diff --git a/pkg/ffapi/restfilter_json_builder.go b/pkg/ffapi/restfilter_json_builder.go
index d3919df..8ce4511 100644
--- a/pkg/ffapi/restfilter_json_builder.go
+++ b/pkg/ffapi/restfilter_json_builder.go
@@ -17,6 +17,9 @@
 package ffapi
 
 import (
+	"encoding/json"
+	"math/big"
+
 	"github.com/hyperledger-firefly/common/pkg/fftypes"
 )
 
@@ -121,6 +124,18 @@ func toSimpleValue(v any) SimpleFilterValue {
 	switch vt := v.(type) {
 	case string:
 		return SimpleFilterValue(vt)
+	case json.Number:
+		return SimpleFilterValue(vt)
+	case *big.Int:
+		if vt == nil {
+			return ""
+		}
+		return SimpleFilterValue(vt.String())
+	case *fftypes.FFBigInt:
+		if vt == nil {
+			return ""
+		}
+		return SimpleFilterValue(vt.String())
 	default:
 		// As the interface requires everything to be a string, we try and support a range
 		// of values using existing functions. Could be improved in the future.
diff --git a/pkg/ffapi/restfilter_json_builder_bigint_test.go b/pkg/ffapi/restfilter_json_builder_bigint_test.go
new file mode 100644
index 0000000..be334d5
--- /dev/null
+++ b/pkg/ffapi/restfilter_json_builder_bigint_test.go
@@ -0,0 +1,79 @@
+// Copyright © 2026 Kaleido, Inc.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ffapi
+
+import (
+	"encoding/json"
+	"math/big"
+	"testing"
+
+	"github.com/hyperledger-firefly/common/pkg/fftypes"
+	"github.com/stretchr/testify/assert"
+)
+
+const uint256Max = "115792089237316195423570985008687907853269984665640564039457584007913129639935"
+
+func mustBigInt(s string) *big.Int {
+	i, ok := new(big.Int).SetString(s, 10)
+	if !ok {
+		panic(s)
+	}
+	return i
+}
+
+// Values passed to the QueryBuilder go through toSimpleValue, which must render big number
+// types as their full decimal string rather than falling back to fftypes.JSONObject.GetString
+// (which only understands string/bool/float/int/uint, and returns "" for anything else).
+func TestQueryBuilderValueTypes(t *testing.T) {
+	for name, tc := range map[string]struct {
+		value    any
+		expected string
+	}{
+		// Controls - these already work
+		"string": {"abc", "abc"},
+		"int":    {42, "42"},
+		"uint64": {uint64(18446744073709551615), "18446744073709551615"},
+		"bool":   {true, "true"},
+
+		// Big number types
+		"*big.Int small":       {big.NewInt(42), "42"},
+		"*big.Int uint256 max": {mustBigInt(uint256Max), uint256Max},
+		"*fftypes.FFBigInt":    {(*fftypes.FFBigInt)(mustBigInt(uint256Max)), uint256Max},
+		"json.Number":          {json.Number("9007199254740993"), "9007199254740993"},
+	} {
+		t.Run(name, func(t *testing.T) {
+			// Single value ops (Equal, GreaterThan, ...)
+			q := NewQueryBuilder().Equal("amount", tc.value).Query()
+			assert.Equal(t, tc.expected, q.Eq[0].Value.String(), "Equal")
+
+			// Multi value ops (In, NotIn)
+			q = NewQueryBuilder().In("amount", []any{tc.value}).Query()
+			assert.Equal(t, tc.expected, q.In[0].Values[0].String(), "In")
+
+			// Value is `json:"value,omitempty"`, so an empty value would be dropped entirely
+			// when the query is serialized
+			b, err := json.Marshal(NewQueryBuilder().Equal("amount", tc.value).Query())
+			assert.NoError(t, err)
+			assert.JSONEq(t, `{"eq":[{"field":"amount","value":"`+tc.expected+`"}]}`, string(b), "JSON")
+		})
+	}
+}
+
+func TestQueryBuilderNilBigInts(t *testing.T) {
+	assert.Empty(t, toSimpleValue((*big.Int)(nil)))
+	assert.Empty(t, toSimpleValue((*fftypes.FFBigInt)(nil)))
+}