diff --git a/rules/common/private/utils.bzl b/rules/common/private/utils.bzl
index c755d588..7efa8480 100644
--- a/rules/common/private/utils.bzl
+++ b/rules/common/private/utils.bzl
@@ -65,7 +65,11 @@ def write_launcher(
     # runfiles_enabled = ctx.configuration.runfiles_enabled()
     runfiles_enabled = False
 
-    java_runtime_info = ctx.attr._target_jdk[java_common.JavaRuntimeInfo]
+    # See https://bazel.build/extending/config#accessing-attributes-with-transitions:
+    # "When attaching a transition to an outgoing edge (regardless of whether the transition is a
+    # 1:1 or 1:2+ transition), `ctx.attr` is forced to be a list if it isn't already. The order of
+    # elements in this list is unspecified."
+    java_runtime_info = ctx.attr._target_jdk[0][java_common.JavaRuntimeInfo]
     java_executable = java_runtime_info.java_executable_runfiles_path
     if not paths.is_absolute(java_executable):
         java_executable = workspace_name + "/" + java_executable
diff --git a/rules/private/phases/phase_binary_launcher.bzl b/rules/private/phases/phase_binary_launcher.bzl
index 5d969cea..2c9116f0 100644
--- a/rules/private/phases/phase_binary_launcher.bzl
+++ b/rules/private/phases/phase_binary_launcher.bzl
@@ -35,7 +35,12 @@ def phase_binary_launcher(ctx, g):
             files = inputs + files,
             transitive_files = depset(
                 order = "default",
-                transitive = [ctx.attr._target_jdk[java_common.JavaRuntimeInfo].files, g.javainfo.java_info.transitive_runtime_jars],
+
+                # See https://bazel.build/extending/config#accessing-attributes-with-transitions:
+                # "When attaching a transition to an outgoing edge (regardless of whether the
+                # transition is a 1:1 or 1:2+ transition), `ctx.attr` is forced to be a list if it
+                # isn't already. The order of elements in this list is unspecified."
+                transitive = [ctx.attr._target_jdk[0][java_common.JavaRuntimeInfo].files, g.javainfo.java_info.transitive_runtime_jars],
             ),
             collect_default = True,
         ),
diff --git a/rules/private/phases/phase_javainfo.bzl b/rules/private/phases/phase_javainfo.bzl
index 6362b50b..f3dce17c 100644
--- a/rules/private/phases/phase_javainfo.bzl
+++ b/rules/private/phases/phase_javainfo.bzl
@@ -3,6 +3,7 @@ load(
     "find_java_runtime_toolchain",
     "find_java_toolchain",
 )
+load("@rules_java//java/common:java_common.bzl", "java_common")
 load(
     "@rules_scala_annex//rules:providers.bzl",
     _ScalaConfiguration = "ScalaConfiguration",
@@ -34,7 +35,12 @@ def phase_javainfo(ctx, g):
                 ctx.actions,
                 jar = ctx.outputs.jar,
                 target_label = ctx.label,
-                java_toolchain = find_java_toolchain(ctx, ctx.attr._java_toolchain),
+
+                # See https://bazel.build/extending/config#accessing-attributes-with-transitions:
+                # "When attaching a transition to an outgoing edge (regardless of whether the
+                # transition is a 1:1 or 1:2+ transition), `ctx.attr` is forced to be a list if it
+                # isn't already. The order of elements in this list is unspecified."
+                java_toolchain = find_java_toolchain(ctx, ctx.attr._java_toolchain[0]),
             )
 
         source_jar_name = ctx.outputs.jar.basename.replace(".jar", "-src.jar")
@@ -47,7 +53,7 @@ def phase_javainfo(ctx, g):
             ctx.actions,
             output_source_jar = output_source_jar,
             sources = ctx.files.srcs,
-            java_toolchain = find_java_toolchain(ctx, ctx.attr._java_toolchain),
+            java_toolchain = find_java_toolchain(ctx, ctx.attr._java_toolchain[0]),
         )
 
         java_info = JavaInfo(
diff --git a/rules/private/phases/phase_test_launcher.bzl b/rules/private/phases/phase_test_launcher.bzl
index 11854c69..4694572c 100644
--- a/rules/private/phases/phase_test_launcher.bzl
+++ b/rules/private/phases/phase_test_launcher.bzl
@@ -16,7 +16,11 @@ load(
 #
 
