diff --git a/.travis.yml b/.travis.yml
index 0e100f7cf..e140375a6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,32 +8,37 @@
 language: go
 
 # Test Go 1.11 and 1.12 across linux, osx, windows.
-# Test Go tip on linux, osx (and explicitly set GO111MODULE=on given Travis sets GO111MODULE=auto).
+# Test Go tip on linux, osx (and explicitly set GO111MODULE=auto for tip, given default might change).
 matrix:
   include:
     - os: linux
       go: tip
       env: SET_GO111MODULE=1
     - os: linux
-      go: "1.12.x"
+      go: tip
     - os: linux
-      go: "1.11.x"
+      go: "1.13.x"
+    - os: linux
+      go: "1.12.x"
     - os: osx
       go: tip
       env: SET_GO111MODULE=1
     - os: osx
-      go: "1.12.x"
+      go: tip
+    - os: osx
+      go: "1.13.x"
     - os: osx
-      go: "1.11.x"
-    - os: windows
       go: "1.12.x"
     - os: windows
-      go: "1.11.x"
+      go: "1.13.x"
+    - os: windows
+      go: "1.12.x"
 
 # Install coreutils for the 'timeout(1)' utility on windows and osx.
 before_install:
   - if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then choco install gnuwin32-coreutils.install; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then ln -s /usr/local/bin/gtimeout /usr/local/bin/timeout ; fi
+  - go get -u github.com/rogpeppe/go-internal/cmd/testscript
 
 # Set the import path (including to help with forks).
 go_import_path: github.com/dvyukov/go-fuzz
@@ -44,6 +49,21 @@ script:
   - "echo 'verify the timeout utility works, including that it exits with status 124 on timeout.'"
   -  "(timeout 2 sleep 10; ret=$?; echo timeout ret=$ret; if [[ $ret -eq 124 ]]; then exit 0; fi; exit $ret)"
 
+  # Sanity check that the 'testscript' cmd seems to function at all.
+  # If all the testscripts fail, this is a good one to troubleshoot first.
+  - cd $GOPATH/src/github.com/dvyukov/go-fuzz
+  - testscript -v testscripts/fuzz_help.txt
+
+  # Run our tests for fuzzing modules.
+  # If multiple modules testcripts fail, probably makes sense to start by troubleshooting
+  # the earliest failing testscript.
+  # TODO: probably makes more sense to move this further down, but place here for now for faster iterations.
+  - testscript -v testscripts/mod_go_fuzz_dep.txt
+  - testscript -v testscripts/mod_outside_gopath.txt
+  - testscript -v testscripts/mod_inside_gopath.txt
+  - testscript -v testscripts/mod_v2.txt
+  - testscript -v testscripts/mod_vendor.txt
+
   # Prepare to test the png example from dvyukov/go-fuzz-corpus.
   - go get -v -d github.com/dvyukov/go-fuzz-corpus/png
   - cd $GOPATH/src/github.com/dvyukov/go-fuzz-corpus/
@@ -53,9 +73,11 @@ script:
   # Reduce chances of future surprises due to any caching.
   - rm -rf fuzz.zip ./freshworkdir
 
-  # Explicitly set GO111MODULE=on if requested (given Travis sets GO111MODULE=auto).
-  # This is currently intended to test tip / 1.13, which default to GO111MODULE=on.
-  - if [[ ! -z "$SET_GO111MODULE" ]]; then export GO111MODULE=on; fi 
+  # Explicitly set GO111MODULE=auto if requested.
+  # Travis and/or tip might change the default value of GO111MODULE at some point.
+  # As of 2019-08-31, travis sets GO111MODULE=auto, and tip defaults to 'auto' if
+  # GO111MODULE is unset.
+  - if [[ ! -z "$SET_GO111MODULE" ]]; then export GO111MODULE=auto; fi 
   - echo "GO111MODULE=$GO111MODULE"
 
   # Instrument using go-fuzz-build on the png example Fuzz function.
diff --git a/README.md b/README.md
index 2f0ffa3e3..e5bd65913 100755
--- a/README.md
+++ b/README.md
@@ -5,6 +5,8 @@ Fuzzing is mainly applicable to packages that parse complex inputs (both text
 and binary), and is especially useful for hardening of systems that parse inputs
 from potentially malicious users (e.g. anything accepted over a network).
