Reject ambiguous configuration and reference bindings

internal/core/config/file_config.go

@@ -3,6 +3,7 @@ package config
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"sort"
@@ -349,6 +350,12 @@ func ParseFileConfigYAML(data []byte) (FileConfig, error) {
 	if err := decoder.Decode(&fileCfg); err != nil {
 		return FileConfig{}, fmt.Errorf("decode yaml: %w", err)
 	}
+	var trailing any
+	if err := decoder.Decode(&trailing); err == nil {
+		return FileConfig{}, fmt.Errorf("config must contain exactly one YAML document")
+	} else if err != io.EOF {
+		return FileConfig{}, fmt.Errorf("decode trailing yaml document: %w", err)
+	}
 	return fileCfg, nil
 }
 

internal/core/config/file_config_contract_test.go

@@ -50,6 +50,28 @@ func TestFileConfigMinimalVersion4AppliesOverDefaults(t *testing.T) {
 	}
 }
 
+func TestParseFileConfigYAMLRejectsAdditionalDocuments(t *testing.T) {
+	tests := []struct {
+		name    string
+		source  string
+		wantErr bool
+	}{
+		{name: "trailing whitespace", source: "version: 4\n\n  \t", wantErr: false},
+		{name: "trailing comment", source: "version: 4\n# trailing comment\n", wantErr: false},
+		{name: "second valid document", source: "version: 4\n---\nversion: 4\n", wantErr: true},
+		{name: "second empty document", source: "version: 4\n---\n", wantErr: true},
+		{name: "second malformed document", source: "version: 4\n---\nversion: [\n", wantErr: true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := ParseFileConfigYAML([]byte(tt.source))
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("ParseFileConfigYAML() error = %v, want error=%t", err, tt.wantErr)
+			}
+		})
+	}
+}
+
 func TestFilePipelineLLMProfileIsPresenceAwareAndDetached(t *testing.T) {
 	const pipelineYAML = `version: 4
 pipelines: