lifecycle/git/database mutations; parity instrumentation emits `workflow:parity-observed` or `workflow:parity-drift` when dual-observe is enabled.
 - **Priority:** MVP/blocking.
 
diff --git a/docs/workflow-steps.md b/docs/workflow-steps.md
index 1ea4c0c414..e298352b08 100644
--- a/docs/workflow-steps.md
+++ b/docs/workflow-steps.md
@@ -55,6 +55,26 @@ Decision-only or investigation tasks can also declare `noCommitsExpected` / `**N
 
 Use the dashboard [Workflow Editor](./workflow-editor.md) to inspect built-ins, tune built-in prompts, duplicate workflows, or author custom workflows. Custom workflows can declare graph nodes and edges, columns/traits, task fields, typed workflow settings, model lanes, optional workflow-step templates, and author-time validation. Use this page for runtime semantics; use the editor guide for the visual authoring surface.
 
+### Workflow graph integrity validation
+
+<!--
+FNXC:WorkflowValidation 2026-06-27-00:00:
+FN-7113 makes graph integrity a save-and-run invariant. The central `parseWorkflowIr`/`validateV2` gate rejects malformed author/plugin graphs before persistence, and `WorkflowGraphTaskRunner` re-validates the resolved IR before side-effecting seams so stale or plugin-supplied invalid graphs fail closed instead of partially running or falling back into the wrong legacy workflow.
+-->
+
+Workflow definitions are validated through the same central IR gate before they can be saved, imported, AI-designed, selected/materialized for a task, or launched by the graph interpreter. Dashboard routes and workflow tools surface `WorkflowIrError` / `WorkflowCompileError` messages as author-facing validation failures instead of persisting partial definitions.
+
+The enforced integrity classes include:
+
+- exactly one `start` node and exactly one `end` node;
+- unique top-level node ids and unique column/field/setting ids;
+- every top-level edge endpoint references a declared top-level node;
+- no illegal non-rework cycles in DAG-required regions;
+- required reachability/dominance rules: every required top-level node must be reachable from `start`, and `parse-steps` must dominate `foreach(source:"task-steps")`; interpreter-owned recovery entry primitives remain valid even when they are re-entered from persisted runtime state instead of the author-facing start path;
+- valid node-specific references, including `parse-steps` artifacts, `loop.exitWhen.nodeId`, foreach/loop/optional-group template entry/exit references, and registered plugin workflow-extension keys.
+
+At run time, `WorkflowGraphTaskRunner` resolves the selected built-in or custom workflow, re-runs this integrity validation before any seam, primitive, or custom-node side effect, and fails closed with an `invalid-ir: <message>` reason when the resolved IR is malformed. Once a node side effect has run, runtime failures keep the existing failed-run behavior rather than re-running the legacy pipeline.
+
 ### Overriding built-in workflow prompts
 
 <!--
diff --git a/packages/core/src/__tests__/workflow-ir-extension-metadata.test.ts b/packages/core/src/__tests__/workflow-ir-extension-metadata.test.ts
index 651de9caa4..49672f060f 100644
--- a/packages/core/src/__tests__/workflow-ir-extension-metadata.test.ts
+++ b/packages/core/src/__tests__/workflow-ir-extension-metadata.test.ts
@@ -29,7 +29,22 @@ describe("workflow IR extension metadata", () => {
     __resetWorkflowExtensionRegistryForTests();
   });
 
-  it("accepts plugin-namespaced column and node extension metadata", () => {
+  it("accepts registered plugin-namespaced column and node extension metadata", () => {
+    getWorkflowExtensionRegistry().register("workflow-pack", {
+      extensionId: "role",
+      name: "Role",
+      kind: "column-metadata",
+      schemaVersion: WORKFLOW_EXTENSION_SCHEMA_VERSION,
+      fallback: "failClosed",
+    });
+    getWorkflowExtensionRegistry().register("workflow-pack", {
+      extensionId: "node-handler",
+      name: "Node handler",
+      kind: "node-handler",
+      schemaVersion: WORKFLOW_EXTENSION_SCHEMA_VERSION,
+      fallback: "failClosed",
+    });
+
     const parsed = parseWorkflowIr(ir({
       columns: [
         {
@@ -60,6 +75,22 @@ describe("workflow IR extension metadata", () => {
     expect(parsed.nodes[0].extensions?.["plugin:workflow-pack:node-handler"]).toEqual({ handler: "plan" });
   });
 
