diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index f8bfaf21b..f9ce20e37 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -35,7 +35,7 @@
 - Stopped runaway exact response cycles across model providers and kept persistent loops fail-closed after bounded guarded retries ([#8669](https://github.com/can1357/oh-my-pi/pull/8669) by [@pstarkgit](https://github.com/pstarkgit)).
 ### Fixed
 
-- Fixed OpenAI Responses / Completions and xAI tool conversion 400ing the whole turn on MCP schemas whose root is an object plus a typeless exclusive-required `anyOf` (e.g. codebase-memory `check_index_coverage`). Those fragments now flatten in `toolWireSchema` so every other tool stays callable.
+- Fixed OpenAI Responses / Completions and xAI tool conversion 400ing the whole turn on MCP schemas whose root is an object plus a typeless exclusive-required `anyOf` (e.g. codebase-memory `check_index_coverage`). Flatten only the **tool root** — nested unions (e.g. `task.outputSchema`) stay intact so Grok still sees valid `task`/`edit` schemas.
 
 
 ## [17.3.4] - 2026-08-14
diff --git a/packages/ai/src/utils/schema/normalize.ts b/packages/ai/src/utils/schema/normalize.ts
index 0a08b6158..9cc224397 100644
--- a/packages/ai/src/utils/schema/normalize.ts
+++ b/packages/ai/src/utils/schema/normalize.ts
@@ -1472,7 +1472,11 @@ const OPENAI_RESPONSES_SCHEMA_VALUE_KEYS = new Set([
  * would not survive).
  */
 export function sanitizeSchemaForOpenAIResponses(schema: JsonObject): JsonObject {
-	return normalizeOpenAIResponsesSchemaNode(schema, new WeakMap()) as JsonObject;
+	const sanitized = normalizeOpenAIResponsesSchemaNode(schema, new WeakMap()) as JsonObject;
+	if (isJsonObject(sanitized)) {
+		flattenOpenAIResponsesObjectConstraintUnion(sanitized);
+	}
+	return sanitized;
 }
 
 /**
@@ -1575,10 +1579,6 @@ function normalizeOpenAIResponsesSchemaNode(value: unknown, cache: WeakMap<JsonO
 		changed = true;
 	}
 
-	if (flattenOpenAIResponsesObjectConstraintUnion(output)) {
-		changed = true;
-	}
-
 
 	// Safe to overwrite the seed: any cyclic re-entry above already observed
 	// the seeded partial and set `changed = true` for that node, so a node
diff --git a/packages/ai/src/utils/schema/wire.ts b/packages/ai/src/utils/schema/wire.ts
index 0f88c7964..b265e9787 100644
--- a/packages/ai/src/utils/schema/wire.ts
+++ b/packages/ai/src/utils/schema/wire.ts
@@ -114,6 +114,9 @@ const kStrippedSchema = Symbol("pi.schema.descriptions.stripped");
 
 function postProcessJsonSchema(schema: Record<string, unknown>): Record<string, unknown> {
 	walk(schema);
+	// xAI 400s only when the *tool root* is object + typeless exclusive-required
+	// anyOf. Nested unions (task.outputSchema, etc.) must stay intact.
+	flattenExclusiveRequiredUnion(schema);
 	normalizeArkPropertyComments(schema);
 	normalizeEmptySchemas(schema);
 	return schema;
@@ -440,7 +443,6 @@ function walk(node: unknown): void {
 	if (!node || typeof node !== "object") return;
 	const obj = node as Record<string, unknown>;
 	rewriteNullableScalarAnyOf(obj);
-	flattenExclusiveRequiredUnion(obj);
 	inferBareEnumScalarType(obj);
 	collapseConstUnionAnyOf(obj);
 	for (const k in obj) walk(obj[k]);
diff --git a/packages/ai/test/schema-normalization.test.ts b/packages/ai/test/schema-normalization.test.ts
index 872287cdf..60a5d79a3 100644
--- a/packages/ai/test/schema-normalization.test.ts
+++ b/packages/ai/test/schema-normalization.test.ts
@@ -630,6 +630,28 @@ describe("sanitizeSchemaForOpenAIResponses", () => {
 		});
 	});
 
+	it("does not flatten nested exclusive-required anyOf (xAI only rejects the tool root)", () => {
+		const schema = {
+			type: "object",
+			properties: {
+				outputSchema: {
+					type: "object",
+					properties: {
+						paths: { type: "array", items: { type: "string" } },
+						scopes: { type: "array", items: { type: "string" } },
+					},
+					anyOf: [{ required: ["paths"] }, { required: ["scopes"] }],
+				},
+			},
+			required: ["outputSchema"],
+		};
+		const sanitized = sanitizeSchemaForOpenAIResponses(schema);
+		expect(sanitized.anyOf).toBeUndefined();
+		const outputSchema = (sanitized.properties as Record<string, unknown>).outputSchema as Record<string, unknown>;
+		expect(outputSchema.anyOf).toEqual([{ required: ["paths"] }, { required: ["scopes"] }]);
+	});
+
+
 });
 
 // ---------------------------------------------------------------------------
diff --git a/packages/ai/test/schema-wire.test.ts b/packages/ai/test/schema-wire.test.ts
index 9e11bfa1a..17f92c3f7 100644
--- a/packages/ai/test/schema-wire.test.ts
+++ b/packages/ai/test/schema-wire.test.ts
@@ -105,6 +105,29 @@ describe("toolWireSchema — raw JSON Schema normalization", () => {
 		expect(wire.required).toEqual(["project"]);
 	});
 
+	it("does not flatten nested exclusive-required anyOf (only the tool root 400s xAI)", () => {
+		const wire = toolWireSchema(
+			jsonTool({
+				type: "object",
+				properties: {
+					outputSchema: {
+						type: "object",
+						properties: {
+							paths: { type: "array", items: { type: "string" } },
+							scopes: { type: "array", items: { type: "string" } },
+						},
+						anyOf: [{ required: ["paths"] }, { required: ["scopes"] }],
+					},
+				},
+				required: ["outputSchema"],
+			}),
+		);
+		expect(wire.anyOf).toBeUndefined();
+		const outputSchema = (wire.properties as Record<string, unknown>).outputSchema as Record<string, unknown>;
+		expect(outputSchema.anyOf).toEqual([{ required: ["paths"] }, { required: ["scopes"] }]);
+	});
+
+
 
 	it("preserves raw JSON Schema required defaults and safe-integer bounds", () => {
 		const wire = toolWireSchema(