diff --git a/internal/testrun/testrun.go b/internal/testrun/testrun.go
index ce6f188..57cc027 100644
--- a/internal/testrun/testrun.go
+++ b/internal/testrun/testrun.go
@@ -40,6 +40,9 @@ type Options struct {
 	Arm string
 	// Generator selects the action picker: "llm" or the default seeded picker.
 	Generator string
+	// LabelSource selects how candidates are named to the model picker, and is
+	// recorded in meta.json as part of the run's cell.
+	LabelSource string
 
 	// iosUDID, iosIsSimulator, and iosCoreDeviceID are filled by Execute after
 	// resolving the iOS target, then read by buildDriver to choose the simulator
@@ -66,6 +69,7 @@ func buildRunMeta(options Options, bundleSHA256 string, seed int64, host string,
 		SanderlingVersion: "0.0.1",
 		Arm:               options.Arm,
 		Generator:         options.Generator,
+		LabelSource:       options.LabelSource,
 		MaxSteps:          options.MaxSteps,
 		DurationMillis:    options.Duration.Milliseconds(),
 		Host:              host,
@@ -198,6 +202,7 @@ func Execute(ctx context.Context, options Options, stdout io.Writer) error {
 		TraceWriter: traceWriter,
 		Logger:      newProgressLogger(stdout),
 		Generator:   options.Generator,
+		LabelSource: options.LabelSource,
 	})
 
 	terminateCtx, terminateCancel := context.WithTimeout(context.Background(), 5*time.Second)
diff --git a/internal/testrun/testrun_test.go b/internal/testrun/testrun_test.go
index 2df82c4..6be7777 100644
--- a/internal/testrun/testrun_test.go
+++ b/internal/testrun/testrun_test.go
@@ -187,13 +187,14 @@ func TestResolveSpecAPIPath_ReturnsEmptyWhenMissing(t *testing.T) {
 
 func TestBuildRunMeta_RecordsArmMembership(t *testing.T) {
 	options := Options{
-		Spec:      "spec.ts",
-		BundleID:  "com.example",
-		Platform:  "android",
-		Duration:  3 * time.Minute,
-		MaxSteps:  300,
-		Arm:       "llm-visible-text",
-		Generator: "llm",
+		Spec:        "spec.ts",
+		BundleID:    "com.example",
+		Platform:    "android",
+		Duration:    3 * time.Minute,
+		MaxSteps:    300,
+		Arm:         "llm-visible-text",
+		Generator:   "llm",
+		LabelSource: verifier.LabelSourceVisibleText,
 	}
 	meta := buildRunMeta(options, "deadbeef", 7, "farm-01",
 		verifier.LLMConfig{Model: "claude-sonnet-5", Instructions: "exercise the outbox"}, true)
@@ -201,6 +202,9 @@ func TestBuildRunMeta_RecordsArmMembership(t *testing.T) {
 	if meta.Arm != "llm-visible-text" || meta.Generator != "llm" {
 		t.Errorf("arm membership: got arm=%q generator=%q", meta.Arm, meta.Generator)
 	}
+	if meta.LabelSource != verifier.LabelSourceVisibleText {
+		t.Errorf("label source: got %q, want %q", meta.LabelSource, verifier.LabelSourceVisibleText)
+	}
 	if meta.Model != "claude-sonnet-5" || meta.Instructions != "exercise the outbox" {
 		t.Errorf("llm config: got model=%q instructions=%q", meta.Model, meta.Instructions)
 	}
@@ -223,6 +227,25 @@ func TestBuildRunMeta_OmitsModelWhenSeededPickerRuns(t *testing.T) {
 	}
 }
 
+// TestBuildRunMeta_RecordsLabelSourceForASeededRun is the deliberate difference
+// from Model and Instructions above. The seeded picker never reads a label, but
+// the run still belongs to a labelling cell, and the pair of seeded runs across
+// the two cells is the manipulation check. Omitting it here would leave those
+// two runs indistinguishable in the artifact.
+func TestBuildRunMeta_RecordsLabelSourceForASeededRun(t *testing.T) {
+	options := Options{
+		Platform:    "android",
+		Generator:   "seeded",
+		Duration:    time.Minute,
+		LabelSource: verifier.LabelSourceResourceID,
+	}
+	meta := buildRunMeta(options, "deadbeef", 1, "farm-01", verifier.LLMConfig{}, false)
+
+	if meta.LabelSource != verifier.LabelSourceResourceID {
+		t.Errorf("label source: got %q, want %q", meta.LabelSource, verifier.LabelSourceResourceID)
+	}
+}
+
 func TestBuildRunMeta_OmitsModelWhenSpecDeclaresNoLLMGenerator(t *testing.T) {
 	options := Options{Platform: "android", Generator: "llm", Duration: time.Minute}
 	meta := buildRunMeta(options, "deadbeef", 1, "farm-01", verifier.LLMConfig{}, false)
diff --git a/internal/trace/writer.go b/internal/trace/writer.go
index 30e6ce2..d2f6b3f 100644
--- a/internal/trace/writer.go
+++ b/internal/trace/writer.go
@@ -140,6 +140,12 @@ type Meta struct {
 	Generator    string `json:"generator,omitempty"`
 	Model        string `json:"model,omitempty"`
 	Instructions string `json:"instructions,omitempty"`
+	// LabelSource records how candidates were named to the picker. It is written
+	// for a seeded run too, even though that picker selects by index and never
+	// reads a label: it is the cell the run was assigned to, and the pair of
+	// seeded runs across the two label modes is the manipulation check that says
+	// how much of any difference is just application nondeterminism.
+	LabelSource string `json:"label_source,omitempty"`
 	// MaxSteps and DurationMillis are the budget the run was given, which has
 	// to be identical across arms for a comparison to mean anything.
 	MaxSteps       int   `json:"max_steps,omitempty"`
diff --git a/internal/trace/writer_test.go b/internal/trace/writer_test.go
index 1d50b70..119c339 100644
--- a/internal/trace/writer_test.go
+++ b/internal/trace/writer_test.go
@@ -448,6 +448,7 @@ func TestWriteMeta_ArmMembershipRoundTrip(t *testing.T) {
 		SanderlingVersion: "0.0.1",
 		Arm:               "llm-visible-text",
 		Generator:         "llm",
+		LabelSource:       "visible-text",
 		Model:             "claude-sonnet-5",
 		Instructions:      "exercise the outbox",
 		MaxSteps:          300,
@@ -486,7 +487,7 @@ func TestWriteMeta_OmitsArmMembershipWhenUnset(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	for _, key := range []string{"arm", "generator", "model", "instructions", "max_steps", "duration_millis", "host"} {
+	for _, key := range []string{"arm", "generator", "label_source", "model", "instructions", "max_steps", "duration_millis", "host"} {
 		if strings.Contains(string(body), `"`+key+`"`) {
 			t.Errorf("meta.json carries %q when unset:\n%s", key, body)
 		}