diff --git a/src/__tests__/ollama.test.ts b/src/__tests__/ollama.test.ts
index e409825e..48ef877f 100644
--- a/src/__tests__/ollama.test.ts
+++ b/src/__tests__/ollama.test.ts
@@ -1,5 +1,6 @@
 import { describe, it, expect, afterEach } from "vitest";
 import * as http from "node:http";
+import * as net from "node:net";
 import type { Fixture, HandlerDefaults } from "../types.js";
 import { createServer, type ServerInstance } from "../server.js";
 import { ollamaToCompletionRequest, handleOllama, handleOllamaGenerate } from "../ollama.js";
@@ -106,6 +107,66 @@ function postRaw(url: string, raw: string): Promise<{ status: number; body: stri
   });
 }
 
+// Sends a raw request with `Upgrade: h2c` + `Connection: Upgrade` headers, as
+// OkHttp (and thus langchain4j) speculatively does even for plain requests.
+// http.request() can't produce this from the client side, so we use a raw
+// socket and parse the response ourselves.
+function requestWithH2cUpgradeHeaders(
+  url: string,
+  method: string,
+  body?: string,
+): Promise<{ status: number; body: string }> {
+  return new Promise((resolve, reject) => {
+    const parsed = new URL(url);
+    const port = parsed.port ? Number(parsed.port) : 80;
+    const socket = net.connect(port, parsed.hostname, () => {
+      const bodyHeaders = body
+        ? `Content-Type: application/json\r\nContent-Length: ${Buffer.byteLength(body)}\r\n`
+        : "";
+      socket.write(
+        `${method} ${parsed.pathname} HTTP/1.1\r\n` +
+          `Host: ${parsed.host}\r\n` +
+          bodyHeaders +
+          `Upgrade: h2c\r\n` +
+          `Connection: Upgrade\r\n` +
+          `\r\n` +
+          (body ?? ""),
+      );
+    });
+    let data = "";
+    // The server keeps the connection alive (we don't send a real
+    // `Connection: close`), so completion must be detected from the response
+    // framing itself rather than waiting for the socket to close.
+    socket.on("data", (chunk: Buffer) => {
+      data += chunk.toString();
+      const headerEnd = data.indexOf("\r\n\r\n");
+      if (headerEnd === -1) return;
+      const head = data.slice(0, headerEnd);
+      const rest = data.slice(headerEnd + 4);
+      const isChunked = /transfer-encoding:\s*chunked/i.test(head);
+      const contentLengthMatch = head.match(/content-length:\s*(\d+)/i);
+      const done = isChunked
+        ? rest.endsWith("0\r\n\r\n")
+        : contentLengthMatch
+          ? Buffer.byteLength(rest) >= Number(contentLengthMatch[1])
+          : false;
+      if (!done) return;
+
+      const statusLine = head.split("\r\n")[0] ?? "";
+      const status = Number(statusLine.split(" ")[1] ?? 0);
+      const responseBody = isChunked
+        ? rest
+            .split("\r\n")
+            .filter((line) => !/^[0-9a-f]+$/i.test(line.trim()))
+            .join("")
+        : rest.slice(0, Number(contentLengthMatch![1]));
+      socket.destroy();
+      resolve({ status, body: responseBody });
+    });
+    socket.on("error", reject);
+  });
+}
+
 function parseNDJSON(body: string): object[] {
   return body
     .split("\n")
@@ -882,6 +943,42 @@ describe("GET /api/tags", () => {
     const names = body.models.map((m: { name: string }) => m.name);
     expect(names).toContain("gpt-4");
   });
+
+  // Regression test: langchain4j's OkHttp-based client probes for HTTP/2
+  // cleartext support by sending `Upgrade: h2c` + `Connection: Upgrade` on
+  // its plain GET /api/tags request. Node's http module treats any
+  // `Connection: Upgrade` header as a protocol-upgrade request, so this must
+  // not be swallowed by the WebSocket-upgrade path and 404.
+  it("responds normally when the request carries h2c upgrade probe headers", async () => {
+    instance = await createServer(allFixtures);
+    const res = await requestWithH2cUpgradeHeaders(`${instance.url}/api/tags`, "GET");
+
+    expect(res.status).toBe(200);
+    const body = JSON.parse(res.body);
+    const names = body.models.map((m: { name: string }) => m.name);
+    expect(names).toContain("llama3");
+  });
+});
+
+describe("POST /api/chat (h2c upgrade probe)", () => {
+  // Regression test: the same h2c probe headers on a POST with a body used to
+  // reach the request handler with an empty body (Node detaches its parser
+  // from `req` once "upgrade" fires, so bytes buffered in `head` never became
+  // part of `req`'s stream), producing a "Malformed JSON body" error instead
+  // of the real validation error for the request that was actually sent.
+  it("still parses the JSON body and returns the real validation error", async () => {
+    instance = await createServer(allFixtures);
+    const res = await requestWithH2cUpgradeHeaders(
+      `${instance.url}/api/chat`,
+      "POST",
+      JSON.stringify({ model: "llama3.2" }),
+    );
+
+    expect(res.status).toBe(400);
+    const body = JSON.parse(res.body);
+    expect(body.error.message).toMatch(/messages/i);
+    expect(body.error.message).not.toMatch(/Malformed JSON/i);
+  });
 });
 
 // ─── Integration tests: journal ─────────────────────────────────────────────
diff --git a/src/server.ts b/src/server.ts
index 7a77f46f..97296f0d 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -3145,6 +3145,32 @@ export async function createServerWithResolvedAuth(
     socket: import("node:net").Socket,
     head: Buffer,
   ): Promise<void> {
+    // `Upgrade: h2c` + `Connection: Upgrade` on plain requests — including
+    // ones with a body — to probe for HTTP/2 cleartext support. Only actual
+    // WebSocket upgrades belong on this path.
+    //
+    // Node detaches its HTTP parser from the socket the moment "upgrade"
+    // fires, so `req` never receives a body: any bytes already read land in
+    // `head` instead, and reconnecting them to `req` (e.g. via `req.push`)
+    // would still leave chunked bodies and further framing unhandled. Rather
+    // than reimplementing body parsing, rebuild the request line and headers
+    // with `Upgrade`/`Connection` dropped — the only thing that made this
+    // look like an upgrade — replay them plus `head` onto the socket, and
+    // let Node re-parse the connection from scratch as an ordinary request.
+    if ((req.headers.upgrade ?? "").toLowerCase() !== "websocket") {
+      const requestLine = `${req.method} ${req.url} HTTP/${req.httpVersion}\r\n`;
+      const headerLines: string[] = [];
+      for (let i = 0; i < req.rawHeaders.length; i += 2) {
+        if (/^(?:upgrade|connection)$/i.test(req.rawHeaders[i])) continue;
+        headerLines.push(`${req.rawHeaders[i]}: ${req.rawHeaders[i + 1]}`);
+      }
+      const rebuilt = Buffer.from(requestLine + headerLines.join("\r\n") + "\r\n\r\n");
+      socket.unshift(head);
+      socket.unshift(rebuilt);
+      server.emit("connection", socket);
+      return;
+    }
+
     const parsedUrl = new URL(req.url ?? "/", `http://${req.headers.host ?? "localhost"}`);
     let pathname = parsedUrl.pathname;