+  it("rejects unknown plugin-namespaced extension metadata keys", () => {
+    expect(() =>
+      parseWorkflowIr(ir({
+        nodes: [
+          {
+            id: "start",
+            kind: "start",
+            column: "todo",
+            extensions: { "plugin:workflow-pack:missing": {} },
+          },
+          { id: "end", kind: "end", column: "todo" },
+        ],
+      })),
+    ).toThrow(/Workflow node 'start' extension key 'plugin:workflow-pack:missing' is not registered/);
+  });
+
   it("rejects extension metadata keys outside the plugin namespace", () => {
     expect(() =>
       parseWorkflowIr(ir({
@@ -76,6 +107,14 @@ describe("workflow IR extension metadata", () => {
   });
 
   it("rejects non-object extension metadata values", () => {
+    getWorkflowExtensionRegistry().register("workflow-pack", {
+      extensionId: "node-handler",
+      name: "Node handler",
+      kind: "node-handler",
+      schemaVersion: WORKFLOW_EXTENSION_SCHEMA_VERSION,
+      fallback: "failClosed",
+    });
+
     expect(() =>
       parseWorkflowIr(ir({
         nodes: [
@@ -140,6 +179,14 @@ describe("workflow IR extension metadata", () => {
   });
 
   it("keeps v2 when otherwise-pure workflows carry extension metadata", () => {
+    getWorkflowExtensionRegistry().register("workflow-pack", {
+      extensionId: "role",
+      name: "Role",
+      kind: "column-metadata",
+      schemaVersion: WORKFLOW_EXTENSION_SCHEMA_VERSION,
+      fallback: "failClosed",
+    });
+
     const parsed = parseWorkflowIr(ir({
       columns: [
         {
diff --git a/packages/core/src/__tests__/workflow-ir.test.ts b/packages/core/src/__tests__/workflow-ir.test.ts
index 6285828f7f..b1f28d1277 100644
--- a/packages/core/src/__tests__/workflow-ir.test.ts
+++ b/packages/core/src/__tests__/workflow-ir.test.ts
@@ -13,6 +13,7 @@ import type {
   WorkflowIrNode,
   WorkflowIrEdge,
 } from "../workflow-ir-types.js";
+import { BUILTIN_WORKFLOWS } from "../builtin-workflows.js";
 
 function v2(
   columns: WorkflowIrV2["columns"],
@@ -153,6 +154,25 @@ describe("parseWorkflowIr — v2 columns & placement", () => {
     );
     expect(() => parseWorkflowIr(ir)).toThrow(/duplicate column id 'dup'/);
   });
+
+  it("rejects duplicate top-level node ids before Map de-duplication can mask them", () => {
+    const ir = v2(
+      [{ id: "only", name: "Only", traits: [] }],
+      [
+        { id: "start", kind: "start", column: "only" },
+        { id: "dup", kind: "prompt", column: "only" },
+        { id: "dup", kind: "script", column: "only" },
+        { id: "end", kind: "end", column: "only" },
+      ],
+      [
+        { from: "start", to: "dup" },
+        { from: "dup", to: "end" },
+      ],
+    );
+
+    expect(() => parseWorkflowIr(ir)).toThrow(WorkflowIrError);
+    expect(() => parseWorkflowIr(ir)).toThrow(/Workflow IR has duplicate node id 'dup'/);
+  });
 });
 
 // FNXC:WorkflowOptionalGroup 2026-06-21-18:00:
@@ -659,8 +679,92 @@ describe("parseWorkflowIr — version & shape guards", () => {
     );
   });
 
