diff --git a/internal/libevm/errs/errs.go b/internal/libevm/errs/errs.go
new file mode 100644
index 000000000000..0254537c255b
--- /dev/null
+++ b/internal/libevm/errs/errs.go
@@ -0,0 +1,93 @@
+// Copyright 2025 the libevm authors.
+//
+// The libevm additions to go-ethereum are free software: you can redistribute
+// them and/or modify them under the terms of the GNU Lesser General Public License
+// as published by the Free Software Foundation, either version 3 of the License,
+// or (at your option) any later version.
+//
+// The libevm additions are distributed in the hope that they will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser
+// General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see
+// <http://www.gnu.org/licenses/>.
+
+// Package errs provides a mechanism for [testing error semantics] through
+// unique identifiers, instead of depending on error messages that may result in
+// change-detector tests.
+//
+// [testing error semantics]: https://google.github.io/styleguide/go/decisions#test-error-semantics
+package errs
+
+import (
+	"errors"
+	"fmt"
+)
+
+// An ID is a distinct numeric identifier for an error. Any two errors with the
+// same ID will result in [errors.Is] returning true, regardless of their
+// messages.
+type ID int
+
+// An identifier performs ID comparison, for embedding in all error types to
+// provide their Is() method.
+type identifier struct {
+	id ID
+}
+
+func (id identifier) errorID() ID { return id.id }
+
+func (id identifier) Is(target error) bool {
+	t, ok := target.(interface{ errorID() ID })
+	if !ok {
+		return false
+	}
+	return t.errorID() == id.errorID()
+}
+
+func (id ID) asIdentifier() identifier { return identifier{id} }
+
+// WithID returns a new error with the ID and message.
+func WithID(id ID, msg string) error {
+	return noWrap{errors.New(msg), id.asIdentifier()}
+}
+
+type noWrap struct {
+	error
+	identifier
+}
+
+// WithIDf is the formatted equivalent of [WithID], supporting the same
+// wrapping semantics as [fmt.Errorf].
+func WithIDf(id ID, format string, a ...any) error {
+	switch err := fmt.Errorf(format, a...).(type) {
+	case singleWrapper:
+		return single{err, id.asIdentifier()}
+	case multiWrapper:
+		return multi{err, id.asIdentifier()}
+	default:
+		return noWrap{err, id.asIdentifier()}
+	}
+}
+
+type singleWrapper interface {
+	error
+	Unwrap() error
+}
+
+type single struct {
+	singleWrapper
+	identifier
+}
+
+type multiWrapper interface {
+	error
+	Unwrap() []error
+}
+
+type multi struct {
+	multiWrapper
+	identifier
+}
diff --git a/internal/libevm/errs/errs_test.go b/internal/libevm/errs/errs_test.go
new file mode 100644
index 000000000000..bfa6597475dd
--- /dev/null
+++ b/internal/libevm/errs/errs_test.go
@@ -0,0 +1,71 @@
+// Copyright 2025 the libevm authors.
+//
+// The libevm additions to go-ethereum are free software: you can redistribute
+// them and/or modify them under the terms of the GNU Lesser General Public License
+// as published by the Free Software Foundation, either version 3 of the License,
+// or (at your option) any later version.
+//
+// The libevm additions are distributed in the hope that they will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser
+// General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see
+// <http://www.gnu.org/licenses/>.
+
+package errs
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIs(t *testing.T) {
+	ids := []ID{0, 42}
+	errsByID := make(map[ID][]error)
+	for _, id := range ids {
+		errsByID[id] = []error{
+			WithID(id, "WithID()"),
+			WithIDf(id, "WithIDf() no wrapping"),
+			WithIDf(id, "WithIDf() wrap one %w", errors.New("x")),
+			WithIDf(id, "WithIDf() wrap multi %w + %w", errors.New("x"), errors.New("y")),
+		}
+	}
+
+	unidentified := []error{
+		errors.New("errors.New()"),
+		fmt.Errorf("fmt.Errorf()"),
+	}
+
+	for id, errs := range errsByID {
+		for _, err := range errs {
+			for targetID, targets := range errsByID {
+				want := id == targetID
+				for _, target := range targets {
+					assert.Equalf(t, want, errors.Is(err, target), "errors.Is(%v [ID %d], %v [ID %d])", err, id, target, targetID)
+				}
+			}
+
+			for _, target := range unidentified {
+				assert.NotErrorIsf(t, err, target, "error ID %d", id)
+			}
+		}
+	}
+}
+
+func Example() {
+	id42 := WithID(42, "hello")
+	alsoWithID42 := WithIDf(42, "%s", "world")
+	unidentified := errors.New("hello")
+
+	fmt.Println(errors.Is(id42, alsoWithID42))
+	fmt.Println(errors.Is(id42, unidentified))
+
+	// Output:
+	// true
+	// false
+}
diff --git a/params/json.libevm.go b/params/json.libevm.go
index 639bfe18f67e..376581b338f2 100644
--- a/params/json.libevm.go
+++ b/params/json.libevm.go
@@ -19,6 +19,8 @@ package params
 import (
 	"encoding/json"
 	"fmt"
+
+	"github.com/ava-labs/libevm/internal/libevm/errs"
 )
 
 var _ interface {
@@ -43,12 +45,22 @@ func (c *ChainConfig) UnmarshalJSON(data []byte) (err error) {
 	return UnmarshalChainConfigJSON(data, c, c.extra, ec.reuseJSONRoot)
 }
 
