diff --git a/src/cmd/go/internal/gover/toolchain.go b/src/cmd/go/internal/gover/toolchain.go
index 48bc86b568..f27e313524 100644
--- a/src/cmd/go/internal/gover/toolchain.go
+++ b/src/cmd/go/internal/gover/toolchain.go
@@ -18,7 +18,6 @@ import (
 // Examples:
 //
 //	FromToolchain("go1.2.3") == "1.2.3"
-//	FromToolchain("go1.2.3+bigcorp") == "1.2.3"
 //	FromToolchain("go1.2.3-bigcorp") == "1.2.3"
 //	FromToolchain("gccgo-go1.23rc4") == "1.23rc4"
 //	FromToolchain("invalid") == ""
@@ -28,9 +27,11 @@ func FromToolchain(name string) string {
 		v = name[2:]
 	} else if i := strings.Index(name, "-go"); i >= 0 {
 		v = name[i+3:]
+	} else {
+		return ""
 	}
 	// Some builds use custom suffixes; strip them.
-	if i := strings.IndexAny(v, " \t+-"); i >= 0 {
+	if i := strings.IndexAny(v, " \t-"); i >= 0 {
 		v = v[:i]
 	}
 	if !IsValid(v) {
@@ -68,14 +69,10 @@ func (e *TooNewError) Error() string {
 	}
 	if Startup.AutoFile != "" && (Startup.AutoGoVersion != "" || Startup.AutoToolchain != "") {
 		explain += fmt.Sprintf("; %s sets ", base.ShortPath(Startup.AutoFile))
-		if Startup.AutoGoVersion != "" {
-			explain += "go " + Startup.AutoGoVersion
-			if Startup.AutoToolchain != "" {
-				explain += ", "
-			}
-		}
 		if Startup.AutoToolchain != "" {
 			explain += "toolchain " + Startup.AutoToolchain
+		} else {
+			explain += "go " + Startup.AutoGoVersion
 		}
 	}
 	return fmt.Sprintf("%v requires go >= %v (running go %v%v)", e.What, e.GoVersion, Local(), explain)
diff --git a/src/cmd/go/internal/gover/toolchain_test.go b/src/cmd/go/internal/gover/toolchain_test.go
new file mode 100644
index 0000000000..7d05f1d0c3
--- /dev/null
+++ b/src/cmd/go/internal/gover/toolchain_test.go
@@ -0,0 +1,19 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gover
+
+import "testing"
+
+func TestFromToolchain(t *testing.T) { test1(t, fromToolchainTests, "FromToolchain", FromToolchain) }
+
+var fromToolchainTests = []testCase1[string, string]{
+	{"go1.2.3", "1.2.3"},
+	{"1.2.3", ""},
+	{"go1.2.3+bigcorp", ""},
+	{"go1.2.3-bigcorp", "1.2.3"},
+	{"go1.2.3-bigcorp more text", "1.2.3"},
+	{"gccgo-go1.23rc4", "1.23rc4"},
+	{"gccgo-go1.23rc4-bigdwarf", "1.23rc4"},
+}
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index db407b88a7..2e833a979a 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -652,7 +652,7 @@ func ReadWorkFile(path string) (*modfile.WorkFile, error) {
 	if err != nil {
 		return nil, err
 	}
-	if f.Go != nil && gover.Compare(f.Go.Version, gover.Local()) > 0 {
+	if f.Go != nil && gover.Compare(f.Go.Version, gover.Local()) > 0 && cfg.CmdName != "work edit" {
 		base.Fatalf("go: %v", &gover.TooNewError{What: base.ShortPath(path), GoVersion: f.Go.Version})
 	}
 	return f, nil
diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
index 77d2d7f86a..e0261d2c1f 100644
--- a/src/cmd/go/internal/modload/modfile.go
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -80,7 +80,7 @@ func ReadModFile(gomod string, fix modfile.VersionFixer) (data []byte, f *modfil
 		// Errors returned by modfile.Parse begin with file:line.
