Proposal: Multi-Module Workspaces in cmd/go

Author(s): Michael Matloob

Last updated: 2021-04-22

Discussion at


This proposal describes a new workspace mode in the go command for editing multiple modules. The presence of a file in the working directory or a containing directory will put the go command into workspace mode. The file specifies a set of local modules that comprise a workspace. When invoked in workspace mode, the go command will always select these modules and a consistent set of dependencies.


These terms are used often in this document. The Go Modules Reference and its Glossary provide more detail.

  • Main modules: The module the user is working in. Before this proposal, this is the single module containing the directory where the go command is invoked. This module is used as the starting point when running MVS. This proposal proposes allowing multiple main modules.
  • Module version: From the perspective of the go command, a module version is a particular instance of a module. This can be a released version or pseudo version of a module, or a directory with a go.mod file.
  • Build list: The build list is the list of module versions used for a build command such as go build, go list, or go test. The build list is determined from the main module's go.mod file and go.mod files in transitively required modules using minimal version selection. The build list contains versions for all modules in the module graph, not just those relevant to a specific command.
  • MVS or Minimal Version Selection: The algorithm used to determine the versions of all modules that will be used in a build. See the Minimal Version Selection section in the Go Modules Reference for more information.
  • mode: This document references module mode and workspace mode. The modes are the different ways the go command determines which modules and packages it's building and how dependencies are resolved. For example the -mod=readonly mode uses the versions of the modules listed in the go.mod file and fails if it would need to add in a new module dependency, and the -mod=vendor mode uses the modules in the vendor directory.


Users often want to make changes across multiple modules: for instance, to introduce a new interface in a package in one module along with a usage of that interface in another module. Normally, the go command recognizes a single “main” module the user can edit. Other modules are read-only and are loaded from the module cache. The replace directive is the exception: it allows users to replace the resolved version of a module with a working version on disk. But working with the replace directive can often be awkward: each module developer might have working versions at different location on disk, so having the directive in a file that needs to be distributed with the module isn't a good fit for all use cases.

gopls offers users a convenient way to make changes across modules without needing to manipulate replacements. When multiple modules are opened in a gopls workspace, it synthesizes a single go.mod file, called a supermodule that pulls in each of the modules being worked on. The supermodule results in a single build list allowing the tooling to surface changes made in a dependency module to a dependent module. But this means that gopls is building with a different set of versions than an invocation of the go command from the command line, potentially producing different results. Users would have a better experience if they could create a configuration that could be used by gopls as well as their direct invocations of cmd/go and other tools. See the Multi-project gopls workspaces document and proposal issues #37720 and #32394.


This proposal specifically tries to improve the experience in the go command (and the tools using it) for working in multi-module workspaces. That means the following are out of scope:

Tagging and releasing new versions of a module

This proposal does not address the problem of tagging and releasing new versions of modules so that new versions of dependent modules depend on new versions of the dependency modules. But these sorts of features don't belong in the go command. Even so, the workspace file can be useful for a future tool or feature that solves the tagging and releasing problem: the workspace would help the tool know the set of modules the user is working on, and together with the module dependency graph, the tool would be able to determine versions for the new modules.

Building and testing a module with the user's configuration

It would be useful for module developers to build and test their modules with the same build list seen by users of their modules. Unfortunately, there are many such build lists because those build lists depend on the set of modules the user‘s module requires, and the user needs to know what those modules are. So this proposal doesn’t try to solve that problem. But this proposal can make it easier to switch between multiple configurations, which opens the door for other tools for testing modules in different configurations.


The -workfile flag

The new -workfile flag will be accepted by module-aware build commands and most go mod subcommands. The following is a table of which commands can operate in workspace mode and which can operate in module mode. Commands that can operate in workspace mode will accept -workfile and follow the workspace resolution steps below.

go mod download, go mod graph, go mod verify and go mod why all have meanings based on the build list, so they will all work in workspace mode according to the build list. go mod edit, go mod init go mod tidy and go mod vendor only make sense in a single module context, so they will ignore the workspace. go get could make sense in workspace mode but not in all contexts, so it will also ignore the workspace.

mod inito
mod initworko
mod downloadoo
mod graphoo
mod verifyoo
mod whyoo
mod edito
mod tidyo
mod vendoro
test oo

If -workfile is set to off, workspace mode will be disabled. If it is auto (the default), workspace mode will be enabled if a file named is found in the current directory (or any of its parent directories), and disabled otherwise. If -workfile names a path to an existing file that ends in .work, workspace mode will be enabled. Any other value is an error.