 
+**Note:** go-fuzz has recently added preliminary support for fuzzing [Go Modules](github.com/golang/go/wiki/Modules).  See the [section below](https://github.com/dvyukov/go-fuzz/blob/master/README.md#modules-support) for more details. If you encounter a problem with go-fuzz's module support, a workaround might be to disable modules via `export GO111MODULE=off`.
+
 ## Usage
 
 First, you need to write a test function of the form:
@@ -102,9 +104,6 @@ $ go-fuzz-build
 ```
 This will produce png-fuzz.zip archive.
 
-Note that go-fuzz [does not support modules yet](https://github.com/dvyukov/go-fuzz/issues/195).
-`go-fuzz-build` disables modules by setting environment variable `GO111MODULE=off` during the build.
-
 Now we are ready to go:
 ```
 $ go-fuzz
@@ -136,6 +135,22 @@ value should be less than ~5000, otherwise fuzzer can miss new interesting input
 due to hash collisions. And finally ```uptime``` is uptime of the process. This same
 information is also served via http (see the ```-http``` flag).
 
+## Modules support
+
+go-fuzz has preliminary support for fuzzing [Go Modules](github.com/golang/go/wiki/Modules). 
+go-fuzz respects the standard `GO111MODULE` environment variable, which can be set to `on`, `off`, or `auto`. 
+
+You must have a `require` directive for `github.com/dvyukov/go-fuzz` in your go.mod file. Two approaches are:
+
+1. Prior to invoking go-fuzz-build, add a `require` for `github.com/dvyukov/go-fuzz` in your go.mod. A common approach for tracking tool dependencies like this is [via a tools.go file](https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module). Alternatively, you could for example manually add `require github.com/dvyukov/go-fuzz latest` to your go.mod.
+
+2. If you do not have a `require` for `github.com/dvyukov/go-fuzz` in your go.mod, go-fuzz-build will add one automatically. If desired, you can revert your go.mod file when you are done.
+
+Vendoring with modules is not yet supported. A `vendor` directory will be ignored, and go-fuzz will report an error if `GOFLAGS=-mod=vendor` is set.
+
+Note that while modules are used to prepare the build, the final instrumented build is still done in GOPATH mode.
+For most code, this should not matter.
+
 ## libFuzzer support
 
 go-fuzz-build can also generate an archive file
diff --git a/go-fuzz-build/main.go b/go-fuzz-build/main.go
index 324fc7c6c..3021a7dd4 100644
--- a/go-fuzz-build/main.go
+++ b/go-fuzz-build/main.go
@@ -58,10 +58,34 @@ func makeTags() string {
 // that clients can then modify and use for calls to go/packages.
 func basePackagesConfig() *packages.Config {
 	cfg := new(packages.Config)
-	cfg.Env = append(os.Environ(), "GO111MODULE=off")
+
+	// Preliminary modules support: note that we do not set 
+	// GO111MODULE here in order to respect any GO111MODULE setting by the user
+	// as we are finding dependencies. Note, however, that we are
+	// still setting up a GOPATH to build, so we later will force 
+	// GO111MODULE to be off when building so that we are in GOPATH mode.
+	// If the user has not set GO111MODULE, the meaning here is
+	// left up to cmd/go (defaulting to 'auto' in Go 1.11-1.13,
+	// but likely defaulting to 'on' at some point during Go 1.14
+	// development cycle).
+	// Also note that we are leaving the overall cfg structure
+	// in place to support future experimentation, etc.
+	cfg.Env = os.Environ()
 	return cfg
 }
 
+// checkModVendor reports if the GOFLAGS env variable
+// contains -mod=vendor, which enables vendoring for modules.