 		return nil, nil, fmt.Errorf("errors parsing go.mod:\n%s\n", err)
 	}
-	if f.Go != nil && gover.Compare(f.Go.Version, gover.Local()) > 0 {
+	if f.Go != nil && gover.Compare(f.Go.Version, gover.Local()) > 0 && cfg.CmdName != "mod edit" {
 		base.Fatalf("go: %v", &gover.TooNewError{What: base.ShortPath(gomod), GoVersion: f.Go.Version})
 	}
 	if f.Module == nil {
diff --git a/src/cmd/go/internal/toolchain/toolchain.go b/src/cmd/go/internal/toolchain/toolchain.go
index 04d5474484..3d565021e7 100644
--- a/src/cmd/go/internal/toolchain/toolchain.go
+++ b/src/cmd/go/internal/toolchain/toolchain.go
@@ -59,12 +59,14 @@ func Switch() {
 
 	sw := os.Getenv(gotoolchainSwitchEnv)
 	os.Unsetenv(gotoolchainSwitchEnv)
+	// The sw == "1" check is delayed until later so that we still fill in gover.Startup for use in errors.
 
-	if !modload.WillBeEnabled() || sw == "1" {
+	if !modload.WillBeEnabled() {
 		return
 	}
 
 	gotoolchain := cfg.Getenv("GOTOOLCHAIN")
+	gover.Startup.GOTOOLCHAIN = gotoolchain
 	if gotoolchain == "" {
 		// cfg.Getenv should fall back to $GOROOT/go.env,
 		// so this should not happen, unless a packager
@@ -74,7 +76,6 @@ func Switch() {
 		// and diagnose the problem.
 		return
 	}
-	gover.Startup.GOTOOLCHAIN = gotoolchain
 
 	var minToolchain, minVers string
 	if x, y, ok := strings.Cut(gotoolchain, "+"); ok { // go1.2.3+auto
@@ -103,11 +104,17 @@ func Switch() {
 				// that a non-default toolchain version is being used here.
 				// (Normally you can run "go version", but go install m@v ignores the
 				// context that "go version" works in.)
-				fmt.Fprintf(os.Stderr, "go: using go%s for %v\n", goVers, m)
-				gotoolchain = "go" + goVers
+				var err error
+				gotoolchain, err = NewerToolchain(context.Background(), goVers)
+				if err != nil {
+					fmt.Fprintf(os.Stderr, "go: %v\n", err)
+					gotoolchain = "go" + goVers
+				}
+				fmt.Fprintf(os.Stderr, "go: using %s for %v\n", gotoolchain, m)
 			}
 		} else {
 			file, goVers, toolchain := modGoToolchain()
+			gover.Startup.AutoFile = file
 			if toolchain == "local" {
 				// Local means always use the default local toolchain,
 				// which is already set, so nothing to do here.
@@ -121,24 +128,31 @@ func Switch() {
 				// That's what people who use toolchain local want:
 				// only ever use the toolchain configured in the local system
 				// (including its environment and go env -w file).
-			} else if toolchain != "" {
-				// Accept toolchain only if it is >= our min.
-				toolVers := gover.FromToolchain(toolchain)
-				if gover.Compare(toolVers, minVers) >= 0 {
-					gotoolchain = toolchain
-				}
+				gover.Startup.AutoToolchain = toolchain
+				gotoolchain = "local"
 			} else {
+				if toolchain != "" {
+					// Accept toolchain only if it is >= our min.
+					toolVers := gover.FromToolchain(toolchain)
+					if toolVers == "" || (!strings.HasPrefix(toolchain, "go") && !strings.Contains(toolchain, "-go")) {
+						base.Fatalf("invalid toolchain %q in %s", toolchain, base.ShortPath(file))
+					}
+					if gover.Compare(toolVers, minVers) >= 0 {
+						gotoolchain = toolchain
+						minVers = toolVers
+						gover.Startup.AutoToolchain = toolchain
+					}
+				}
 				if gover.Compare(goVers, minVers) > 0 {
 					gotoolchain = "go" + goVers
+					gover.Startup.AutoGoVersion = goVers
+					gover.Startup.AutoToolchain = "" // in case we are overriding it for being too old
 				}
 			}
-			gover.Startup.AutoFile = file
-			gover.Startup.AutoGoVersion = goVers
-			gover.Startup.AutoToolchain = toolchain
 		}
 	}
 
-	if gotoolchain == "local" || gotoolchain == "go"+gover.Local() {
+	if sw == "1" || gotoolchain == "local" || gotoolchain == "go"+gover.Local() {
 		// Let the current binary handle the command.
 		return
 	}
@@ -147,8 +161,6 @@ func Switch() {
 	// We want to allow things like go1.20.3 but also gccgo-go1.20.3.
