diff --git a/go/packages/external.go b/go/packages/external.go
index 53cc080d..9d369a30 100644
--- a/go/packages/external.go
+++ b/go/packages/external.go
@@ -16,6 +16,17 @@ import (
 	"strings"
 )
 
+// Driver
+type driverRequest struct {
+	// TODO(matloob): Add a "command" option, so the "list" argument
+	// to the command is instead supplied in the driverRequest?
+	Mode       LoadMode          `json:"mode"`
+	Env        []string          `json:"env"`
+	BuildFlags []string          `json:"build_flags"`
+	Tests      bool              `json:"tests"`
+	Overlay    map[string][]byte `json:"overlay"`
+}
+
 // findExternalTool returns the file path of a tool that supplies
 // the build system package structure, or "" if not found."
 // If GOPACKAGESDRIVER is set in the environment findExternalTool returns its
@@ -39,21 +50,22 @@ func findExternalDriver(cfg *Config) driver {
 		}
 	}
 	return func(cfg *Config, words ...string) (*driverResponse, error) {
+		req, err := json.Marshal(driverRequest{
+			Mode:       cfg.Mode,
+			Env:        cfg.Env,
+			BuildFlags: cfg.BuildFlags,
+			Tests:      cfg.Tests,
+			Overlay:    cfg.Overlay,
+		})
+		if err != nil {
+			return nil, fmt.Errorf("failed to encode message to driver tool: %v", err)
+		}
+
 		buf := new(bytes.Buffer)
-		fullargs := []string{
-			"list",
-			fmt.Sprintf("-test=%t", cfg.Tests),
-			fmt.Sprintf("-export=%t", usesExportData(cfg)),
-			fmt.Sprintf("-deps=%t", cfg.Mode >= LoadImports),
-		}
-		for _, f := range cfg.BuildFlags {
-			fullargs = append(fullargs, fmt.Sprintf("-buildflag=%v", f))
-		}
-		fullargs = append(fullargs, "--")
-		fullargs = append(fullargs, words...)
-		cmd := exec.CommandContext(cfg.Context, tool, fullargs...)
-		cmd.Env = cfg.Env
+		cmd := exec.CommandContext(cfg.Context, tool, words...)
 		cmd.Dir = cfg.Dir
+		cmd.Env = cfg.Env
+		cmd.Stdin = bytes.NewReader(req)
 		cmd.Stdout = buf
 		cmd.Stderr = new(bytes.Buffer)
 		if err := cmd.Run(); err != nil {