+func checkModVendor() bool {
+	val := os.Getenv("GOFLAGS")
+	for _, s := range strings.Split(val, " ") {
+		if s == "-mod=vendor" {
+			return true
+		}
+	}
+	return false
+}
+
 // main copies the package with all dependent packages into a temp dir,
 // instruments Go source files there, and builds setting GOROOT to the temp dir.
 func main() {
@@ -82,6 +106,12 @@ func main() {
 	if *flagLibFuzzer && *flagRace {
 		c.failf("-race and -libfuzzer are incompatible")
 	}
+	if checkModVendor() {
+		// Preliminary modules support: we don't support -mod=vendor.
+		// Part of the issue is go-fuzz-dep and go-fuzz-defs
+		// won't be in the user's vendor directory.
+		c.failf("GOFLAGS with -mod=vendor is not supported")
+	}
 
 	c.startProfiling()  // start pprof as requested
 	c.loadPkg(pkg)      // load and typecheck pkg
@@ -486,10 +516,14 @@ func (c *Context) buildInstrumentedBinary(blocks *[]CoverBlock, sonar *[]CoverBl
 	}
 	args = append(args, "-o", outf, mainPkg)
 	cmd := exec.Command("go", args...)
+
+	// Preliminary modules support: we are constructing a GOPATH
+	// environment, so while building we force GOPATH mode here
+	// via GO111MODULE=off.
 	cmd.Env = append(os.Environ(),
 		"GOROOT="+filepath.Join(c.workdir, "goroot"),
 		"GOPATH="+filepath.Join(c.workdir, "gopath"),
-		"GO111MODULE=off", // temporary measure until we have proper module support
+		"GO111MODULE=off",
 	)
 	if out, err := cmd.CombinedOutput(); err != nil {
 		c.failf("failed to execute go build: %v\n%v", err, string(out))
diff --git a/go-fuzz/main.go b/go-fuzz/main.go
index 15c66b224..149fa590d 100644
--- a/go-fuzz/main.go
+++ b/go-fuzz/main.go
@@ -99,7 +99,11 @@ func main() {
 			// Try the default. Best effort only.
 			var bin string
 			cfg := new(packages.Config)
-			cfg.Env = append(os.Environ(), "GO111MODULE=off")
+			// Preliminary modules support: note that we do not set 
+			// GO111MODULE here in order to respect any GO111MODULE setting by the user
+			// as we are finding dependencies. See preliminary modules support 
+			// comments in go-fuzz-build/main.go for more details.
+			cfg.Env = os.Environ()
 			pkgs, err := packages.Load(cfg, ".")
 			if err == nil && len(pkgs) == 1 {
 				bin = pkgs[0].Name + "-fuzz.zip"
diff --git a/testscripts/fuzz_help.txt b/testscripts/fuzz_help.txt
new file mode 100644
index 000000000..c00622bc6
--- /dev/null
+++ b/testscripts/fuzz_help.txt
@@ -0,0 +1,8 @@
+# Start by seeing if anything works at all.
+# This is intended to excercise travis and the testscript cmd invoking go-fuzz.
+# If this fails, likely everything else will fail too, but if so might be easier to 
+# start by troubleshooting this.
+
+# Ask go-fuzz-build to emit its help message, which should be a failing status code.
+! exec go-fuzz-build -h
+stderr 'Usage of.*go-fuzz-build'
diff --git a/testscripts/mod_go_fuzz_dep.txt b/testscripts/mod_go_fuzz_dep.txt
new file mode 100644
index 000000000..e9cb1e053
--- /dev/null
+++ b/testscripts/mod_go_fuzz_dep.txt
@@ -0,0 +1,56 @@
+# These steps validate that go-fuzz/go-fuzz-dep and go-fuzz/go-fuzz-defs are found,
+# along with validating how they are currently found, and where they end up being located.
+
+# Enter a simple module with a fuzz function.
+# Note that we are outside of GOPATH, so the presence of the 'go.mod' here will
+# enable module-module for cmd/go (which is true in Go 1.11-1.13, and likely will be true in 1.14 as well).
+cd testmod
+
+# Sanity check the module seems well formed.
