diff --git a/changelog.d/fixes/antigravity-replayable-request-body.md b/changelog.d/fixes/antigravity-replayable-request-body.md
new file mode 100644
index 00000000000..c642e07a4c0
--- /dev/null
+++ b/changelog.d/fixes/antigravity-replayable-request-body.md
@@ -0,0 +1 @@
+- **fix(antigravity):** send the finite JSON upload as a replayable fixed request body even when the upstream response is streamed, avoiding the one-shot `ReadableStream`/`duplex` upload path and keeping retries replay-safe under large Codex/Responses payloads.
diff --git a/open-sse/executors/antigravity/executeAttempt.ts b/open-sse/executors/antigravity/executeAttempt.ts
index 276412efaae..793bdb436d1 100644
--- a/open-sse/executors/antigravity/executeAttempt.ts
+++ b/open-sse/executors/antigravity/executeAttempt.ts
@@ -196,21 +196,6 @@ export function buildAntigravity429ErrorMessage(errorJson: unknown): string {
   return errorMessage;
 }
 
-function getChunkedOrFixedBody(bodyStr: string, stream: boolean): BodyInit {
-  if (stream) {
-    return new ReadableStream(
-      {
-        async start(controller) {
-          controller.enqueue(new TextEncoder().encode(bodyStr));
-          controller.close();
-        },
-      },
-      { highWaterMark: 16384 }
-    );
-  }
-  return bodyStr;
-}
-
 function cloneAntigravityRequestBody(body: unknown): unknown {
   if (!body || typeof body !== "object") {
     return body;
@@ -330,7 +315,7 @@ export async function sendAntigravityRequest(
   headers: Record<string, string>,
   transformedBody: Record<string, unknown>,
   credentials: AntigravityCredentials,
-  stream: boolean,
+  _stream: boolean,
   signal: AbortSignal | null | undefined,
   log: SafeAntigravityLog,
   retryAttempt: number,
@@ -363,11 +348,13 @@ export async function sendAntigravityRequest(
     "TELEMETRY",
     `[Antigravity] PhysicalSend - RequestId: ${correlationId ?? "none"}, URL: ${url}, Model: ${model}, PhysicalSend: ${physicalSendOrdinal}, RetryAttempt: ${retryAttempt}`
   );
+  // The Antigravity request payload is finite JSON even when the response is streamed.
+  // Keep the upload replayable instead of wrapping it in a one-shot ReadableStream; proxyFetch
+  // can then use its normal replay/fallback path without retaining a duplex upload stream.
   let response = await fetchAntigravityWithReadinessTimeout(url, {
     method: "POST",
     headers: finalHeaders,
-    body: getChunkedOrFixedBody(serializedRequest.bodyString, stream),
-    ...(stream ? { duplex: "half" } : {}),
+    body: serializedRequest.bodyString,
     signal,
   });
 
@@ -384,8 +371,7 @@ export async function sendAntigravityRequest(
     response = await fetchAntigravityWithReadinessTimeout(url, {
       method: "POST",
       headers: retryHeaders,
-      body: getChunkedOrFixedBody(serializedRequest.bodyString, stream),
-      ...(stream ? { duplex: "half" } : {}),
+      body: serializedRequest.bodyString,
       signal,
     });
     finalHeaders = retryHeaders;
@@ -455,8 +441,7 @@ export async function tryCreditsRetry(
     const creditsResp = await fetchAntigravityWithReadinessTimeout(url, {
       method: "POST",
       headers: finalCreditsHeaders,
-      body: getChunkedOrFixedBody(serializedCreditsRequest.bodyString, stream),
-      ...(stream ? { duplex: "half" } : {}),
+      body: serializedCreditsRequest.bodyString,
       signal,
     });
     if (creditsResp.ok || creditsResp.status !== HTTP_STATUS.RATE_LIMITED) {
diff --git a/tests/unit/antigravity-replayable-request-body.test.ts b/tests/unit/antigravity-replayable-request-body.test.ts
new file mode 100644
index 00000000000..c44751e8993
--- /dev/null
+++ b/tests/unit/antigravity-replayable-request-body.test.ts
@@ -0,0 +1,108 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  sendAntigravityRequest,
+  tryCreditsRetry,
+} from "../../open-sse/executors/antigravity/executeAttempt.ts";
+
+// The Antigravity request payload is finite JSON even when the response is streamed, so every
+// physical send (first attempt, 403 retry, Google One AI credits retry) must carry a replayable
+// string body with no `duplex` upload stream (#5770 follow-up).
