diff --git a/changelog.d/fixes/13591-antigravity-opaque-400-error-surfaced.md b/changelog.d/fixes/13591-antigravity-opaque-400-error-surfaced.md
new file mode 100644
index 0000000000..37c11a42fe
--- /dev/null
+++ b/changelog.d/fixes/13591-antigravity-opaque-400-error-surfaced.md
@@ -0,0 +1,3 @@
+- **fix(providers):** Antigravity error responses and logs now surface the real upstream
+  message (e.g. Gemini field-path rejections) instead of the generic "Antigravity upstream
+  error (400)" placeholder (#13591) — thanks @afonsoft
diff --git a/open-sse/executors/antigravityUpstreamError.ts b/open-sse/executors/antigravityUpstreamError.ts
index 074824ef17..2807baca9f 100644
--- a/open-sse/executors/antigravityUpstreamError.ts
+++ b/open-sse/executors/antigravityUpstreamError.ts
@@ -21,6 +21,20 @@ const GEO_BLOCKED_HINT =
   "call the model API. Route antigravity/agy egress through a proxy in a " +
   "supported region (e.g. US/EU) or use a different provider.";
 
+/**
+ * Extract the real upstream error message (e.g. Google's Gemini-dialect field-path
+ * rejection) from a parsed Antigravity `upstream_details`-shaped body, so callers can
+ * surface it directly in `error.message` instead of only nesting it under
+ * `upstream_details` — the generic `parseUpstreamError()` re-parser used by the shared
+ * chatCore failure path only reads the outer `error.message` (#13591).
+ */
+function extractUpstreamMessage(details: unknown): string | null {
+  if (!details || typeof details !== "object") return null;
+  const err = (details as { error?: { message?: unknown } }).error;
+  const msg = err && typeof err.message === "string" ? err.message : null;
+  return msg && msg.trim() ? msg.trim() : null;
+}
+
 export function buildAntigravityUpstreamError(status: number, statusText: string, rawBody: string) {
   let upstreamDetails: unknown;
   try {
@@ -36,5 +50,9 @@ export function buildAntigravityUpstreamError(status: number, statusText: string
       upstreamDetails
     );
   }
-  return buildErrorBody(status, `Antigravity upstream error (${status})${suffix}`, upstreamDetails);
+  const upstreamMessage = extractUpstreamMessage(upstreamDetails);
+  const message = upstreamMessage
+    ? `Antigravity upstream error (${status}): ${upstreamMessage}`
+    : `Antigravity upstream error (${status})${suffix}`;
+  return buildErrorBody(status, message, upstreamDetails);
 }
diff --git a/tests/unit/antigravityUpstreamError.test.ts b/tests/unit/antigravityUpstreamError.test.ts
new file mode 100644
index 0000000000..4329b4fb8d
--- /dev/null
+++ b/tests/unit/antigravityUpstreamError.test.ts
@@ -0,0 +1,75 @@
+// Regression coverage for issue #13591: Antigravity double-wraps its own errors, so
+// parseUpstreamError() (the shared chatCore failure-classification path) only ever saw
+// the generic "Antigravity upstream error (400)" template instead of the real Gemini
+// upstream detail buried under `upstream_details`. buildAntigravityUpstreamError() must
+// surface the real upstream message as `error.message` directly.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { buildAntigravityUpstreamError } from "../../open-sse/executors/antigravityUpstreamError.ts";
+import { parseUpstreamError } from "../../open-sse/utils/error.ts";
+
+test("issue #13591: parseUpstreamError surfaces the real upstream detail for an Antigravity-wrapped 400", async () => {
+  const rawUpstreamGeminiBody = JSON.stringify({
+    error: {
+      code: 400,
+      message:
+        "Invalid value at 'tools[0].function_declarations[0].parameters.properties[0].value' " +
+        '(type.googleapis.com/google.ai.generativelanguage.v1beta.Schema), "string"',
+      status: "INVALID_ARGUMENT",
+    },
+  });
+
+  const wrappedErrorBody = buildAntigravityUpstreamError(400, "", rawUpstreamGeminiBody);
+
+  assert.ok(
+    JSON.stringify(wrappedErrorBody).includes("function_declarations"),
+    "sanity: buildAntigravityUpstreamError should embed the real upstream detail somewhere in the body"
+  );
+  assert.ok(
+    (wrappedErrorBody as { error: { message: string } }).error.message.includes(
+      "function_declarations"
+    ),
+    "buildAntigravityUpstreamError should surface the real upstream detail directly in error.message"
+  );
+
+  const wrappedResponse = new Response(JSON.stringify(wrappedErrorBody), {
+    status: 400,
+    headers: { "Content-Type": "application/json" },
+  });
+
+  const parsed = await parseUpstreamError(wrappedResponse, "antigravity");
+
+  assert.ok(
+    parsed.message.includes("function_declarations"),
+    `expected parseUpstreamError to surface the real upstream detail, but got: ${JSON.stringify(parsed.message)}`
+  );
+});
+
+test("issue #13591: geo-blocked branch keeps its explicit hint message untouched", () => {
+  const geoBlockedBody = JSON.stringify({
+    error: {
+      message: "User location is not supported for the API use.",
+    },
+  });
+
+  const wrappedErrorBody = buildAntigravityUpstreamError(400, "Bad Request", geoBlockedBody) as {
+    error: { message: string };
+  };
+
+  assert.ok(
+    wrappedErrorBody.error.message.includes(
+      "not offered from this server's current egress location"
+    ),
+    "geo-blocked responses must keep the operator-facing hint, not the raw upstream text"
+  );
+});
+
+test("issue #13591: non-JSON upstream body falls back to the generic templated message without throwing", () => {
+  const htmlErrorPage = "<html><body>502 Bad Gateway</body></html>";
+
+  const wrappedErrorBody = buildAntigravityUpstreamError(502, "Bad Gateway", htmlErrorPage) as {
+    error: { message: string };
+  };
+
+  assert.equal(wrappedErrorBody.error.message, "Antigravity upstream error (502): Bad Gateway");
+});