Preserve large-integer precision when decoding tool arguments

agent/format/jsonformat/encoding.go

@@ -3,8 +3,10 @@
 package jsonformat
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
+	"io"
 
 	"github.com/google/jsonschema-go/jsonschema"
 )
@@ -69,9 +71,18 @@ func (f *Format) Normalize(v any) error {
 func applySchema(data json.RawMessage, resolved *jsonschema.Resolved) (json.RawMessage, error) {
 	var v any
 	if len(data) > 0 {
-		if err := json.Unmarshal(data, &v); err != nil {
+		// Decode with UseNumber so integers beyond 2^53 are not silently
+		// truncated by being decoded into float64 and re-marshalled.
+		dec := json.NewDecoder(bytes.NewReader(data))
+		dec.UseNumber()
+		if err := dec.Decode(&v); err != nil {
 			return nil, fmt.Errorf("unmarshaling arguments: %w", err)
 		}
+		// Unlike json.Unmarshal, json.Decoder tolerates trailing data after the
+		// first value; reject it so validation is not looser than before.
+		if _, err := dec.Token(); err != io.EOF {
+			return nil, fmt.Errorf("unmarshaling arguments: unexpected trailing data after JSON value")
+		}
 	}
 	if err := resolved.ApplyDefaults(&v); err != nil {
 		return nil, fmt.Errorf("applying schema defaults: %w", err)

agent/format/jsonformat/encoding_test.go

@@ -133,3 +133,30 @@ func TestNormalizeEmptyStruct(t *testing.T) {
 		t.Fatalf("Normalize: %v", err)
 	}
 }
+
+// Integer arguments beyond 2^53 must survive Unmarshal: decoding through an
+// interface{} as float64 would silently truncate them.
+func TestFormat_Unmarshal_PreservesLargeIntegerPrecision(t *testing.T) {
+	type output struct {
+		N int64 `json:"n"`
+	}
+	format := requireFormat(t, jsonformat.MustFor[output]())
+	var out output
+	if err := format.Unmarshal([]byte(`{"n":9007199254740993}`), &out); err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if out.N != 9007199254740993 {
+		t.Errorf("N = %d, want 9007199254740993 (large-integer precision lost)", out.N)
+	}
+}
+
+func TestFormat_Unmarshal_RejectsTrailingData(t *testing.T) {
+	type output struct {
+		N int `json:"n"`
+	}
+	format := requireFormat(t, jsonformat.MustFor[output]())
+	var out output
+	if err := format.Unmarshal([]byte(`{"n":1} {"n":2}`), &out); err == nil {
+		t.Fatal("expected an error for trailing data after the JSON value, got nil")
+	}
+}