If workspace mode is on, -mod=readonly must be specified either implictly or explicitly. Otherwise, the go command will return an error. If -mod is not explicitly set and file is found, -mod=readonly is set. (That is, it takes precedence over the existence of a vendor/module.txt which would normally imply -mod=vendor.)

If workspace mode is on, the file (either named by -workfile or the nearest one found when -workfile is auto) will be parsed to determine the three parameters for workspace mode: a Go version, a list of directories, and a list of replacements.

If workspace mode is on, the selected workspace file will show up in the go env variable GOWORK. When not in workspace mode, GOWORK will be off.

The file

The following is an example of a valid file:

go 1.17

directory (
    ./baz //
    ./tools //

replace => v1.4.5

The file will have a similar syntax as the go.mod file. Restrictions in go.mod lexical elements still apply to the file

The file has three directives: the go directive, the directory directive, and the replace directive.

The go Directive

The file requires a go directive. The go directive accepts a version just as it does in a go.mod file. The go directive is used to allow adding new semantics to the files without breaking previous users. It does not override go versions in invididual modules.


go 1.17

The directory directive

The directory directive takes an absolute or relative path to a directory containing a go.mod file as an argument. The syntax of the path is the same as directory replacements in replace directives. The path must be to a module directory containing a go.mod file. The file must contain at least one directory directive. The go command may optionally edit the comments on the directory directive when doing any operation in workspace mode to add the module path from the directory's go.mod file.

Note that the directory directive has no restriction on where the directory is located: module directories listed in file can be located outside the directory the file itself is in.


directory (
    ./tools //
    ./mod   //

Each directory listed (in this example ./tools and ./mod) refers to a single module: the module specified by the go.mod file in that directory. It does not refer to any other modules specified by go.mod files in subdirectories of that directory.

The modules specified by directory directives in the file are the workspace modules. The workspace modules will collectively be the main modules when doing a build in workspace mode. These modules are always selected by MVS with the version "", and their replace and exclude directives are applied.

The replace directive

The replace directive has the same syntax and semantics as the replace directive in a go.mod file.


replace ( => ../tools v0.4.1  => v0.5

The replace directives in the are applied in addition to and with higher precedence than replaces in the workspace modules. A replace directive in the file overrides replace directives in workspace modules applying to the same module or module version. If two or more workspace modules replace the same module or module version with different module versions or directories, and there is not an overriding replace in the file, the go command will report an error. The go command will report errors for replacements of workspace modules that don't refer to the same directory as the workspace module. If any of those exist in a workspace module replacing another workspace module, the user will have to explicitly replace that workspace module with its path on disk.

Semantics of workspace mode

If workspace mode is on and the file has valid syntax, the Go version provided by the file is used to control the exact behavior of workspace mode. For the first version of Go supporting workspace mode and unless changes are made in following versions the following semantics apply:

When doing a build operation under workspace mode the go command will try to find a go.mod file. If a go.mod file is found, its containing directory must be declared with a directory directive in the file. Because the build list is determined by the workspace rather than a go.mod file, outside of a module, the go command will proceed as normal to build any non-relative package paths or patterns. Outside of a module, a package composed of .go files listed on the command line resolves its imports according to the workspace, and the package‘s imports will be resolved according to the workspace’s build list.

The all pattern in workspace mode resolves to the union of all for over the set of workspace modules. all is the set of packages needed to build and test packages in the workspace modules.

To construct the build list, each of the workspace modules are main modules and are selected by MVS and their replace and exclude directives will be applied. replace directives in the file override the replaces in the workspace modules. Similar to a single main module in module mode, each of the main modules will have version "", but MVS will traverse other versions of the main modules that are depended on by transitive module dependencies. For the purposes of lazy loading, we load the explicit dependencies of each workspace module when doing the deepening scan.

Module vendor directories are ignored in workspace mode because of the requirement of -mod=readonly.