 def phase_test_launcher(ctx, g):
-    files = ctx.attr._target_jdk[java_common.JavaRuntimeInfo].files.to_list() + [g.compile.zinc_info.analysis_store]
+    # See https://bazel.build/extending/config#accessing-attributes-with-transitions:
+    # "When attaching a transition to an outgoing edge (regardless of whether the transition is a
+    # 1:1 or 1:2+ transition), `ctx.attr` is forced to be a list if it isn't already. The order of
+    # elements in this list is unspecified."
+    files = ctx.attr._target_jdk[0][java_common.JavaRuntimeInfo].files.to_list() + [g.compile.zinc_info.analysis_store]
 
     coverage_replacements = {}
     coverage_runner_jars = depset(direct = [])
@@ -31,7 +35,7 @@ def phase_test_launcher(ctx, g):
         coverage_replacements[jar] if jar in coverage_replacements else jar
         for jar in g.javainfo.java_info.transitive_runtime_jars.to_list()
     ])
-    runner_jars = depset(transitive = [ctx.attr.runner[JavaInfo].transitive_runtime_jars, coverage_runner_jars])
+    runner_jars = depset(transitive = [ctx.attr.runner[0][JavaInfo].transitive_runtime_jars, coverage_runner_jars])
     all_jars = [test_jars, runner_jars]
 
     args = ctx.actions.args()
@@ -43,7 +47,7 @@ def phase_test_launcher(ctx, g):
         args.add_all("--shared_classpath", shared_deps.transitive_runtime_jars, map_each = _test_launcher_short_path)
     elif ctx.attr.isolation == "process":
         subprocess_executable = ctx.actions.declare_file("{}/subprocess".format(ctx.label.name))
-        subprocess_runner_jars = ctx.attr.subprocess_runner[JavaInfo].transitive_runtime_jars
+        subprocess_runner_jars = ctx.attr.subprocess_runner[0][JavaInfo].transitive_runtime_jars
         all_jars.append(subprocess_runner_jars)
         files += _write_launcher(
             ctx,
diff --git a/rules/private/phases/phase_zinc_compile.bzl b/rules/private/phases/phase_zinc_compile.bzl
index 59976f34..58246420 100644
--- a/rules/private/phases/phase_zinc_compile.bzl
+++ b/rules/private/phases/phase_zinc_compile.bzl
@@ -30,7 +30,11 @@ def phase_zinc_compile(ctx, g):
     javacopts = [
         ctx.expand_location(option, ctx.attr.data)
         for option in ctx.attr.javacopts + java_common.default_javac_opts(
-            java_toolchain = find_java_toolchain(ctx, ctx.attr._java_toolchain),
+            # See https://bazel.build/extending/config#accessing-attributes-with-transitions:
+            # "When attaching a transition to an outgoing edge (regardless of whether the transition
+            # is a 1:1 or 1:2+ transition), `ctx.attr` is forced to be a list if it isn't already.
+            # The order of elements in this list is unspecified."
+            java_toolchain = find_java_toolchain(ctx, ctx.attr._java_toolchain[0]),
         )
     ]
 
diff --git a/rules/register_toolchain.bzl b/rules/register_toolchain.bzl
index 9f8112dd..0b02bb38 100644
--- a/rules/register_toolchain.bzl
+++ b/rules/register_toolchain.bzl
@@ -14,6 +14,7 @@ load(
     "phase_zinc_compile",
     "phase_zinc_depscheck",
 )
+load("@rules_scala_annex_scala_toolchain//:default.bzl", "default_scala_toolchain_name")
 
 def _bootstrap_configuration_impl(ctx):
     return [
@@ -205,7 +206,7 @@ def _make_register_toolchain(configuration_rule):
 register_bootstrap_toolchain = _make_register_toolchain(_bootstrap_configuration)
 register_zinc_toolchain = _make_register_toolchain(_zinc_configuration)
 
-def _scala_toolchain_transition_impl(_, attr):
+def _scala_toolchain_incoming_transition_impl(settings, attr):
     if attr.scala_toolchain_name == "":
         return {}
 
@@ -213,13 +214,24 @@ def _scala_toolchain_transition_impl(_, attr):
         "@rules_scala_annex_scala_toolchain//:scala-toolchain": attr.scala_toolchain_name,
     }
 