 	// We want to disallow mistakes / bad ideas like GOTOOLCHAIN=bash,
 	// since we will find that in the path lookup.
-	// gover.FromToolchain has already done this check (except for the 1)
-	// but doing it again makes sure we don't miss it on unexpected code paths.
 	if !strings.HasPrefix(gotoolchain, "go1") && !strings.Contains(gotoolchain, "-go1") {
 		base.Fatalf("invalid GOTOOLCHAIN %q", gotoolchain)
 	}
diff --git a/src/cmd/go/testdata/script/gotoolchain.txt b/src/cmd/go/testdata/script/gotoolchain.txt
deleted file mode 100644
index abfa5c8cc0..0000000000
--- a/src/cmd/go/testdata/script/gotoolchain.txt
+++ /dev/null
@@ -1,301 +0,0 @@
-# Plain go version
-go version
-! stdout go1\.999
-
-# Default should be auto
-env GOTOOLCHAIN=
-go env GOTOOLCHAIN
-stdout auto
-go env
-stdout GOTOOLCHAIN=.?auto.?
-
-# GOTOOLCHAIN from network, does not exist
-env GOTOOLCHAIN=go1.9999x
-! go version
-stderr 'go: download go1.9999x for .*: toolchain not available'
-
-[short] skip
-
-env GOTOOLCHAIN=
-mkdir $WORK/bin
-[!GOOS:plan9] env PATH=$WORK/bin${:}$PATH
-[GOOS:plan9] env path=$WORK/bin${:}$path
-go build -o $WORK/bin/ ./go1.999testpath.go  # adds .exe extension implicitly on Windows
-cp $WORK/bin/go1.999testpath$GOEXE $WORK/bin/custom-go1.999.0$GOEXE
-cp $WORK/bin/go1.999testpath$GOEXE $WORK/bin/go1.999.0-custom$GOEXE
-
-# GOTOOLCHAIN from PATH
-env GOTOOLCHAIN=go1.999testpath
-go version
-stdout 'go1.999testpath here!'
-
-# GOTOOLCHAIN from PATH, with forced subprocess
-env GOTOOLCHAIN=go1.999testpath
-env GODEBUG=gotoolchainexec=0
-go version
-stdout 'go1.999testpath here!'
-env GODEBUG=
-
-# GOTOOLCHAIN from network
-[!exec:/bin/sh] stop 'the fake proxy serves shell scripts instead of binaries'
-env GOTOOLCHAIN=go1.999testmod
-go version
-stderr 'go: downloading go1.999testmod \(.*/.*\)'
-
-# GOTOOLCHAIN=auto
-env GOTOOLCHAIN=auto
-env TESTGO_VERSION=go1.100
-
-# toolchain line in go.mod
-cp go119toolchain1999 go.mod
-go version
-stdout go1.999
-
-# custom toolchain line in go.mod
-env TESTGO_VERSION=go1.999
-go version
-stdout testpath # go1.999 < go1.999testpath
-
-env TESTGO_VERSION=go1.999.0
-go version
-! stdout testpath # go1.999testpath < go1.999.0
-
-cp go119customtoolchain1999 go.mod
-go version
-stdout go1.999testpath # custom-go1.999.0 >= go1.999.0
-
-cp go119customtoolchain1999b go.mod
-go version
-stdout go1.999testpath # go1.999.0-custom >= go1.999.0
-
-env TESTGO_VERSION=go1.100
-
-# toolchain local in go.mod
-cp go1999toolchainlocal go.mod
-! go build
-stderr '^go: go.mod requires go >= 1.999 \(running go 1.100; go.mod sets go 1.999, toolchain local\)$'
-
-# toolchain local in go.work
-cp empty go.mod
-cp go1999toolchainlocal go.work
-! go build
-stderr '^go: go.work requires go >= 1.999 \(running go 1.100; go.work sets go 1.999, toolchain local\)$'
-rm go.work
-
-# toolchain line in go.work
-cp empty go.mod
-cp go119toolchain1999 go.work
-go version
-stdout go1.999
-rm go.work
-
-# go version in go.mod
-cp go1999 go.mod
-go version
-stdout go1.999
-
-# go version in go.work
-cp empty go.mod
-cp go1999 go.work
-go version
-stdout go1.999
-rm go.work
-
-# GOTOOLCHAIN=auto falls back to local toolchain if newer than go or toolchain line