-  it("rejects missing start/end nodes", () => {
-    const ir = v2([{ id: "c", name: "C", traits: [] }], [{ id: "start", kind: "start", column: "c" }], []);
+  it("rejects missing start nodes", () => {
+    const ir = v2([{ id: "c", name: "C", traits: [] }], [{ id: "end", kind: "end", column: "c" }], []);
+    expect(() => parseWorkflowIr(ir)).toThrow(WorkflowIrError);
     expect(() => parseWorkflowIr(ir)).toThrow(/exactly one start and one end/);
   });
+
+  it("rejects missing end nodes", () => {
+    const ir = v2([{ id: "c", name: "C", traits: [] }], [{ id: "start", kind: "start", column: "c" }], []);
+    expect(() => parseWorkflowIr(ir)).toThrow(WorkflowIrError);
+    expect(() => parseWorkflowIr(ir)).toThrow(/exactly one start and one end/);
+  });
+
+  it("rejects illegal non-rework cycles", () => {
+    const ir = v2(
+      [{ id: "c", name: "C", traits: [] }],
+      [
+        { id: "start", kind: "start", column: "c" },
+        { id: "a", kind: "prompt", column: "c" },
+        { id: "b", kind: "prompt", column: "c" },
+        { id: "end", kind: "end", column: "c" },
+      ],
+      [
+        { from: "start", to: "a" },
+        { from: "a", to: "b" },
+        { from: "b", to: "a" },
+      ],
+    );
+
+    expect(() => parseWorkflowIr(ir)).toThrow(WorkflowIrError);
+    expect(() => parseWorkflowIr(ir)).toThrow(/illegal cycle.*edge 'b' -> 'a'/);
+  });
+
+  it("rejects unreachable required top-level nodes with the offending node id", () => {
+    const ir = v2(
+      [{ id: "c", name: "C", traits: [] }],
+      [
+        { id: "start", kind: "start", column: "c" },
+        { id: "reachable", kind: "prompt", column: "c" },
+        { id: "orphan", kind: "prompt", column: "c" },
+        { id: "end", kind: "end", column: "c" },
+      ],
+      [
+        { from: "start", to: "reachable" },
+        { from: "reachable", to: "end" },
+      ],
+    );
+
+    expect(() => parseWorkflowIr(ir)).toThrow(WorkflowIrError);
+    expect(() => parseWorkflowIr(ir)).toThrow(/Workflow node 'orphan' is not reachable from the start node/);
+  });
+
+  it("rejects invalid parse-steps artifact references with the offending node and artifact", () => {
+    const ir = v2(
+      [{ id: "c", name: "C", traits: [] }],
+      [
+        { id: "start", kind: "start", column: "c" },
+        { id: "parse", kind: "parse-steps", column: "c", config: { artifact: "missing.md", parser: "step-headings" } },
+        { id: "end", kind: "end", column: "c" },
+      ],
+      [
+        { from: "start", to: "parse" },
+        { from: "parse", to: "end" },
+      ],
+    );
+
+    expect(() => parseWorkflowIr(ir)).toThrow(WorkflowIrError);
+    expect(() => parseWorkflowIr(ir)).toThrow(/parse-steps node 'parse' references artifact 'missing.md'/);
+  });
+
+  it("parses valid v2 graphs and every built-in workflow without false rejection", () => {
+    const valid = v2(
+      [{ id: "c", name: "C", traits: [] }],
+      [
+        { id: "start", kind: "start", column: "c" },
+        { id: "custom", kind: "prompt", column: "c" },
+        { id: "end", kind: "end", column: "c" },
+      ],
+      [
+        { from: "start", to: "custom" },
+        { from: "custom", to: "end" },
+      ],
+    );
+
+    expect(() => parseWorkflowIr(valid)).not.toThrow();
+    for (const workflow of BUILTIN_WORKFLOWS) {
+      expect(() => parseWorkflowIr(workflow.ir)).not.toThrow();
+    }
+  });
 });
diff --git a/packages/core/src/workflow-ir.ts b/packages/core/src/workflow-ir.ts
index cc45b90b93..afba851d13 100644
--- a/packages/core/src/workflow-ir.ts
+++ b/packages/core/src/workflow-ir.ts
@@ -307,6 +307,36 @@ function reachableFrom(
   return seen;
 }
 