-scala_toolchain_transition = transition(
-    implementation = _scala_toolchain_transition_impl,
+scala_toolchain_incoming_transition = transition(
+    implementation = _scala_toolchain_incoming_transition_impl,
+    inputs = ["@rules_scala_annex_scala_toolchain//:scala-toolchain"],
+    outputs = ["@rules_scala_annex_scala_toolchain//:scala-toolchain"],
+)
+
+def _scala_toolchain_outgoing_transition_impl(_1, _2):
+    return {
+        "@rules_scala_annex_scala_toolchain//:scala-toolchain": default_scala_toolchain_name,
+    }
+
+scala_toolchain_outgoing_transition = transition(
+    implementation = _scala_toolchain_outgoing_transition_impl,
     inputs = [],
     outputs = ["@rules_scala_annex_scala_toolchain//:scala-toolchain"],
 )
 
-scala_toolchain_transition_attributes = {
+scala_toolchain_attributes = {
     "scala_toolchain_name": attr.string(
         doc = "The name of the Scala toolchain to use for this target (as provided to `register_*_toolchain`)",
     ),
diff --git a/rules/scala.bzl b/rules/scala.bzl
index 0fa4a5f6..ea998d53 100644
--- a/rules/scala.bzl
+++ b/rules/scala.bzl
@@ -41,12 +41,14 @@ load(":jvm.bzl", _labeled_jars = "labeled_jars")
 load(":providers.bzl", _ScalaRulePhase = "ScalaRulePhase")
 load(
     ":register_toolchain.bzl",
-    _scala_toolchain_transition = "scala_toolchain_transition",
-    _scala_toolchain_transition_attributes = "scala_toolchain_transition_attributes",
+    _scala_toolchain_incoming_transition = "scala_toolchain_incoming_transition",
+    _scala_toolchain_outgoing_transition = "scala_toolchain_outgoing_transition",
+    _scala_toolchain_attributes = "scala_toolchain_attributes",
 )
 
 _compile_private_attributes = {
     "_java_toolchain": attr.label(
+        cfg = _scala_toolchain_outgoing_transition,
         default = Label("@bazel_tools//tools/jdk:current_java_toolchain"),
     ),
     "_host_javabase": attr.label(
@@ -75,6 +77,7 @@ _compile_private_attributes = {
 
 _compile_attributes = {
     "srcs": attr.label_list(
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The source Scala and Java files (and `-sources.jar` `.srcjar` `-src.jar` files of those).",
         allow_files = [
             ".scala",
@@ -86,10 +89,12 @@ _compile_attributes = {
         flags = ["DIRECT_COMPILE_TIME_INPUT"],
     ),
     "data": attr.label_list(
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The additional runtime files needed by this library.",
         allow_files = True,
     ),
     "deps": attr.label_list(
+        cfg = _scala_toolchain_outgoing_transition,
         aspects = [
             _labeled_jars,
             _coverage_replacements_provider.aspect,
@@ -98,14 +103,17 @@ _compile_attributes = {
         providers = [JavaInfo],
     ),
     "deps_used_whitelist": attr.label_list(
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The JVM library dependencies to always consider used for `scala_deps_used` checks.",
         providers = [JavaInfo],
     ),
     "deps_unused_whitelist": attr.label_list(
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The JVM library dependencies to always consider unused for `scala_deps_direct` checks.",
         providers = [JavaInfo],
     ),
     "runtime_deps": attr.label_list(
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The JVM runtime-only library dependencies.",
         providers = [JavaInfo],
     ),
@@ -113,6 +121,7 @@ _compile_attributes = {
         doc = "The Javac options.",
     ),
     "plugins": attr.label_list(
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The Scalac plugins.",
         providers = [JavaInfo],
     ),
@@ -121,10 +130,12 @@ _compile_attributes = {
     ),
     "resources": attr.label_list(
         allow_files = True,
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The files to include as classpath resources.",
     ),
     "resource_jars": attr.label_list(
         allow_files = [".jar"],
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The JARs to merge into the output JAR.",
     ),
     "scalacopts": attr.string_list(
@@ -137,6 +148,7 @@ _library_attributes = {
         aspects = [
             _coverage_replacements_provider.aspect,
         ],
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The JVM libraries to add as dependencies to any libraries dependent on this one.",
         providers = [JavaInfo],
     ),
@@ -155,6 +167,7 @@ _runtime_attributes = {
         doc = "The JVM runtime flags.",
     ),
     "runtime_deps": attr.label_list(
+        cfg = _scala_toolchain_outgoing_transition,
         doc = "The JVM runtime-only library dependencies.",
         providers = [JavaInfo],
     ),
@@ -162,10 +175,12 @@ _runtime_attributes = {
 
