diff --git a/src/cmd/go/internal/envcmd/env.go b/src/cmd/go/internal/envcmd/env.go
index 5c45e343309..d23d5391411 100644
--- a/src/cmd/go/internal/envcmd/env.go
+++ b/src/cmd/go/internal/envcmd/env.go
@@ -148,7 +148,7 @@ func ExtraEnvVars() []cfg.EnvVar {
 	gomod := ""
 	modload.Init()
 	if modload.HasModRoot() {
-		gomod = filepath.Join(modload.ModRoot(), "go.mod")
+		gomod = modload.ModFilePath()
 	} else if modload.Enabled() {
 		gomod = os.DevNull
 	}
diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index 774fc3052fb..a51ac21751a 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -74,7 +74,7 @@ func runVendor(ctx context.Context, cmd *base.Command, args []string) {
 	}
 	_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
 
-	vdir := filepath.Join(modload.ModRoot(), "vendor")
+	vdir := filepath.Join(modload.VendorDir())
 	if err := os.RemoveAll(vdir); err != nil {
 		base.Fatalf("go mod vendor: %v", err)
 	}
diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
index 6eae44f1a42..3d831a14d80 100644
--- a/src/cmd/go/internal/modget/get.go
+++ b/src/cmd/go/internal/modget/get.go
@@ -724,7 +724,16 @@ func (r *resolver) performLocalQueries(ctx context.Context) {
 			// restricted to matching packages in the main module.
 			pkgPattern, mainModule := modload.MainModules.DirImportPath(ctx, q.pattern)
 			if pkgPattern == "." {
-				return errSet(fmt.Errorf("%s%s is not within module rooted at %s", q.pattern, absDetail, modload.ModRoot()))
+				modload.MustHaveModRoot()
+				var modRoots []string
+				for _, m := range modload.MainModules.Versions() {
+					modRoots = append(modRoots, modload.MainModules.ModRoot(m))
+				}
+				var plural string
+				if len(modRoots) != 1 {
+					plural = "s"
+				}
+				return errSet(fmt.Errorf("%s%s is not within module%s rooted at %s", q.pattern, absDetail, plural, strings.Join(modRoots, ", ")))
 			}
 
 			match := modload.MatchInModule(ctx, pkgPattern, mainModule, imports.AnyTags())
@@ -737,7 +746,8 @@ func (r *resolver) performLocalQueries(ctx context.Context) {
 					return errSet(fmt.Errorf("no package in current directory"))
 				}
 				if !q.isWildcard() {
-					return errSet(fmt.Errorf("%s%s is not a package in module rooted at %s", q.pattern, absDetail, modload.ModRoot()))
+					modload.MustHaveModRoot()
+					return errSet(fmt.Errorf("%s%s is not a package in module rooted at %s", q.pattern, absDetail, modload.MainModules.ModRoot(mainModule)))
 				}
 				search.WarnUnmatched([]*search.Match{match})
 				return pathSet{}
diff --git a/src/cmd/go/internal/modload/build.go b/src/cmd/go/internal/modload/build.go
index 29735864790..3f2160d52da 100644
--- a/src/cmd/go/internal/modload/build.go
+++ b/src/cmd/go/internal/modload/build.go
@@ -323,7 +323,7 @@ func moduleInfo(ctx context.Context, rs *Requirements, m module.Version, mode Li
 		if filepath.IsAbs(r.Path) {
 			info.Replace.Dir = r.Path
 		} else {
-			info.Replace.Dir = filepath.Join(ModRoot(), r.Path)
+			info.Replace.Dir = filepath.Join(replacedFrom, r.Path)
 		}
 		info.Replace.GoMod = filepath.Join(info.Replace.Dir, "go.mod")
 	}
diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
index 5741299281a..de47974b9b7 100644
--- a/src/cmd/go/internal/modload/import.go
+++ b/src/cmd/go/internal/modload/import.go
@@ -294,7 +294,7 @@ func importFromModules(ctx context.Context, path string, rs *Requirements, mg *M
 		if mainErr != nil {
 			return module.Version{}, "", mainErr
 		}
-		readVendorList()
+		readVendorList(mainModule)
 		return vendorPkgModule[path], vendorDir, nil
 	}
 