+const INTERPRETER_ENTRY_NODE_KINDS: ReadonlySet<WorkflowIrNodeKind> = new Set([
+  "merge-gate",
+  "merge-attempt",
+  "manual-merge-hold",
+  "retry-backoff",
+  "recovery-router",
+  "branch-group-member-integration",
+  "branch-group-promotion",
+  "pr-create",
+  "pr-respond",
+  "pr-merge",
+]);
+
+function validateRequiredTopLevelReachability(
+  nodes: WorkflowIrNode[],
+  outgoing: Map<string, WorkflowIrEdge[]>,
+): void {
+  /*
+  FNXC:WorkflowValidation 2026-06-27-07:40:
+  FN-7113 requires required top-level workflow nodes to be reachable from start at parse time, including interpreter-deferred branch graphs. Engine-owned recovery entry primitives stay exempt because they can be re-entered by persisted runtime state rather than by the author-facing start path.
+  */
+  const startNode = nodes.find((node) => node.kind === "start");
+  if (!startNode) return;
+  const reachable = reachableFrom(startNode.id, outgoing);
+  for (const node of nodes) {
+    if (reachable.has(node.id) || INTERPRETER_ENTRY_NODE_KINDS.has(node.kind)) continue;
+    throw new WorkflowIrError(`Workflow node '${node.id}' is not reachable from the start node`);
+  }
+}
+
 /**
  * Validate a foreach `template` subgraph recursively (KTD-3):
  *  - non-empty;
@@ -1234,7 +1264,14 @@ function validateRegisteredExtensionMetadata(
   value: Record<string, unknown>,
 ): void {
   const definition = getWorkflowExtensionRegistry().get(key);
-  const fields = definition?.extension.configSchema?.fields;
+  /*
+  FNXC:WorkflowValidation 2026-06-27-00:00:
+  FN-7113 requires plugin-referencing workflow graphs to validate against the same central gate as built-in/custom graphs. Reject unknown workflow extension keys by name so authoring surfaces cannot persist a graph whose plugin node/column contract is missing at save or launch time.
+  */
+  if (!definition) {
+    throw new WorkflowIrError(`${owner} extension key '${key}' is not registered`);
+  }
+  const fields = definition.extension.configSchema?.fields;
   if (!fields || fields.length === 0) return;
   for (const field of fields) {
     if (field.required && !(field.key in value)) {
@@ -1314,6 +1351,17 @@ function validateV2(ir: WorkflowIrV2): void {
   validateColumns(ir);
 
   const columnIds = new Set(ir.columns.map((c) => c.id));
+  const nodeIds = new Set<string>();
+  for (const node of ir.nodes) {
+    /*
+    FNXC:WorkflowValidation 2026-06-27-00:00:
+    FN-7113 requires top-level duplicate node ids to fail before persistence or launch. Keep this check before nodesById is built so Map de-duplication cannot silently mask a malformed author/plugin workflow graph.
+    */
+    if (nodeIds.has(node.id)) {
+      throw new WorkflowIrError(`Workflow IR has duplicate node id '${node.id}'`);
+    }
+    nodeIds.add(node.id);
+  }
   const nodesById = new Map(ir.nodes.map((n) => [n.id, n]));
 
   for (const node of ir.nodes) {
@@ -1394,6 +1442,7 @@ function validateV2(ir: WorkflowIrV2): void {
   }
 
   validateNoIllegalCycles(ir.nodes, outgoing);
+  validateRequiredTopLevelReachability(ir.nodes, outgoing);
   validateForeachDominance(ir.nodes, ir.edges, outgoing);
 }
 
diff --git a/packages/engine/src/__tests__/workflow-graph-task-runner.test.ts b/packages/engine/src/__tests__/workflow-graph-task-runner.test.ts
index a4ee4f6414..d170340870 100644
--- a/packages/engine/src/__tests__/workflow-graph-task-runner.test.ts
+++ b/packages/engine/src/__tests__/workflow-graph-task-runner.test.ts
@@ -1,6 +1,10 @@
 import { describe, expect, it, vi } from "vitest";
 import { EventEmitter } from "node:events";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
 import type { Settings, TaskDetail, WorkflowDefinition, WorkflowIr } from "@fusion/core";
+import { TaskStore } from "@fusion/core";
 
 import { NotificationService } from "../notification/notification-service.js";
 import { WorkflowGraphTaskRunner, type WorkflowGraphRunnerStore } from "../workflow-graph-task-runner.js";
@@ -229,6 +233,56 @@ describe("WorkflowGraphTaskRunner (CU-U2)", () => {
     expect(result.reason).toMatch(/workflow-missing/);
   });
 