+exec go list -m all
+stdout '^example.com/testmod$'
+exec go build
+
+# Sanity check go-fuzz is not in our go.mod, nor reported by 'go list -m all'.
+! grep 'github.com/dvyukov/go-fuzz' go.mod
+exec go list -m all
+! stdout 'github.com/dvyukov/go-fuzz'
+
+# Ask go-fuzz-build to build, including specifying the fuzz function for mod.
+exec go-fuzz-build -func=FuzzMod
+exists testmod-fuzz.zip
+
+# Sanity check github.com/dvyukov/go-fuzz was added by go-fuzz-build
+# to our go.mod and is now visible via 'go list -m all'.
+# This is not necessarily a requirement for all time,
+# but this is the way the current modules approach for go-fuzz works.
+# This is important to make sure the go-fuzz-dep source code is
+# findable by go-fuzz-build (when it invokes 'go list').
+grep -count=1 '^require github.com/dvyukov/go-fuzz v[^ ]+ // indirect$' go.mod
+exec go list -m all
+stdout '^example.com/testmod$'
+stdout '^github.com/dvyukov/go-fuzz v[^ ]+$'
+
+# Also output directories for go-fuzz-defs and go-fuzz-dep 
+# in case we need to debug this at some point in the future,
+# or in case cmd/go or go-fuzz change in the future 
+# in some way that moves these out of $GOPATH/pkg/testmod or
+# otherwise alters where these are located.
+# The exact location might not be critical, but we should be aware of what it is, 
+# so capture the location here in this test so we know if it changes later.
+exec go list -f {{.Dir}} github.com/dvyukov/go-fuzz/go-fuzz-defs
+stdout 'gopath.pkg.mod.github.com.dvyukov.go-fuzz@v[^ ]+.go-fuzz-defs$'
+exec go list -f {{.Dir}} github.com/dvyukov/go-fuzz/go-fuzz-dep
+stdout 'gopath.pkg.mod.github.com.dvyukov.go-fuzz@v[^ ]+.go-fuzz-dep$'
+
+# Define a simple module 'mod' that has a fuzz function.
+
+-- testmod/go.mod --
+module example.com/testmod
+
+-- testmod/fuzz.go --
+package testmod
+
+func FuzzMod(data []byte) int {
+    return 0
+}
diff --git a/testscripts/mod_inside_gopath.txt b/testscripts/mod_inside_gopath.txt
new file mode 100644
index 000000000..f9970f21b
--- /dev/null
+++ b/testscripts/mod_inside_gopath.txt
@@ -0,0 +1,99 @@
+# These steps validate we can fuzz inside of GOPATH
+# with GO111MODULE on and off.
+
+# Here, we purposefully do not test with GO111MODULE auto and unset,
+# because the meaning of auto within GOPATH has changed with
+# Go 1.13, and the meaning of unset will likely change in Go 1.14
+# (and some chance auto could change meaning again in Go 1.14).
+# However, the Go 1.13 and 1.14 changes ultimately translate
+# to what external conditions enable or disable module mode
+# for cmd/go. Testing here with GO111MODULE on and off allows
+# us to explicitly test with cmd/go's module mode enabled and disabled in GOPATH,
+# which is our current goal here. (These are not cmd/go tests after all).
+ 
+# The foo module being fuzzed depends on a 'replace' 
+# in its go.mod to find one of its dependencies, and that dependency bar
+# is located *outside* of GOPATH, which is a way to check that
+# module-mode is required to compile successfully.
+# (Other go-fuzz tests validate that non-module targets work in GOPATH with 
+# cmd/go's module mode disabled; that has been the status quo).
+
+# Enter a simple module with a fuzz function.
+# This is inside GOPATH.
+cd gopath/src/example.com/foo
+
+# Copy a pristine go.mod file.
+cp go.mod_PRISTINE go.mod
+
+# First, we test with GO111MODULE=on, which will likely be the default in Go 1.14.
+env GO111MODULE=on
+
+# Sanity check the module seems well formed.
