diff --git a/patches/picohttpparser/strict-chunk-size.patch b/patches/picohttpparser/strict-chunk-size.patch
new file mode 100644
index 000000000000..ece6f7c902a6
--- /dev/null
+++ b/patches/picohttpparser/strict-chunk-size.patch
@@ -0,0 +1,15 @@
+--- a/picohttpparser.c
++++ b/picohttpparser.c
+@@ -557,6 +557,12 @@
+                         ret = -1;
+                         goto Exit;
+                     }
++                    /* chunk-size is 1*HEXDIG followed by chunk-ext (";") or CRLF; reject anything
++                     * else so tokens like "0x5" are not misread as the last chunk */
++                    if (buf[src] != ';' && buf[src] != '\015' && buf[src] != '\012') {
++                        ret = -1;
++                        goto Exit;
++                    }
+                     break;
+                 }
+                 if (decoder->_hex_count == sizeof(size_t) * 2) {
diff --git a/scripts/build/deps/picohttpparser.ts b/scripts/build/deps/picohttpparser.ts
index 5b5dea6d1ba0..2c65d9d3632f 100644
--- a/scripts/build/deps/picohttpparser.ts
+++ b/scripts/build/deps/picohttpparser.ts
@@ -22,6 +22,8 @@ export const picohttpparser: Dependency = {
     commit: PICOHTTPPARSER_COMMIT,
   }),
 
+  patches: ["patches/picohttpparser/strict-chunk-size.patch"],
+
   build: () => ({ kind: "none" }),
 
   provides: () => ({
diff --git a/test/js/web/fetch/fetch-chunked-size.test.ts b/test/js/web/fetch/fetch-chunked-size.test.ts
new file mode 100644
index 000000000000..4e306f3fa4f1
--- /dev/null
+++ b/test/js/web/fetch/fetch-chunked-size.test.ts
@@ -0,0 +1,55 @@
+import { describe, expect, it } from "bun:test";
+import net from "node:net";
+
+async function serveChunked(body: string) {
+  const { promise, resolve } = Promise.withResolvers<net.AddressInfo>();
+  const server = net
+    .createServer(socket => {
+      socket.on("error", () => {});
+      socket.once("data", () => {
+        socket.end(`HTTP/1.1 200 OK\r\nTransfer-Encoding: chunked\r\n\r\n${body}`);
+      });
+    })
+    .listen(0, "127.0.0.1", () => resolve(server.address() as net.AddressInfo));
+  const address = await promise;
+  return { server, url: `http://127.0.0.1:${address.port}/` };
+}
+
+describe("fetch: chunked chunk-size token validation", () => {
+  // RFC 9112 7.1: chunk-size is 1*HEXDIG followed by ";" (chunk-ext) or CRLF.
+  // node/llhttp rejects every token below with HPE_INVALID_CHUNK_SIZE.
+  describe("rejects malformed chunk-size", () => {
+    it.each([
+      ["0x5", ""], // was misread as size 0: resolved 200 with empty body, data dropped
+      ["5g", "hello"], // was misread as size 5
+      ["5 ", "hello"],
+      ["5\t", "hello"],
+      ["5.0", "hello"],
+      ["5-", "hello"],
+    ])("token %j", async (token, _previouslyResolvedAs) => {
+      const { server, url } = await serveChunked(`${token}\r\nhello\r\n0\r\n\r\n`);
+      await using _s = server;
+      const result = await fetch(url)
+        .then(res => res.text())
+        .then(body => ({ resolved: body }))
+        .catch(e => e);
+      expect(result?.code).toBe("InvalidHTTPResponse");
+    });
+  });
+
+  describe("accepts well-formed chunk-size", () => {
+    it.each([
+      ["5", "hello"],
+      ["5;ext", "hello"],
+      ["5;ext=1", "hello"],
+      ["05", "hello"],
+      ["A", "0123456789"],
+    ])("token %j", async (token, payload) => {
+      const { server, url } = await serveChunked(`${token}\r\n${payload}\r\n0\r\n\r\n`);
+      await using _s = server;
+      const res = await fetch(url);
+      expect(await res.text()).toBe(payload);
+      expect(res.status).toBe(200);
+    });
+  });
+});