fix: inline local $ref in tool inputSchema for LLM consumption

.changeset/inline-ref-in-tool-schema.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/core': patch
+---
+
+Inline local `$ref` pointers in tool `inputSchema` so schemas are self-contained and LLM-consumable. LLMs cannot resolve JSON Schema `$ref` — they serialize referenced parameters as strings instead of objects. Recursive schemas are handled gracefully — cyclic `$ref` pointers are left in place with only their `$defs` entries preserved, while all non-cyclic refs are fully inlined.

packages/core/src/util/schema.ts

@@ -20,6 +20,122 @@ export type AnyObjectSchema = z.core.$ZodObject;
  */
 export type SchemaOutput<T extends AnySchema> = z.output<T>;
 
+/**
+ * Resolves all local `$ref` pointers in a JSON Schema by inlining the
+ * referenced definitions.
+ *
+ * - Caches resolved defs to avoid redundant work with diamond references
+ *   (A→B→D, A→C→D — D is resolved once and reused).
+ * - Gracefully handles cycles — cyclic `$ref` are left in place with their
+ *   `$defs` entries preserved. Non-cyclic refs in the same schema are still
+ *   fully inlined. This avoids breaking existing servers that have recursive
+ *   schemas which work (degraded) today.
+ * - Preserves sibling keywords alongside `$ref` per JSON Schema 2020-12
+ *   (e.g. `{ "$ref": "...", "description": "override" }`).
+ *
+ * @internal Exported for testing only.
+ */
+export function dereferenceLocalRefs(schema: Record<string, unknown>): Record<string, unknown> {
+    // "$defs" is the standard keyword since JSON Schema 2019-09.
+    // See: https://json-schema.org/draft/2020-12/json-schema-core#section-8.2.4
+    // "definitions" is the legacy equivalent from drafts 04–07.
+    // See: https://json-schema.org/draft-07/json-schema-validation#section-9
+    // If both exist (malformed schema), "$defs" takes precedence.
+    const defsKey = '$defs' in schema ? '$defs' : 'definitions' in schema ? 'definitions' : undefined;
+    const defs: Record<string, unknown> = defsKey ? (schema[defsKey] as Record<string, unknown>) : {};
+
+    // No definitions container — nothing to inline.
+    // Note: $ref: "#" (root self-reference) is intentionally not handled — no schema
+    // library produces it, no other MCP SDK handles it, and it's always cyclic.
+    if (!defsKey) return schema;
+
+    // Cache resolved defs to avoid redundant traversal on diamond references
+    // (A→B→D, A→C→D — D is resolved once and reused). Cached values are shared
+    // by reference, which is safe because schemas are immutable after generation.
+    const resolvedDefs = new Map<string, unknown>();
+    // Def names where a cycle was detected — these $ref are left in place
+    // and their $defs entries must be preserved in the output.
+    const cyclicDefs = new Set<string>();
+
+    /**
+     * Recursively inlines `$ref` pointers in a JSON Schema node by replacing
+     * them with the referenced definition content.
+     *
+     * @param node - The current schema node being traversed.
+     * @param stack - Def names currently being inlined (ancestor chain). If a
+     *   def is encountered while already on the stack, it's a cycle — the
+     *   `$ref` is left in place and the def name is added to `cyclicDefs`.
+     */
+    function inlineRefs(node: unknown, stack: Set<string>): unknown {
+        if (node === null || typeof node !== 'object') return node;
+        if (Array.isArray(node)) return node.map(item => inlineRefs(item, stack));
+
+        const obj = node as Record<string, unknown>;
+
+        // JSON Schema 2020-12 allows keywords alongside $ref (e.g. description, default).
+        // Destructure to get the ref target and any sibling keywords to merge later.
+        const { $ref: ref, ...siblings } = obj;
+        if (typeof ref === 'string') {
+            const hasSiblings = Object.keys(siblings).length > 0;
+
+            let resolved: unknown;
+
+            // Local definition reference: #/$defs/Name or #/definitions/Name
+            const prefix = `#/${defsKey}/`;
+            if (!ref.startsWith(prefix)) return obj; // Non-local $ref (external URL, etc.) — leave as-is
+
+            const defName = ref.slice(prefix.length);
+            const def = defs[defName];
+            if (def === undefined) return obj; // Unknown def — leave as-is
+            if (stack.has(defName)) {
+                cyclicDefs.add(defName);
+                return obj; // Cycle — leave $ref in place
+            }
+
+            if (resolvedDefs.has(defName)) {
+                resolved = resolvedDefs.get(defName);
+            } else {
+                stack.add(defName);
+                resolved = inlineRefs(def, stack);
+                stack.delete(defName);
+                resolvedDefs.set(defName, resolved);
+            }
+
+            // Merge sibling keywords onto the resolved definition.
+            // Note: boolean JSON Schemas (true/false) skip this merge — siblings are dropped.
+            // This is acceptable: the SDK's JsonSchemaType excludes boolean schemas by design,
+            // and no schema library (Zod v4, ArkType, Valibot) produces boolean $defs entries.
+            if (hasSiblings && resolved !== null && typeof resolved === 'object' && !Array.isArray(resolved)) {
+                const resolvedSiblings = Object.fromEntries(Object.entries(siblings).map(([k, v]) => [k, inlineRefs(v, stack)]));
+                return { ...(resolved as Record<string, unknown>), ...resolvedSiblings };
+            }
+            return resolved;
+        }
+
+        // Regular object — recurse into values, skipping root-level $defs container
+        const result: Record<string, unknown> = {};
+        for (const [key, value] of Object.entries(obj)) {
+            if (obj === schema && (key === '$defs' || key === 'definitions')) continue;
+            result[key] = inlineRefs(value, stack);
+        }
+        return result;
+    }
+
+    const resolved = inlineRefs(schema, new Set()) as Record<string, unknown>;
+
+    // Re-attach $defs only for cyclic definitions, using their resolved/cached
+    // versions so that any non-cyclic refs inside them are already inlined.
+    if (defsKey && cyclicDefs.size > 0) {
+        const prunedDefs: Record<string, unknown> = {};
+        for (const name of cyclicDefs) {
+            prunedDefs[name] = resolvedDefs.get(name) ?? defs[name];
+        }
+        resolved[defsKey] = prunedDefs;
+    }
+
+    return resolved;
+}
+
 /**
  * Parses data against a Zod schema (synchronous).
  * Returns a discriminated union with success/error.

packages/core/src/util/standardSchema.ts

@@ -6,6 +6,8 @@
 
 /* eslint-disable @typescript-eslint/no-namespace */
 
+import { dereferenceLocalRefs } from './schema.js';
+
 // Standard Schema interfaces — vendored from https://standardschema.dev (spec v1, Jan 2025)
 
 export interface StandardTypedV1<Input = unknown, Output = Input> {
@@ -156,7 +158,7 @@ export function standardSchemaToJsonSchema(schema: StandardJSONSchemaV1, io: 'in
                 `Wrap your schema in z.object({...}) or equivalent.`
         );
     }
-    return { type: 'object', ...result };
+    return dereferenceLocalRefs({ type: 'object', ...result });
 }
 
 // Validation