Creating and editing files

Two new subcommands will be added to go mod: go mod initwork and go mod editwork.

go mod initwork will take as arguments a (potentially empty) list of directories it will use to write out a file in the working directory with a go statement and a directory statement listing each of the directories. go mod initwork will take an optional -o flag to specify a different output file path, which can be used to create workspace files for other configurations.

go mod editwork will work similarly to go mod edit and take the following flags:

  • -fmt will reformat the file
  • -go=version will set the file's go directive to version
  • -directory=path and -dropdirectory=path will add and drop a directory directive for the given path
  • -replace and -dropreplace will work exactly as they do for go mod edit


This proposal addresses these workflows among others:


A change in one module that requires a change in another module

One common workflow is when a user wants to add a feature in an upstream module and make use of the feature in their own module. Currently, they might open the two modules in their editor through gopls, which will create a supermodule requiring and replacing both of the modules, and creating a single build list used for both of the modules. The editor tooling and builds done through the editor will use that build list, but the user will not have access to the ‘supermodule’ outside their editor: go command invocations run in their terminal outside the editor will use a different build list. The user can change their go.mod to add a replace, which will be reflected in both the editor and their go command invocations, but this is a change they will need to remember to revert before submitting.

When these changes are done often, for example because a project's code base is split among several modules, a user might want to have a consistent configuration used to join the modules together. In that case the user will want to configure their editor and the go command to always use a single build list when working in those modules. One way to do this is to work in a top level module that transitively requires the others, if it exists, and replace the dependencies. But they then need to remember to not check in the replace and always need to run their go commands from that designated module.


As an example, the gopls code base in might want to add a new function to package and start using it. If the user has the and repos in the same directory they might run:

go mod initwork ./mod ./tools

which will produce this file:

go 1.17

directory (
    ./mod //
    ./tools //

Then they could work on the new function in and its usage in and when run from any directory in the workspace the go command would present a consistent build list. When they were satisfied with their change, they could release a new version of, update's go.mod to require the new vesion of, and then turn off workspace mode with -workfile=off to make sure the change behaves as expected.

Multiple modules in the same repository that depend on each other

A further variant of the above is a module that depends on another module in the same repository. In this case checking in go.mod files that require and replace each other is not as much of a problem, but especially as the number of modules grows keeping them in sync becomes more difficult. If a user wants to keep the same build list as they move between directories so that they can continue to test against the same configuration, they will need to make sure all the modules replace each other, which is error prone. It would be far more convenient to have a single configuration linking all the modules together. Of course, this use case has the additional problem of updating the requirements on the replaced modules in the repository. This is a case of the problem of updating version requirements on released modules which is out of scope for this proposal.

Our goal is that when there are several tightly coupled modules in the same repository, users would choose to check files defining the workspace into the repository instead of replaces in the go.mod files. A monorepo with multiple modules might choose to have a file in the root, and users can change the workspace with the -workfile flag to confirm the build configurations when the modules are depended on from outside. Because the go command will operate in workspace mode when run within the repository, CI/CD systems should be configured to test with -workfile=off. If not, the CI/CD systems will not test that version requirements among the repository's modules are properly incremented to use changes in the modules.

As a counterpoint to the above, most files should exist outside of any repository: files should only be used in repositories if they contain multiple tightly-coupled modules. If a repository contains a single module, or unrelated modules, there's not much utility to adding a file because each user may have a different directory structure on their computer outside of that repository.


As a simple example the gopls binary is in the module which depends on other packages in the module. Currently, building and testing the top level gopls code is done by entering the directory of the module which replaces its usage of the


go 1.12

require (
    ... v0.1.0

replace  => ../

This replace can be removed and replaced with a file in the top level of the repo that includes both modules:

go 1.17

directory (

This allows any of the tests in either module to be run from anywhere in the repo. Of course, to release the modules, the module needs to be tagged and released, and then the module needs to require that new release.

Switching between multiple configurations

Users might want to easily be able to test their modules with different configurations of dependencies. For instance, they might want to test their module using the development versions of the dependencies, using the build list determined using the module as a single main module, and using a build list with alternate versions of dependencies that are commonly used. By making a workspace with the development versions of the dependencies and another adding the alternative versions of the dependencies with replaces, it's easy to switch between the three configurations.

Users who want to test using a subset of the workspace modules can also easily comment out some of the directory directives in their workspace file instead of making separate workspace files with the appropriate subset of workspace modules, if that works better for their workflows.

