diff --git a/scripts/generate-schemas.ts b/scripts/generate-schemas.ts
index 858ab914..109f3524 100644
--- a/scripts/generate-schemas.ts
+++ b/scripts/generate-schemas.ts
@@ -8,10 +8,15 @@
  *
  * ts-to-zod is a powerful tool but has limitations that require post-processing:
  *
- * ### 1. Zod Import Path (keep standard `"zod"` for version agnosticism)
+ * ### 1. Zod Import Path (`"zod"` → `"zod/v4"`)
  *
- * ts-to-zod generates `import { z } from "zod"` which works with both
- * Zod v3.25+ and v4. We keep this standard import to support both versions.
+ * ts-to-zod generates `import { z } from "zod"`. We rewrite this to
+ * `import { z } from "zod/v4"` because the generated schemas compose with
+ * schemas imported from `@modelcontextprotocol/sdk/types.js`, which the SDK
+ * constructs via `zod/v4`. Mixing schemas from the v3 and v4 APIs at runtime
+ * fails with errors like `keyValidator._parse is not a function` (v3 internals
+ * calling into v4 objects, or vice versa). The `zod/v4` subpath is exported by
+ * both zod 3.25+ and zod 4.x, so the peerDependency range is preserved.
  *
  * ### 2. External Type References (`z.any()` → actual schemas)
  *
@@ -179,11 +184,13 @@ async function generateJsonSchema() {
  * Post-process generated schemas for project compatibility.
  */
 function postProcess(content: string): string {
-  // 1. Add MCP SDK schema imports (keep standard zod import for v3/v4 compatibility)
+  // 1. Rewrite to zod/v4 and add MCP SDK schema imports.
+  // zod/v4 aligns with the SDK's own zod import — composing v3 and v4
+  // schema instances throws at parse time. See header comment for details.
   const mcpImports = EXTERNAL_TYPE_SCHEMAS.join(",\n  ");
   content = content.replace(
     'import { z } from "zod";',
-    `import { z } from "zod";
+    `import { z } from "zod/v4";
 import {
   ${mcpImports},
 } from "@modelcontextprotocol/sdk/types.js";`,
@@ -257,7 +264,12 @@ function replaceRecordAndWithPassthrough(content: string): string {
  * Post-process generated integration tests.
  */
 function postProcessTests(content: string): string {
-  // Keep standard zod import for v3/v4 compatibility
+  // Rewrite to zod/v4 so z.infer matches the v4 schemas in schema.ts.
+  content = content.replace(
+    'import { z } from "zod";',
+    'import { z } from "zod/v4";',
+  );
+
   content = content.replace(
     "// Generated by ts-to-zod",
     `// Generated by ts-to-zod
diff --git a/src/generated/schema.test.ts b/src/generated/schema.test.ts
index 47e59788..cebad70b 100644
--- a/src/generated/schema.test.ts
+++ b/src/generated/schema.test.ts
@@ -1,7 +1,7 @@
 // Generated by ts-to-zod
 // Integration tests verifying schemas match TypeScript types
 // Run: npm run generate:schemas
-import { z } from "zod";
+import { z } from "zod/v4";
 
 import * as spec from "../spec.types.js";
 import * as generated from "./schema.js";
diff --git a/src/generated/schema.ts b/src/generated/schema.ts
index 88797b50..8eb12b5a 100644
--- a/src/generated/schema.ts
+++ b/src/generated/schema.ts
@@ -1,7 +1,7 @@
 // Generated by ts-to-zod
 // Post-processed for Zod v3/v4 compatibility and MCP SDK integration
 // Run: npm run generate:schemas
-import { z } from "zod";
+import { z } from "zod/v4";
 import {
   ContentBlockSchema,
   CallToolResultSchema,