diff --git a/example/convex/_generated/api.d.ts b/example/convex/_generated/api.d.ts
index a01f78be..3c3163ae 100644
--- a/example/convex/_generated/api.d.ts
+++ b/example/convex/_generated/api.d.ts
@@ -14,6 +14,7 @@ import type * as e2e from "../e2e.js";
 import type * as example from "../example.js";
 import type * as inlineTest from "../inlineTest.js";
 import type * as nestedWorkflow from "../nestedWorkflow.js";
+import type * as oversized from "../oversized.js";
 import type * as passingSignals from "../passingSignals.js";
 import type * as test_oldSyntax from "../test/oldSyntax.js";
 import type * as transcription from "../transcription.js";
@@ -32,6 +33,7 @@ declare const fullApi: ApiFromModules<{
   example: typeof example;
   inlineTest: typeof inlineTest;
   nestedWorkflow: typeof nestedWorkflow;
+  oversized: typeof oversized;
   passingSignals: typeof passingSignals;
   "test/oldSyntax": typeof test_oldSyntax;
   transcription: typeof transcription;
diff --git a/example/convex/oversized.test.ts b/example/convex/oversized.test.ts
new file mode 100644
index 00000000..45444e13
--- /dev/null
+++ b/example/convex/oversized.test.ts
@@ -0,0 +1,111 @@
+/// <reference types="vite/client" />
+
+import { expect, describe, test, vi, beforeEach, afterEach } from "vitest";
+import { initConvexTest } from "./setup.test";
+import { components, internal } from "./_generated/api";
+import { assert } from "convex-helpers";
+import { getStatus } from "@convex-dev/workflow";
+import { workflow } from "./example";
+
+describe("oversized values", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  test("large step return value fails workflow and calls onComplete", async () => {
+    const t = initConvexTest();
+    const workflowId = await t.mutation(async (ctx) => {
+      const wfId = await workflow.start(
+        ctx,
+        internal.oversized.largeReturnWorkflow,
+        {},
+        {
+          onComplete: internal.oversized.onComplete,
+          context: {},
+          startAsync: true,
+        },
+      );
+      await ctx.db.insert("flows", {
+        workflowId: wfId,
+        in: "largeReturn",
+        out: null,
+      });
+      return wfId;
+    });
+    await t.finishAllScheduledFunctions(vi.runAllTimers);
+
+    const status = await t.run((ctx) =>
+      getStatus(ctx, components.workflow, workflowId),
+    );
+    expect(status.type).toBe("failed");
+    assert(status.type === "failed");
+    expect(status.error).toContain("Step return value too large");
+
+    const flow = await t.query(async (ctx) => {
+      return ctx.db
+        .query("flows")
+        .withIndex("workflowId", (q) => q.eq("workflowId", workflowId))
+        .first();
+    });
+    expect(flow).not.toBeNull();
+    assert(flow);
+    expect(flow.out).not.toBeNull();
+    expect(flow.out.kind).toBe("failed");
+    expect(flow.out.error).toContain("Step return value too large");
+  });
+
+  test("large event value fails workflow and calls onComplete", async () => {
+    const t = initConvexTest();
+    const workflowId = await t.mutation(async (ctx) => {
+      const wfId = await workflow.start(
+        ctx,
+        internal.oversized.eventWorkflow,
+        {},
+        {
+          onComplete: internal.oversized.onComplete,
+          context: {},
+          startAsync: true,
+        },
+      );
+      await ctx.db.insert("flows", {
+        workflowId: wfId,
+        in: "eventWorkflow",
+        out: null,
+      });
+      return wfId;
+    });
+    // Let the workflow start and reach the awaitEvent point
+    await t.finishAllScheduledFunctions(vi.runAllTimers);
+
+    const status = await t.run((ctx) =>
+      getStatus(ctx, components.workflow, workflowId),
+    );
+    expect(status.type).toBe("inProgress");
+
+    // Send the oversized event
+    await t.mutation(internal.oversized.sendBigEvent, { workflowId });
+    await t.finishAllScheduledFunctions(vi.runAllTimers);
+
+    const status2 = await t.run((ctx) =>
+      getStatus(ctx, components.workflow, workflowId),
+    );
+    expect(status2.type).toBe("failed");
+    assert(status2.type === "failed");
+    expect(status2.error).toContain("Step return value too large");
+    expect(status2.error).toContain("900002 bytes");
+
+    const flow = await t.query(async (ctx) => {
+      return ctx.db
+        .query("flows")
+        .withIndex("workflowId", (q) => q.eq("workflowId", workflowId))
+        .first();
+    });
+    expect(flow).not.toBeNull();
+    expect(flow!.out).not.toBeNull();
+    expect(flow!.out.kind).toBe("failed");
+    expect(flow!.out.error).toContain("Step return value too large");
+  });
+});
diff --git a/example/convex/oversized.ts b/example/convex/oversized.ts
new file mode 100644
index 00000000..03ec9f4c
--- /dev/null
+++ b/example/convex/oversized.ts
@@ -0,0 +1,74 @@
+import { v } from "convex/values";
+import { sendEvent, defineEvent } from "@convex-dev/workflow";
+import { components, internal } from "./_generated/api.js";
+import { internalAction, internalMutation } from "./_generated/server.js";
+import { vWorkflowId } from "@convex-dev/workflow";
+import { vResultValidator } from "@convex-dev/workpool";
+import { workflow } from "./example.js";
+
+// Action that returns a value larger than 800KB.
