diff --git a/pkg/runs/preparation.go b/pkg/runs/preparation.go
index a77c4e8d..b0430490 100644
--- a/pkg/runs/preparation.go
+++ b/pkg/runs/preparation.go
@@ -2,7 +2,6 @@ package runs
 
 import (
 	"context"
-	"encoding/json"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -15,6 +14,7 @@ import (
 	"agent-compose/pkg/projects"
 	"agent-compose/pkg/storage/sandboxstore"
 	agentcomposev2 "agent-compose/proto/agentcompose/v2"
+	"google.golang.org/protobuf/encoding/protojson"
 )
 
 type PreparationStore interface {
@@ -151,7 +151,8 @@ func DecodeRevisionSpec(raw string) (*agentcomposev2.ProjectSpec, error) {
 	if err != nil {
 		return nil, fmt.Errorf("decode project revision spec: %w", err)
 	}
-	if err := json.Unmarshal(normalizedData, &spec); err != nil {
+	opts := protojson.UnmarshalOptions{DiscardUnknown: true}
+	if err := opts.Unmarshal(normalizedData, &spec); err != nil {
 		return nil, fmt.Errorf("decode project revision spec: %w", err)
 	}
 	if err := restoreCanonicalRevisionWorkspaces(data, &spec); err != nil {
diff --git a/pkg/runs/preparation_driver_oneof_test.go b/pkg/runs/preparation_driver_oneof_test.go
new file mode 100644
index 00000000..b246a74d
--- /dev/null
+++ b/pkg/runs/preparation_driver_oneof_test.go
@@ -0,0 +1,25 @@
+package runs
+
+import (
+	"testing"
+
+	agentcomposev2 "agent-compose/proto/agentcompose/v2"
+)
+
+// DecodeRevisionSpec must preserve oneof fields (e.g. driver runtime config).
+// Regression test for the GetProject → PatchProject round-trip where a driver
+// decoded via encoding/json lost its config oneof, so PatchProject rejected the
+// returned spec with "driver requires exactly one runtime config".
+func TestDecodeRevisionSpecPreservesDriverRuntimeConfig(t *testing.T) {
+	spec, err := DecodeRevisionSpec(`{"name":"demo","agents":[{"name":"worker","driver":{"name":"docker","docker":{}}}]}`)
+	if err != nil {
+		t.Fatalf("DecodeRevisionSpec returned error: %v", err)
+	}
+	driver := spec.GetAgents()[0].GetDriver()
+	if driver.GetName() != "docker" {
+		t.Fatalf("driver name = %q, want docker", driver.GetName())
+	}
+	if _, ok := driver.GetConfig().(*agentcomposev2.DriverSpec_Docker); !ok {
+		t.Fatalf("driver config = %T, want *agentcomposev2.DriverSpec_Docker", driver.GetConfig())
+	}
+}