diff --git a/src/agents/tools/web-fetch.cf-markdown.test.ts b/src/agents/tools/web-fetch.cf-markdown.test.ts
index ab46d136afeb..8be80256eaa5 100644
--- a/src/agents/tools/web-fetch.cf-markdown.test.ts
+++ b/src/agents/tools/web-fetch.cf-markdown.test.ts
@@ -6,7 +6,7 @@ import * as logger from "../../logger.js";
 import { withFetchPreconnect } from "../../test-utils/fetch-mock.js";
 import "./web-fetch.test-mocks.js";
 import { createWebFetchTool } from "./web-fetch.js";
-import { createBaseWebFetchToolConfig, makeFetchHeaders } from "./web-fetch.test-harness.js";
+import { createBaseWebFetchToolConfig } from "./web-fetch.test-harness.js";
 
 const lookupMock = vi.fn();
 const baseToolConfig = createBaseWebFetchToolConfig({
@@ -14,24 +14,20 @@ const baseToolConfig = createBaseWebFetchToolConfig({
 });
 
 function markdownResponse(body: string, extraHeaders: Record<string, string> = {}): Response {
-  return {
-    ok: true,
+  return new Response(body, {
     status: 200,
-    headers: makeFetchHeaders({
+    headers: {
       "content-type": "text/markdown; charset=utf-8",
       ...extraHeaders,
-    }),
-    text: async () => body,
-  } as Response;
+    },
+  });
 }
 
 function htmlResponse(body: string): Response {
-  return {
-    ok: true,
+  return new Response(body, {
     status: 200,
-    headers: makeFetchHeaders({ "content-type": "text/html; charset=utf-8" }),
-    text: async () => body,
-  } as Response;
+    headers: { "content-type": "text/html; charset=utf-8" },
+  });
 }
 
 describe("web_fetch Cloudflare Markdown for Agents", () => {
diff --git a/src/agents/tools/web-shared.test.ts b/src/agents/tools/web-shared.test.ts
index 85c82001ab19..358588ba44a4 100644
--- a/src/agents/tools/web-shared.test.ts
+++ b/src/agents/tools/web-shared.test.ts
@@ -146,4 +146,48 @@ describe("readResponseText", () => {
     expect(cancel).toHaveBeenCalledTimes(1);
     expect(releaseLock).toHaveBeenCalledTimes(1);
   });
+
+  it("does not invoke whole-body fallbacks when maxBytes is set", async () => {
+    const arrayBuffer = vi.fn(async () => new TextEncoder().encode("hello").buffer);
+    const text = vi.fn(async () => "hello");
+    const response = {
+      arrayBuffer,
+      headers: new Headers(),
+      text,
+    } as unknown as Response;
+
+    await expect(readResponseText(response, { maxBytes: 4 })).resolves.toEqual({
+      text: "",
+      truncated: true,
+      bytesRead: 0,
+    });
+    expect(arrayBuffer).not.toHaveBeenCalled();
+    expect(text).not.toHaveBeenCalled();
+  });
+
+  it("treats a native bodyless response as empty when maxBytes is set", async () => {
+    await expect(
+      readResponseText(new Response(null, { status: 204 }), { maxBytes: 4 }),
+    ).resolves.toEqual({
+      text: "",
+      truncated: false,
+      bytesRead: 0,
+    });
+  });
+
+  it("preserves uncapped text-only fallback byte accounting", async () => {
+    const value = "中文🔥";
+    const text = vi.fn(async () => value);
+    const response = {
+      headers: new Headers(),
+      text,
+    } as unknown as Response;
+
+    await expect(readResponseText(response)).resolves.toEqual({
+      text: value,
+      truncated: false,
+      bytesRead: new TextEncoder().encode(value).byteLength,
+    });
+    expect(text).toHaveBeenCalledTimes(1);
+  });
 });
diff --git a/src/agents/tools/web-shared.ts b/src/agents/tools/web-shared.ts
index ea41f49cd298..523cc6694ae5 100644
--- a/src/agents/tools/web-shared.ts
+++ b/src/agents/tools/web-shared.ts
@@ -297,6 +297,15 @@ export async function readResponseText(
     return { text: decodeResponseBytes(res, bytes), truncated, bytesRead };
   }
 
