diff --git a/artifact_ownership_contract_test.go b/artifact_ownership_contract_test.go
new file mode 100644
index 0000000..ba5cfad
--- /dev/null
+++ b/artifact_ownership_contract_test.go
@@ -0,0 +1,43 @@
+package promptkit_test
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"testing"
+
+	"go.maximumdirect.net/configkit"
+	"go.maximumdirect.net/promptkit"
+)
+
+type independentArtifactReader struct{}
+
+func (independentArtifactReader) Read(_ context.Context, ref promptkit.ArtifactRef) (*promptkit.Artifact, error) {
+	return &promptkit.Artifact{Body: []byte(ref.Body), Hash: ref.Body}, nil
+}
+
+func TestConcurrentPreparationWithTransferredArtifacts(t *testing.T) {
+	input := configkit.Input{Source: configkit.Mapping("ownership", map[string]any{
+		"profiles": []any{map[string]any{"id": "profile", "endpoint": "https://example.invalid/v1", "model": "model"}},
+		"prompts":  []any{map[string]any{"id": "prompt", "default_profile": "profile", "inputs": []any{map[string]any{"name": "document", "required": true}}, "messages": []any{map[string]any{"role": "user", "content": `{{input "document"}}`}}, "output": map[string]any{"format": "text", "validation_mode": "none"}}},
+	})}
+	engine, err := promptkit.NewEngine(promptkit.Config{Sources: promptkit.ConfigurationSources{Main: &input}}, promptkit.WithArtifactReader(independentArtifactReader{}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var group sync.WaitGroup
+	for i := range 24 {
+		group.Go(func() {
+			value := fmt.Sprintf("body-%d", i)
+			prepared, err := engine.Prepare(context.Background(), promptkit.RunRequest{PromptID: "prompt", Inputs: map[string]promptkit.ArtifactRef{"document": promptkit.Inline(value)}})
+			if err != nil {
+				t.Errorf("prepare: %v", err)
+				return
+			}
+			if len(prepared.Messages) != 1 || prepared.Messages[0].Content != value || prepared.InputHashes["document"] != value {
+				t.Errorf("prepared = %+v", prepared)
+			}
+		})
+	}
+	group.Wait()
+}
diff --git a/docs/consumers/pkg-promptkit.md b/docs/consumers/pkg-promptkit.md
index 0e1e25a..7b5e93d 100644
--- a/docs/consumers/pkg-promptkit.md
+++ b/docs/consumers/pkg-promptkit.md
@@ -891,6 +891,11 @@ of collaborator errors. Implementations must honor cancellation, safely manage
 copies they retain, avoid unsafe logging of content or credentials, and enforce
 the application policy that motivated the injection.
 
+A successful `ArtifactReader.Read` transfers the returned artifact and mutable
+body storage to Promptkit. Readers that previously reused a returned buffer
+must instead return independent storage and stop mutating it after return;
+private scratch buffers may still be reused when the returned bytes are copied.
+
 ## Handle Errors
 
 Use `errors.Is` with the
diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index 17ec1dc..d3e2261 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -244,6 +244,10 @@ Focused authentication and public smoke tests pass on Go 1.27.3.
 the accepted handoff, without promising safety for contract-violating readers.
 Explain migration in consumer guidance; no borrowing/release protocol is added.
 
+Completion: ArtifactReader ownership and migration guidance require independent
+transferred storage. Focused concurrent preparation, adapter isolation, and
+cause-propagation tests pass with the race detector on Go 1.27.3.
+
 ## Stage 9: Detach Prepared Ownership And Session Storage
 
 **Catalog:** 16, 18. **Files:** runner/prepared state and domain session normalization.
diff --git a/types.go b/types.go
index 29703d7..533df8c 100644
--- a/types.go
+++ b/types.go
@@ -295,15 +295,17 @@ type Artifact struct {
 //
 // Read may be called concurrently. It must honor ctx cancellation to make
 // Prepare, PrepareExecution, and Run responsive to cancellation. The engine
-// passes a copied ref and immediately copies the returned Artifact.Body; it
-// does not retain either value. Readers supply artifact metadata, and the
+// passes a copied ref and defensively copies the returned Artifact.Body.
+// A successful Read transfers the returned artifact and its mutable body
+// storage to Promptkit. The reader must not mutate them through retained
+// aliases after return. Private buffers may be reused only when the returned
+// storage is independent. Readers supply artifact metadata, and the
 // engine assigns an input-map name only when the returned artifact name is
 // empty.
 //
 // An injected reader owns any application-specific path containment,
 // authorization, content-size, and content-type policy. It must protect
 // sensitive references and bodies in its logging and in any copies it retains.
-// It may reuse or mutate the returned artifact and body after Read returns.
 //
 // Returning a non-nil error makes the engine return an error matching
 // ErrArtifactLoad while preserving the reader error through errors.Is.