Workspaces in gopls

With this change, users will be able to configure gopls to use files describing their workspace. gopls can pass the workspace to the go command in its invocations if it's running a version of Go that supports workspaces, or can easily rewrite the workspace file into a supermodule for earlier versions. The semantics of workspace mode are not quite the same as for a supermodule in general (for instance ... and all have different meanings) but are the same or close enough for the cases that matter.

A GOPATH-like setup

While this proposal does not aim to completely recreate all GOPATH workflows, it can be used to create a setup that shares some aspects of the GOPATH setup: A user who is working with a set of modules in GOPATH, but in GOPATH mode so that all dependencies are resolved from the GOPATH tree can add a file to the base of a GOPATH directory that lists all the modules in that GOPATH (and even those in other GOPATH directories, if their path has multiple elements). Then all their dependencies that are under that GOPATH directory will continue to be resolved from those locations.

Of course there are caveats to this workflow: GOPATH packages that are not contained in a module can't be added to the workspace, and the file needs to be manually maintained to add modules instead of walking a directory tree like GOPATH mode does. And opting into workspace mode piecemeal by adding modules one by one can be frustrating because the modules outside of the new workspace will require -modfile to be set to off or another file that includes it. But even with these differences, used this way, can recreate some of the convenience of GOPATH while still providing the benefits of modules.

The workfile flag

One alternative that was considered for disabling module mode would be to have module mode be an option for the -mod flag. -mod=work would be the default and users could set any other value to turn off workspace mode. This removes the redundant knob that exists in this proposal where workspace mode is set independently of the -mod flag, but only -mod=readonly is allowed. The reason this alternative was adopted for this proposal is that it could be unintuitive and hard for users to remember to set -mod=readonly to turn workspace mode off. Users might think to set -mod=mod to turn workspace mode off even though they don't intend to modify their go.mod file.

This also avoids conflicting defaults: the existence of a file implies workspace mode, but the existence of vendor/module.txt implies -mod=vendor. Separating the configurations makes it clear that the file takes precedence.

But regardless of the above, it‘s useful to have a way to specify the path to a different file similar to the -modfile flag for the same reasons that -modfile exists. Given that -workfile exists it’s natural to add a -workfile=off option to turn off workspace mode.

The file

The configuration of multi-module workspaces is put in a file rather than being passed through an environment variable or flag because there are multiple parameters for configuration that would be difficult to put into a single flag or environment variable and unwieldy to put into multiple.

The go command locates files the same way it locates go.mod files to make it easy for users already familiar with modules to learn the rules for whether their current directory is in a workspace and which one. files allow users to operate in directories outside of any modules but still use the workspace build list. This makes it easy for users to have a GOPATH-like user experience by placing a file in their home directory linking their modules together.

Like the go.mod file, we want the format of the configuration for multi-module workspaces to be machine writable and human readable. Though there are other popular configuration formats such as yaml and json, they can often be confusing or annoying to write. The format used by the go.mod file is already familar to Go programmers, and is easy for both humans and computers to read and write.

Modules are listed by the directory containing the module‘s go.mod file rather than listing the paths to the go.mod files themselves to avoid the redundant basename in every path. Alternatively, if the go.mod files were listed directly it would be more clear that directories aren’t being searched for all modules contained under them but rather refer to a single module. Modules are required to be listed explicitly instead of allowing for patterns that match all modules under a directory because those entries would require slow directory walks each time the go command would need to load a workspace. Because a module's path is not always clear from its directory name, we will allow the go command add comments on the directory directive with the module path.

Requiring the directories listed in the file to have go.mod files means that projects without go.mod files can‘t be added to a workspace even though they can be required as implicit modules in go.mod files. To support these we would have to add to the file some way of associating the directories with go.mod files. But these projects are already getting more rare and the missing go.mod can be worked around by adding a temporary go.mod file to the project’s directory.

The naming of the go and replace directives is straightforward: they are the same as in go.mod. The directory directive is called directory because that is its argument. Using module to list the module directories could be confusing because there is already a module directive in go.mod that has a different meaning. On the other hand, names like modvers and moddir are awkward.