+exec go list -m all
+stdout '^example.com/foo$'
+exec go build
+
+# Ask go-fuzz-build to build, including specifying the fuzz function for mod.
+exec go-fuzz-build -func=FuzzMod
+exists foo-fuzz.zip
+
+# Validate we can start fuzzing.
+# Note that 'timeout(1)' will error here, so we preface the invocation with '!'.
+# For travis on Windows, we install 'timeout(1)' as part of our travis setup steps.
+# To test this locally on Windows, you might need to change 'timeout' to '\cygwin64\bin\timeout' or similar.
+! exec timeout 5 go-fuzz -procs=1 -func=FuzzMod
+stderr 'workers: \d+, corpus: '
+
+# Clean up.
+cp go.mod_PRISTINE go.mod
+rm foo-fuzz.zip
+
+# Second, we test with GO111MODULE=off.
+# The meaning of this is unlikely to change in Go 1.14,
+# altough in some (distant?) future, GO111MODULE=off might
+# no longer be supported.
+env GO111MODULE=off
+
+# Confirm 'go list -m' and 'go build' fail.
+! exec go list -m all
+! exec go build
+
+# Confirm go-fuzz-build fails.
+! exec go-fuzz-build -func=FuzzMod
+! exists foo-fuzz.zip
+
+# Clean up (mainly in case we later add another test below).
+cp go.mod_PRISTINE go.mod
+rm foo-fuzz.zip
+
+# Define two modules.
+# example.com/foo has a fuzz function, and depends on example.com/bar.
+# foo is inside GOPATH, and bar is outside of GOPATH.
+
+-- gopath/src/example.com/foo/go.mod_PRISTINE --
+module example.com/foo
+
+require example.com/bar v0.0.0
+
+replace example.com/bar => ../../../../bar
+
+-- gopath/src/example.com/foo/fuzz.go --
+package foo
+
+import "example.com/bar"
+
+func FuzzMod(data []byte) int {
+	bar.Bar()
+	return 0
+}
+
+-- bar/go.mod --
+module example.com/bar
+
+-- bar/bar.go --
+package bar
+
+func Bar() string {
+	return "hello from bar"
+}
+
diff --git a/testscripts/mod_outside_gopath.txt b/testscripts/mod_outside_gopath.txt
new file mode 100644
index 000000000..e90e5b24b
--- /dev/null
+++ b/testscripts/mod_outside_gopath.txt
@@ -0,0 +1,131 @@
+# These steps validate we can fuzz outside of GOPATH
+# with GO111MODULE auto, on, unset, and off.
+ 
+# The foo module being fuzzed depends on a 'replace' 
+# in its go.mod to find one of its dependencies,
+# which is a way to check that module-mode is required to compile successfully.
+
+# Enter a simple module with a fuzz function.
+# This is outside of GOPATH.
+cd foo
+
+# Copy a pristine go.mod file.
+cp go.mod_PRISTINE go.mod
+
+# First, we test with GO111MODULE=auto, which is the default
+# in Go 1.11-1.13. (In 1.14, the default will likely be GO111MODULE=on).
+env GO111MODULE=auto
+
+# Sanity check the module seems well formed.
+exec go list -m all
+stdout '^example.com/foo$'
+exec go build
+
+# Because we are outside GOPATH, the presence of the 'go.mod' here will
+# enable module-module for cmd/go with GO111MODULE=auto.
+# This is true in Go 1.11-1.13, and likely will be true in 1.14 as well.
+# Ask go-fuzz-build to build, including specifying the fuzz function for mod.
+exec go-fuzz-build -func=FuzzMod
+exists foo-fuzz.zip
+
+# Validate we can start fuzzing.
+# Note that 'timeout(1)' will error here, so we preface the invocation with '!'.
+# For travis on Windows, we install 'timeout(1)' as part of our travis setup steps.
+# To test this locally on Windows, you might need to change 'timeout' to '\cygwin64\bin\timeout' or similar.
+! exec timeout 5 go-fuzz -procs=1 -func=FuzzMod
+stderr 'workers: \d+, corpus: '
+
+# Clean up.
