diff --git a/src/cmd/api/goapi.go b/src/cmd/api/goapi.go
index efc2696f8f..b07a238d67 100644
--- a/src/cmd/api/goapi.go
+++ b/src/cmd/api/goapi.go
@@ -215,8 +215,7 @@ func main() {
 	}
 	optional := fileFeatures(*nextFile)
 	exception := fileFeatures(*exceptFile)
-	fail = !compareAPI(bw, features, required, optional, exception,
-		*allowNew && strings.Contains(runtime.Version(), "devel"))
+	fail = !compareAPI(bw, features, required, optional, exception, *allowNew)
 }
 
 // export emits the exported package features.
diff --git a/src/cmd/api/run.go b/src/cmd/api/run.go
index 8c9fb723a5..81979de191 100644
--- a/src/cmd/api/run.go
+++ b/src/cmd/api/run.go
@@ -10,8 +10,11 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	exec "internal/execabs"
+	"internal/goversion"
+	"io/fs"
 	"log"
 	"os"
 	"path/filepath"
@@ -43,6 +46,7 @@ func main() {
 	apiDir := filepath.Join(goroot, "api")
 	out, err := exec.Command(goCmd(), "tool", "api",
 		"-c", findAPIDirFiles(apiDir),
+		allowNew(apiDir),
 		"-next", filepath.Join(apiDir, "next.txt"),
 		"-except", filepath.Join(apiDir, "except.txt")).CombinedOutput()
 	if err != nil {
@@ -71,3 +75,35 @@ func findAPIDirFiles(apiDir string) string {
 	}
 	return strings.Join(apiFiles, ",")
 }
+
+// allowNew returns the -allow_new flag to use for the 'go tool api' invocation.
+func allowNew(apiDir string) string {
+	// Verify that the api/go1.n.txt for previous Go version exists.
+	// It definitely should, otherwise it's a signal that the logic below may be outdated.
+	if _, err := os.Stat(filepath.Join(apiDir, fmt.Sprintf("go1.%d.txt", goversion.Version-1))); err != nil {
+		log.Fatalln("Problem with api file for previous release:", err)
+	}
+
+	// See whether the api/go1.n.txt for this Go version has been created.
+	// (As of April 2021, it gets created during the release of the first Beta.)
+	_, err := os.Stat(filepath.Join(apiDir, fmt.Sprintf("go1.%d.txt", goversion.Version)))
+	if errors.Is(err, fs.ErrNotExist) {
+		// It doesn't exist, so we're in development or before Beta 1.
+		// At this stage, unmentioned API additions are deemed okay.
+		// (They will be quietly shown in API check output, but the test won't fail).
+		return "-allow_new=true"
+	} else if err == nil {
+		// The api/go1.n.txt for this Go version has been created,
+		// so we're definitely past Beta 1 in the release cycle.
+		//
+		// From this point, enforce that api/go1.n.txt is an accurate and complete
+		// representation of what's going into the release by failing API check if
+		// there are API additions (a month into the freeze, there shouldn't be many).
+		//
+		// See golang.org/issue/43956.
+		return "-allow_new=false"
+	} else {
+		log.Fatal(err)
+	}
+	panic("unreachable")
+}