diff --git a/changelog.d/fixes/2461-antigravity-streaming-403-raw-bytes.md b/changelog.d/fixes/2461-antigravity-streaming-403-raw-bytes.md
new file mode 100644
index 0000000000..84cf50b3de
--- /dev/null
+++ b/changelog.d/fixes/2461-antigravity-streaming-403-raw-bytes.md
@@ -0,0 +1 @@
+- **fix(sse):** Antigravity streaming requests that hit a non-ok upstream response (e.g. a 403) no longer pipe the raw upstream bytes straight through to the client — a binary/non-UTF8 error body (observed as gzip-magic-byte garbage) is now routed through the same sanitized `buildAntigravityUpstreamError()` path the non-streaming branch already used, instead of corrupting the client-visible error message. Regression guard: `tests/unit/antigravity-streaming-error-body-sanitized-2461.test.ts` — thanks @Duongkhanhtool
diff --git a/open-sse/executors/antigravity.ts b/open-sse/executors/antigravity.ts
index 494d750836..49b1d8ff37 100644
--- a/open-sse/executors/antigravity.ts
+++ b/open-sse/executors/antigravity.ts
@@ -1591,6 +1591,34 @@ export class AntigravityExecutor extends BaseExecutor {
           };
         }
 
+        // #2461: a non-ok upstream response (e.g. 403) must never be piped through the
+        // streaming pass-through below as if it were an SSE body. Google occasionally
+        // returns non-UTF8/binary error bodies (observed: gzip-magic-byte payloads) for
+        // 403s on this endpoint; reading/forwarding those raw bytes corrupts the
+        // client-visible error message. Mirror the non-streaming branch above and build
+        // a sanitized JSON error via buildAntigravityUpstreamError (hard rule #12)
+        // instead of streaming unknown bytes straight through.
+        if (!response.ok) {
+          const rawBody = await response
+            .clone()
+            .text()
+            .catch(() => "");
+          const errorBody = buildAntigravityUpstreamError(
+            response.status,
+            response.statusText,
+            rawBody
+          );
+          return {
+            response: new Response(JSON.stringify(errorBody), {
+              status: response.status,
+              headers: { "Content-Type": "application/json" },
+            }),
+            url,
+            headers: finalHeaders,
+            transformedBody: attachToolNameMap(transformedBody, requestToolNameMap),
+          };
+        }
+
         // Streaming path: wrap the response body in a pass-through TransformStream
         // that extracts remainingCredits from the final SSE chunk(s) without
         // consuming the stream. The client receives the unmodified SSE data.
diff --git a/tests/unit/antigravity-streaming-error-body-sanitized-2461.test.ts b/tests/unit/antigravity-streaming-error-body-sanitized-2461.test.ts
new file mode 100644
index 0000000000..41d242c1f5
--- /dev/null
+++ b/tests/unit/antigravity-streaming-error-body-sanitized-2461.test.ts
@@ -0,0 +1,63 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import { AntigravityExecutor } from "../../open-sse/executors/antigravity.ts";
+import {
+  clearAntigravityVersionCache,
+  seedAntigravityVersionCache,
+} from "../../open-sse/services/antigravityVersion.ts";
+
+// Ports decolua/9router#2461: a non-ok (e.g. 403) Antigravity upstream response in the
+// STREAMING path was piped straight through to the client via a raw pass-through
+// TransformStream, with no `response.ok` check at all — unlike the non-streaming path,
+// which already builds a sanitized error via buildAntigravityUpstreamError. When the
+// upstream 403 body is gzip-compressed (or otherwise binary/non-UTF8), those raw bytes
+// end up surfaced verbatim in the client-visible error message, corrupting it (reporters
+// saw literal control-byte garbage after "[ERROR] [403]:").
+test.afterEach(() => {
+  clearAntigravityVersionCache();
+});
+
+test("AntigravityExecutor.execute (stream=true) sanitizes a non-ok upstream body instead of piping raw bytes", async () => {
+  const executor = new AntigravityExecutor();
+  const originalFetch = globalThis.fetch;
+  seedAntigravityVersionCache("2026.04.17-test");
+
+  // Simulate a gzip-compressed 403 body (magic bytes 0x1f 0x8b), the exact shape
+  // reported upstream — reading it as text without decoding produces garbage.
+  const binaryBody = new Uint8Array([0x1f, 0x8b, 0x08, 0x00, 0x02, 0xff, 0x52, 0x41, 0x4e]);
+
+  globalThis.fetch = async () =>
+    new Response(binaryBody, {
+      status: 403,
+      headers: { "Content-Type": "application/json" },
+    });
+
+  try {
+    const result = await executor.execute({
+      model: "antigravity/gemini-2.5-flash",
+      body: { request: { contents: [] } },
+      stream: true,
+      credentials: { accessToken: "token", projectId: "project-1" },
+      log: { debug() {}, warn() {} },
+    });
+
+    assert.equal(result.response.status, 403);
+
+    const bodyText = await result.response.text();
+
+    // The raw gzip magic bytes must never reach the client-visible error text.
+    assert.ok(
+      !bodyText.includes("\x1f\x8b"),
+      `expected sanitized error body, got raw bytes leaking through: ${JSON.stringify(bodyText)}`
+    );
+
+    // Must be routed through buildErrorBody()/buildAntigravityUpstreamError() — a clean,
+    // parseable JSON error shape (hard rule #12), not an arbitrary pass-through stream.
+    const parsed = JSON.parse(bodyText) as { error?: { message?: string } };
+    assert.ok(parsed.error?.message, "expected a structured error.message");
+    assert.match(parsed.error.message, /Antigravity upstream error \(403\)/);
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});