+cp go.mod_PRISTINE go.mod
+rm foo-fuzz.zip
+
+# Second, we test with GO111MODULE=on, which will likely be the default in Go 1.14.
+env GO111MODULE=on
+
+# Sanity check we can still interact with the module.
+exec go list -m all
+stdout '^example.com/foo$'
+exec go build
+
+# Ask go-fuzz-build to build, including specifying the fuzz function for mod.
+exec go-fuzz-build -func=FuzzMod
+exists foo-fuzz.zip
+
+# Validate we can start fuzzing.
+! exec timeout 5 go-fuzz -procs=1 -func=FuzzMod
+stderr 'workers: \d+, corpus: '
+
+# Clean up.
+cp go.mod_PRISTINE go.mod
+rm foo-fuzz.zip
+
+# Third, we test with GO111MODULE unset.
+# The meaning of this will likely change in Go 1.14, but given we are 
+# outside of GOPATH, in theory these steps will still work even in Go 1.14.
+env GO111MODULE=
+
+# Sanity check we can still interact with the module.
+exec go list -m all
+stdout '^example.com/foo$'
+exec go build
+
+# Ask go-fuzz-build to build, including specifying the fuzz function for mod.
+exec go-fuzz-build -func=FuzzMod
+exists foo-fuzz.zip
+
+# Validate we can start fuzzing.
+! exec timeout 5 go-fuzz -procs=1 -func=FuzzMod
+stderr 'workers: \d+, corpus: '
+
+# Clean up.
+cp go.mod_PRISTINE go.mod
+rm foo-fuzz.zip
+
+# Fourth, we test with GO111MODULE=off.
+# The meaning of this is unlikely to change in Go 1.14,
+# altough in some (distant?) future, GO111MODULE=off might
+# no longer be supported.
+env GO111MODULE=off
+
+# Confirm 'go list -m' and 'go build' fail.
+! exec go list -m all
+! exec go build
+
+# Confirm go-fuzz-build fails.
+! exec go-fuzz-build -func=FuzzMod
+! exists foo-fuzz.zip
+
+# Clean up (mainly in case we later add another test below).
+cp go.mod_PRISTINE go.mod
+rm foo-fuzz.zip
+
+# Define two modules.
+# example.com/foo has a fuzz function, and depends on example.com/bar.
+
+-- foo/go.mod_PRISTINE --
+module example.com/foo
+
+require example.com/bar v0.0.0
+
+replace example.com/bar => ../bar
+
+-- foo/fuzz.go --
+package foo
+
+import "example.com/bar"
+
+func FuzzMod(data []byte) int {
+	bar.Bar()
+	return 0
+}
+
+-- bar/go.mod --
+module example.com/bar
+
+-- bar/bar.go --
+package bar
+
+func Bar() string {
+	return "hello from bar"
+}
+
diff --git a/testscripts/mod_v2.txt b/testscripts/mod_v2.txt
new file mode 100644
index 000000000..811ada271
--- /dev/null
+++ b/testscripts/mod_v2.txt
@@ -0,0 +1,76 @@
+# These steps validate we can fuzz a module that depends on a v2 module.
+# v2 modules are particularly problematic if not handled properly.
+# We validate that 'go-fuzz' and 'go-fuzz-build' work with and without 
+# the package path and fuzz function being specified.
+
+# Enter a module that depends on a v2+ module.
+# Note that we are outside of GOPATH, so the presence of the 'go.mod' here will
+# enable module-module for cmd/go (which is true in Go 1.11-1.13, and likely will be true in 1.14 as well).
+cd foo 
+
+# Sanity check the module seems well formed.
+exec go list -m all
+stdout '^example.com/foo$'
+stdout '^example.com/bar/v2 v2.0.0 => ../bar$'
+exec go build
+
+# Ask go-fuzz-build to build, first specifying the package path and fuzz function for foo.
+# foo is a module itself, and foo also depends on a v2 module bar.