+
+test("Antigravity streaming sends finite JSON as a replayable fixed request body", async () => {
+  const originalFetch = globalThis.fetch;
+  const sends: Array<{ body: BodyInit | null | undefined; duplex: unknown }> = [];
+
+  globalThis.fetch = (async (_url: string | URL | Request, init?: RequestInit) => {
+    sends.push({
+      body: init?.body,
+      duplex: (init as (RequestInit & { duplex?: unknown }) | undefined)?.duplex,
+    });
+    return new Response(
+      'data: {"response":{"candidates":[{"content":{"parts":[{"text":"ok"}]},"finishReason":"STOP"}]}}\n\n',
+      { status: sends.length === 1 ? 403 : 200, headers: { "Content-Type": "text/event-stream" } }
+    );
+  }) as typeof fetch;
+
+  try {
+    const body = {
+      project: "project-1",
+      requestId: "agent-test",
+      request: { contents: [{ role: "user", parts: [{ text: "hello" }] }] },
+      model: "gemini-2.5-flash",
+      userAgent: "antigravity",
+      requestType: "agent",
+    };
+    const result = await sendAntigravityRequest(
+      "antigravity",
+      "https://cloudcode-pa.googleapis.com/v1internal:streamGenerateContent?alt=sse",
+      "gemini-2.5-flash",
+      { "Content-Type": "application/json" },
+      body,
+      { accessToken: "token", projectId: "project-1" },
+      true,
+      null,
+      { debug() {}, info() {}, warn() {}, error() {} },
+      0,
+      { value: 0 },
+      "agent-test"
+    );
+
+    assert.equal(result.response.status, 200);
+    assert.equal(sends.length, 2, "403 retry must perform a second physical send");
+    for (const send of sends) {
+      assert.equal(typeof send.body, "string");
+      assert.equal(send.duplex, undefined);
+    }
+    assert.equal(String(sends[0]?.body), String(sends[1]?.body));
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("Antigravity credits retry also keeps a streaming request body replayable", async () => {
+  const originalFetch = globalThis.fetch;
+  let capturedBody: BodyInit | null | undefined;
+  let capturedDuplex: unknown;
+
+  globalThis.fetch = (async (_url: string | URL | Request, init?: RequestInit) => {
+    capturedBody = init?.body;
+    capturedDuplex = (init as (RequestInit & { duplex?: unknown }) | undefined)?.duplex;
+    return new Response(
+      'data: {"response":{"candidates":[{"content":{"parts":[{"text":"ok"}]},"finishReason":"STOP"}]}}\n\n',
+      { status: 200, headers: { "Content-Type": "text/event-stream" } }
+    );
+  }) as typeof fetch;
+
+  try {
+    const result = await tryCreditsRetry(
+      "antigravity",
+      "https://cloudcode-pa.googleapis.com/v1internal:streamGenerateContent?alt=sse",
+      { "Content-Type": "application/json" },
+      {
+        project: "project-1",
+        requestId: "agent-test-credits",
+        request: { contents: [{ role: "user", parts: [{ text: "hello" }] }] },
+        model: "gemini-2.5-flash",
+        userAgent: "antigravity",
+        requestType: "agent",
+      },
+      { accessToken: "token", projectId: "project-1" },
+      true,
+      null,
+      { debug() {}, info() {}, warn() {}, error() {} },
+      "account-1",
+      () => {},
+      { value: 0 },
+      "agent-test-credits"
+    );
+
+    assert.equal(result?.response.status, 200);
+    assert.equal(typeof capturedBody, "string");
+    assert.equal(capturedDuplex, undefined);
+    assert.match(String(capturedBody), /"enabledCreditTypes":\["GOOGLE_ONE_AI"\]/);
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});