diff --git a/convex/_generated/api.d.ts b/convex/_generated/api.d.ts
index 0b6416a..d182901 100644
--- a/convex/_generated/api.d.ts
+++ b/convex/_generated/api.d.ts
@@ -19,6 +19,7 @@ import type * as messages from "../messages.js";
 import type * as mutations from "../mutations.js";
 import type * as pagination from "../pagination.js";
 import type * as queries from "../queries.js";
+import type * as returnsValidation from "../returnsValidation.js";
 import type * as scheduler from "../scheduler.js";
 import type * as storage from "../storage.js";
 import type * as textSearch from "../textSearch.js";
@@ -42,6 +43,7 @@ declare const fullApi: ApiFromModules<{
   mutations: typeof mutations;
   pagination: typeof pagination;
   queries: typeof queries;
+  returnsValidation: typeof returnsValidation;
   scheduler: typeof scheduler;
   storage: typeof storage;
   textSearch: typeof textSearch;
diff --git a/convex/returnsValidation.test.ts b/convex/returnsValidation.test.ts
new file mode 100644
index 0000000..b237dc5
--- /dev/null
+++ b/convex/returnsValidation.test.ts
@@ -0,0 +1,109 @@
+import { expect, test } from "vitest";
+import { convexTest } from "../index";
+import { api } from "./_generated/api";
+import schema from "./schema";
+
+test("query with incomplete return validator should fail", async () => {
+  const t = convexTest(schema);
+
+  await t.run(async (ctx) => {
+    await ctx.db.insert("messages", { author: "Alice", body: "Hello" });
+  });
+
+  await expect(
+    t.query(api.returnsValidation.queryWithIncompleteReturnValidator),
+  ).rejects.toThrowError(/Return value validation failed/);
+});
+
+test("query with correct return validator should pass", async () => {
+  const t = convexTest(schema);
+
+  await t.run(async (ctx) => {
+    await ctx.db.insert("messages", { author: "Bob", body: "World" });
+  });
+
+  const result = await t.query(
+    api.returnsValidation.queryWithCorrectReturnValidator,
+  );
+  expect(result).not.toBeNull();
+  expect(result?.author).toBe("Bob");
+  expect(result?._creationTime).toBeDefined();
+});
+
+test("query returning correct primitive type should pass", async () => {
+  const t = convexTest(schema);
+  const result = await t.query(api.returnsValidation.queryReturningNumber);
+  expect(result).toBe(42);
+});
+
+test("query returning wrong primitive type should fail", async () => {
+  const t = convexTest(schema);
+  await expect(
+    t.query(api.returnsValidation.queryReturningWrongType),
+  ).rejects.toThrowError(/Return value validation failed/);
+});
+
+test("mutation with incomplete return validator should fail", async () => {
+  const t = convexTest(schema);
+
+  await expect(
+    t.mutation(api.returnsValidation.mutationWithIncompleteReturnValidator, {
+      author: "Charlie",
+      body: "Test",
+    }),
+  ).rejects.toThrowError(/Return value validation failed/);
+});
+
+test("mutation with correct return validator should pass", async () => {
+  const t = convexTest(schema);
+
+  const result = await t.mutation(
+    api.returnsValidation.mutationWithCorrectReturnValidator,
+    { author: "Diana", body: "Test" },
+  );
+  expect(result).not.toBeNull();
+  expect(result.author).toBe("Diana");
+  expect(result._creationTime).toBeDefined();
+});
+
+test("query without return validator should pass", async () => {
+  const t = convexTest(schema);
+
+  await t.run(async (ctx) => {
+    await ctx.db.insert("messages", { author: "Eve", body: "No validator" });
+  });
+
+  const result = await t.query(
+    api.returnsValidation.queryWithoutReturnValidator,
+  );
+  expect(result).not.toBeNull();
+});
+
+test("query returning array with incomplete item validator should fail", async () => {
+  const t = convexTest(schema);
+
+  await t.run(async (ctx) => {
+    await ctx.db.insert("messages", { author: "Frank", body: "Array test" });
+  });
+
+  await expect(
+    t.query(api.returnsValidation.queryReturningArrayWithIncompleteValidator),
+  ).rejects.toThrowError(/Return value validation failed/);
+});
+
+test("action with incomplete return validator should fail", async () => {
+  const t = convexTest(schema);
+
+  await expect(
+    t.action(api.returnsValidation.actionWithIncompleteReturnValidator),
+  ).rejects.toThrowError(/Return value validation failed/);
+});
+
+test("action with correct return validator should pass", async () => {
+  const t = convexTest(schema);
+
+  const result = await t.action(
+    api.returnsValidation.actionWithCorrectReturnValidator,
