refactor: add isCompletable type guard for cleaner usage

Encapsulates the type checking logic in a dedicated type guard function,
making the code more readable and maintainable compared to verbose
inline type assertions.

The type guard properly narrows the TypeScript type to CompletableSchema<T>,
allowing direct access to _def.complete without additional casting.

Author: felixweinberger

src/server/completable.ts

@@ -22,6 +22,14 @@ export interface CompletableDef<T extends ZodTypeAny = ZodTypeAny> {
  */
 export type CompletableSchema<T extends ZodTypeAny> = T & { _def: T['_def'] & CompletableDef<T> };
 
+/**
+ * Type guard to check if a schema has been wrapped with completion capabilities.
+ */
+export function isCompletable<T extends ZodTypeAny>(schema: ZodTypeAny): schema is CompletableSchema<T> {
+    const defLike = (schema as any)._def;
+    return defLike?.typeName === McpZodTypeKind.Completable;
+}
+
 /**
  * Wraps a Zod type to provide autocompletion capabilities. Useful for, e.g., prompt arguments in MCP.
  *

src/server/mcp.ts

@@ -32,7 +32,7 @@ import {
     ToolAnnotations,
     LoggingMessageNotification
 } from '../types.js';
-import { CompletableDef, McpZodTypeKind } from './completable.js';
+import { CompletableDef, isCompletable } from './completable.js';
 import { UriTemplate, Variables } from '../shared/uriTemplate.js';
 import { RequestHandlerExtra } from '../shared/protocol.js';
 import { Transport } from '../shared/transport.js';
@@ -238,13 +238,11 @@ export class McpServer {
         }
 
         const field = prompt.argsSchema.shape[request.params.argument.name];
-        const defLike = (field as unknown as { _def?: { typeName?: unknown } })._def;
-        if (!defLike || defLike.typeName !== McpZodTypeKind.Completable) {
+        if (!isCompletable<ZodString>(field)) {
             return EMPTY_COMPLETION_RESULT;
         }
 
-        const def: CompletableDef<ZodString> = (field as unknown as { _def: CompletableDef<ZodString> })._def;
-        const suggestions = await def.complete(request.params.argument.value, request.params.context);
+        const suggestions = await field._def.complete(request.params.argument.value, request.params.context);
         return createCompletionResult(suggestions);
     }