 _runtime_private_attributes = {
     "_target_jdk": attr.label(
+        cfg = _scala_toolchain_outgoing_transition,
         default = Label("@bazel_tools//tools/jdk:current_java_runtime"),
         providers = [java_common.JavaRuntimeInfo],
     ),
     "_java_stub_template": attr.label(
+        cfg = _scala_toolchain_outgoing_transition,
         default = Label("@anx_java_stub_template//file"),
         allow_single_file = True,
     ),
@@ -235,11 +250,11 @@ def make_scala_library(*extras):
             _compile_attributes,
             _compile_private_attributes,
             _library_attributes,
-            _scala_toolchain_transition_attributes,
+            _scala_toolchain_attributes,
             _extras_attributes(extras),
             *[extra["attrs"] for extra in extras]
         ),
-        cfg = _scala_toolchain_transition,
+        cfg = _scala_toolchain_incoming_transition,
         doc = "Compiles a Scala JVM library.",
         implementation = _scala_library_implementation,
         outputs = _dicts.add(
@@ -263,7 +278,7 @@ def make_scala_binary(*extras):
             _compile_private_attributes,
             _runtime_attributes,
             _runtime_private_attributes,
-            _scala_toolchain_transition_attributes,
+            _scala_toolchain_attributes,
             {
                 "main_class": attr.string(
                     doc = "The main class. If not provided, it will be inferred by its type signature.",
@@ -272,7 +287,7 @@ def make_scala_binary(*extras):
             _extras_attributes(extras),
             *[extra["attrs"] for extra in extras]
         ),
-        cfg = _scala_toolchain_transition,
+        cfg = _scala_toolchain_incoming_transition,
         doc = """
 Compiles and links a Scala JVM executable.
 
@@ -309,7 +324,7 @@ def make_scala_test(*extras):
             _compile_private_attributes,
             _runtime_attributes,
             _runtime_private_attributes,
-            _scala_toolchain_transition_attributes,
+            _scala_toolchain_attributes,
             _testing_private_attributes,
             {
                 "isolation": attr.string(
@@ -323,6 +338,7 @@ def make_scala_test(*extras):
                 ),
                 "scalacopts": attr.string_list(),
                 "shared_deps": attr.label_list(
+                    cfg = _scala_toolchain_outgoing_transition,
                     doc = "If isolation is \"classloader\", the list of deps to keep loaded between tests",
                     providers = [JavaInfo],
                 ),
@@ -336,13 +352,19 @@ def make_scala_test(*extras):
                         "com.novocode.junit.JUnitFramework",
                     ],
                 ),
-                "runner": attr.label(default = "@rules_scala_annex//src/main/scala/higherkindness/rules_scala/workers/zinc/test"),
-                "subprocess_runner": attr.label(default = "@rules_scala_annex//src/main/scala/higherkindness/rules_scala/common/sbt-testing:subprocess"),
+                "runner": attr.label(
+                    cfg = _scala_toolchain_outgoing_transition,
+                    default = "@rules_scala_annex//src/main/scala/higherkindness/rules_scala/workers/zinc/test",
+                ),
+                "subprocess_runner": attr.label(
+                    cfg = _scala_toolchain_outgoing_transition,
+                    default = "@rules_scala_annex//src/main/scala/higherkindness/rules_scala/common/sbt-testing:subprocess"
+                ),
             },
             _extras_attributes(extras),
             *[extra["attrs"] for extra in extras]
         ),
-        cfg = _scala_toolchain_transition,
+        cfg = _scala_toolchain_incoming_transition,
         doc = """
 Compiles and links a collection of Scala tests.
 
@@ -387,13 +409,17 @@ _scala_repl_private_attributes = _dicts.add(
 scala_repl = rule(
     attrs = _dicts.add(
         _scala_repl_private_attributes,
-        _scala_toolchain_transition_attributes,
+        _scala_toolchain_attributes,
         {
             "data": attr.label_list(
+                cfg = _scala_toolchain_outgoing_transition,
                 doc = "The additional runtime files needed by this REPL.",
                 allow_files = True,
             ),
-            "deps": attr.label_list(providers = [JavaInfo]),
+            "deps": attr.label_list(
+                cfg = _scala_toolchain_outgoing_transition,
+                providers = [JavaInfo],
+            ),
             "jvm_flags": attr.string_list(
                 doc = "The JVM runtime flags.",
             ),
@@ -402,7 +428,7 @@ scala_repl = rule(
             ),
         },
     ),
-    cfg = _scala_toolchain_transition,
+    cfg = _scala_toolchain_incoming_transition,
     doc = """
 Launches a REPL with all given dependencies available.
 