+  );
+  expect(result).toEqual({ name: "test", value: 42 });
+});
diff --git a/convex/returnsValidation.ts b/convex/returnsValidation.ts
new file mode 100644
index 0000000..c44234d
--- /dev/null
+++ b/convex/returnsValidation.ts
@@ -0,0 +1,142 @@
+import { v } from "convex/values";
+import { action, mutation, query } from "./_generated/server";
+
+// Query that returns a document but the return validator is INCOMPLETE
+// (missing _creationTime which Convex auto-adds to all documents)
+export const queryWithIncompleteReturnValidator = query({
+  args: {},
+  returns: v.union(
+    v.null(),
+    v.object({
+      _id: v.id("messages"),
+      author: v.string(),
+      body: v.string(),
+      embedding: v.optional(v.array(v.number())),
+      score: v.optional(v.number()),
+      // _creationTime: v.number(), <-- INTENTIONALLY MISSING
+    }),
+  ),
+  handler: async (ctx) => {
+    return await ctx.db.query("messages").first();
+  },
+});
+
+// Query with correct return validator (includes _creationTime)
+export const queryWithCorrectReturnValidator = query({
+  args: {},
+  returns: v.union(
+    v.null(),
+    v.object({
+      _id: v.id("messages"),
+      _creationTime: v.number(),
+      author: v.string(),
+      body: v.string(),
+      embedding: v.optional(v.array(v.number())),
+      score: v.optional(v.number()),
+    }),
+  ),
+  handler: async (ctx) => {
+    return await ctx.db.query("messages").first();
+  },
+});
+
+// Query returning primitive
+export const queryReturningNumber = query({
+  args: {},
+  returns: v.number(),
+  handler: async () => {
+    return 42;
+  },
+});
+
+// Query with wrong primitive type
+export const queryReturningWrongType = query({
+  args: {},
+  returns: v.string(),
+  handler: async (): Promise<any> => {
+    return 42;
+  },
+});
+
+// Mutation with incomplete return validator
+export const mutationWithIncompleteReturnValidator = mutation({
+  args: { author: v.string(), body: v.string() },
+  returns: v.object({
+    _id: v.id("messages"),
+    author: v.string(),
+    body: v.string(),
+    embedding: v.optional(v.array(v.number())),
+    score: v.optional(v.number()),
+    // _creationTime: v.number(), <-- INTENTIONALLY MISSING
+  }),
+  handler: async (ctx, args) => {
+    const id = await ctx.db.insert("messages", args);
+    return (await ctx.db.get(id))!;
+  },
+});
+
+// Mutation with correct return validator
+export const mutationWithCorrectReturnValidator = mutation({
+  args: { author: v.string(), body: v.string() },
+  returns: v.object({
+    _id: v.id("messages"),
+    _creationTime: v.number(),
+    author: v.string(),
+    body: v.string(),
+    embedding: v.optional(v.array(v.number())),
+    score: v.optional(v.number()),
+  }),
+  handler: async (ctx, args) => {
+    const id = await ctx.db.insert("messages", args);
+    return (await ctx.db.get(id))!;
+  },
+});
+
+// Query without return validator (should pass - no validation)
+export const queryWithoutReturnValidator = query({
+  args: {},
+  handler: async (ctx) => {
+    return await ctx.db.query("messages").first();
+  },
+});
+
+// Query returning array with incomplete item validator
+export const queryReturningArrayWithIncompleteValidator = query({
+  args: {},
+  returns: v.array(
+    v.object({
+      _id: v.id("messages"),
+      author: v.string(),
+      body: v.string(),
+      embedding: v.optional(v.array(v.number())),
+      score: v.optional(v.number()),
+      // Missing _creationTime
+    }),
+  ),
+  handler: async (ctx) => {
+    return await ctx.db.query("messages").collect();
+  },
+});
+
+// Action with incomplete return validator
+export const actionWithIncompleteReturnValidator = action({
+  args: {},
+  returns: v.object({
+    name: v.string(),
+  }),
+  handler: async () => {
+    return { name: "test", extraField: "unexpected" } as { name: string };
+  },
+});
+
+// Action with correct return validator
+export const actionWithCorrectReturnValidator = action({
+  args: {},
+  returns: v.object({
+    name: v.string(),
+    value: v.number(),
+  }),
+  handler: async () => {
+    return { name: "test", value: 42 };
+  },
+});
diff --git a/index.ts b/index.ts
index aa0e05f..2580bf0 100644
--- a/index.ts
+++ b/index.ts
@@ -2353,12 +2353,14 @@ function withAuth(auth: AuthFake = new AuthFake()) {
       const func = await getFunctionFromPath(functionPath, "query");
       validateValidator(JSON.parse((func as any).exportArgs()), args ?? {});
 
