diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index 05ee094ea7..ccf5605a63 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -104,6 +104,7 @@
 //		Change to dir before running the command.
 //		Any files named on the command line are interpreted after
 //		changing directories.
+//		If used, this flag must be the first one in the command line.
 //	-a
 //		force rebuilding of packages that are already up-to-date.
 //	-n
@@ -1865,7 +1866,7 @@
 //
 // Usage:
 //
-//	go vet [-C dir] [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]
+//	go vet [build flags] [-vettool prog] [vet flags] [packages]
 //
 // Vet runs the Go vet command on the packages named by the import paths.
 //
@@ -1874,10 +1875,6 @@
 // For a list of checkers and their flags, see 'go tool vet help'.
 // For details of a specific checker such as 'printf', see 'go tool vet help printf'.
 //
-// The -C flag changes to dir before running the 'go vet' command.
-// The -n flag prints commands that would be executed.
-// The -x flag prints commands as they are executed.
-//
 // The -vettool=prog flag selects a different analysis tool with alternative
 // or additional checks.
 // For example, the 'shadow' analyzer can be built and run using these commands:
@@ -1886,7 +1883,7 @@
 //	go vet -vettool=$(which shadow)
 //
 // The build flags supported by go vet are those that control package resolution
-// and execution, such as -n, -x, -v, -tags, and -toolexec.
+// and execution, such as -C, -n, -x, -v, -tags, and -toolexec.
 // For more about these flags, see 'go help build'.
 //
 // See also: go fmt, go fix.
diff --git a/src/cmd/go/internal/base/base.go b/src/cmd/go/internal/base/base.go
index f3774ae2f0..2171d13909 100644
--- a/src/cmd/go/internal/base/base.go
+++ b/src/cmd/go/internal/base/base.go
@@ -57,6 +57,20 @@ var Go = &Command{
 	// Commands initialized in package main
 }
 
+// Lookup returns the subcommand with the given name, if any.
+// Otherwise it returns nil.
+//
+// Lookup ignores subcommands that have len(c.Commands) == 0 and c.Run == nil.
+// Such subcommands are only for use as arguments to "help".
+func (c *Command) Lookup(name string) *Command {
+	for _, sub := range c.Commands {
+		if sub.Name() == name && (len(c.Commands) > 0 || c.Runnable()) {
+			return sub
+		}
+	}
+	return nil
+}
+
 // hasFlag reports whether a command or any of its subcommands contain the given
 // flag.
 func hasFlag(c *Command, name string) bool {
diff --git a/src/cmd/go/internal/base/flag.go b/src/cmd/go/internal/base/flag.go
index 9d8d1c0c8d..74e1275cfd 100644
--- a/src/cmd/go/internal/base/flag.go
+++ b/src/cmd/go/internal/base/flag.go
@@ -6,7 +6,7 @@ package base
 
 import (
 	"flag"
-	"os"
+	"fmt"
 
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/fsys"
@@ -62,7 +62,7 @@ func AddBuildFlagsNX(flags *flag.FlagSet) {
 func AddChdirFlag(flags *flag.FlagSet) {
 	// The usage message is never printed, but it's used in chdir_test.go
 	// to identify that the -C flag is from AddChdirFlag.
-	flags.Func("C", "AddChdirFlag", os.Chdir)
+	flags.Func("C", "AddChdirFlag", ChdirFlag)
 }
 
 // AddModFlag adds the -mod build flag to the flag set.
@@ -77,3 +77,9 @@ func AddModCommonFlags(flags *flag.FlagSet) {
 	flags.StringVar(&cfg.ModFile, "modfile", "", "")
 	flags.StringVar(&fsys.OverlayFile, "overlay", "", "")
 }
+
+func ChdirFlag(s string) error {
+	// main handles -C by removing it from the command line.
+	// If we see one during flag parsing, that's an error.
+	return fmt.Errorf("-C flag must be first flag on command line")
+}
diff --git a/src/cmd/go/internal/vet/vet.go b/src/cmd/go/internal/vet/vet.go
index f59994c7c9..2d42097120 100644
--- a/src/cmd/go/internal/vet/vet.go
+++ b/src/cmd/go/internal/vet/vet.go
@@ -25,7 +25,7 @@ func init() {
 
 var CmdVet = &base.Command{
 	CustomFlags: true,
-	UsageLine:   "go vet [-C dir] [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]",
+	UsageLine:   "go vet [build flags] [-vettool prog] [vet flags] [packages]",
 	Short:       "report likely mistakes in packages",
 	Long: `
 Vet runs the Go vet command on the packages named by the import paths.
@@ -35,10 +35,6 @@ For more about specifying packages, see 'go help packages'.
 For a list of checkers and their flags, see 'go tool vet help'.
 For details of a specific checker such as 'printf', see 'go tool vet help printf'.
 