Semantics of workspace mode

A single build list is constructed from the set of workspace modules to give developers consistent results wherever they are in their workspace. Further, the single build list allows tooling to present a consistent view of the workspace, so that editor operations and information doesn't change in surprising ways when moving between files.

replace directives are respected when building the build list because many modules already have many replaces in them that are necessary to properly build them. Not respecting them would break users unnessesarily. replace directives exist in the workspace file to allow for resolving conflicts between replaces in workspace modules. Because all workspace modules exist as co-equals in the workspace, there is no clear and intuitive way to resolve replace conflicts without explicit input from the user. One alternative is to add special syntax for overriding replaces to make the overriding behavior more explicit, and an additional option is to add an option to add syntax to nullify replaces without overriding them.

Working in modules not listed in the workspace file is disallowed to avoid what could become a common source of confusion: if the go command stayed in workspace mode, it's possible that a command line query could resolve to a different version of the module the directory contains. Users could be confused about a go build or go list command completing successfully but not respecting changes made in the current module. On the other hand, a user could be confused about the go command implicitly ignoring the workspace if they intended the current module to be in the workspace. It is better to make the situation clear to the user to allow them either to add the current module to the workspace or explicitly turn workspace mode off according to their preference.

Module vendoring is ignored in workspace mode because it is not clear which modules' vendor directories should be respected if there are multiple workpace modules with vendor directories containing the same dependencies. Worse, if module A vendors and module B vendors, then a workspace that combines A and B would select v0.2.0 in the overall build list, but would not have any vendored copy of for that version. As the modules spec says, “Vendoring may be used to allow interoperation with older versions of Go, or to ensure that all files used for a build are stored in a single file tree.”. Because developers in workspace mode are necessarily not using an older version of Go, and the build list used by the workspace is different than that used in the module, vendoring is not as useful for workspaces as it is for individual modules. files

The go command will use the collective set of go.sum files that exist across the workspace modules to verify dependency modules, but there are cases where the go.sum files in the workspace modules collectively do not contain all sums needed to verify the build: The simpler case is if the workspace go.mod files themselves are incomplete, the go command will add missing sums to the workspace‘s file rather than to the module’s go.sum. But even if all workspace go.sum files are complete, they may still not contain all necessary sums:

If the workspace includes modules X and Y, and X imports a package from, and Y has a transitive requirement on (but does not import any packages from it), then X/go.sum will contain a checksum only for v1.0.0/go.sum and v1.0.0, and Y will contain a checksum only for v1.1.0/go.sum. No individual module will have a checksum for the source code for v1.1.0, because no module in isolation actually uses that source code.

Creating and editing files

The go mod initwork and go mod editwork subcommands are being added for the same reasons that the go go mod init and go mod edit commands exist: they make it more convenient to create and edit files. The names are awkward, but it's not clear that it would be worth making the commands named go work init and go work edit if go work would only have two subcommands.


Tools based on the go command, either directly through go list or via will work without changes with workspaces.

This change does not affect the Go language or its core libraries. But we would like to maintain the semantics of a file across versions of Go to avoid causing unnecessary churn and surprise for users.

This is why all valid files provide a Go version. Newer versions of Go will continue to respect the workspace semantics of the version of Go listed in the file. This will make it possible (if necessary) to make changes in the of workspace files in future versions of Go for users who create new workspaces or explicitly increase the Go version of their file.


The implementation for this would all be in the go command. It would need to be able to read files, which we could easily implement reusing parts of the go.mod parser. We would need to add the new -workfile flag to the Go command and modify the go command to look for the file to determine if it's in workspace mode. The most substantial part of the implementation would be to modify the module loader to be able to accept multiple main modules rather than a single main module, and run MVS with the multiple main modules when it is in workspace mode.

To avoid issues with the release cycle, if the implementation is not finished before a release, the behavior to look for a file and to turn on workspace mode can be guarded behind a GOEXPERIMENT. Without the experiment turned on it will be possible to work on the implementation even if it can‘t be completed in time because it will never be active in the release. We could also set the -workfile flag’s default to off in the first version and change it to its automatic behavior later.

Related issues

#32394 x/tools/gopls: support multi-module workspaces

Issue #32394 is about gopls' support for multi-module workspaces. gopls currently allows users to provide a “workspace root” which is a directory it searches for go.mod files to build a supermodule from. Alternatively, users can create a gopls.mod file in their workspace root that gopls will use as its supermodule. This proposal creates a concept of a workspace that is similar to that gopls that is understood by the go command so that users can have a consistent configuration across their editor and direct invocations of the go command.

