diff --git a/packages/codev-sandbox/src/broker.test.ts b/packages/codev-sandbox/src/broker.test.ts
index f1ef99fb13..c246953a18 100644
--- a/packages/codev-sandbox/src/broker.test.ts
+++ b/packages/codev-sandbox/src/broker.test.ts
@@ -16,7 +16,7 @@ test("resolves allowed prefixes onto the upstream and denies everything else", (
   expect(resolveBrokerTarget(routes, "/etc/passwd")).toBe(null)
 })
 
-test("strips hop-by-hop and host headers before forwarding", () => {
+test("strips hop-by-hop, host, and browser-controlled headers before forwarding", () => {
   expect(
     forwardableRequestHeaders({
       host: "127.0.0.1:9999",
@@ -25,6 +25,11 @@ test("strips hop-by-hop and host headers before forwarding", () => {
       authorization: "Bearer token",
       "x-api-key": "key",
       accept: undefined,
+      // Chromium refuses to send a request carrying these, so a host that
+      // forwards through Electron's net stack fails outright on them.
+      "sec-fetch-mode": "cors",
+      "Sec-Fetch-Site": "cross-site",
+      "sec-ch-ua": '"Chromium";v="1"',
     }),
   ).toEqual({ authorization: "Bearer token", "x-api-key": "key" })
 })
diff --git a/packages/codev-sandbox/src/broker.ts b/packages/codev-sandbox/src/broker.ts
index 84986e41b8..1f75c1d94f 100644
--- a/packages/codev-sandbox/src/broker.ts
+++ b/packages/codev-sandbox/src/broker.ts
@@ -77,10 +77,20 @@ const REQUEST_HEADER_DROP = new Set([
 // body we no longer send.
 const RESPONSE_HEADER_DROP = new Set(["content-encoding", "content-length", "transfer-encoding", "connection"])
 
+// Browser-controlled headers a client may not set. A host that forwards with
+// Chromium's network stack (the desktop's Electron net.fetch) rejects the
+// entire request with net::ERR_INVALID_ARGUMENT if one is present, so they are
+// dropped here rather than at one caller.
+const REQUEST_HEADER_DROP_PREFIX = ["sec-fetch-", "sec-ch-", "proxy-"]
+
 export function forwardableRequestHeaders(headers: Record<string, string | string[] | undefined>) {
   return Object.fromEntries(
     Object.entries(headers)
-      .filter(([name, value]) => value !== undefined && !REQUEST_HEADER_DROP.has(name.toLowerCase()))
+      .filter(([name, value]) => {
+        if (value === undefined) return false
+        const lower = name.toLowerCase()
+        return !REQUEST_HEADER_DROP.has(lower) && !REQUEST_HEADER_DROP_PREFIX.some((p) => lower.startsWith(p))
+      })
       .map(([name, value]) => [name, Array.isArray(value) ? value.join(", ") : (value as string)]),
   )
 }