diff --git a/internal/stdlib/stdlib.go b/internal/stdlib/stdlib.go
index 3b05de4..1f82179 100644
--- a/internal/stdlib/stdlib.go
+++ b/internal/stdlib/stdlib.go
@@ -325,12 +325,9 @@
 	if UseTestData {
 		refNames = testRefs
 	} else {
-		re := git.NewRemote(memory.NewStorage(), &config.RemoteConfig{
-			URLs: []string{GoRepoURL},
-		})
-		refs, err := re.List(&git.ListOptions{})
+		refs, err := remoteRefs()
 		if err != nil {
-			return nil, fmt.Errorf("re.List: %v", err)
+			return nil, err
 		}
 		for _, r := range refs {
 			refNames = append(refNames, r.Name())
@@ -347,6 +344,34 @@
 	return versions, nil
 }
 
+// ResolveSupportedBranches returns the current hashes for each ref in
+// SupportedBranches.
+func ResolveSupportedBranches() (_ map[string]string, err error) {
+	defer derrors.Wrap(&err, "ResolveSupportedBranches")
+
+	refs, err := remoteRefs()
+	if err != nil {
+		return nil, err
+	}
+	m := map[string]string{}
+	for _, r := range refs {
+		name := r.Name().Short()
+		if SupportedBranches[name] {
+			m[name] = r.Hash().String()
+		}
+	}
+	return m, nil
+}
+
+func remoteRefs() (_ []*plumbing.Reference, err error) {
+	defer derrors.Wrap(&err, "remoteRefs")
+
+	re := git.NewRemote(memory.NewStorage(), &config.RemoteConfig{
+		URLs: []string{GoRepoURL},
+	})
+	return re.List(&git.ListOptions{})
+}
+
 // Directory returns the directory of the standard library relative to the repo root.
 func Directory(v string) string {
 	if semver.Compare(v, "v1.4.0-beta.1") >= 0 ||
@@ -456,8 +481,19 @@
 	return cdir, resolvedVersion, commitTime, nil
 }
 
+const pseudoHashLen = 12
+
 func newPseudoVersion(version string, commitTime time.Time, hash plumbing.Hash) string {
-	return fmt.Sprintf("%s-%s-%s", version, commitTime.Format("20060102150405"), hash.String()[:12])
+	return fmt.Sprintf("%s-%s-%s", version, commitTime.Format("20060102150405"), hash.String()[:pseudoHashLen])
+}
+
+// VersionMatchesHash reports whether v is a pseudo-version whose hash
+// part matches the prefix of the given hash.
+func VersionMatchesHash(v, hash string) bool {
+	if !version.IsPseudo(v) {
+		return false
+	}
+	return v[len(v)-pseudoHashLen:] == hash[:pseudoHashLen]
 }
 
 // semanticVersion returns the semantic version corresponding to the
diff --git a/internal/stdlib/stdlib_test.go b/internal/stdlib/stdlib_test.go
index 811c8c1..969be4f 100644
--- a/internal/stdlib/stdlib_test.go
+++ b/internal/stdlib/stdlib_test.go
@@ -335,3 +335,31 @@
 		}
 	}
 }
+
+func TestVersionMatchesHash(t *testing.T) {
+	v := "v0.0.0-20210910212848-c8dfa306babb"
+	h := "c8dfa306babb91e88f8ba25329b3ef8aa11944e1"
+	if !VersionMatchesHash(v, h) {
+		t.Error("got false, want true")
+	}
+	h = "c8dfa306babXb91e88f8ba25329b3ef8aa11944e1"
+	if VersionMatchesHash(v, h) {
+		t.Error("got true, want false")
+	}
+}
+
+func TestResolveSupportedBranches(t *testing.T) {
+	got, err := ResolveSupportedBranches()
+	if err != nil {
+		t.Fatal(err)
+	}
+	// We can't check the hashes because they change, but we can check the keys.
+	for key := range got {
+		if !SupportedBranches[key] {
+			t.Errorf("got key %q not in SupportedBranches", key)
+		}
+	}
+	if g, w := len(got), len(SupportedBranches); g != w {
+		t.Errorf("got %d hashes, want %d", g, w)
+	}
+}
diff --git a/internal/worker/server.go b/internal/worker/server.go
index 6bcee22..8051007 100644
--- a/internal/worker/server.go
+++ b/internal/worker/server.go
@@ -504,22 +504,25 @@
 
 func (s *Server) handleFetchStdSupportedBranches(w http.ResponseWriter, r *http.Request) (err error) {
 	defer derrors.Wrap(&err, "handleFetchStdSupportedBranches")
+	resolvedHashes, err := stdlib.ResolveSupportedBranches()
+	if err != nil {
+		return err
+	}
 	for requestedVersion := range stdlib.SupportedBranches {
-		_, resolvedVersion, _, err := stdlib.ContentDir(requestedVersion)
-		if err != nil {
-			return err
-		}
-		var resolvedVersionDB string
+		var schedule bool
+		resolvedHash := resolvedHashes[requestedVersion]
 		vm, err := s.db.GetVersionMap(r.Context(), stdlib.ModulePath, requestedVersion)
 		switch {
 		case err == nil:
-			resolvedVersionDB = vm.ResolvedVersion
+			schedule = !stdlib.VersionMatchesHash(vm.ResolvedVersion, resolvedHash)
+			log.Debugf(r.Context(), "stdlib branch %s: have %s, remote is %q; scheduling = %t",
+				requestedVersion, vm.ResolvedVersion, resolvedHash, schedule)
 		case errors.Is(err, derrors.NotFound):
-			resolvedVersionDB = ""
+			schedule = true
 		default:
 			return err
 		}
-		if resolvedVersionDB != resolvedVersion {
+		if schedule {
 			if _, err := s.queue.ScheduleFetch(r.Context(), stdlib.ModulePath, requestedVersion, nil); err != nil {
 				return fmt.Errorf("error scheduling fetch for %s: %w", requestedVersion, err)
 			}