fix(tools): strip xAI-unsupported JSON Schema keywords from tool definitions

xAI rejects minLength, maxLength, minItems, maxItems, minContains, and
maxContains in tool schemas with a 502 error instead of ignoring them.
This causes all requests to fail when any tool definition includes these
validation-constraint keywords (e.g. sessions_spawn uses maxLength and
maxItems on its attachment fields).

Add stripXaiUnsupportedKeywords() in schema/clean-for-xai.ts, mirroring
the existing cleanSchemaForGemini() pattern. Apply it in normalizeToolParameters()
when the provider is xai directly, or openrouter with an x-ai/* model id.

Fixes tool calls for x-ai/grok-* models both direct and via OpenRouter.

src/agents/pi-tools.schema.ts

@@ -1,5 +1,6 @@
 import type { AnyAgentTool } from "./pi-tools.types.js";
 import { cleanSchemaForGemini } from "./schema/clean-for-gemini.js";
+import { isXaiProvider, stripXaiUnsupportedKeywords } from "./schema/clean-for-xai.js";
 
 function extractEnumValues(schema: unknown): unknown[] | undefined {
   if (!schema || typeof schema !== "object") {
@@ -64,7 +65,7 @@ function mergePropertySchemas(existing: unknown, incoming: unknown): unknown {
 
 export function normalizeToolParameters(
   tool: AnyAgentTool,
-  options?: { modelProvider?: string },
+  options?: { modelProvider?: string; modelId?: string },
 ): AnyAgentTool {
   const schema =
     tool.parameters && typeof tool.parameters === "object"
@@ -79,6 +80,7 @@ export function normalizeToolParameters(
   // - OpenAI rejects function tool schemas unless the *top-level* is `type: "object"`.
   //   (TypeBox root unions compile to `{ anyOf: [...] }` without `type`).
   // - Anthropic expects full JSON Schema draft 2020-12 compliance.
+  // - xAI rejects validation-constraint keywords (minLength, maxLength, etc.) outright.
   //
   // Normalize once here so callers can always pass `tools` through unchanged.
 
@@ -86,13 +88,24 @@ export function normalizeToolParameters(
     options?.modelProvider?.toLowerCase().includes("google") ||
     options?.modelProvider?.toLowerCase().includes("gemini");
   const isAnthropicProvider = options?.modelProvider?.toLowerCase().includes("anthropic");
+  const isXai = isXaiProvider(options?.modelProvider, options?.modelId);
+
+  function applyProviderCleaning(s: unknown): unknown {
+    if (isGeminiProvider && !isAnthropicProvider) {
+      return cleanSchemaForGemini(s);
+    }
+    if (isXai) {
+      return stripXaiUnsupportedKeywords(s);
+    }
+    return s;
+  }
 
   // If schema already has type + properties (no top-level anyOf to merge),
-  // clean it for Gemini compatibility (but only if using Gemini, not Anthropic)
+  // clean it for Gemini/xAI compatibility as appropriate.
   if ("type" in schema && "properties" in schema && !Array.isArray(schema.anyOf)) {
     return {
       ...tool,
-      parameters: isGeminiProvider && !isAnthropicProvider ? cleanSchemaForGemini(schema) : schema,
+      parameters: applyProviderCleaning(schema),
     };
   }
 
@@ -107,10 +120,7 @@ export function normalizeToolParameters(
     const schemaWithType = { ...schema, type: "object" };
     return {
       ...tool,
-      parameters:
-        isGeminiProvider && !isAnthropicProvider
-          ? cleanSchemaForGemini(schemaWithType)
-          : schemaWithType,
+      parameters: applyProviderCleaning(schemaWithType),
     };
   }
 
@@ -184,10 +194,7 @@ export function normalizeToolParameters(
     // - OpenAI rejects schemas without top-level `type: "object"`.
     // - Anthropic accepts proper JSON Schema with constraints.
     // Merging properties preserves useful enums like `action` while keeping schemas portable.
-    parameters:
-      isGeminiProvider && !isAnthropicProvider
-        ? cleanSchemaForGemini(flattenedSchema)
-        : flattenedSchema,
+    parameters: applyProviderCleaning(flattenedSchema),
   };
 }