+  it("persists a valid workflow through the store and launches it through the graph runner", async () => {
+    const rootDir = mkdtempSync(join(tmpdir(), "fn-7113-workflow-run-"));
+    const globalDir = mkdtempSync(join(tmpdir(), "fn-7113-workflow-global-"));
+    const store = new TaskStore(rootDir, globalDir, { inMemoryDb: true });
+    try {
+      const invalidIr: WorkflowIr = {
+        version: "v2",
+        name: "invalid-save",
+        columns: [{ id: "todo", name: "Todo", traits: [] }],
+        nodes: [
+          { id: "start", kind: "start", column: "todo" },
+          { id: "dup", kind: "prompt", column: "todo" },
+          { id: "dup", kind: "script", column: "todo" },
+          { id: "end", kind: "end", column: "todo" },
+        ],
+        edges: [
+          { from: "start", to: "dup" },
+          { from: "dup", to: "end" },
+        ],
+      };
+      await expect(store.createWorkflowDefinition({ name: "Invalid", ir: invalidIr })).rejects.toThrow(
+        /Workflow IR has duplicate node id 'dup'/,
+      );
+
+      const workflow = await store.createWorkflowDefinition({ name: "Valid", ir: fullLifecycleIr() });
+      const persisted = await store.getWorkflowDefinition(workflow.id);
+      expect(persisted?.id).toBe(workflow.id);
+      const savedTask = await store.createTask({ description: "save run", enabledWorkflowSteps: [] });
+      await store.selectTaskWorkflow(savedTask.id, workflow.id);
+
+      const calls: string[] = [];
+      const runner = new WorkflowGraphTaskRunner({
+        store,
+        seams: recordingSeams(calls),
+        runCustomNode: async (node) => {
+          calls.push(`custom:${node.id}`);
+          return { outcome: "success" };
+        },
+      });
+
+      const result = await runner.run(savedTask, flagOn);
+
+      expect(result.disposition).toBe("completed");
+      expect(calls).toEqual(["custom:lint", "execute", "review", "merge", "custom:notify"]);
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+      rmSync(globalDir, { recursive: true, force: true });
+    }
+  });
+
   it("resolves built-in workflow selections without requiring the store to return a definition", async () => {
     const calls: string[] = [];
     const getWorkflowDefinition = vi.fn(async () => undefined);
@@ -252,61 +306,92 @@ describe("WorkflowGraphTaskRunner (CU-U2)", () => {
     expect(getWorkflowDefinition).not.toHaveBeenCalled();
   });
 
-  it("falls back (never strands the task) when the interpreter throws", async () => {
-    // Malformed graph: edge references unknown node → WorkflowIrError inside run().
+  it("fails closed with invalid-ir before any side-effect seam when resolved IR is malformed", async () => {
     const badIr: WorkflowIr = {
-      version: "v1",
+      version: "v2",
       name: "bad",
+      columns: [{ id: "c", name: "C", traits: [] }],
       nodes: [
-        { id: "start", kind: "start" },
-        { id: "end", kind: "end" },
+        { id: "start", kind: "start", column: "c" },
+        { id: "dup", kind: "prompt", column: "c" },
+        { id: "dup", kind: "script", column: "c" },
+        { id: "end", kind: "end", column: "c" },
+      ],
+      edges: [
+        { from: "start", to: "dup" },
+        { from: "dup", to: "end" },
       ],
-      edges: [{ from: "start", to: "ghost" }],
     };
+    const calls: string[] = [];
     const events: string[] = [];
     const runner = new WorkflowGraphTaskRunner({
       store: storeWith(definition(badIr)),
-      seams: recordingSeams([]),
-      runCustomNode: async () => ({ outcome: "success" }),
-      onEvent: (e) => events.push(e.type),
+      seams: recordingSeams(calls),
+      runCustomNode: async (node) => {
+        calls.push(`custom:${node.id}`);
+        return { outcome: "success" };
+      },
+      onEvent: (e) => events.push(`${e.type}:${e.detail}`),
     });
+
     const result = await runner.run(task, flagOn);
-    expect(result.disposition).toBe("fell-back");
-    expect(result.reason).toMatch(/interpreter-error/);
-    expect(events).toContain("fallback");
+
+    expect(result.disposition).toBe("failed");
+    expect(result.outcome).toBe("failure");
+    expect(result.reason).toMatch(/invalid-ir: Workflow IR has duplicate node id 'dup'/);
+    expect(result.visitedNodeIds).toEqual([]);
+    expect(calls).toEqual([]);
+    expect(events.some((event) => event.includes("terminal:invalid-ir"))).toBe(true);
   });
 