-      return await runQueryWithHandler(
+      const result = await runQueryWithHandler(
         (ctx, a) => getHandler(func)(ctx, a),
         args,
         functionPath,
         isNested,
       );
+      validateReturnValue(func, result, "query", functionPath);
+      return result;
     },
 
     mutationFromPath: async (
@@ -2369,20 +2371,22 @@ function withAuth(auth: AuthFake = new AuthFake()) {
       const func = await getFunctionFromPath(functionPath, "mutation");
       validateValidator(JSON.parse((func as any).exportArgs()), args ?? {});
 
-      return await runMutationWithHandler(
+      const result = await runMutationWithHandler(
         getHandler(func),
         args,
         {},
         functionPath,
         isNested,
       );
+      validateReturnValue(func, result, "mutation", functionPath);
+      return result;
     },
 
     actionFromPath: async (functionPath: FunctionPath, args: any) => {
       const func = await getFunctionFromPath(functionPath, "action");
       validateValidator(JSON.parse((func as any).exportArgs()), args ?? {});
 
-      return await runActionWithHandler(
+      const result = await runActionWithHandler(
         getHandler(func),
         args,
         functionPath,
@@ -2390,6 +2394,8 @@ function withAuth(auth: AuthFake = new AuthFake()) {
         byTypeWithPath.runMutation,
         byTypeWithPath.runAction,
       );
+      validateReturnValue(func, result, "action", functionPath);
+      return result;
     },
     runQuery: async (
       functionReference: FunctionReference<any, any, any, any>,
@@ -2616,6 +2622,24 @@ function parseArgs(
   return args;
 }
 
+function validateReturnValue(
+  func: any,
+  result: any,
+  functionType: "query" | "mutation" | "action",
+  functionPath: FunctionPath,
+) {
+  const returnsValidator = JSON.parse(func.exportReturns());
+  if (returnsValidator !== null) {
+    try {
+      validateValidator(returnsValidator, result);
+    } catch (e) {
+      throw new Error(
+        `Return value validation failed for ${functionType} "${functionPath.udfPath}":\n${(e as Error).message}`,
+      );
+    }
+  }
+}
+
 function createFunctionHandle(functionPath: FunctionPath) {
   return `function://${functionPath.componentPath};${functionPath.udfPath}`;
 }