-env TESTGO_VERSION=go1.1000
-
-# toolchain line in go.mod
-cp go119toolchain1999 go.mod
-go version
-stdout go1.1000
-
-# toolchain line in go.work
-cp empty go.mod
-cp go119toolchain1999 go.work
-go version
-stdout go1.1000
-rm go.work
-
-# go version in go.mod
-cp go1999 go.mod
-go version
-stdout go1.1000
-
-# go version in go.work
-cp empty go.mod
-cp go1999 go.work
-go version
-stdout go1.1000
-rm go.work
-
-# GOTOOLCHAIN=auto uses different toolchain when instructed and newer
-env TESTGO_VERSION=go1.100
-
-# toolchain line in go.mod
-cp go119toolchain1999 go.mod
-go version
-stdout go1.999
-
-# toolchain line in go.work
-cp empty go.mod
-cp go119toolchain1999 go.work
-go version
-stdout go1.999
-rm go.work
-
-# go version in go.mod
-cp go1999 go.mod
-go version
-stdout go1.999
-
-# go version in go.work
-cp empty go.mod
-cp go1999 go.work
-go version
-stdout go1.999
-rm go.work
-
-# go1.999 should handle go1.998 without a download
-env TESTGO_VERSION=go1.999
-cp go1998 go.mod
-go version
-! stdout go1.998 # local toolchain instead
-
-# go1.998 should handle go1.998 without a download too
-env TESTGO_VERSION=go1.999
-go version
-stdout go1.999 # local toolchain instead
-
-# go1.998+foo should handle go1.998 without a download too
-env TESTGO_VERSION=go1.998+foo
-go version
-stdout 'go1.998\+foo' # local toolchain instead
-
-# go1.998-foo should handle go1.998 without a download too
-env TESTGO_VERSION=go1.998-foo
-go version
-stdout go1.998-foo # local toolchain instead
-
-# 'go1.998 foo' should handle go1.998 without a download too
-env TESTGO_VERSION='go1.998 foo'
-go version
-stdout 'go1.998 foo' # local toolchain instead
-
-# go1.997-foo should download go1.998
-env TESTGO_VERSION=go1.997-foo
-! go version
-stderr go1.998
-
-# GOTOOLCHAIN=go1.1000+auto falls back to go1.1000 if newer than go line
-env TESTGO_VERSION=go1.1
-env GOTOOLCHAIN=go1.1000+auto
-
-# toolchain line in go.mod
-cp go119toolchain1999 go.mod
-! go version
-stderr go1.1000
-
-# toolchain line in go.work
-cp empty go.mod
-cp go119toolchain1999 go.work
-! go version
-stderr go1.1000
-rm go.work
-
-# go version in go.mod
-cp go1999 go.mod
-! go version
-stderr go1.1000
-
-# go version in go.work
-cp empty go.mod
-cp go1999 go.work
-! go version
-stderr go1.1000
-rm go.work
-
-# GOTOOLCHAIN=path refuses to download
-env GOTOOLCHAIN=path
-env TESTGO_VERSION=go1.19
-
-cp go1999 go.mod
-go version
-stdout go1.999
-
-cp go1999mod go.mod
-! go version
-stderr '^go: cannot find "go1.999mod" in PATH$'
-
-# go install m@v should use go version in m@v's go.mod
-env GOTOOLCHAIN=path
-env TESTGO_VERSION=go1.19
-cp go1999 go.mod
-! go install rsc.io/fortune/nonexist@v0.0.1
-stderr '^go: cannot find "go1.21rc999" in PATH$'
-
-# go install m@v should handle queries
-! go install rsc.io/fortune/nonexist@v0.0
-stderr '^go: cannot find "go1.21rc999" in PATH$'
-
-# go run m@v should use go version in m@v's go.mod
-env GOTOOLCHAIN=path
-env TESTGO_VERSION=go1.19
-cp go1999 go.mod
-! go run -unknownflag=here rsc.io/fortune/nonexist@v0.0.1 args here
-stderr '^go: cannot find "go1.21rc999" in PATH$'
-go run -unknownflag here rsc.io/fortune/nonexist@v0.0.1
-stdout 'go1.999testpath here!'