+// Internal error identifiers for precise testing.
+const (
+	errIDDecodeJSONIntoCombination errs.ID = iota
+	errIDDecodeJSONIntoExtra
+	errIDEncodeJSONCombination
+	errIDEncodeExtraToRawJSON
+	errIDEncodeDuplicateJSONKey
+	errIDNilExtra
+)
+
 // UnmarshalChainConfigJSON is equivalent to [ChainConfig.UnmarshalJSON]
 // had [Extras] with `C` been registered, but without the need to call
 // [RegisterExtras]. The `extra` argument MUST NOT be nil.
 func UnmarshalChainConfigJSON[C any](data []byte, config *ChainConfig, extra *C, reuseJSONRoot bool) (err error) {
 	if extra == nil {
-		return fmt.Errorf("%T argument is nil; use %T.UnmarshalJSON() directly", extra, config)
+		return errs.WithIDf(errIDNilExtra, "%T argument is nil; use %T.UnmarshalJSON() directly", extra, config)
 	}
 
 	if reuseJSONRoot {
@@ -56,7 +68,7 @@ func UnmarshalChainConfigJSON[C any](data []byte, config *ChainConfig, extra *C,
 			return fmt.Errorf("decoding JSON into %T: %s", config, err)
 		}
 		if err := json.Unmarshal(data, extra); err != nil {
-			return fmt.Errorf("decoding JSON into %T: %s", extra, err)
+			return errs.WithIDf(errIDDecodeJSONIntoExtra, "decoding JSON into %T: %s", extra, err)
 		}
 		return nil
 	}
@@ -69,7 +81,7 @@ func UnmarshalChainConfigJSON[C any](data []byte, config *ChainConfig, extra *C,
 		extra,
 	}
 	if err := json.Unmarshal(data, &combined); err != nil {
-		return fmt.Errorf(`decoding JSON into combination of %T and %T (as "extra" key): %s`, config, extra, err)
+		return errs.WithIDf(errIDDecodeJSONIntoCombination, `decoding JSON into combination of %T and %T (as "extra" key): %s`, config, extra, err)
 	}
 	return nil
 }
@@ -100,7 +112,7 @@ func MarshalChainConfigJSON[C any](config ChainConfig, extra C, reuseJSONRoot bo
 		}
 		data, err = json.Marshal(jsonExtra)
 		if err != nil {
-			return nil, fmt.Errorf(`encoding combination of %T and %T (as "extra" key) to JSON: %s`, config, extra, err)
+			return nil, errs.WithIDf(errIDEncodeJSONCombination, `encoding combination of %T and %T (as "extra" key) to JSON: %s`, config, extra, err)
 		}
 		return data, nil
 	}
@@ -116,13 +128,13 @@ func MarshalChainConfigJSON[C any](config ChainConfig, extra C, reuseJSONRoot bo
 	}
 	extraJSONRaw, err := toJSONRawMessages(extra)
 	if err != nil {
-		return nil, fmt.Errorf("converting extra config to JSON raw messages: %s", err)
+		return nil, errs.WithIDf(errIDEncodeExtraToRawJSON, "converting extra config to JSON raw messages: %s", err)
 	}
 
 	for k, v := range extraJSONRaw {
 		_, ok := configJSONRaw[k]
 		if ok {
-			return nil, fmt.Errorf("duplicate JSON key %q in ChainConfig and extra %T", k, extra)
+			return nil, errs.WithIDf(errIDEncodeDuplicateJSONKey, "duplicate JSON key %q in ChainConfig and extra %T", k, extra)
 		}
 		configJSONRaw[k] = v
 	}
diff --git a/params/json.libevm_test.go b/params/json.libevm_test.go
index 84b04b354278..5fa21318bd4d 100644
--- a/params/json.libevm_test.go
+++ b/params/json.libevm_test.go
@@ -13,6 +13,7 @@
 // You should have received a copy of the GNU Lesser General Public License
 // along with the go-ethereum library. If not, see
 // <http://www.gnu.org/licenses/>.
+
 package params
 
 import (
@@ -24,6 +25,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"github.com/ava-labs/libevm/internal/libevm/errs"
 	"github.com/ava-labs/libevm/libevm/pseudo"
 )
 