+exec go-fuzz-build -func=FuzzDependOnV2Mod example.com/foo
+exists foo-fuzz.zip
+
+# Validate we can start fuzzing, first with the fuzz function specified.
+# Note that 'timeout(1)' will error here, so we preface the invocation with '!'.
+# For travis on Windows, we install 'timeout(1)' as part of our travis setup steps.
+# To test this locally on Windows, you might need to change 'timeout' to '\cygwin64\bin\timeout' or similar.
+! exec timeout 5 go-fuzz -procs=1 -func=FuzzDependOnV2Mod
+stderr 'workers: \d+, corpus: '
+
+# Validate we can start fuzzing, now without a fuzz function specified.
+! exec timeout 5 go-fuzz -procs=1
+stderr 'workers: \d+, corpus: '
+
+# Ask go-fuzz-build to build again, but now do not specify the fuzz function.
+rm foo-fuzz.zip
+exec go-fuzz-build
+exists foo-fuzz.zip
+
+# Validate we can start fuzzing with the new zip, first with a fuzz function specified.
+! exec timeout 5 go-fuzz -procs=1 -func=FuzzDependOnV2Mod
+stderr 'workers: \d+, corpus: '
+
+# Validate we can start fuzzing with the new zip, now without a fuzz function specified.
+! exec timeout 5 go-fuzz -procs=1
+stderr 'workers: \d+, corpus: '
+
+# Define two modules.
+# example.com/foo has a fuzz function, and depends on example.com/bar/v2.
+# The v2 module is following the 'major branch' approach for v2+ modules, 
+# not 'major subdirectory' approach. (Details: https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher)
+
+-- foo/go.mod --
+module example.com/foo
+
+require example.com/bar/v2 v2.0.0
+
+replace example.com/bar/v2 => ../bar
+
+-- foo/fuzz.go --
+package foo
+
+import "example.com/bar/v2"
+
+func FuzzDependOnV2Mod(data []byte) int {
+    bar.Bar()
+    return 0
+}
+
+-- bar/go.mod --
+module example.com/bar/v2
+
+-- bar/bar.go --
+package bar
+
+func Bar() string {
+    return "hello from bar"
+}
diff --git a/testscripts/mod_vendor.txt b/testscripts/mod_vendor.txt
new file mode 100644
index 000000000..7773bc122
--- /dev/null
+++ b/testscripts/mod_vendor.txt
@@ -0,0 +1,133 @@
+# These steps validate we get a proper error
+# if we try to fuzz a module using GOFLAGS=-mod=vendor.
+# It then stops.
+
+# For the future, we have some unexecuted steps after the stop.
+# These don't run now, but attempt to validate fuzzing a module using -mod=vendor,
+# including validating that the actual code in the 'vendor' directory is used.
+
+# TODO: -mod=vendor is not yet supported with go-fuzz, so the majority of this test is disabled with a 'stop' command.
+# If you change go-fuzz-build to remove the check for -mod=vendor, it currently fails with error:
+#    > exec go-fuzz-build -func=FuzzDependOnVendor
+#    [stderr]
+#    could not load packages: go [list -e -json -compiled=true -test=false -export=false -deps=true -find=false -tags gofuzz -- . 
+#      github.com/dvyukov/go-fuzz/go-fuzz-dep]: exit status 1: build github.com/dvyukov/go-fuzz/go-fuzz-dep: 
+#      cannot load github.com/dvyukov/go-fuzz/go-fuzz-dep: 
+#      open $WORK\foo\vendor\github.com\dvyukov\go-fuzz\go-fuzz-dep: The system cannot find the path specified.
+
+# TODO: this test currently relies on latest go-fuzz/go-fuzz-dep and go-fuzz-defs
+# versions on master being downloaded from github as part of the tests.
+# If this test is run in a go-fuzz branch and there have been signficant
+# changes to go-fuzz/go-fuzz-dep and go-fuzz-defs in the branch, this test 
+# will use the versions from master and might incorrectly fail.
+# If that happens, the foo/go.mod below can temporarily be modified to point 
+# to the branch, or this test can temporarily be ignored or removed.
