A Bazel toolchain is a configuration provider that tells a build rule what build tools, such as compilers and linkers, to use and how to configure them using parameters defined by the rule's creator.

When a build runs, Bazel performs toolchain resolution based on the specified execution and target platforms to determine and apply the toolchain most appropriate to that build. It does so by matching the constraints specified in the project's BUILD file(s) with the constraints specified in the toolchain definition.

Toolchain Resolution

Inputs to the resolution mechanism include the required toolchain types (including none) and the target platform. The resolution mechanism outputs a single execution platform and a toolchain type-to-toolchain map. Toolchain resolution works as follows:

  1. Collect all available execution platforms, including the host. This is an ordered list.
  2. Collect all available toolchains. This is also an ordered list.
    • Toolchains are collected from the following sources:
      1. Any extra toolchains given on the command line via the --extra_toolchains flag.
      2. Any toolchains in the WORKSPACE file, via the register_toolchains function.
  3. For each toolchain type and execution platform, select the first toolchain that matches the execution platform and target platform.
  4. With the full set of toolchains and execution platforms for each type, select the first execution platform that can satisfy all toolchain types.

Execution platforms listed first are preferred, and toolchains listed first are preferred. Every configured target has the same execution platform for all actions that target generates.

Because Bazel always selects the first matching toolchain, order the toolchains by preference if you expect the possibility of multiple matches.

Note: Some Bazel rules do not yet support toolchain resolution.

Defining a toolchain

Defining a toolchain requires the following:

  • Toolchain rule - a rule invoked in a custom build or test rule that specifies the build tool configuration options particular to the toolchain and supported platforms (for example, go_toolchain). This rule must return a ToolchainInfo provider. The toolchain rule is lazily instantiated by Bazel on an as-needed basis. Because of this, a toolchain rule's dependencies can be as complex as needed, including reliance on remote repositories, without affecting builds that do not use them.

  • Toolchain definition - tells Bazel which platform constraints apply to the toolchain using the toolchain() rule. This rule must specify a unique toolchain type label, which is used as input during toolchain resolution.

  • Toolchain registration - makes the toolchain available to a Bazel project using the register_toolchains() function in the project's WORKSPACE file.

Creating a toolchain rule

Toolchain rules are rules that create and return providers. To define a toolchain rule, first determine the information that the new rule will require.

In the example below, we are adding support for a new programming language, so we need to specify paths to the compiler and the system libraries, plus a flag that determines the CPU architecture for which Bazel builds the output.

def _my_toolchain_impl(ctx):
  toolchain = platform_common.ToolchainInfo(
    compiler = ctx.attr.compiler,
    system_lib = ctx.attr.system_lib,
    arch_flags = ctx.attr.arch_flags,
  return [toolchain]

my_toolchain = rule(
  attrs = {
    'compiler': attr.string(),
    'system_lib': attr.string(),
    'arch_flags': attr.string_list(),

An example invocation of the rule looks as follows:

  name = 'linux_toolchain_impl',
  compiler = '@remote_linux_repo//compiler:compiler_binary',
  system_lib = '@remote_linux_repo//library:system_library',
  arch_flags = [

  name = 'darwin_toolchain_impl',
  compiler = '@remote_darwin_repo//compiler:compiler_binary',
  system_lib = '@remote_darwin_repo//library:system_library',
  arch_flags = [
    #'--debug_everything', # --debug_everything currently broken on Darwin

Creating a toolchain definition

The toolchain definition is an instance of the toolchain() rule that specifies the toolchain type, execution and target constraints, and the label of the actual rule-specific toolchain. The use of the toolchain() rule enables the lazy loading of toolchains.

Below is an example toolchain definition:

toolchain_type(name = 'my_toolchain_type')

  name = 'linux_toolchain',
  toolchain_type = '//path/to:my_toolchain_type',
  exec_compatible_with = [
  target_compatible_with = [
  toolchain = ':linux_toolchain_impl',

Registering a toolchain

Once the toolchain rule and definition exist, register the toolchain to make Bazel aware of it. You can register a toolchain either via the project's WORKSPACE file or specify it in the --extra_toolchains flag.

Below is an example toolchain registration in a WORKSPACE file:


Using a toolchain in a rule

To use a toolchain in a rule, add the toolchain type to the rule definition. For example:

my_library = rule(
  toolchains = ['//path/to:my_toolchain_type']

When using the ctx.toolchains rule, Bazel checks the execution and target platforms, and select the first toolchain that matches. The rule implementation can then access the toolchain as follows:

def _my_library_impl(ctx):
  toolchain = ctx.toolchains['//path/to:my_toolchain_type']
  command = '%s -l %s %s' % (toolchain.compiler, toolchain.system_lib, toolchain.arch_flags)

Debugging a toolchain

When adding toolchain support to an existing rule, use the --toolchain_resolution_debug flag to make toolchain resolution verbose. Bazel will output names of toolchains it is checking and skipping during the resolution process.