diff --git a/tvx/builders/generator.go b/tvx/builders/generator.go
index a3e91e33c..1f7c32cdd 100644
--- a/tvx/builders/generator.go
+++ b/tvx/builders/generator.go
@@ -41,6 +41,14 @@ type Generator struct {
 	wg sync.WaitGroup
 }
 
+// genData is the generation data to stamp into vectors.
+// TODO in the future this should contain the commit of this tool and
+//  the builder api.
+var genData = schema.GenerationData{
+	Source:  "script",
+	Version: "v0",
+}
+
 type MessageVectorGenItem struct {
 	Metadata *schema.Metadata
 	Func     func(*Builder)
@@ -100,7 +108,7 @@ func (g *Generator) MessageVectorGroup(group string, vectors ...*MessageVectorGe
 				w = os.Stdout
 			} else {
 				file := filepath.Join(g.OutputPath, fmt.Sprintf("%s--%s.json", group, item.Metadata.ID))
-				out, err := os.OpenFile(file, os.O_WRONLY|os.O_CREATE, 0644)
+				out, err := os.OpenFile(file, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
 				if err != nil {
 					log.Printf("failed to write to file %s: %s", file, err)
 					return
@@ -122,6 +130,9 @@ func (g *Generator) MessageVectorGroup(group string, vectors ...*MessageVectorGe
 func (g *Generator) generateOne(w io.Writer, b *MessageVectorGenItem, indent bool) {
 	log.Printf("generating test vector: %s", b.Metadata.ID)
 
+	// stamp with our generation data.
+	b.Metadata.Gen = genData
+
 	vector := MessageVector(b.Metadata)
 
 	// TODO: currently if an assertion fails, we call os.Exit(1), which
diff --git a/tvx/schema/schema.go b/tvx/schema/schema.go
index 87d58078e..7083c2350 100644
--- a/tvx/schema/schema.go
+++ b/tvx/schema/schema.go
@@ -30,16 +30,16 @@ type Selector string
 // Metadata provides information on the generation of this test case
 type Metadata struct {
 	ID      string         `json:"id"`
-	Version string         `json:"version"`
-	Desc    string         `json:"description"`
-	Comment string         `json:"comment"`
+	Version string         `json:"version,omitempty"`
+	Desc    string         `json:"description,omitempty"`
+	Comment string         `json:"comment,omitempty"`
 	Gen     GenerationData `json:"gen"`
 }
 
 // GenerationData tags the source of this test case
 type GenerationData struct {
-	Source  string `json:"source"`
-	Version string `json:"version"`
+	Source  string `json:"source,omitempty"`
+	Version string `json:"version,omitempty"`
 }
 
 // StateTree represents a state tree within preconditions and postconditions.
@@ -92,7 +92,7 @@ func (beb *Base64EncodedBytes) UnmarshalJSON(v []byte) error {
 // TestVector is a single test case
 type TestVector struct {
 	Class    `json:"class"`
-	Selector `json:"selector"`
+	Selector `json:"selector,omitempty"`
 	Meta     *Metadata `json:"_meta"`
 
 	// CAR binary data to be loaded into the test environment, usually a CAR