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mcp: enforce input schema type "object" (#349)

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The spec makes it clear that input schemas must have type "object".
Enforce that.
Allow "any" as an input argument type by special-casing it.

Fixes #283.
This commit is contained in:
Jonathan Amsterdam 2025-08-22 11:04:44 -04:00 • committed by GitHub
commit 18c96d6912
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1 changed files with 18 additions and 9 deletions
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9
mcp/server.go
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@ -162,6 +162,9 @@ func (s *Server) AddTool(t *Tool, h ToolHandler) {
// discovered until runtime, when the LLM sent bad data.
panic(fmt.Errorf("AddTool %q: missing input schema", t.Name))
}
if t.InputSchema.Type != "object" {
panic(fmt.Errorf(`AddTool %q: input schema must have type "object"`, t.Name))
}
st := &serverTool{tool: t, handler: h}
// Assume there was a change, since add replaces existing tools.
// (It's possible a tool was replaced with an identical one, but not worth checking.)
@ -190,6 +193,12 @@ func ToolFor[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHandle
// TODO(v0.3.0): test
func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHandler, error) {
tt := *t
// Special handling for an "any" input: treat as an empty object.
if reflect.TypeFor[In]() == reflect.TypeFor[any]() && t.InputSchema == nil {
tt.InputSchema = &jsonschema.Schema{Type: "object"}
}
var inputResolved *jsonschema.Resolved
if _, err := setSchema[In](&tt.InputSchema, &inputResolved); err != nil {
return nil, nil, fmt.Errorf("input schema: %w", err)