+
+# Enter a module that has a vendor directory, where the code in the vendor
+# directory has been edited compared (so that we can validate the vendored
+# code is in fact used).
+# Note that we are outside of GOPATH, so the presence of the 'go.mod' here will
+# enable module-module for cmd/go (which is true in Go 1.11-1.13, and likely will be true in 1.14 as well).
+cd foo 
+
+# Sanity check the module seems well formed.
+exec go list -m all
+stdout '^example.com/foo$'
+stdout '^example.com/bar v0.0.0 => ../bar$'
+exec go build
+
+# Do an intial 'go mod vendor'
+exec go mod vendor
+
+# Overwrite the original bar with bad code that will not compile,
+# while leaving the vendored version of bar alone 
+# so that vendored code still compiles.
+cp ../bar/bar.go_BAD ../bar/bar.go 
+
+# Sanity check the good code compiles, and bad code fails.
+# Successful compilation here relies on -mod=vendor being enabled.
+env GOFLAGS=-mod=vendor
+exec go build
+env GOFLAGS=
+! exec go build
+
+# Validate we get the expected error if we try to run go-fuzz-build
+# with -mod=vendor in GOFLAGS.
+env GOFLAGS=-mod=vendor
+! exec go-fuzz-build -func=FuzzDependOnVendor
+stderr 'GOFLAGS with -mod=vendor is not supported'
+env GOFLAGS='-v -mod=vendor'
+! exec go-fuzz-build -func=FuzzDependOnVendor
+stderr 'GOFLAGS with -mod=vendor is not supported'
+
+# This is as far as we can currently go here, given go-fuzz does not support -mod=vendor.
+# The remainder of the test has never passed, but we will leave here in case useful
+# in the future. The rest of the tests are set to pass as if GOFLAGS=-mod=vendor worked,
+# but they have never run, so might have a typo or other mistake.
+stop 'do not run actual vendoring test. -mod=vendor not yet supported.'
+
+# Ask go-fuzz-build to build, specifying the fuzz function for foo.
+# First, we rely on -mod=vendor being set.
+env GOFLAGS=-mod=vendor
+exec go-fuzz-build -func=FuzzDependOnVendor
+exists foo-fuzz.zip
+
+# Validate we can start fuzzing.
+# Note that 'timeout(1)' will error here, so we preface the invocation with '!'.
+# For travis on Windows, we install 'timeout(1)' as part of our travis setup steps.
+# To test this locally on Windows, you might need to change 'timeout' to '\cygwin64\bin\timeout' or similar.
+! exec timeout 5 go-fuzz -procs=1 -func=FuzzUsingVendor
+stderr 'workers: \d+, corpus: '
+
+# Ask go-fuzz-build to build again, but now we do not specify -mod=vendor, 
+# which results in a compilation failure because the non-vendored code
+# deliberately does not compile (as a way to validate we are using the vendored code above).
+env GOFLAGS=
+rm foo-fuzz.zip
+! exec go-fuzz-build
+! exists foo-fuzz.zip
+
+# Define two modules.
+# example.com/foo has a fuzz function, and depends on example.com/bar.
+# foo has vendored bar and (in theory) made a code change inside foo's vendor 
+# directory. The bar code inside foo's vendor compiles, but the original
+# bar code does not compile.
+
+-- foo/fuzz.go --
+package foo
+
+import "example.com/bar"
+
+func FuzzUsingVendor(data []byte) int {
+	bar.Bar()
+	return 0
+}
+
+-- foo/go.mod --
+module example.com/foo
+
+require (
+    example.com/bar v0.0.0
+    github.com/dvyukov/go-fuzz latest
+)
+
+replace example.com/bar => ../bar
+
+-- bar/bar.go_BAD --
+package bar
+
+// This will not compile.
+func BarWillNotCompile() string {
+
+}
+
+-- bar/bar.go --
+package bar
+
+// This compiles.
+func Bar() string {
+	return "hello from bar"
+}
+
+-- bar/go.mod --
+module example.com/bar