+  if (maxBytes) {
+    if (res instanceof Response && res.body === null) {
+      return { text: "", truncated: false, bytesRead: 0 };
+    }
+    // Whole-body fallbacks allocate before returning, so they cannot honor a byte cap.
+    // Fail closed instead of making maxBytes a returned-text limit only.
+    return { text: "", truncated: true, bytesRead: 0 };
+  }
+
   const readBytes = (res as { arrayBuffer?: () => Promise<ArrayBuffer> }).arrayBuffer;
   if (typeof readBytes === "function") {
     try {
@@ -313,7 +322,8 @@ export async function readResponseText(
 
   try {
     const text = await res.text();
-    return { text, truncated: false, bytesRead: text.length };
+    const bytes = new TextEncoder().encode(text);
+    return { text, truncated: false, bytesRead: bytes.byteLength };
   } catch {
     return { text: "", truncated: false, bytesRead: 0 };
   }
diff --git a/src/agents/tools/web-tools.fetch.test.ts b/src/agents/tools/web-tools.fetch.test.ts
index b0b0ef9bfdb6..d5bd448fde03 100644
--- a/src/agents/tools/web-tools.fetch.test.ts
+++ b/src/agents/tools/web-tools.fetch.test.ts
@@ -6,7 +6,6 @@ import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import type { LookupFn } from "../../infra/net/ssrf.js";
 import { resolveRequestUrl } from "../../plugin-sdk/request-url.js";
 import { withFetchPreconnect } from "../../test-utils/fetch-mock.js";
-import { makeFetchHeaders } from "./web-fetch.test-harness.js";
 const { extractReadableContentMock, resolveWebFetchDefinitionMock } = vi.hoisted(() => ({
   extractReadableContentMock: vi.fn(),
   resolveWebFetchDefinitionMock: vi.fn(),
@@ -29,37 +28,36 @@ import { WEB_FETCH_SPILL_MAX_CHARS } from "./web-fetch.js";
 
 const lookupMock = vi.fn();
 
-type MockResponse = {
-  ok: boolean;
-  status: number;
-  url?: string;
-  headers?: { get: (key: string) => string | null };
-  text?: () => Promise<string>;
-  json?: () => Promise<unknown>;
-};
+function responseWithUrl(body: BodyInit, init: ResponseInit, url: string): Response {
+  const response = new Response(body, init);
+  Object.defineProperty(response, "url", { value: url });
+  return response;
+}
 
-function htmlResponse(html: string, url = "https://example.com/"): MockResponse {
-  return {
-    ok: true,
-    status: 200,
+function htmlResponse(html: string, url = "https://example.com/"): Response {
+  return responseWithUrl(
+    html,
+    {
+      status: 200,
+      headers: { "content-type": "text/html; charset=utf-8" },
+    },
     url,
-    headers: makeFetchHeaders({ "content-type": "text/html; charset=utf-8" }),
-    text: async () => html,
-  };
+  );
 }
 
 function textResponse(
   text: string,
   url = "https://example.com/",
   contentType = "text/plain; charset=utf-8",
-): MockResponse {
-  return {
-    ok: true,
-    status: 200,
+): Response {
+  return responseWithUrl(
+    text,
+    {
+      status: 200,
+      headers: { "content-type": contentType },
+    },
     url,
-    headers: makeFetchHeaders({ "content-type": contentType }),
-    text: async () => text,
-  };
+  );
 }
 