-
-# go run m@v should handle known flags correctly
-! go run -gcflags foo rsc.io/fortune/nonexist@v0.0.1 args here
-stderr '^go: cannot find "go1.21rc999" in PATH$'
-! go run -x rsc.io/fortune/nonexist@v0.0.1 args here
-stderr '^go: cannot find "go1.21rc999" in PATH$'
-
-# go run m@v should handle queries
-! go run rsc.io/fortune/nonexist@v0
-stderr '^go: cannot find "go1.21rc999" in PATH$'
-
-# go install m@v should work if not upgrading
-go install rsc.io/fortune/nonexist@v1
-stderr '^go: downloading rsc.io/fortune v1.0.0$'
-stdout '^go1.999testpath here!'
-
--- empty --
-
--- go1999 --
-go 1.999testpath
-
--- go1998 --
-go 1.998
-
--- go1999mod --
-go 1.999mod
-
--- go119 ---
-go 1.19
-
--- go119toolchain1999 --
-go 1.19
-toolchain go1.999testpath
-
--- go119customtoolchain1999 --
-go 1.19
-toolchain custom-go1.999.0
-
--- go119customtoolchain1999b --
-go 1.19
-toolchain go1.999.0-custom
-
--- go1999toolchainlocal --
-go 1.999
-toolchain local
-
--- go1.999testpath.go --
-package main
-
-import "os"
-
-func main() {
-	os.Stdout.WriteString("go1.999testpath here!\n")
-}
diff --git a/src/cmd/go/testdata/script/gotoolchain_local.txt b/src/cmd/go/testdata/script/gotoolchain_local.txt
new file mode 100644
index 0000000000..a7e2b36cc2
--- /dev/null
+++ b/src/cmd/go/testdata/script/gotoolchain_local.txt
@@ -0,0 +1,267 @@
+# This test uses the fake toolchain switch support in cmd/go/internal/toolchain.Switch
+# to exercise all the version selection logic without needing actual toolchains.
+# See gotoolchain_net.txt and gotoolchain_path.txt for tests of network and PATH toolchains.
+
+env TESTGO_VERSION=go1.500
+env TESTGO_VERSION_SWITCH=1
+
+# Default setting should be auto
+env GOTOOLCHAIN=
+go env GOTOOLCHAIN
+stdout auto
+go env
+stdout GOTOOLCHAIN=.?auto.?  # maybe quoted
+
+# GOTOOLCHAIN=auto runs default toolchain without a go.mod or go.work
+go version
+stdout go1.500
+
+# GOTOOLCHAIN=path runs default toolchain without a go.mod or go.work
+env GOTOOLCHAIN=path
+go version
+stdout go1.500
+
+# GOTOOLCHAIN=asdf is a syntax error
+env GOTOOLCHAIN=asdf
+! go version
+stderr '^go: invalid GOTOOLCHAIN "asdf"$'
+
+# GOTOOLCHAIN=version is used directly.
+env GOTOOLCHAIN=go1.600
+go version
+stdout go1.600
+
+env GOTOOLCHAIN=go1.400
+go version
+stdout go1.400
+
+# GOTOOLCHAIN=version+auto sets a minimum.
+env GOTOOLCHAIN=go1.600+auto
+go version
+stdout go1.600
+
+env GOTOOLCHAIN=go1.400+auto
+go version
+stdout go1.400
+
+# GOTOOLCHAIN=version+path sets a minimum too.
+env GOTOOLCHAIN=go1.600+path
+go version
+stdout go1.600
+
+env GOTOOLCHAIN=go1.400+path
+go version
+stdout go1.400
+
+# Create a go.mod file and test interactions with auto and path.
+
+# GOTOOLCHAIN=auto uses go line if newer than local toolchain.
+env GOTOOLCHAIN=auto
+go mod init m
+go mod edit -go=1.700 -toolchain=none
+go version
+stdout 1.700
+
+go mod edit -go=1.300 -toolchain=none
+go version
+stdout 1.500 # local toolchain is newer
+
+go mod edit -go=1.700 -toolchain=go1.300
+go version
+stdout go1.700 # toolchain too old, ignored
+
+go mod edit -go=1.300 -toolchain=local
+go version
+stdout go1.500
+
+go mod edit -go=1.700 -toolchain=local
+go version
+stdout go1.500 # toolchain local is like GOTOOLCHAIN=local and wins
+! go build
+stderr '^go: go.mod requires go >= 1.700 \(running go 1.500; go.mod sets toolchain local\)'
+
+# GOTOOLCHAIN=path does the same.