-The -C flag changes to dir before running the 'go vet' command.
-The -n flag prints commands that would be executed.
-The -x flag prints commands as they are executed.
-
 The -vettool=prog flag selects a different analysis tool with alternative
 or additional checks.
 For example, the 'shadow' analyzer can be built and run using these commands:
@@ -47,7 +43,7 @@ For example, the 'shadow' analyzer can be built and run using these commands:
   go vet -vettool=$(which shadow)
 
 The build flags supported by go vet are those that control package resolution
-and execution, such as -n, -x, -v, -tags, and -toolexec.
+and execution, such as -C, -n, -x, -v, -tags, and -toolexec.
 For more about these flags, see 'go help build'.
 
 See also: go fmt, go fix.
diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index e39e499930..e2e0e07299 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -61,6 +61,7 @@ and test commands:
 		Change to dir before running the command.
 		Any files named on the command line are interpreted after
 		changing directories.
+		If used, this flag must be the first one in the command line.
 	-a
 		force rebuilding of packages that are already up-to-date.
 	-n
diff --git a/src/cmd/go/main.go b/src/cmd/go/main.go
index d050792998..00b0a2b78b 100644
--- a/src/cmd/go/main.go
+++ b/src/cmd/go/main.go
@@ -18,6 +18,7 @@ import (
 	"path/filepath"
 	"runtime"
 	rtrace "runtime/trace"
+	"slices"
 	"strings"
 
 	"cmd/go/internal/base"
@@ -92,6 +93,7 @@ var _ = go11tag
 
 func main() {
 	log.SetFlags(0)
+	handleChdirFlag()
 	toolchain.Select()
 
 	flag.Usage = base.Usage
@@ -156,43 +158,61 @@ func main() {
 		os.Exit(2)
 	}
 
-BigCmdLoop:
-	for bigCmd := base.Go; ; {
-		for _, cmd := range bigCmd.Commands {
-			if cmd.Name() != args[0] {
-				continue
-			}
-			if len(cmd.Commands) > 0 {
-				bigCmd = cmd
-				args = args[1:]
-				if len(args) == 0 {
-					help.PrintUsage(os.Stderr, bigCmd)
-					base.SetExitStatus(2)
-					base.Exit()
-				}
-				if args[0] == "help" {
-					// Accept 'go mod help' and 'go mod help foo' for 'go help mod' and 'go help mod foo'.
-					help.Help(os.Stdout, append(strings.Split(cfg.CmdName, " "), args[1:]...))
-					return
-				}
-				cfg.CmdName += " " + args[0]
-				continue BigCmdLoop
-			}
-			if !cmd.Runnable() {
-				continue
-			}
-			invoke(cmd, args)
+	cmd, used := lookupCmd(args)
+	cfg.CmdName = strings.Join(args[:used], " ")
+	if len(cmd.Commands) > 0 {
+		if used >= len(args) {
+			help.PrintUsage(os.Stderr, cmd)
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+		if args[used] == "help" {
+			// Accept 'go mod help' and 'go mod help foo' for 'go help mod' and 'go help mod foo'.
+			help.Help(os.Stdout, append(slices.Clip(args[:used]), args[used+1:]...))
 			base.Exit()
-			return
 		}
 		helpArg := ""
-		if i := strings.LastIndex(cfg.CmdName, " "); i >= 0 {
-			helpArg = " " + cfg.CmdName[:i]
+		if used > 0 {
+			helpArg += " " + strings.Join(args[:used], " ")
 		}
 		fmt.Fprintf(os.Stderr, "go %s: unknown command\nRun 'go help%s' for usage.\n", cfg.CmdName, helpArg)
 		base.SetExitStatus(2)
 		base.Exit()
 	}
+	invoke(cmd, args[used-1:])
+	base.Exit()
+}
+
+// lookupCmd interprets the initial elements of args
+// to find a command to run (cmd.Runnable() == true)
+// or else a command group that ran out of arguments
+// or had an unknown subcommand (len(cmd.Commands) > 0).
+// It returns that command and the number of elements of args
+// that it took to arrive at that command.
+func lookupCmd(args []string) (cmd *base.Command, used int) {
+	cmd = base.Go
+	for used < len(args) {
+		c := cmd.Lookup(args[used])
+		if c == nil {
+			break
+		}
+		if c.Runnable() {
+			cmd = c
+			used++
+			break
+		}
+		if len(c.Commands) > 0 {
+			cmd = c
+			used++
+			if used >= len(args) || args[0] == "help" {
+				break
+			}
+			continue
+		}
+		// len(c.Commands) == 0 && !c.Runnable() => help text; stop at "help"
+		break
+	}
+	return cmd, used
 }
 
