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mcp: validate user-provided output schemas (#408)

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toolForErr was ignoring the output schema if the output type was any.
That neglected the case where the user provided their own output schema.

Fixes #371.
This commit is contained in:
Jonathan Amsterdam 2025-09-04 14:04:27 -04:00 • committed by GitHub
commit 30261728f7
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1 changed files with 59 additions and 3 deletions
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4
mcp/server.go
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@ -212,7 +212,7 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHan
elemZero any // only non-nil if Out is a pointer type
outputResolved *jsonschema.Resolved
)
if reflect.TypeFor[Out]() != reflect.TypeFor[any]() {
if t.OutputSchema != nil || reflect.TypeFor[Out]() != reflect.TypeFor[any]() {
var err error
elemZero, err = setSchema[Out](&tt.OutputSchema, &outputResolved)
if err != nil {
@ -302,8 +302,8 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHan
// TODO(rfindley): we really shouldn't ever return 'null' results. Maybe we
// should have a jsonschema.Zero(schema) helper?
func setSchema[T any](sfield **jsonschema.Schema, rfield **jsonschema.Resolved) (zero any, err error) {
rt := reflect.TypeFor[T]()
if *sfield == nil {
rt := reflect.TypeFor[T]()
if rt.Kind() == reflect.Pointer {
rt = rt.Elem()
zero = reflect.Zero(rt).Interface()