+env GOTOOLCHAIN=path
+go mod edit -go=1.700 -toolchain=none
+go version
+stdout 1.700
+
+go mod edit -go=1.300 -toolchain=none
+go version
+stdout 1.500 # local toolchain is newer
+
+go mod edit -go=1.700 -toolchain=go1.300
+go version
+stdout go1.700 # toolchain too old, ignored
+
+go mod edit -go=1.300 -toolchain=local
+go version
+stdout go1.500
+
+go mod edit -go=1.700 -toolchain=local
+go version
+stdout go1.500 # toolchain applies even if older than go line
+! go build
+stderr '^go: go.mod requires go >= 1.700 \(running go 1.500; GOTOOLCHAIN=path; go.mod sets toolchain local\)'
+
+# GOTOOLCHAIN names can have prefix- or -suffix
+env GOTOOLCHAIN=go1.800-bigcorp
+go version
+stdout go1.800-bigcorp
+
+env GOTOOLCHAIN=bigcorp-go1.100
+go version
+stdout bigcorp-go1.100
+
+env GOTOOLCHAIN=auto
+go mod edit -go=1.999 -toolchain=go1.800-bigcorp
+go version
+stdout go1.999
+
+go mod edit -go=1.777 -toolchain=go1.800-bigcorp
+go version
+stdout go1.800-bigcorp
+
+go mod edit -go=1.999 -toolchain=bigcorp-go1.800
+go version
+stdout go1.999
+
+go mod edit -go=1.777 -toolchain=bigcorp-go1.800
+go version
+stdout bigcorp-go1.800
+
+# go.work takes priority over go.mod
+go mod edit -go=1.700 -toolchain=go1.999-wrong
+go work init
+go work edit -go=1.400 -toolchain=go1.600-right
+go version
+stdout go1.600-right
+
+go work edit -go=1.400 -toolchain=local
+go version
+stdout go1.500
+
+# go.work misconfiguration does not break go work edit
+# ('go 1.600 / toolchain local' forces use of 1.500 which can't normally load that go.work; allow work edit to fix it.)
+go work edit -go=1.600 -toolchain=local
+go version
+stdout go1.500
+
+go work edit -toolchain=none
+go version
+stdout go1.600
+
+rm go.work
+
+# go.mod misconfiguration does not break go mod edit
+go mod edit -go=1.600 -toolchain=local
+go version
+stdout go1.500
+
+go mod edit -toolchain=none
+go version
+stdout go1.600
+
+# toolchain built with a custom version should know how it compares to others
+
+env TESTGO_VERSION=go1.500-bigcorp
+go mod edit -go=1.499 -toolchain=none
+go version
+stdout go1.500-bigcorp
+
+go mod edit -go=1.500 -toolchain=none
+go version
+stdout go1.500-bigcorp
+
+go mod edit -go=1.501 -toolchain=none
+go version
+stdout go1.501
+
+env TESTGO_VERSION=bigcorp-go1.500
+go mod edit -go=1.499 -toolchain=none
+go version
+stdout bigcorp-go1.500
+
+go mod edit -go=1.500 -toolchain=none
+go version
+stdout bigcorp-go1.500
+
+go mod edit -go=1.501 -toolchain=none
+go version
+stdout go1.501
+
+env TESTGO_VERSION='go1.500 (bigcorp)'
+go mod edit -go=1.499 -toolchain=none
+go version
+stdout 'go1.500 \(bigcorp\)'
+
+go mod edit -go=1.500 -toolchain=none
+go version
+stdout 'go1.500 \(bigcorp\)'
+
+go mod edit -go=1.501 -toolchain=none
+go version
+stdout go1.501
+
+# go install m@v and go run m@v should ignore go.mod and use m@v
+env TESTGO_VERSION=go1.2.3
+go mod edit -go=1.999 -toolchain=go1.998
+
+! go install rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: using go1.22.9 for rsc.io/fortune@v0.0.1'
+stderr '^go: rsc.io/fortune/nonexist@v0.0.1: module rsc.io/fortune@v0.0.1 found, but does not contain package rsc.io/fortune/nonexist'
+
+! go run rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: using go1.22.9 for rsc.io/fortune@v0.0.1'
+stderr '^go: rsc.io/fortune/nonexist@v0.0.1: module rsc.io/fortune@v0.0.1 found, but does not contain package rsc.io/fortune/nonexist'
+
+# go install should handle unknown flags to find m@v
+! go install -unknownflag rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: using go1.22.9 for rsc.io/fortune@v0.0.1'
