From c86500ec501cd9d5690d006546585dc322628b20 Mon Sep 17 00:00:00 2001
From: Sheng Kun Chang <nothingchang@mirrorstack.ai>
Date: Tue, 16 Jun 2026 00:09:12 +0800
Subject: [PATCH] fix(mcp): inline tool schemas so inputSchema is type:object

deriveSchema used jsonschema.Reflect's default, which emits a top-level
{"$ref":"#/$defs/..."} with the object schema nested in $defs. The MCP
spec requires inputSchema/outputSchema to be an object schema (type:"object"),
and clients reject the ENTIRE tools/list when any tool's top-level schema is a
$ref. Setting DoNotReference inlines the struct schema as a concrete
{"type":"object",...}.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 internal/core/mcp.go | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/internal/core/mcp.go b/internal/core/mcp.go
index 4057c87..3037243 100644
--- a/internal/core/mcp.go
+++ b/internal/core/mcp.go
@@ -104,7 +104,12 @@ func MCPResource[Out any](name, description string, handler func(ctx context.Con
 
 func deriveSchema[T any]() (json.RawMessage, error) {
 	var zero T
-	return json.Marshal(jsonschema.Reflect(zero))
+	// DoNotReference inlines the struct schema so the top level is a concrete
+	// {"type":"object",...} rather than invopop's default {"$ref":"#/$defs/..."}.
+	// The MCP spec requires inputSchema/outputSchema to be an object schema with
+	// type:"object"; a top-level $ref makes clients reject the whole tools/list.
+	r := &jsonschema.Reflector{DoNotReference: true}
+	return json.Marshal(r.Reflect(zero))
 }
 
 // wrapMCPToolHandler adapts a typed handler into the type-erased registry form.