@@ -653,11 +653,11 @@ func fetch(ctx context.Context, mod module.Version, needSum bool) (dir string, i
 	if modRoot := MainModules.ModRoot(mod); modRoot != "" {
 		return modRoot, true, nil
 	}
-	if r, _ := Replacement(mod); r.Path != "" {
+	if r, replacedFrom := Replacement(mod); r.Path != "" {
 		if r.Version == "" {
 			dir = r.Path
 			if !filepath.IsAbs(dir) {
-				dir = filepath.Join(ModRoot(), dir)
+				dir = filepath.Join(replacedFrom, dir)
 			}
 			// Ensure that the replacement directory actually exists:
 			// dirInModule does not report errors for missing modules,
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 5dd946215bf..1a91b83148a 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -374,7 +374,6 @@ func Init() {
 	}
 
 	if inWorkspaceMode() {
-
 		_ = TODOWorkspaces("go.work.sum, and also allow modfetch to fall back to individual go.sums")
 		_ = TODOWorkspaces("replaces")
 		var err error
@@ -403,8 +402,7 @@ func Init() {
 		//
 		// See golang.org/issue/32027.
 	} else {
-		_ = TODOWorkspaces("Instead of modfile path, find modfile OR workfile path depending on mode")
-		modfetch.GoSumFile = strings.TrimSuffix(ModFilePath(), ".mod") + ".sum"
+		modfetch.GoSumFile = strings.TrimSuffix(modFilePath(modRoots[0]), ".mod") + ".sum"
 	}
 }
 
@@ -463,21 +461,8 @@ func Enabled() bool {
 	return modRoots != nil || cfg.ModulesEnabled
 }
 
-// ModRoot returns the root of the main module.
-// It calls base.Fatalf if there is no main module.
-func ModRoot() string {
-	if !HasModRoot() {
-		die()
-	}
-	if inWorkspaceMode() {
-		panic("ModRoot called in workspace mode")
-	}
-	// This is similar to MustGetSingleMainModule but we can't call that
-	// because MainModules may not yet exist when ModRoot is called.
-	if len(modRoots) != 1 {
-		panic("not in workspace mode but there are multiple ModRoots")
-	}
-	return modRoots[0]
+func VendorDir() string {
+	return filepath.Join(MainModules.ModRoot(MainModules.mustGetSingleMainModule()), "vendor")
 }
 
 func inWorkspaceMode() bool {
@@ -495,12 +480,21 @@ func HasModRoot() bool {
 	return modRoots != nil
 }
 
-// ModFilePath returns the effective path of the go.mod file. Normally, this
-// "go.mod" in the directory returned by ModRoot, but the -modfile flag may
-// change its location. ModFilePath calls base.Fatalf if there is no main
+// MustHaveModRoot checks that a main module or main modules are present,
+// and calls base.Fatalf if there are no main modules.
+func MustHaveModRoot() {
+	Init()
+	if !HasModRoot() {
+		die()
+	}
+}
+
+// ModFilePath returns the path that would be used for the go.mod
+// file, if in module mode. ModFilePath calls base.Fatalf if there is no main
 // module, even if -modfile is set.
 func ModFilePath() string {
-	return modFilePath(ModRoot())
+	MustHaveModRoot()
+	return modFilePath(findModuleRoot(base.Cwd()))
 }
 
 func modFilePath(modRoot string) string {
@@ -674,7 +668,7 @@ func loadModFile(ctx context.Context) (rs *Requirements, needCommit bool) {
 	mainModule := MainModules.mustGetSingleMainModule()
 
 	if cfg.BuildMod == "vendor" {
-		readVendorList()
+		readVendorList(mainModule)
 		index := MainModules.Index(mainModule)
 		modFile := MainModules.ModFile(mainModule)
 		checkVendorConsistency(index, modFile)
@@ -719,7 +713,7 @@ func CreateModFile(ctx context.Context, modPath string) {
 	modRoot := base.Cwd()
 	modRoots = []string{modRoot}
 	Init()
-	modFilePath := ModFilePath()
+	modFilePath := modFilePath(modRoot)
 	if _, err := fsys.Stat(modFilePath); err == nil {
 		base.Fatalf("go: %s already exists", modFilePath)
 	}
@@ -1344,6 +1338,7 @@ func commitRequirements(ctx context.Context, goVersion string, rs *Requirements)
 		return
 	}
 	mainModule := MainModules.Versions()[0]
+	modFilePath := modFilePath(MainModules.ModRoot(mainModule))
 	modFile := MainModules.ModFile(mainModule)
 
 	var list []*modfile.Require
@@ -1383,8 +1378,7 @@ func commitRequirements(ctx context.Context, goVersion string, rs *Requirements)
 		}
 		return
 	}
-	gomod := ModFilePath()
-	if _, ok := fsys.OverlayPath(gomod); ok {
+	if _, ok := fsys.OverlayPath(modFilePath); ok {
 		if dirty {
 			base.Fatalf("go: updates to go.mod needed, but go.mod is part of the overlay specified with -overlay")
 		}
@@ -1422,7 +1416,7 @@ func commitRequirements(ctx context.Context, goVersion string, rs *Requirements)
 
 	errNoChange := errors.New("no update needed")
 