+stderr '^flag provided but not defined: -unknownflag'
+
+! go install -unknownflag arg rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: using go1.22.9 for rsc.io/fortune@v0.0.1'
+stderr '^flag provided but not defined: -unknownflag'
+
+# go run should handle unknown boolean flags and flags with =arg
+! go run -unknownflag rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: using go1.22.9 for rsc.io/fortune@v0.0.1'
+stderr '^flag provided but not defined: -unknownflag'
+
+! go run -unknown=flag rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: using go1.22.9 for rsc.io/fortune@v0.0.1'
+stderr '^flag provided but not defined: -unknown'
+
+# go run assumes unknown flags don't take arguments
+! go run -unknownflag rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: using go1.22.9 for rsc.io/fortune@v0.0.1'
+stderr '^flag provided but not defined: -unknownflag'
+
+! go run -unknownflag oops rsc.io/fortune/nonexist@v0.0.1  # lost parse, cannot find m@v
+! stderr go1.22.9
+! stderr '^go: using'
+stderr '^flag provided but not defined: -unknownflag'
+
+# go install m@v should handle queries
+! go install rsc.io/fortune/nonexist@v0.0
+stderr '^go: using go1.22.9 for rsc.io/fortune@v0.0.1'
+stderr '^go: rsc.io/fortune/nonexist@v0.0: module rsc.io/fortune@v0.0 found \(v0.0.1\), but does not contain package rsc.io/fortune/nonexist'
+
+# go run m@v should handle queries
+! go install rsc.io/fortune/nonexist@v0
+stderr '^go: using go1.22.9 for rsc.io/fortune@v0.0.1'
+stderr '^go: rsc.io/fortune/nonexist@v0: module rsc.io/fortune@v0 found \(v0.0.1\), but does not contain package rsc.io/fortune/nonexist'
+
+# go install m@v should use local toolchain if not upgrading
+! go install rsc.io/fortune/nonexist@v1
+! stderr go1.22.9
+! stderr '^go: using'
+stderr '^go: downloading rsc.io/fortune v1.0.0$'
+stderr '^go: rsc.io/fortune/nonexist@v1: module rsc.io/fortune@v1 found \(v1.0.0\), but does not contain package rsc.io/fortune/nonexist'
+
+# go run m@v should use local toolchain if not upgrading
+! go run rsc.io/fortune/nonexist@v1
+! stderr go1.22.9
+! stderr '^go: using'
+stderr '^go: rsc.io/fortune/nonexist@v1: module rsc.io/fortune@v1 found \(v1.0.0\), but does not contain package rsc.io/fortune/nonexist'
diff --git a/src/cmd/go/testdata/script/gotoolchain_net.txt b/src/cmd/go/testdata/script/gotoolchain_net.txt
new file mode 100644
index 0000000000..7d92b72315
--- /dev/null
+++ b/src/cmd/go/testdata/script/gotoolchain_net.txt
@@ -0,0 +1,29 @@
+# This test only checks that basic network lookups work.
+# The full test of toolchain version selection is in gotoolchain.txt.
+
+[short] skip
+
+env TESTGO_VERSION=go1.21actual
+
+# GOTOOLCHAIN from network, does not exist
+env GOTOOLCHAIN=go1.9999x
+! go version
+stderr 'go: download go1.9999x for .*: toolchain not available'
+
+# GOTOOLCHAIN from network
+[!exec:/bin/sh] stop 'the fake proxy serves shell scripts instead of binaries'
+env GOTOOLCHAIN=go1.999testmod
+go version
+stderr 'go: downloading go1.999testmod \(.*/.*\)'
+
+# Test a real GOTOOLCHAIN
+[!net:golang.org] skip
+[!GOOS:darwin] [!GOOS:windows] [!GOOS:linux] skip
+[!GOARCH:amd64] [!GOARCH:arm64] skip
+
+env GOPROXY=
+env GOSUMDB=
+env GOTOOLCHAIN=go1.20.1
+go version
+stderr '^go: downloading go1.20.1 '
+stdout go1.20.1
diff --git a/src/cmd/go/testdata/script/gotoolchain_path.txt b/src/cmd/go/testdata/script/gotoolchain_path.txt
new file mode 100644
index 0000000000..f0e7ab9123
--- /dev/null
+++ b/src/cmd/go/testdata/script/gotoolchain_path.txt
@@ -0,0 +1,64 @@
+# This test only checks that basic PATH lookups work.