-  it("an interpreter error AFTER side effects terminates as failed, not fell-back", async () => {
-    // Cycle reached only after custom nodes execute: re-running legacy would
-    // repeat the implementation, so the runner must not signal fallback.
-    const cyclicIr: WorkflowIr = {
-      version: "v1",
-      name: "cyclic",
+  it("fails closed with invalid-ir before any custom node when resolved IR has a dangling edge", async () => {
+    const badIr: WorkflowIr = {
+      version: "v2",
+      name: "bad-edge",
+      columns: [{ id: "c", name: "C", traits: [] }],
       nodes: [
-        { id: "start", kind: "start" },
-        { id: "a", kind: "prompt", config: { prompt: "a" } },
-        { id: "b", kind: "prompt", config: { prompt: "b" } },
-        { id: "end", kind: "end" },
+        { id: "start", kind: "start", column: "c" },
+        { id: "a", kind: "prompt", column: "c" },
+        { id: "end", kind: "end", column: "c" },
       ],
       edges: [
-        { from: "start", to: "a", condition: "success" },
-        { from: "a", to: "b", condition: "success" },
-        { from: "b", to: "a", condition: "success" },
+        { from: "start", to: "a" },
+        { from: "a", to: "ghost" },
       ],
     };
     const calls: string[] = [];
     const runner = new WorkflowGraphTaskRunner({
-      store: storeWith(definition(cyclicIr)),
+      store: storeWith(definition(badIr)),
       seams: recordingSeams(calls),
       runCustomNode: async (node) => {
         calls.push(`custom:${node.id}`);
         return { outcome: "success" };
       },
     });
+
     const result = await runner.run(task, flagOn);
-    expect(calls.length).toBeGreaterThan(0);
+
     expect(result.disposition).toBe("failed");
-    expect(result.reason).toMatch(/interpreter-error/);
+    expect(result.outcome).toBe("failure");
+    expect(result.reason).toMatch(/invalid-ir: Workflow edge 'a' -> 'ghost' references undefined node 'ghost'/);
+    expect(result.visitedNodeIds).toEqual([]);
+    expect(calls).toEqual([]);
+  });
+
+  it("a custom-node failure AFTER side effects terminates as failed, not fell-back", async () => {
+    const calls: string[] = [];
+    const runner = new WorkflowGraphTaskRunner({
+      store: storeWith(definition(fullLifecycleIr())),
+      seams: recordingSeams(calls),
+      runCustomNode: async (node) => {
+        throw new Error(`custom boom: ${node.id}`);
+      },
+    });
+    const result = await runner.run(task, flagOn);
+    expect(calls).toEqual([]);
+    expect(result.visitedNodeIds).toEqual(["start", "lint"]);
+    expect(result.disposition).toBe("failed");
+    expect(result.reason).toBeUndefined();
   });
 
   it("exposes node outcomes in the shared context for downstream consumers", async () => {
diff --git a/packages/engine/src/workflow-graph-task-runner.ts b/packages/engine/src/workflow-graph-task-runner.ts
index 6589a92ae8..166e8a92ad 100644
--- a/packages/engine/src/workflow-graph-task-runner.ts
+++ b/packages/engine/src/workflow-graph-task-runner.ts
@@ -1,5 +1,11 @@
-import type { Settings, TaskDetail, WorkflowDefinition, WorkflowStepResult } from "@fusion/core";
-import { getBuiltinWorkflow, isBuiltinWorkflowId } from "@fusion/core";
+import type { Settings, TaskDetail, WorkflowDefinition, WorkflowIr, WorkflowStepResult } from "@fusion/core";
+import {
+  compileWorkflowToSteps,
+  getBuiltinWorkflow,
+  isBuiltinWorkflowId,
+  parseWorkflowIr,
+  WorkflowCompileError,
+} from "@fusion/core";
 
