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mcp: validate tool output (#352)

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Validate the handler's returned output against the output schema of a tool.

Fixes #301.
This commit is contained in:
Jonathan Amsterdam 2025-09-03 16:25:48 -04:00 • committed by GitHub
commit 8314ec099b
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1 changed files with 35 additions and 7 deletions
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14
mcp/tool.go
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@ -9,6 +9,7 @@ import (
"context"
"encoding/json"
"fmt"
// "log"
"github.com/google/jsonschema-go/jsonschema"
)
@ -42,13 +43,16 @@ func unmarshalSchema(data json.RawMessage, resolved *jsonschema.Resolved, v any)
if err := dec.Decode(v); err != nil {
return fmt.Errorf("unmarshaling: %w", err)
}
// TODO: test with nil args.
return validateSchema(resolved, v)
}
func validateSchema(resolved *jsonschema.Resolved, value any) error {
if resolved != nil {
if err := resolved.ApplyDefaults(v); err != nil {
return fmt.Errorf("applying defaults from \n\t%s\nto\n\t%s:\n%w", schemaJSON(resolved.Schema()), data, err)
if err := resolved.ApplyDefaults(value); err != nil {
return fmt.Errorf("applying defaults from \n\t%s\nto\n\t%v:\n%w", schemaJSON(resolved.Schema()), value, err)
}
if err := resolved.Validate(v); err != nil {
return fmt.Errorf("validating\n\t%s\nagainst\n\t %s:\n %w", data, schemaJSON(resolved.Schema()), err)
if err := resolved.Validate(value); err != nil {
return fmt.Errorf("validating\n\t%v\nagainst\n\t %s:\n %w", value, schemaJSON(resolved.Schema()), err)
}
}
return nil