 func invoke(cmd *base.Command, args []string) {
@@ -271,3 +291,44 @@ func maybeStartTrace(pctx context.Context) context.Context {
 
 	return ctx
 }
+
+// handleChdirFlag handles the -C flag before doing anything else.
+// The -C flag must be the first flag on the command line, to make it easy to find
+// even with commands that have custom flag parsing.
+// handleChdirFlag handles the flag by chdir'ing to the directory
+// and then removing that flag from the command line entirely.
+//
+// We have to handle the -C flag this way for two reasons:
+//
+//   1. Toolchain selection needs to be in the right directory to look for go.mod and go.work.
+//
+//   2. A toolchain switch later on reinvokes the new go command with the same arguments.
+//      The parent toolchain has already done the chdir; the child must not try to do it again.
+func handleChdirFlag() {
+	_, used := lookupCmd(os.Args[1:])
+	used++ // because of [1:]
+	if used >= len(os.Args) {
+		return
+	}
+
+	var dir string
+	switch a := os.Args[used]; {
+	default:
+		return
+
+	case a == "-C", a == "--C":
+		if used+1 >= len(os.Args) {
+			return
+		}
+		dir = os.Args[used+1]
+		os.Args = slices.Delete(os.Args, used, used+2)
+
+	case strings.HasPrefix(a, "-C="), strings.HasPrefix(a, "--C="):
+		_, dir, _ = strings.Cut(a, "=")
+		os.Args = slices.Delete(os.Args, used, used+1)
+	}
+
+	if err := os.Chdir(dir); err != nil {
+		base.Fatalf("go: %v", err)
+	}
+}
diff --git a/src/cmd/go/testdata/script/chdir.txt b/src/cmd/go/testdata/script/chdir.txt
index 8952d18a72..a6feed6b45 100644
--- a/src/cmd/go/testdata/script/chdir.txt
+++ b/src/cmd/go/testdata/script/chdir.txt
@@ -17,15 +17,19 @@ go doc -C ../strings HasPrefix
 go env -C $OLD/custom GOMOD
 stdout 'custom[\\/]go.mod'
 ! go env -C ../nonexist
-stderr '^invalid value "../nonexist" for flag -C: chdir ../nonexist:.*$'
+stderr '^go: chdir ../nonexist: '
 
 # go test
-go test -n -C ../strings
+go test -C ../strings -n
 stderr 'strings\.test'
 
 # go vet
-go vet -n -C ../strings
+go vet -C ../strings -n
 stderr strings_test
 
+# -C must be first on command line (as of Go 1.21)
+! go test -n -C ../strings
+stderr '^invalid value "../strings" for flag -C: -C flag must be first flag on command line$'
+
 -- custom/go.mod --
 module m
diff --git a/src/cmd/go/testdata/script/mod_get_exec_toolchain.txt b/src/cmd/go/testdata/script/mod_get_exec_toolchain.txt
index f78d517c87..497fe36f40 100644
--- a/src/cmd/go/testdata/script/mod_get_exec_toolchain.txt
+++ b/src/cmd/go/testdata/script/mod_get_exec_toolchain.txt
@@ -122,6 +122,21 @@ stderr '^go: rsc.io/needgo124@v0.0.1 requires go >= 1.24; switching to go1.24rc1
 stderr '^go: upgraded go 1.1 => 1.24$'
 stderr '^go: added toolchain go1.24rc1$'
 
+# The -C flag should not happen more than once due to switching.
+mkdir dir dir/dir
+cp go.mod.new go.mod
+cp go.mod.new dir/go.mod
+cp go.mod.new dir/dir/go.mod
+cp p.go dir/p.go
+cp p.go dir/dir/p.go
+go get -C dir rsc.io/needgo124
+stderr '^go: rsc.io/needgo124@v0.0.1 requires go >= 1.24; switching to go1.24rc1$'
+stderr '^go: upgraded go 1.1 => 1.24$'
+stderr '^go: added toolchain go1.24rc1$'
+cmp go.mod.new go.mod
+cmp go.mod.new dir/dir/go.mod
+grep 'go 1.24$' dir/go.mod
+
 -- go.mod.new --
 module m
 go 1.1