+# The full test of toolchain version selection is in gotoolchain.txt.
+
+[short] skip
+
+env TESTGO_VERSION=go1.21pre3
+
+# Compile a fake toolchain to put in the path under various names.
+env GOTOOLCHAIN=
+mkdir $WORK/bin
+[!GOOS:plan9] env PATH=$WORK/bin${:}$PATH
+[GOOS:plan9] env path=$WORK/bin${:}$path
+go build -o $WORK/bin/ ./fakego.go  # adds .exe extension implicitly on Windows
+cp $WORK/bin/fakego$GOEXE $WORK/bin/go1.50.0$GOEXE
+
+go version
+stdout go1.21pre3
+
+# GOTOOLCHAIN=go1.50.0
+env GOTOOLCHAIN=go1.50.0
+go version
+stdout 'running go1.50.0 from PATH'
+
+# GOTOOLCHAIN=path with toolchain line
+env GOTOOLCHAIN=path
+go mod init m
+go mod edit -toolchain=go1.50.0
+go version
+stdout 'running go1.50.0 from PATH'
+
+# GOTOOLCHAIN=path with go line
+env GOTOOLCHAIN=path
+go mod edit -toolchain=none -go=go1.50.0
+go version
+stdout 'running go1.50.0 from PATH'
+
+# GOTOOLCHAIN=auto with toolchain line
+env GOTOOLCHAIN=auto
+go mod edit -toolchain=go1.50.0 -go=1.21
+go version
+stdout 'running go1.50.0 from PATH'
+
+# GOTOOLCHAIN=auto with go line
+env GOTOOLCHAIN=auto
+go mod edit -toolchain=none -go=go1.50.0
+go version
+stdout 'running go1.50.0 from PATH'
+
+-- fakego.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func main() {
+	exe, _ := os.Executable()
+	name := filepath.Base(exe)
+	name = strings.TrimSuffix(name, ".exe")
+	fmt.Printf("running %s from PATH\n", name)
+}
diff --git a/src/cmd/go/testdata/script/mod_get_future.txt b/src/cmd/go/testdata/script/mod_get_future.txt
index 6f2985af86..72c0b97804 100644
--- a/src/cmd/go/testdata/script/mod_get_future.txt
+++ b/src/cmd/go/testdata/script/mod_get_future.txt
@@ -1,6 +1,6 @@
 env TESTGO_VERSION=go1.21
 ! go mod download rsc.io/future@v1.0.0
-stderr '^go: rsc.io/future@v1.0.0 requires go >= 1.999 \(running go 1.21; go.mod sets go 1.21\)$'
+stderr '^go: rsc.io/future@v1.0.0 requires go >= 1.999 \(running go 1.21\)$'
 
 -- go.mod --
 module m
diff --git a/src/cmd/go/testdata/script/mod_go_version.txt b/src/cmd/go/testdata/script/mod_go_version.txt
index 4e6baf89f1..b5350fc3e1 100644
--- a/src/cmd/go/testdata/script/mod_go_version.txt
+++ b/src/cmd/go/testdata/script/mod_go_version.txt
@@ -4,9 +4,9 @@ env GO111MODULE=on
 env TESTGO_VERSION=go1.21
 
 ! go list
-stderr -count=1 '^go: sub@v1.0.0: sub requires go >= 1.999 \(running go 1.21; go.mod sets go 1.1\)$'
+stderr -count=1 '^go: sub@v1.0.0: sub requires go >= 1.999 \(running go 1.21\)$'
 ! go build sub
-stderr -count=1 '^go: sub@v1.0.0: sub requires go >= 1.999 \(running go 1.21; go.mod sets go 1.1\)$'
+stderr -count=1 '^go: sub@v1.0.0: sub requires go >= 1.999 \(running go 1.21\)$'
 
 -- go.mod --
 module m