diff --git a/apps/server/src/server.ts b/apps/server/src/server.ts
index d4ad3bff4d..be879e0da0 100644
--- a/apps/server/src/server.ts
+++ b/apps/server/src/server.ts
@@ -1487,13 +1487,7 @@ async function forwardOpencodeV2Request(input: {
     target.searchParams.set("location[directory]", input.workspace.path);
   }
 
-  const headers = new Headers(input.request.headers);
-  headers.delete("authorization");
-  headers.delete("x-openwork-host-token");
-  headers.delete("x-openwork-client-id");
-  headers.delete("host");
-  headers.delete("origin");
-  headers.delete("transfer-encoding");
+  const headers = forwardableProxyRequestHeaders(input.request.headers);
   headers.set("authorization", `Basic ${Buffer.from(`opencode:${input.connection.password}`).toString("base64")}`);
 
   // Like the v1 proxy: an engine that is down or restarting is an expected
@@ -1978,13 +1972,7 @@ async function forwardOpencodeRequest(input: {
     throw new ApiError(400, "opencode_unconfigured", "OpenCode base URL is missing for this workspace");
   }
 
-  let headers = new Headers(input.request.headers);
-  headers.delete("authorization");
-  headers.delete("x-openwork-host-token");
-  headers.delete("x-openwork-client-id");
-  headers.delete("host");
-  headers.delete("origin");
-  headers.delete("transfer-encoding");
+  let headers = forwardableProxyRequestHeaders(input.request.headers);
 
   const directory = workspace ? resolveOpencodeDirectory(workspace) : null;
   let search = input.url.search;
@@ -2409,6 +2397,35 @@ async function proxyEngineEventStreams(input: {
   });
 }
 
+/**
+ * Copy client request headers for forwarding to the engine, dropping
+ * hop-by-hop and transport-level headers. Node's fetch (undici) rejects some
+ * of these outright (e.g. `expect: 100-continue` throws NotSupportedError
+ * "expect header not supported", surfacing as `TypeError: fetch failed`), and
+ * none of them are meaningful across a proxy hop.
+ */
+const PROXY_STRIPPED_REQUEST_HEADERS = [
+  "authorization",
+  "x-openwork-host-token",
+  "x-openwork-client-id",
+  "host",
+  "origin",
+  "transfer-encoding",
+  "expect",
+  "connection",
+  "keep-alive",
+  "proxy-connection",
+  "upgrade",
+  "te",
+  "trailer",
+] as const;
+
+export function forwardableProxyRequestHeaders(source: HeadersInit): Headers {
+  const headers = new Headers(source);
+  for (const name of PROXY_STRIPPED_REQUEST_HEADERS) headers.delete(name);
+  return headers;
+}
+
 /**
  * Strip hop-by-hop and transport-level headers that Bun's native fetch keeps
  * in the upstream response even after it has already decoded the body for us.
diff --git a/apps/server/test/proxy-request-headers.test.ts b/apps/server/test/proxy-request-headers.test.ts
new file mode 100644
index 0000000000..bb216e4769
--- /dev/null
+++ b/apps/server/test/proxy-request-headers.test.ts
@@ -0,0 +1,38 @@
+import { describe, expect, test } from "bun:test";
+import { forwardableProxyRequestHeaders } from "../src/server.js";
+
+describe("forwardableProxyRequestHeaders", () => {
+  test("drops expect and hop-by-hop headers that Node fetch (undici) rejects", () => {
+    const headers = forwardableProxyRequestHeaders({
+      "Expect": "100-continue",
+      "Connection": "keep-alive",
+      "Keep-Alive": "timeout=5",
+      "Transfer-Encoding": "chunked",
+      "Upgrade": "h2c",
+      "TE": "gzip",
+      "Host": "127.0.0.1:1234",
+      "Origin": "http://localhost",
+      "Authorization": "Bearer client",
+      "X-OpenWork-Host-Token": "secret",
+      "X-OpenWork-Client-Id": "client",
+      "Content-Type": "application/json",
+      "X-Custom": "kept",
+    });
+    for (const name of ["expect", "connection", "keep-alive", "transfer-encoding", "upgrade", "te", "host", "origin", "authorization", "x-openwork-host-token", "x-openwork-client-id"]) {
+      expect(headers.has(name)).toBe(false);
+    }
+    expect(headers.get("content-type")).toBe("application/json");
+    expect(headers.get("x-custom")).toBe("kept");
+  });
+
+  test("sanitized headers are accepted by fetch", async () => {
+    const server = Bun.serve({ port: 0, fetch: () => new Response("ok") });
+    try {
+      const headers = forwardableProxyRequestHeaders({ "Expect": "100-continue", "Content-Type": "application/json" });
+      const response = await fetch(`http://127.0.0.1:${server.port}/`, { method: "POST", headers, body: "{}" });
+      expect(await response.text()).toBe("ok");
+    } finally {
+      server.stop(true);
+    }
+  });
+});