From 0ff508bc0381b8a0e358e5a25e3a81aab0851cac Mon Sep 17 00:00:00 2001
From: Limehawk <128890849+limehawk@users.noreply.github.com>
Date: Fri, 30 Jan 2026 15:04:54 -0500
Subject: [PATCH 1/2] fix: default to JSON Schema draft-2020-12 for Zod v3
 schemas

---
 .changeset/fix-zod3-json-schema-2020-12.md    | 7 +++++++
 packages/core/src/util/zodJsonSchemaCompat.ts | 1 +
 2 files changed, 8 insertions(+)
 create mode 100644 .changeset/fix-zod3-json-schema-2020-12.md

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..249bc92d5 100644
--- a/packages/core/src/util/zodJsonSchemaCompat.ts
+++ b/packages/core/src/util/zodJsonSchemaCompat.ts
@@ -39,6 +39,7 @@ export function toJsonSchemaCompat(schema: AnyObjectSchema, opts?: CommonOpts):
 
     // v3 branch — use vendored converter
     return zodToJsonSchema(schema as z3.ZodTypeAny, {
+        target: 'jsonSchema2020-12',
         strictUnions: opts?.strictUnions ?? true,
         pipeStrategy: opts?.pipeStrategy ?? 'input'
     }) as JsonSchema;

From 74cb8ced017ef8bd147c7d4f0b2bdb4510b21230 Mon Sep 17 00:00:00 2001
From: Limehawk <128890849+limehawk@users.noreply.github.com>
Date: Fri, 30 Jan 2026 15:46:09 -0500
Subject: [PATCH 2/2] fix: add type cast for jsonSchema2020-12 target

---
 packages/core/src/util/zodJsonSchemaCompat.ts | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/packages/core/src/util/zodJsonSchemaCompat.ts b/packages/core/src/util/zodJsonSchemaCompat.ts
index 249bc92d5..54bad03bd 100644
--- a/packages/core/src/util/zodJsonSchemaCompat.ts
+++ b/packages/core/src/util/zodJsonSchemaCompat.ts
@@ -38,8 +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',
+        target: 'jsonSchema2020-12' as 'jsonSchema2019-09',
         strictUnions: opts?.strictUnions ?? true,
         pipeStrategy: opts?.pipeStrategy ?? 'input'
     }) as JsonSchema;