+export const largeReturnAction = internalAction({
+  args: {},
+  returns: v.string(),
+  handler: async (): Promise<string> => {
+    return "x".repeat(900_000);
+  },
+});
+
+export const largeReturnWorkflow = workflow
+  .define({
+    args: {},
+  })
+  .handler(async (step) => {
+    const result = await step.runAction(
+      internal.oversized.largeReturnAction,
+      {},
+    );
+    return result;
+  });
+
+export const bigEvent = defineEvent({
+  name: "bigEvent",
+  validator: v.string(),
+});
+
+export const eventWorkflow = workflow
+  .define({
+    args: {},
+  })
+  .handler(async (step) => {
+    const result = await step.awaitEvent(bigEvent);
+    return result;
+  });
+
+export const sendBigEvent = internalMutation({
+  args: {
+    workflowId: vWorkflowId,
+  },
+  returns: v.null(),
+  handler: async (ctx, args) => {
+    await sendEvent(ctx, components.workflow, {
+      ...bigEvent,
+      workflowId: args.workflowId,
+      value: "y".repeat(900_000),
+    });
+  },
+});
+
+export const onComplete = internalMutation({
+  args: {
+    workflowId: vWorkflowId,
+    result: vResultValidator,
+    context: v.any(),
+  },
+  returns: v.null(),
+  handler: async (ctx, args) => {
+    const flow = await ctx.db
+      .query("flows")
+      .withIndex("workflowId", (q) => q.eq("workflowId", args.workflowId))
+      .first();
+    if (!flow) return null;
+    await ctx.db.patch(flow._id, { out: args.result });
+    return null;
+  },
+});
diff --git a/src/component/_generated/api.ts b/src/component/_generated/api.ts
index 9a930507..3e00bc83 100644
--- a/src/component/_generated/api.ts
+++ b/src/component/_generated/api.ts
@@ -12,6 +12,7 @@ import type * as event from "../event.js";
 import type * as journal from "../journal.js";
 import type * as logging from "../logging.js";
 import type * as model from "../model.js";
+import type * as oversizedValues from "../oversizedValues.js";
 import type * as pool from "../pool.js";
 import type * as utils from "../utils.js";
 import type * as workflow from "../workflow.js";
@@ -28,6 +29,7 @@ const fullApi: ApiFromModules<{
   journal: typeof journal;
   logging: typeof logging;
   model: typeof model;
+  oversizedValues: typeof oversizedValues;
   pool: typeof pool;
   utils: typeof utils;
   workflow: typeof workflow;
diff --git a/src/component/event.ts b/src/component/event.ts
index a002316c..f0fef08f 100644
--- a/src/component/event.ts
+++ b/src/component/event.ts
@@ -6,6 +6,7 @@ import { vResultValidator } from "@convex-dev/workpool";
 import type { Doc, Id } from "./_generated/dataModel.js";
 import { assert } from "convex-helpers";
 import { enqueueWorkflow, getWorkpool, workpoolOptions } from "./pool.js";
+import { checkForOversizedResult } from "./oversizedValues.js";
 
 export async function awaitEvent(
   ctx: MutationCtx,
@@ -40,7 +41,7 @@ export async function awaitEvent(
           stepId: entry._id,
         },
       });
-      entry.step.runResult = event.state.result;
+      entry.step.runResult = checkForOversizedResult(event.state.result);
       entry.step.inProgress = false;
       entry.step.completedAt = Date.now();
       break;
@@ -146,7 +147,7 @@ export const send = mutation({
         );
         assert(step.step.kind === "event", "Step is not an event");
         step.step.eventId = event._id;
-        step.step.runResult = args.result;
+        step.step.runResult = checkForOversizedResult(args.result);
         step.step.inProgress = false;
         step.step.completedAt = Date.now();