 function errorHtmlResponse(
@@ -67,14 +65,16 @@ function errorHtmlResponse(
   status = 404,
   url = "https://example.com/",
   contentType: string | null = "text/html; charset=utf-8",
-): MockResponse {
-  return {
-    ok: false,
-    status,
+): Response {
+  const body = contentType ? html : new TextEncoder().encode(html);
+  return responseWithUrl(
+    body,
+    {
+      status,
+      headers: contentType ? { "content-type": contentType } : undefined,
+    },
     url,
-    headers: contentType ? makeFetchHeaders({ "content-type": contentType }) : makeFetchHeaders({}),
-    text: async () => html,
-  };
+  );
 }
 function installMockFetch(
   impl: (input: RequestInfo | URL, init?: RequestInit) => Promise<Response>,
@@ -111,13 +111,7 @@ function createFetchTool(fetchOverrides: Record<string, unknown> = {}) {
 
 function installPlainTextFetch(text: string) {
   installMockFetch((input: RequestInfo | URL) =>
-    Promise.resolve({
-      ok: true,
-      status: 200,
-      headers: makeFetchHeaders({ "content-type": "text/plain" }),
-      text: async () => text,
-      url: resolveRequestUrl(input),
-    } as Response),
+    Promise.resolve(textResponse(text, resolveRequestUrl(input))),
   );
 }
 
@@ -415,13 +409,7 @@ describe("web_fetch extraction fallbacks", () => {
   it("enforces maxChars after wrapping", async () => {
     const longText = "x".repeat(5_000);
     installMockFetch((input: RequestInfo | URL) =>
-      Promise.resolve({
-        ok: true,
-        status: 200,
-        headers: makeFetchHeaders({ "content-type": "text/plain" }),
-        text: async () => longText,
-        url: resolveRequestUrl(input),
-      } as Response),
+      Promise.resolve(textResponse(longText, resolveRequestUrl(input))),
     );
 
     const tool = createFetchTool({
@@ -655,13 +643,7 @@ describe("web_fetch extraction fallbacks", () => {
   it("keeps DNS pinning for web_fetch by default even when HTTP_PROXY is configured", async () => {
     vi.stubEnv("HTTP_PROXY", "http://127.0.0.1:7890");
     const mockFetch = installMockFetch((input: RequestInfo | URL) =>
-      Promise.resolve({
-        ok: true,
-        status: 200,
-        headers: makeFetchHeaders({ "content-type": "text/plain" }),
-        text: async () => "proxy body",
-        url: resolveRequestUrl(input),
-      } as Response),
+      Promise.resolve(textResponse("proxy body", resolveRequestUrl(input))),
     );
     const tool = createFetchTool({ firecrawl: { enabled: false } });
 
@@ -678,13 +660,7 @@ describe("web_fetch extraction fallbacks", () => {
   it("uses env proxy dispatch for web_fetch when trusted env proxy is explicitly enabled", async () => {
     vi.stubEnv("HTTP_PROXY", "http://127.0.0.1:7890");
     const mockFetch = installMockFetch((input: RequestInfo | URL) =>
-      Promise.resolve({
-        ok: true,
-        status: 200,
-        headers: makeFetchHeaders({ "content-type": "text/plain" }),
-        text: async () => "proxy body",
-        url: resolveRequestUrl(input),
-      } as Response),
+      Promise.resolve(textResponse("proxy body", resolveRequestUrl(input))),
     );
     const tool = createFetchTool({
       firecrawl: { enabled: false },
@@ -800,13 +776,14 @@ describe("web_fetch extraction fallbacks", () => {
   });
 
   it("uses the provider fallback when direct fetch fails", async () => {
-    installMockFetch((_input: RequestInfo | URL) => {
-      return Promise.resolve({
-        ok: false,
-        status: 403,
-        headers: makeFetchHeaders({ "content-type": "text/html" }),
-        text: async () => "blocked",
-      } as Response);
+    installMockFetch((input: RequestInfo | URL) => {
+      return Promise.resolve(
+        responseWithUrl(
+          "blocked",
+          { status: 403, headers: { "content-type": "text/html" } },
+          resolveRequestUrl(input),
+        ),
+      );
     });
 
     resolveWebFetchDefinitionMock.mockReturnValue({