diff --git a/changelog.d/fixes/8136-antigravity-400-lockout.md b/changelog.d/fixes/8136-antigravity-400-lockout.md
new file mode 100644
index 0000000000..10de14be52
--- /dev/null
+++ b/changelog.d/fixes/8136-antigravity-400-lockout.md
@@ -0,0 +1 @@
+- fix(providers): classify HTTP 400 model-unavailable as MODEL_NOT_FOUND so Antigravity Pro fallback locks out the deprecated model
diff --git a/open-sse/services/errorClassifier.ts b/open-sse/services/errorClassifier.ts
index 9aebe0531d..948ff21144 100644
--- a/open-sse/services/errorClassifier.ts
+++ b/open-sse/services/errorClassifier.ts
@@ -232,8 +232,18 @@ export function classifyProviderError(
   }
   if (statusCode >= 500) return PROVIDER_ERROR_TYPES.SERVER_ERROR;
 
-  if (statusCode === 400 && isContextOverflow(bodyStr)) {
-    return PROVIDER_ERROR_TYPES.CONTEXT_OVERFLOW;
+  if (statusCode === 400) {
+    if (isContextOverflow(bodyStr)) {
+      return PROVIDER_ERROR_TYPES.CONTEXT_OVERFLOW;
+    }
+    // Some providers (e.g. Antigravity's Pro-fallback chain, #8136) return a
+    // plain 400 for a model that is no longer available, instead of 404/401.
+    // Without this check the error falls through to `return null`, so
+    // lockModel() never fires and the same dead model gets retried on every
+    // request. Detect the phrasing here, same as the 401 branch above (#7268).
+    if (containsModelUnavailableMessage(bodyStr)) {
+      return PROVIDER_ERROR_TYPES.MODEL_NOT_FOUND;
+    }
   }
 
   return null;
diff --git a/tests/unit/repro-8136-antigravity-400-model-not-supported-lockout.test.ts b/tests/unit/repro-8136-antigravity-400-model-not-supported-lockout.test.ts
new file mode 100644
index 0000000000..6d040f4554
--- /dev/null
+++ b/tests/unit/repro-8136-antigravity-400-model-not-supported-lockout.test.ts
@@ -0,0 +1,43 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+const { classifyProviderError, PROVIDER_ERROR_TYPES } = await import(
+  "../../open-sse/services/errorClassifier.ts"
+);
+
+test("#8136: classifyProviderError(400, Antigravity 'model is not supported' body) returns MODEL_NOT_FOUND", () => {
+  const body = {
+    error: {
+      code: 400,
+      message: "The model gemini-3.1-pro-low is not supported for this project.",
+      status: "INVALID_ARGUMENT",
+    },
+  };
+  const classified = classifyProviderError(400, body, "antigravity");
+  assert.equal(
+    classified,
+    PROVIDER_ERROR_TYPES.MODEL_NOT_FOUND,
+    `expected MODEL_NOT_FOUND, got ${JSON.stringify(classified)}`
+  );
+});
+
+test("#8136: same phrasing at 401 still works (regression guard for #7268)", () => {
+  const body = { error: { message: "The model gemini-3.1-pro-low is not supported for this project." } };
+  assert.equal(classifyProviderError(401, body, "antigravity"), PROVIDER_ERROR_TYPES.MODEL_NOT_FOUND);
+});
+
+test("#8136: plain 400 context-overflow still classifies as CONTEXT_OVERFLOW, not MODEL_NOT_FOUND", () => {
+  const body = {
+    error: {
+      message: "This model's maximum context length is 128000 tokens. Please reduce the length of the messages.",
+    },
+  };
+  assert.equal(
+    classifyProviderError(400, body, "antigravity"),
+    PROVIDER_ERROR_TYPES.CONTEXT_OVERFLOW
+  );
+});
+
+test("#8136: a generic 400 bad-request body (no model-unavailable phrasing) is not reclassified as MODEL_NOT_FOUND", () => {
+  const body = { error: { message: "Invalid request: missing required field 'messages'." } };
+  assert.equal(classifyProviderError(400, body, "antigravity"), null);
+});