Reject unknown configuration fields

internal/config/config_test.go

@@ -63,6 +63,24 @@ func TestLoadUnknownSchemaVersion(t *testing.T) {
 	}
 }
 
+func TestLoadRejectsUnknownFields(t *testing.T) {
+	content := `{"version":1,"default":{"baseBranch":"main"}}`
+	f, err := os.CreateTemp("", "taskcapsule-*.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(f.Name())
+	if _, err := f.Write([]byte(content)); err != nil {
+		t.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := Load(f.Name()); err == nil {
+		t.Fatal("expected unknown configuration field to be rejected")
+	}
+}
+
 func TestLoadDuplicateService(t *testing.T) {
 	content := `{
 		"version": 1,

internal/config/load.go

@@ -1,8 +1,10 @@
 package config
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
 )
 
@@ -13,7 +15,15 @@ func Load(path string) (*Config, error) {
 	}
 
 	var cfg Config
-	if err := json.Unmarshal(data, &cfg); err != nil {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+	decoder.DisallowUnknownFields()
+	if err := decoder.Decode(&cfg); err != nil {
+		return nil, fmt.Errorf("invalid configuration: %v", err)
+	}
+	if err := decoder.Decode(&struct{}{}); err != io.EOF {
+		if err == nil {
+			err = fmt.Errorf("multiple JSON values")
+		}
 		return nil, fmt.Errorf("invalid configuration: %v", err)
 	}