diff --git a/open-sse/translator/helpers/geminiHelper.ts b/open-sse/translator/helpers/geminiHelper.ts
index fefa882c3525..73ca48cfea88 100644
--- a/open-sse/translator/helpers/geminiHelper.ts
+++ b/open-sse/translator/helpers/geminiHelper.ts
@@ -534,7 +534,16 @@ function removeUnsupportedKeywords(obj: unknown, keywords: Set<string>): void {
   const record = obj as JsonRecord;
   // Delete unsupported *constraint* keywords at the current schema level.
   for (const key of Object.keys(record)) {
-    if (keywords.has(key) || key.startsWith("x-")) {
+    // `~`-prefixed keys are the Standard Schema convention (Zod 4+, Valibot,
+    // ArkType) for internal/vendor metadata namespaced to avoid colliding
+    // with real schema property names -- e.g. a tool built from one of those
+    // libraries can leak a literal `~optional` key into a property's
+    // subschema. The plain `"optional"` entry in the denylist above doesn't
+    // match the tilde-prefixed form, and Gemini 400s the entire tool list on
+    // the unrecognized field ("Unknown name \"~optional\" ... Cannot find
+    // field"), taking down every model behind it. Strip the whole class the
+    // same way `x-` vendor extensions are already stripped below.
+    if (keywords.has(key) || key.startsWith("x-") || key.startsWith("~")) {
       delete record[key];
     }
   }
diff --git a/tests/unit/gemini-standard-schema-tilde-optional.test.ts b/tests/unit/gemini-standard-schema-tilde-optional.test.ts
new file mode 100644
index 000000000000..b892796444cd
--- /dev/null
+++ b/tests/unit/gemini-standard-schema-tilde-optional.test.ts
@@ -0,0 +1,80 @@
+/**
+ * antigravity/gemini returned [400] "Invalid JSON payload received.
+ * Unknown name \"~optional\" at 'tools[0].function_declarations[N].parameters
+ * .properties[M].value': Cannot find field." -- taking down every model behind
+ * it (this exact error killed the entire `default` combo fallback chain live).
+ *
+ * Root cause: `~`-prefixed keys are the Standard Schema convention (Zod 4+,
+ * Valibot, ArkType) for internal/vendor metadata, namespaced with a leading
+ * `~` specifically so it can never collide with a real user-defined schema
+ * property name. A tool built from one of those libraries leaked a literal
+ * `~optional` key into a property's subschema. `GEMINI_UNSUPPORTED_SCHEMA_KEYS`
+ * already listed the plain `"optional"` string, but that exact-match check
+ * doesn't catch the tilde-prefixed form, so `removeUnsupportedKeywords` left
+ * it in place and Gemini's OpenAPI 3.0 schema subset rejects the whole
+ * request on the unrecognized field.
+ *
+ * Fix: strip any `~`-prefixed key at every schema level, the same way `x-`
+ * vendor extensions are already stripped, instead of only matching literal
+ * keys in the denylist. This covers `~optional` and any other Standard
+ * Schema metadata key the same libraries may emit.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { cleanJSONSchemaForAntigravity } from "../../open-sse/translator/helpers/geminiHelper.ts";
+import { openaiToGeminiRequest } from "../../open-sse/translator/request/openai-to-gemini.ts";
+
+test("~optional (and other tilde-prefixed Standard Schema keys) are stripped at all levels for antigravity/gemini schemas", () => {
+  const schema = {
+    type: "object",
+    "~standard": { version: 1, vendor: "zod" },
+    properties: {
+      query: { type: "string" },
+      value: { type: "string", "~optional": true, description: "the value" },
+    },
+  };
+
+  const cleaned = JSON.stringify(cleanJSONSchemaForAntigravity(schema));
+
+  assert.ok(!cleaned.includes("~optional"), "~optional must be removed");
+  assert.ok(!cleaned.includes("~standard"), "~standard must be removed");
+  assert.ok(cleaned.includes("query"), "unrelated properties must be preserved");
+  assert.ok(
+    cleaned.includes("the value"),
+    "unrelated sibling keys on the same subschema must be preserved"
+  );
+});
+
+test("OpenAI -> Gemini request strips ~optional from a tool parameter's subschema", () => {
+  const body = {
+    messages: [{ role: "user", content: "hi" }],
+    tools: [
+      {
+        type: "function",
+        function: {
+          name: "set_config",
+          description: "set a config value",
+          parameters: {
+            type: "object",
+            properties: {
+              key: { type: "string" },
+              value: { type: "string", "~optional": true },
+            },
+          },
+        },
+      },
+    ],
+  };
+
+  const result = openaiToGeminiRequest("gemini-3.7-flash-low", body, false) as {
+    tools?: Array<{ functionDeclarations?: Array<{ parameters: unknown }> }>;
+  };
+
+  const parameters = result.tools?.[0]?.functionDeclarations?.[0]?.parameters;
+  assert.ok(parameters, "expected a translated function declaration");
+  assert.ok(
+    !JSON.stringify(parameters).includes("~optional"),
+    "~optional must not reach the upstream request"
+  );
+});