diff --git a/packages/workflow-executor/src/errors.ts b/packages/workflow-executor/src/errors.ts
index 3c55df97e2..6b9883bc6b 100644
--- a/packages/workflow-executor/src/errors.ts
+++ b/packages/workflow-executor/src/errors.ts
@@ -216,6 +216,13 @@ export class RunNotFoundError extends Error {
   }
 }
 
+export class StepAlreadyExecutedError extends Error {
+  constructor(runId: string, stepIndex: number) {
+    super(`Step ${stepIndex} in run "${runId}" has already been executed`);
+    this.name = 'StepAlreadyExecutedError';
+  }
+}
+
 export class PendingDataNotFoundError extends Error {
   constructor(runId: string, stepIndex: number) {
     super(`Step ${stepIndex} in run "${runId}" not found or has no pending data`);
diff --git a/packages/workflow-executor/src/http/executor-http-server.ts b/packages/workflow-executor/src/http/executor-http-server.ts
index cae4be50cf..adce1f8d00 100644
--- a/packages/workflow-executor/src/http/executor-http-server.ts
+++ b/packages/workflow-executor/src/http/executor-http-server.ts
@@ -9,7 +9,12 @@ import http from 'http';
 import Koa from 'koa';
 import koaJwt from 'koa-jwt';
 
-import { InvalidPendingDataError, PendingDataNotFoundError, RunNotFoundError } from '../errors';
+import {
+  InvalidPendingDataError,
+  PendingDataNotFoundError,
+  RunNotFoundError,
+  StepAlreadyExecutedError,
+} from '../errors';
 
 export interface ExecutorHttpServerOptions {
   port: number;
@@ -176,6 +181,13 @@ export default class ExecutorHttpServer {
     try {
       await this.options.runner.patchPendingData(runId, stepIndex, ctx.request.body);
     } catch (err) {
+      if (err instanceof StepAlreadyExecutedError) {
+        ctx.status = 409;
+        ctx.body = { error: 'Step has already been executed' };
+
+        return;
+      }
+
       if (err instanceof PendingDataNotFoundError) {
         ctx.status = 404;
         ctx.body = { error: 'Step execution not found or has no pending data' };
diff --git a/packages/workflow-executor/src/runner.ts b/packages/workflow-executor/src/runner.ts
index 69e01d283b..d209c0c439 100644
--- a/packages/workflow-executor/src/runner.ts
+++ b/packages/workflow-executor/src/runner.ts
@@ -12,6 +12,7 @@ import {
   InvalidPendingDataError,
   PendingDataNotFoundError,
   RunNotFoundError,
+  StepAlreadyExecutedError,
   causeMessage,
 } from './errors';
 import StepExecutorFactory from './executors/step-executor-factory';
@@ -123,6 +124,10 @@ export default class Runner {
       throw new PendingDataNotFoundError(runId, stepIndex);
     }
 
+    if ('executionResult' in execution && execution.executionResult !== undefined) {
+      throw new StepAlreadyExecutedError(runId, stepIndex);
+    }
+
     const parsed = schema.safeParse(body);
 
     if (!parsed.success) {
@@ -135,8 +140,6 @@ export default class Runner {
       );
     }
 
-    // Cast is safe: the type guard above ensures `execution` is the correct union branch,
-    // and patchBodySchemas[execution.type] only accepts keys valid for that branch.
     await this.config.runStore.saveStepExecution(runId, {
       ...execution,
       pendingData: { ...(execution.pendingData as object), ...(parsed.data as object) },
diff --git a/packages/workflow-executor/test/http/executor-http-server.test.ts b/packages/workflow-executor/test/http/executor-http-server.test.ts
index 17aabf8813..5e52dea0f5 100644
--- a/packages/workflow-executor/test/http/executor-http-server.test.ts
+++ b/packages/workflow-executor/test/http/executor-http-server.test.ts
@@ -8,6 +8,7 @@ import {
   InvalidPendingDataError,
   PendingDataNotFoundError,
   RunNotFoundError,
+  StepAlreadyExecutedError,
 } from '../../src/errors';
 import ExecutorHttpServer from '../../src/http/executor-http-server';
 
