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fix: preserve JSON-looking strings in body mutations #2846

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fix: preserve JSON-looking strings in body mutations #2846

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b1b18aa
fix: preserve JSON-looking strings in body mutations
git-jxj Oct 13, 2026
75f12ff
Merge main to include the upstream MCP benchmark fix
git-jxj Oct 16, 2026
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48 changes: 2 additions & 46 deletions internal/bodymutator/body_mutator.go
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Original file line number Diff line number Diff line change
Expand Up @@ -7,8 +7,8 @@ package bodymutator

import (
"fmt"
"strings"

"github.com/tidwall/gjson"
"github.com/tidwall/sjson"

"github.com/envoyproxy/ai-gateway/internal/filterapi"
Expand Down Expand Up @@ -37,50 +37,6 @@ func (b *BodyMutator) HasMutations() bool {
return b != nil && b.bodyMutations != nil
}

// isJSONValue checks if a string represents a JSON value (not a plain string)
func isJSONValue(value string) bool {
value = strings.TrimSpace(value)

// Check for quoted strings (JSON strings)
if strings.HasPrefix(value, "\"") && strings.HasSuffix(value, "\"") {
return true
}

// Check for numbers (integers or floats)
if value == "0" || value == "true" || value == "false" || value == "null" {
return true
}

// Check for positive/negative numbers
if len(value) > 0 {
first := value[0]
if (first >= '0' && first <= '9') || first == '-' || first == '+' {
// Simple number check - contains only digits, dots, +, -, e, E
isNumber := true
for _, r := range value {
if (r < '0' || r > '9') && r != '.' && r != '-' && r != '+' && r != 'e' && r != 'E' {
isNumber = false
break
}
}
if isNumber {
return true
}
}
}

// Check for objects or arrays
if strings.HasPrefix(value, "{") && strings.HasSuffix(value, "}") {
return true
}
if strings.HasPrefix(value, "[") && strings.HasSuffix(value, "]") {
return true
}

// Default to plain string
return false
}

// Mutate mutates the request body based on the body mutations.
func (b *BodyMutator) Mutate(requestBody []byte) ([]byte, error) {
if b.bodyMutations == nil {
Expand Down Expand Up @@ -109,7 +65,7 @@ func (b *BodyMutator) Mutate(requestBody []byte) ([]byte, error) {
if field.Path != "" {
// Check value type to determine appropriate sjson method
// TODO handle JSON value check in configuration load time too.
if isJSONValue(field.Value) {
if gjson.Valid(field.Value) {
// Use SetRawBytes for JSON values (quoted strings, numbers, booleans, objects, arrays)
mutatedBody, err = sjson.SetRawBytesOptions(mutatedBody, field.Path, []byte(field.Value), &sjson.Options{ReplaceInPlace: true})
} else {
Expand Down
56 changes: 56 additions & 0 deletions internal/bodymutator/body_mutator_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -190,3 +190,59 @@ func TestBodyMutator_Mutate_InvalidJSONValue(t *testing.T) {
require.Equal(t, "not valid json but will be treated as string", result["service_tier"])
require.Equal(t, "valid", result["valid_field"])
}

func TestBodyMutator_Mutate_JSONLookingStrings(t *testing.T) {
for _, value := range []string{
"1.0.0",
"+1",
"01",
"1e",
"-",
`[abc]`,
`{"key":}`,
`"invalid\q"`,
"",
" ",
} {
t.Run(value, func(t *testing.T) {
mutator := NewBodyMutator(&filterapi.HTTPBodyMutation{
Set: []filterapi.HTTPBodyField{{Path: "metadata", Value: value}},
}, nil)
mutated, err := mutator.Mutate([]byte(`{"model":"test-model","messages":[]}`))
require.NoError(t, err)
var result map[string]interface{}
require.NoError(t, json.Unmarshal(mutated, &result), "mutation produced %s", mutated)
require.Equal(t, value, result["metadata"])
require.Equal(t, "test-model", result["model"])
})
}
}

func TestBodyMutator_Mutate_JSONLiterals(t *testing.T) {
for _, value := range []string{
`"quoted"`,
`42`,
`-0.7`,
`1e2`,
`true`,
`false`,
`null`,
`{"nested": [1, true, null]}`,
`["item", 2]`,
` {"key": "value"} `,
} {
t.Run(value, func(t *testing.T) {
mutator := NewBodyMutator(&filterapi.HTTPBodyMutation{
Set: []filterapi.HTTPBodyField{{Path: "metadata", Value: value}},
}, nil)
mutated, err := mutator.Mutate([]byte(`{"model":"test-model"}`))
require.NoError(t, err)
var result map[string]interface{}
require.NoError(t, json.Unmarshal(mutated, &result))
var expected interface{}
require.NoError(t, json.Unmarshal([]byte(value), &expected))
require.Equal(t, expected, result["metadata"])
require.Equal(t, "test-model", result["model"])
})
}
}
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