refactor: use .passthrough() instead of looseObject for base types

Using .passthrough() instead of z.looseObject() because:
- looseObject adds [x: string]: unknown index signature to inferred type
- This breaks TypeScript union narrowing (can't check 'prop' in obj)
- .passthrough() allows extra properties at runtime without affecting type

Note: Capability schemas still use looseObject intentionally since they
need the extensibility semantic and don't participate in union narrowing.

The Infer<typeof> type exports are still needed because:
- spec.types.ts has index signatures for extensibility
- sdk.types.ts inherits these from spec
- Only schema-inferred types are clean (no index signatures)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ochafik

scripts/generate-schemas.ts

@@ -516,7 +516,12 @@ function postProcess(content: string): string {
 }
 
 /**
- * Transform z.record(z.string(), z.unknown()).and(z.object({...})) to z.looseObject({...})
+ * Transform z.record(z.string(), z.unknown()).and(z.object({...})) to z.object({...}).passthrough()
+ *
+ * Using .passthrough() instead of looseObject because:
+ * - looseObject adds [x: string]: unknown index signature to the inferred type
+ * - This breaks TypeScript union narrowing (can't check 'prop' in obj)
+ * - .passthrough() allows extra properties at runtime without affecting the type
  */
 function transformRecordAndToLooseObject(sourceFile: SourceFile): void {
     // Find all call expressions
@@ -541,9 +546,10 @@ function transformRecordAndToLooseObject(sourceFile: SourceFile): void {
         const objectLiteral = objectArgs[0];
         if (!Node.isObjectLiteralExpression(objectLiteral)) return;
 
-        // Replace with z.looseObject({...})
+        // Replace with z.object({...}).passthrough()
+        // This allows extra properties at runtime but doesn't add index signature to type
         const objectContent = objectLiteral.getText();
-        node.replaceWithText(`z.looseObject(${objectContent})`);
+        node.replaceWithText(`z.object(${objectContent}).passthrough()`);
     });
 }
 

src/generated/sdk.schemas.ts

@@ -58,7 +58,7 @@ export const RequestParamsSchema = z
     .object({
         /** @description See [General fields: `_meta`](/specification/draft/basic/index#meta) for notes on `_meta` usage. */
         _meta: z
-            .looseObject({
+            .object({
                 /** @description If specified, the caller is requesting out-of-band progress notifications for this request (as represented by notifications/progress). The value of this parameter is an opaque token that will be attached to any subsequent notifications. The receiver is not obligated to provide these notifications. */
                 progressToken: ProgressTokenSchema.optional().describe(
                     'If specified, the caller is requesting out-of-band progress notifications for this request (as represented by notifications/progress). The value of this parameter is an opaque token that will be attached to any subsequent notifications. The receiver is not obligated to provide these notifications.'
@@ -68,6 +68,7 @@ export const RequestParamsSchema = z
                     'If specified, this request is related to the provided task.'
                 )
             })
+            .passthrough()
             .optional()
             .describe('See [General fields: `_meta`](/specification/draft/basic/index#meta) for notes on `_meta` usage.')
     })
@@ -93,13 +94,15 @@ export const NotificationSchema = z.object({
 /**
  * @category Common Types
  */
-export const ResultSchema = z.looseObject({
-    /** @description See [General fields: `_meta`](/specification/draft/basic/index#meta) for notes on `_meta` usage. */
-    _meta: z
-        .record(z.string(), z.unknown())
-        .optional()
-        .describe('See [General fields: `_meta`](/specification/draft/basic/index#meta) for notes on `_meta` usage.')
-});
+export const ResultSchema = z
+    .object({
+        /** @description See [General fields: `_meta`](/specification/draft/basic/index#meta) for notes on `_meta` usage. */
+        _meta: z
+            .record(z.string(), z.unknown())
+            .optional()
+            .describe('See [General fields: `_meta`](/specification/draft/basic/index#meta) for notes on `_meta` usage.')
+    })
+    .passthrough();
 
 /**
  * @category Common Types
@@ -1991,9 +1994,11 @@ export const URLElicitationRequiredErrorSchema = JSONRPCErrorResponseSchema.omit
         error: ErrorSchema.and(
             z.object({
                 code: z.any(),
-                data: z.looseObject({
-                    elicitations: z.array(ElicitRequestURLParamsSchema)
-                })
+                data: z
+                    .object({
+                        elicitations: z.array(ElicitRequestURLParamsSchema)
+                    })
+                    .passthrough()
             })
         )
     })