@@ -380,6 +381,22 @@ describe('ExecutorHttpServer', () => {
       expect(runner.patchPendingData).toHaveBeenCalledWith('run-1', 2, { userConfirmed: true });
     });
 
+    it('returns 409 when patchPendingData throws StepAlreadyExecutedError', async () => {
+      const runner = createMockRunner({
+        patchPendingData: jest.fn().mockRejectedValue(new StepAlreadyExecutedError('run-1', 0)),
+      });
+      const server = createServer({ runner });
+      const token = signToken({ id: 'user-1' });
+
+      const response = await request(server.callback)
+        .patch('/runs/run-1/steps/0/pending-data')
+        .set('Authorization', `Bearer ${token}`)
+        .send({ userConfirmed: true });
+
+      expect(response.status).toBe(409);
+      expect(response.body).toEqual({ error: 'Step has already been executed' });
+    });
+
     it('returns 404 when patchPendingData throws PendingDataNotFoundError', async () => {
       const runner = createMockRunner({
         patchPendingData: jest.fn().mockRejectedValue(new PendingDataNotFoundError('run-1', 0)),
diff --git a/packages/workflow-executor/test/runner.test.ts b/packages/workflow-executor/test/runner.test.ts
index 462bdf09d0..5842d8012b 100644
--- a/packages/workflow-executor/test/runner.test.ts
+++ b/packages/workflow-executor/test/runner.test.ts
@@ -12,6 +12,7 @@ import {
   InvalidPendingDataError,
   PendingDataNotFoundError,
   RunNotFoundError,
+  StepAlreadyExecutedError,
 } from '../src/errors';
 import BaseStepExecutor from '../src/executors/base-step-executor';
 import ConditionStepExecutor from '../src/executors/condition-step-executor';
@@ -881,6 +882,60 @@ describe('getRunStepExecutions', () => {
 // ---------------------------------------------------------------------------
 
 describe('patchPendingData', () => {
+  it('throws StepAlreadyExecutedError when step has executionResult', async () => {
+    const runStore = createMockRunStore({
+      getStepExecutions: jest.fn().mockResolvedValue([
+        {
+          type: 'update-record',
+          stepIndex: 0,
+          pendingData: { fieldName: 'status', value: 'active' },
+          executionResult: { updatedValues: { status: 'active' } },
+        },
+      ]),
+    });
+    runner = new Runner(createRunnerConfig({ runStore }));
+
+    await expect(runner.patchPendingData('run-1', 0, { userConfirmed: true })).rejects.toThrow(
+      StepAlreadyExecutedError,
+    );
+  });
+
+  it('throws StepAlreadyExecutedError when step was skipped', async () => {
+    const runStore = createMockRunStore({
+      getStepExecutions: jest.fn().mockResolvedValue([
+        {
+          type: 'trigger-action',
+          stepIndex: 0,
+          pendingData: { name: 'send_email', displayName: 'Send Email' },
+          executionResult: { skipped: true },
+        },
+      ]),
+    });
+    runner = new Runner(createRunnerConfig({ runStore }));
+
+    await expect(runner.patchPendingData('run-1', 0, { userConfirmed: true })).rejects.toThrow(
+      StepAlreadyExecutedError,
+    );
+  });
+
+  it('does not throw StepAlreadyExecutedError when executionResult is undefined', async () => {
+    const runStore = createMockRunStore({
+      getStepExecutions: jest.fn().mockResolvedValue([
+        {
+          type: 'update-record',
+          stepIndex: 0,
+          pendingData: { fieldName: 'status', value: 'active' },
+          executionResult: undefined,
+        },
+      ]),
+    });
+    runner = new Runner(createRunnerConfig({ runStore }));
+
+    await runner.patchPendingData('run-1', 0, { userConfirmed: true });
+
+    expect(runStore.saveStepExecution).toHaveBeenCalled();
+  });
+
   it('throws PendingDataNotFoundError when step is not found', async () => {
     const runStore = createMockRunStore({ getStepExecutions: jest.fn().mockResolvedValue([]) });
     runner = new Runner(createRunnerConfig({ runStore }));