 import { WorkflowGraphExecutor, type WorkflowGraphExecutorDeps, type WorkflowNodeOutcome, type WorkflowTaskProjection } from "./workflow-graph-executor.js";
 import type {
@@ -29,11 +35,15 @@ import type { WorkflowPrimitiveContext, WorkflowRuntimePrimitives } from "./runt
  */
 export type WorkflowGraphRunDisposition = "completed" | "failed" | "fell-back";
 
+function isInterpreterDeferredCompileError(error: unknown): boolean {
+  return error instanceof WorkflowCompileError && error.message.includes("require the workflow interpreter (deferred)");
+}
+
 export interface WorkflowGraphTaskRunResult {
   disposition: WorkflowGraphRunDisposition;
   outcome?: WorkflowNodeOutcome;
   visitedNodeIds: string[];
-  /** Why the runner fell back (flag-off, no-selection, workflow-missing, interpreter-error). */
+  /** Why the runner fell back or failed before execution (flag-off, no-selection, workflow-missing, invalid-ir, interpreter-error). */
   reason?: string;
   /** Shared graph context after the run (node outcomes/values). */
   context?: Record<string, unknown>;
@@ -141,6 +151,11 @@ export class WorkflowGraphTaskRunner {
     return { disposition: "fell-back", reason, visitedNodeIds: [] };
   }
 
+  private failBeforeSideEffects(taskId: string, reason: string): WorkflowGraphTaskRunResult {
+    this.emit("terminal", taskId, reason);
+    return { disposition: "failed", outcome: "failure", reason, visitedNodeIds: [] };
+  }
+
   public async run(
     task: TaskDetail,
     settings: Pick<Settings, "experimentalFeatures"> | undefined,
@@ -167,13 +182,28 @@ export class WorkflowGraphTaskRunner {
       return this.fallBack(task.id, `workflow-missing: ${selection.workflowId}`);
     }
 
+    let validatedIr: WorkflowIr;
+    try {
+      /*
+      FNXC:WorkflowExecution 2026-06-27-07:40:
+      FN-7113 requires the interpreter to re-validate the resolved built-in/custom/plugin workflow IR before any seam, primitive, or custom-node side effects. Invalid persisted or plugin-authored graphs fail closed with an author-facing invalid-ir reason instead of partially running or falling back into the wrong legacy workflow.
+      */
+      validatedIr = parseWorkflowIr(definition.ir);
+      try {
+        compileWorkflowToSteps(validatedIr);
+      } catch (err) {
+        if (!isInterpreterDeferredCompileError(err)) throw err;
+      }
+    } catch (err) {
+      return this.failBeforeSideEffects(task.id, `invalid-ir: ${err instanceof Error ? err.message : String(err)}`);
+    }
+
     this.emit("start", task.id, definition.id);
     this.branchProgress.clear();
 
-    // Track whether any node side effects ran. A pre-run interpreter error
-    // (bad IR structure, wiring) can safely fall back to the legacy pipeline;
-    // a mid-run error cannot — re-running legacy would repeat the implementation
-    // session — so it terminates as "failed" for the caller to park instead.
+    // Track whether any node side effects ran. Invalid IR is rejected above as
+    // failed/terminal before this point; once execution starts, no error can
+    // safely fall back to legacy because that could repeat a partial workflow run.
     let sideEffectsRan = false;
     const invoked: string[] = [];
     const seams = this.deps.seams;
@@ -254,7 +284,7 @@ export class WorkflowGraphTaskRunner {
           }
         },
       });
-      const result = await executor.run(task, settings, definition.ir);
+      const result = await executor.run(task, settings, validatedIr);
       if (!result.executed) {
         return this.fallBack(task.id, "not-executed");
       }