-	err = lockedfile.Transform(ModFilePath(), func(old []byte) ([]byte, error) {
+	err = lockedfile.Transform(modFilePath, func(old []byte) ([]byte, error) {
 		if bytes.Equal(old, new) {
 			// The go.mod file is already equal to new, possibly as the result of some
 			// other process.
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index dd69e2afbfe..cb5a2d7a353 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -440,7 +440,16 @@ func matchLocalDirs(ctx context.Context, m *search.Match, rs *Requirements) {
 		if !filepath.IsAbs(dir) {
 			absDir = filepath.Join(base.Cwd(), dir)
 		}
-		if search.InDir(absDir, cfg.GOROOTsrc) == "" && search.InDir(absDir, ModRoot()) == "" && pathInModuleCache(ctx, absDir, rs) == "" {
+
+		modRoot := findModuleRoot(absDir)
+		found := false
+		for _, mod := range MainModules.Versions() {
+			if MainModules.ModRoot(mod) == modRoot {
+				found = true
+				break
+			}
+		}
+		if !found && search.InDir(absDir, cfg.GOROOTsrc) == "" && pathInModuleCache(ctx, absDir, rs) == "" {
 			m.Dirs = []string{}
 			m.AddError(fmt.Errorf("directory prefix %s outside available modules", base.ShortPath(absDir)))
 			return
@@ -513,7 +522,7 @@ func resolveLocalPackage(ctx context.Context, dir string, rs *Requirements) (str
 					return "", fmt.Errorf("without -mod=vendor, directory %s has no package path", absDir)
 				}
 
-				readVendorList()
+				readVendorList(mainModule)
 				pkg := strings.TrimPrefix(suffix, "/vendor/")
 				if _, ok := vendorPkgModule[pkg]; !ok {
 					return "", fmt.Errorf("directory %s is not a package listed in vendor/modules.txt", absDir)
@@ -582,10 +591,10 @@ func pathInModuleCache(ctx context.Context, dir string, rs *Requirements) string
 	tryMod := func(m module.Version) (string, bool) {
 		var root string
 		var err error
-		if repl, _ := Replacement(m); repl.Path != "" && repl.Version == "" {
+		if repl, replModRoot := Replacement(m); repl.Path != "" && repl.Version == "" {
 			root = repl.Path
 			if !filepath.IsAbs(root) {
-				root = filepath.Join(ModRoot(), root)
+				root = filepath.Join(replModRoot, root)
 			}
 		} else if repl.Path != "" {
 			root, err = modfetch.DownloadDir(repl)
diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
index bc5c83dffce..664fc0f91bd 100644
--- a/src/cmd/go/internal/modload/modfile.go
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -528,7 +528,7 @@ func goModSummary(m module.Version) (*modFileSummary, error) {
 
 		// For every module other than the target,
 		// return the full list of modules from modules.txt.
-		readVendorList()
+		readVendorList(MainModules.mustGetSingleMainModule())
 
 		// We don't know what versions the vendored module actually relies on,
 		// so assume that it requires everything.
diff --git a/src/cmd/go/internal/modload/vendor.go b/src/cmd/go/internal/modload/vendor.go
index b2da3783ea1..daa58880754 100644
--- a/src/cmd/go/internal/modload/vendor.go
+++ b/src/cmd/go/internal/modload/vendor.go
@@ -36,13 +36,13 @@ type vendorMetadata struct {
 }
 
 // readVendorList reads the list of vendored modules from vendor/modules.txt.
-func readVendorList() {
+func readVendorList(mainModule module.Version) {
 	vendorOnce.Do(func() {
 		vendorList = nil
 		vendorPkgModule = make(map[string]module.Version)
 		vendorVersion = make(map[string]string)
 		vendorMeta = make(map[module.Version]vendorMetadata)
-		data, err := os.ReadFile(filepath.Join(ModRoot(), "vendor/modules.txt"))
+		data, err := os.ReadFile(filepath.Join(MainModules.ModRoot(mainModule), "vendor/modules.txt"))
 		if err != nil {
 			if !errors.Is(err, fs.ErrNotExist) {
 				base.Fatalf("go: %s", err)
@@ -136,7 +136,7 @@ func readVendorList() {
 // go 1.14) or at least does not contradict (go 1.13 or earlier) the
 // requirements and replacements listed in the main module's go.mod file.
 func checkVendorConsistency(index *modFileIndex, modFile *modfile.File) {
-	readVendorList()
+	readVendorList(MainModules.mustGetSingleMainModule())
 
 	pre114 := false
 	if semver.Compare(index.goVersionV, "v1.14") < 0 {