@@ -157,42 +159,33 @@ func TestUnmarshalChainConfigJSON_Errors(t *testing.T) {
 		jsonData      string // string for convenience
 		extra         *testExtra
 		reuseJSONRoot bool
-		wantConfig    ChainConfig
-		wantExtra     any
-		wantErrRegex  string
+		wantErrID     errs.ID
 	}{
 		"invalid_json": {
-			extra:        &testExtra{},
-			wantExtra:    &testExtra{},
-			wantErrRegex: `^decoding JSON into combination of \*.+\.ChainConfig and \*.+\.testExtra \(as "extra" key\): .+$`,
+			extra:     &testExtra{},
+			wantErrID: errIDDecodeJSONIntoCombination,
 		},
 		"nil_extra_at_root_depth": {
 			jsonData:      `{"chainId": 1}`,
 			extra:         nil,
 			reuseJSONRoot: true,
-			wantExtra:     (*testExtra)(nil),
-			wantErrRegex:  `^\*.+.testExtra argument is nil; use \*.+\.ChainConfig\.UnmarshalJSON\(\) directly$`,
+			wantErrID:     errIDNilExtra,
 		},
 		"nil_extra_at_extra_key": {
-			jsonData:     `{"chainId": 1}`,
-			extra:        nil,
-			wantExtra:    (*testExtra)(nil),
-			wantErrRegex: `^\*.+\.testExtra argument is nil; use \*.+\.ChainConfig.UnmarshalJSON\(\) directly$`,
+			jsonData:  `{"chainId": 1}`,
+			extra:     nil,
+			wantErrID: errIDNilExtra,
 		},
 		"wrong_extra_type_at_extra_key": {
-			jsonData:     `{"chainId": 1, "extra": 1}`,
-			extra:        &testExtra{},
-			wantConfig:   ChainConfig{ChainID: big.NewInt(1)},
-			wantExtra:    &testExtra{},
-			wantErrRegex: `^decoding JSON into combination of \*.+\.ChainConfig and \*.+\.testExtra \(as "extra" key\): .+$`,
+			jsonData:  `{"chainId": 1, "extra": 1}`,
+			extra:     &testExtra{},
+			wantErrID: errIDDecodeJSONIntoCombination,
 		},
 		"wrong_extra_type_at_root_depth": {
 			jsonData:      `{"chainId": 1, "field": 1}`,
 			extra:         &testExtra{},
 			reuseJSONRoot: true,
-			wantConfig:    ChainConfig{ChainID: big.NewInt(1)},
-			wantExtra:     &testExtra{},
-			wantErrRegex:  `^decoding JSON into \*.+\.testExtra: .+`,
+			wantErrID:     errIDDecodeJSONIntoExtra,
 		},
 	}
 
@@ -204,14 +197,7 @@ func TestUnmarshalChainConfigJSON_Errors(t *testing.T) {
 			data := []byte(testCase.jsonData)
 			config := ChainConfig{}
 			err := UnmarshalChainConfigJSON(data, &config, testCase.extra, testCase.reuseJSONRoot)
-			if testCase.wantErrRegex == "" {
-				require.NoError(t, err)
-			} else {
-				require.Error(t, err)
-				require.Regexp(t, testCase.wantErrRegex, err.Error())
-			}
-			assert.Equal(t, testCase.wantConfig, config)
-			assert.Equal(t, testCase.wantExtra, testCase.extra)
+			assert.ErrorIs(t, err, errs.WithID(testCase.wantErrID, ""))
 		})
 	}
 }
@@ -223,31 +209,28 @@ func TestMarshalChainConfigJSON_Errors(t *testing.T) {
 		config        ChainConfig
 		extra         any
 		reuseJSONRoot bool
-		wantJSONData  string // string for convenience
 		wantErrRegex  string
+		wantErrID     errs.ID
 	}{
 		"invalid_extra_at_extra_key": {
 			extra: struct {
 				Field chan struct{} `json:"field"`
 			}{},
-			wantErrRegex: `^encoding combination of .+\.ChainConfig and .+ to JSON: .+$`,
-		},
-		"nil_extra_at_extra_key": {
-			wantJSONData: `{"chainId":null}`,
+			wantErrID: errIDEncodeJSONCombination,
 		},
 		"invalid_extra_at_root_depth": {
 			extra: struct {
 				Field chan struct{} `json:"field"`
 			}{},
 			reuseJSONRoot: true,
-			wantErrRegex:  "^converting extra config to JSON raw messages: .+$",
+			wantErrID:     errIDEncodeExtraToRawJSON,
 		},
 		"duplicate_key": {
 			extra: struct {
 				Field string `json:"chainId"`
 			}{},
 			reuseJSONRoot: true,
-			wantErrRegex:  `^duplicate JSON key "chainId" in ChainConfig and extra struct .+$`,
+			wantErrID:     errIDEncodeDuplicateJSONKey,
 		},
 	}
 
@@ -257,14 +240,8 @@ func TestMarshalChainConfigJSON_Errors(t *testing.T) {
 			t.Parallel()
 
 			config := ChainConfig{}
-			data, err := MarshalChainConfigJSON(config, testCase.extra, testCase.reuseJSONRoot)
-			if testCase.wantErrRegex == "" {
-				require.NoError(t, err)
-			} else {
-				require.Error(t, err)
-				assert.Regexp(t, testCase.wantErrRegex, err.Error())
-			}
-			assert.Equal(t, testCase.wantJSONData, string(data))
+			_, err := MarshalChainConfigJSON(config, testCase.extra, testCase.reuseJSONRoot)
+			assert.ErrorIs(t, err, errs.WithID(testCase.wantErrID, ""))
 		})
 	}
 }