         await ctx.db.replace(step._id, step);
diff --git a/src/component/oversizedValues.ts b/src/component/oversizedValues.ts
new file mode 100644
index 00000000..ee6905ab
--- /dev/null
+++ b/src/component/oversizedValues.ts
@@ -0,0 +1,34 @@
+import { type Value, convexToJson, getConvexSize } from "convex/values";
+import type { RunResult } from "@convex-dev/workpool";
+
+export const MAX_RETURN_VALUE_SIZE = 800 << 10; // 800 KiB
+const PREVIEW_SIZE = 128 << 10; // 128 KB
+
+function truncatedPreview(returnValue: unknown): string {
+  const json = JSON.stringify(convexToJson(returnValue as Value));
+  if (json.length <= PREVIEW_SIZE * 2) {
+    return json;
+  }
+  return json.slice(0, PREVIEW_SIZE) + "..." + json.slice(-PREVIEW_SIZE);
+}
+
+export function checkReturnValueSize(
+  returnValue: Value | undefined,
+): string | null {
+  const size = getConvexSize(returnValue);
+  if (size > MAX_RETURN_VALUE_SIZE) {
+    return `Step return value too large (${size} bytes). Maximum is ${MAX_RETURN_VALUE_SIZE} bytes. Preview: ${truncatedPreview(returnValue)}`;
+  }
+  return null;
+}
+
+export function checkForOversizedResult(result: RunResult): RunResult {
+  if (result.kind !== "success") {
+    return result;
+  }
+  const sizeError = checkReturnValueSize(result.returnValue);
+  if (!sizeError) {
+    return result;
+  }
+  return { kind: "failed", error: sizeError };
+}
diff --git a/src/component/pool.ts b/src/component/pool.ts
index cdaf82fb..87d5a921 100644
--- a/src/component/pool.ts
+++ b/src/component/pool.ts
@@ -22,6 +22,7 @@ import { getDefaultLogger } from "./utils.js";
 import { completeHandler } from "./workflow.js";
 import type { Doc } from "./_generated/dataModel.js";
 import { vWorkflowId, type WorkflowId } from "../types.js";
+import { checkForOversizedResult } from "./oversizedValues.js";
 
 export const workpoolOptions = v.object({
   logLevel: v.optional(logLevel),
@@ -147,32 +148,15 @@ async function onCompleteHandler(
   }
   journalEntry.step.inProgress = false;
   journalEntry.step.completedAt = Date.now();
-  switch (args.result.kind) {
-    case "success":
-      journalEntry.step.runResult = {
-        kind: "success",
-        returnValue: args.result.returnValue,
-      };
-      break;
-    case "failed":
-      journalEntry.step.runResult = {
-        kind: "failed",
-        error: args.result.error,
-      };
-      break;
-    case "canceled":
-      journalEntry.step.runResult = {
-        kind: "canceled",
-      };
-      break;
-  }
+  const runResult = checkForOversizedResult(args.result);
+  journalEntry.step.runResult = runResult;
   await ctx.db.replace(journalEntry._id, journalEntry);
   console.debug(`Completed execution of ${stepId}`, journalEntry);
 
   console.event("stepCompleted", {
     workflowId,
     workflowName: workflow.name,
-    status: args.result.kind,
+    status: journalEntry.step.runResult.kind,
     stepName: journalEntry.step.name,
     stepNumber: journalEntry.stepNumber,
     durationMs: journalEntry.step.completedAt - journalEntry.step.startedAt,
@@ -180,7 +164,7 @@ async function onCompleteHandler(
   if (workflow.runResult !== undefined) {
     if (workflow.runResult.kind !== "canceled") {
       console.error(
-        `Workflow: ${workflowId} already ${workflow.runResult.kind} when completing ${stepId} with status ${args.result.kind}`,
+        `Workflow: ${workflowId} already ${workflow.runResult.kind} when completing ${stepId} with status ${journalEntry.step.runResult.kind}`,
       );
     }
     return;