diff --git a/src/handlers/utils.test.tsx b/src/handlers/utils.test.tsx
index 52ba6071b..5d931b479 100644
--- a/src/handlers/utils.test.tsx
+++ b/src/handlers/utils.test.tsx
@@ -1,7 +1,9 @@
 import { describe, expect, test } from "bun:test";
+import z from "zod";
 import {
   assertMutuallyExclusiveFlags,
   parseJsonArrayFlag,
+  parseJsonFlagWithSchema,
   parseJsonObjectFlag,
   parseTags,
 } from "./utils";
@@ -19,6 +21,49 @@ describe("structured JSON flags", () => {
   });
 });
 
+describe("parseJsonFlagWithSchema", () => {
+  const ModelSchema = z
+    .object({ modelId: z.string(), apiKeyArn: z.string().optional() })
+    .refine((m) => m.modelId !== "needs-key" || m.apiKeyArn !== undefined, {
+      message: "apiKeyArn is required",
+    });
+
+  test("returns undefined when the flag is omitted", () => {
+    expect(parseJsonFlagWithSchema("model", undefined, ModelSchema)).toBeUndefined();
+  });
+
+  test("returns valid input unchanged", () => {
+    expect(parseJsonFlagWithSchema("model", '{"modelId":"anthropic.x"}', ModelSchema)).toEqual({
+      modelId: "anthropic.x",
+    });
+  });
+
+  // The F17 case: without the strict pass this parses, drops `modlId`, and sends
+  // an object missing the field the user was trying to set.
+  test("rejects a key the schema does not declare instead of dropping it", () => {
+    expect(() =>
+      parseJsonFlagWithSchema("model", '{"modelId":"anthropic.x","modlId":"typo"}', ModelSchema),
+    ).toThrow("model");
+  });
+
+  test("keeps the schema's own refinements", () => {
+    expect(() => parseJsonFlagWithSchema("model", '{"modelId":"needs-key"}', ModelSchema)).toThrow(
+      "apiKeyArn is required",
+    );
+  });
+
+  // A record accepts any key by definition, so there is nothing to reject and the
+  // strict pass must leave it alone.
+  test("leaves a record schema permissive", () => {
+    const tags = parseJsonFlagWithSchema(
+      "tags",
+      '{"env":"prod"}',
+      z.record(z.string(), z.string()),
+    );
+    expect(tags).toEqual({ env: "prod" });
+  });
+});
+
 describe("parseTags", () => {
   test("returns undefined for undefined input", () => {
     expect(parseTags(undefined)).toBeUndefined();
diff --git a/src/handlers/utils.tsx b/src/handlers/utils.tsx
index 82d18ae39..fda58f136 100644
--- a/src/handlers/utils.tsx
+++ b/src/handlers/utils.tsx
@@ -1,6 +1,6 @@
 import { createContext, useContext, useEffect } from "react";
 import type { Context } from "../router";
-import type z from "zod";
+import z from "zod";
 import type { CoreOptions } from "../core/types";
 import type { AppIO } from "../io";
 import { AgentCoreCLIError, InputValidationError, SilentCLIError } from "../errors";
@@ -69,7 +69,7 @@ export function parseJsonFlagWithSchema<T>(
   const parsed = parseJsonFlag<unknown>(name, raw);
   if (parsed === undefined) return undefined;
 
-  const result = schema.safeParse(parsed);
+  const result = strictened(schema).safeParse(parsed);
   if (!result.success) {
     throw new InputValidationError(
       `Invalid value for option '--${name}': ${formatZodError(result.error)}`,
@@ -79,6 +79,11 @@ export function parseJsonFlagWithSchema<T>(
   return result.data;
 }
 
+// Top level only: a nested typo (model.bedrockModelConfig.modelIdd) still slips through.
+function strictened<T>(schema: z.ZodType<T>): z.ZodType<T> {
+  return schema instanceof z.ZodObject ? (schema.strict() as unknown as z.ZodType<T>) : schema;
+}
+
 export function parseJsonObjectFlag<T extends object>(name: string, raw: string): T;
 export function parseJsonObjectFlag<T extends object>(
   name: string,