#44347 proposal: cmd/go: support local experiments with interdependent modules; then retire GOPATH

Issue #44347 proposes adding a GOTINKER mode to the go command. Under the proposal, if GOTINKER is set to a directory, the go command will resolve import paths and dependencies in modules by looking first in a GOPATH-structured tree under the GOTINKER directory before looking at the module cache. This would allow users who want to have a GOPATH like workflow to build a GOPATH at GOTINKER, but still resolve most of their dependencies (those not in the GOTINKER tree) using the standard module resolution system. It also provides for a multi-module workflow for users who put their modules under GOTINKER and work in those modules.

This proposal also tries to provide some aspects of the GOPATH workflow and to help with multi-module workflows. A user could put the modules that they would put under GOTINKER in that proposal into their files to get a similar experience to the one they'd get under the GOTINKER proposal. A major difference between the proposals is that in GOTINKER modules would be found by their paths under the GOTINKER tree instead of being explicitly listed in the file. But both proposals provide for a set of replaced module directories that take precedence over the module versions that would normally be resolved by MVS, when working in any of those modules.

#26640 cmd/go: allow go.mod.local to contain replace/exclude lines

The issue of maintaining user-specific replaces in go.mod files was brought up in #26640. It proposes an alternative go.mod.local file so that local changes to the go.mod file could be made adding replaces without needing to risk local changes being committed in go.mod itself. The file provides users a place to put many of the local changes that would be put in the proposed go.mod.local file.

#39005 proposal: cmd/go: introduce a build configurations file

This issue proposes to add a mechanism to specify configurations for builds, such as build tags. This might be something that can be added in a future extension of files.

Open issues

Clearing replaces

We might want to add a mechanism to ignore all replaces of a module or module version.

For example one module in the workspace could have replace => v0.3.4 because v0.4.0 would be selected otherwise and they think it's broken. Another module in the workspace could have require v0.5.0 which fixes the incompatibilities and also adds some features that are necessary.

In that case, the user might just want to knock the replacements away, but they might not want to remove the existing replacements for policy reasons (or because the replacement is actually in a separate repo).

A rough edge with checked-in files

If a repository already has a checked in file in the module root, this can create an inconvenience for developers who want to set up a different workspace. The file will override any file that exists outside the repo, and they will need to either work with their current directory outside the repo or pass -workfile to the go command. In either case it‘s an inconvenience. The hope is that files are rarely checked into repos: checked in files are only useful in multi-module repositories (otherwise they’d point to directories outside the repo or only contain replaces) and most repositories contain only a single module.

Setting the GOWORK environment variable instead of -workfile

GOWORK can‘t be set by users because we don’t want there to be ambiguity about how to enter workspace mode, but an alternative could be to use an environment variable instead of the -workfile flag to change the location of the workspace file. Note that with the proposal as is, -workfile may be set in GOFLAGS, and that may be persisted with go env -w. Developers won't need to type it out every time.

Patterns and Anti-Patterns

If this proposal is accepted, before it is released the documentation should specify a set of patterns and anti-patterns and how to achieve certain workflows using workspaces. For instance, it should mention that single-module repositories should rarely contain files.

Future work

Versioning and releasing dependent modules

As mentioned above, this proposal does not try to solve the problem of versioning and releasing modules so that new versions of dependent modules depend on new versions of the dependency modules. A tool built in the future can use the current workspace as well as the set of dependencies in the module graph to automate this work.

Pushing down dependencies from the build list back to the workspace modules

Even though it's out of scope to update the dependencies between workspace modules because that requires a release, it might be useful to make dependency versions consistent. One idea could be to push the module versions of dependency modules back into the go.mod files of the dependency modules. But this could lead to confusion because while the dependency versions will be consistent, the dependencies between the workspace modules will still need to be updated separately.

Listing the module versions in the workspace

While modules have a single file listing all their root dependencies, the set of workspaces' root dependencies is split among many files, and the same is true of the set of replaces. It may be helpful to add a command to list the effective set of root dependencies and replaces and which go.mod file each of them comes from.

Perhaps there could be a command named go mod workstatus that gives an overview of the status of the modules in the workspace.