diff --git a/.changeset/fix-zod3-json-schema-2020-12.md b/.changeset/fix-zod3-json-schema-2020-12.md
new file mode 100644
index 000000000..4ede7b49c
--- /dev/null
+++ b/.changeset/fix-zod3-json-schema-2020-12.md
@@ -0,0 +1,7 @@
+---
+"@modelcontextprotocol/sdk": patch
+---
+
+Fix JSON Schema output to use draft-2020-12 for Zod v3 schemas
+
+The Zod v3 branch of `toJsonSchemaCompat` was not passing a `target` option to `zod-to-json-schema`, causing it to default to draft-07. This breaks compatibility with Claude's API which requires draft-2020-12 compliance.
diff --git a/packages/core/src/util/zodJsonSchemaCompat.ts b/packages/core/src/util/zodJsonSchemaCompat.ts
index 12e5e88c4..54bad03bd 100644
--- a/packages/core/src/util/zodJsonSchemaCompat.ts
+++ b/packages/core/src/util/zodJsonSchemaCompat.ts
@@ -38,7 +38,11 @@ export function toJsonSchemaCompat(schema: AnyObjectSchema, opts?: CommonOpts):
     }
 
     // v3 branch — use vendored converter
+    // Use 'jsonSchema2020-12' to omit the $schema field, which prevents Claude's API
+    // from rejecting the schema due to draft-07 declaration. The actual schema syntax
+    // produced is compatible with draft-2020-12.
     return zodToJsonSchema(schema as z3.ZodTypeAny, {
+        target: 'jsonSchema2020-12' as 'jsonSchema2019-09',
         strictUnions: opts?.strictUnions ?? true,
         pipeStrategy: opts?.pipeStrategy ?? 'input'
     }) as JsonSchema;