@@ -442,23 +468,32 @@ Use this only for libraries with macros. Otherwise, use `java_import`.
 
 scaladoc = rule(
     attrs = _dicts.add(
-        _scala_toolchain_transition_attributes,
+        _scala_toolchain_attributes,
         _scaladoc_private_attributes,
         {
-            "compiler_deps": attr.label_list(providers = [JavaInfo]),
-            "deps": attr.label_list(providers = [JavaInfo]),
-            "srcs": attr.label_list(allow_files = [
-                ".java",
-                ".scala",
-                ".srcjar",
-                "-sources.jar",
-                "-src.jar",
-            ]),
+            "compiler_deps": attr.label_list(
+                cfg = _scala_toolchain_outgoing_transition,
+                providers = [JavaInfo],
+            ),
+            "deps": attr.label_list(
+                cfg = _scala_toolchain_outgoing_transition,
+                providers = [JavaInfo],
+            ),
+            "srcs": attr.label_list(
+                allow_files = [
+                    ".java",
+                    ".scala",
+                    ".srcjar",
+                    "-sources.jar",
+                    "-src.jar",
+                ],
+                cfg = _scala_toolchain_outgoing_transition,
+            ),
             "scalacopts": attr.string_list(),
             "title": attr.string(),
         },
     ),
-    cfg = _scala_toolchain_transition,
+    cfg = _scala_toolchain_incoming_transition,
     doc = """
 Generates Scaladocs.
 """,
diff --git a/rules/scala/private/repl.bzl b/rules/scala/private/repl.bzl
index f03431c5..e571b80c 100644
--- a/rules/scala/private/repl.bzl
+++ b/rules/scala/private/repl.bzl
@@ -49,7 +49,12 @@ def scala_repl_implementation(ctx):
             runfiles = ctx.runfiles(
                 collect_default = True,
                 collect_data = True,
-                files = ctx.attr._target_jdk[java_common.JavaRuntimeInfo].files.to_list(),
+
+                # See https://bazel.build/extending/config#accessing-attributes-with-transitions:
+                # "When attaching a transition to an outgoing edge (regardless of whether the
+                # transition is a 1:1 or 1:2+ transition), `ctx.attr` is forced to be a list if it
+                # isn't already. The order of elements in this list is unspecified."
+                files = ctx.attr._target_jdk[0][java_common.JavaRuntimeInfo].files.to_list(),
                 transitive_files = files,
             ),
         ),
diff --git a/rules/scala/workspace.bzl b/rules/scala/workspace.bzl
index a9a30550..66afc8a8 100644
--- a/rules/scala/workspace.bzl
+++ b/rules/scala/workspace.bzl
@@ -83,14 +83,22 @@ def _toolchain_configuration_repository_impl(repository_ctx):
     repository_ctx.file(
         "BUILD",
         """\
+load(":default.bzl", "default_scala_toolchain_name")
 load("@bazel_skylib//rules:common_settings.bzl", "string_setting")
 
 string_setting(
     name = "scala-toolchain",
-    build_setting_default = "{}",
+    build_setting_default = default_scala_toolchain_name,
     visibility = ["//visibility:public"],
 )
-""".format(repository_ctx.attr.default_scala_toolchain_name),
+""",
+    )
+
+    repository_ctx.file(
+        "default.bzl",
+        "default_scala_toolchain_name = \"{}\"\n".format(
+            repository_ctx.attr.default_scala_toolchain_name,
+        ),
     )
 
 _toolchain_configuration_repository = repository_rule(
diff --git a/tests/scala/toolchain/BUILD b/tests/scala/toolchain/BUILD
new file mode 100644
index 00000000..e07eefc9
--- /dev/null
+++ b/tests/scala/toolchain/BUILD
@@ -0,0 +1,23 @@
+load("@rules_scala_annex//rules:scala.bzl", "scala_library")
+
+scala_library(
+    name = "child",
+    srcs = ["Child.scala"],
+    tags = ["manual"],
+)
+
+scala_library(
+    name = "parent1",
+    srcs = ["Parent.scala"],
+    scala_toolchain_name = "test_zinc_2_13",
+    tags = ["manual"],
+    deps = [":child"],
+)
+
+scala_library(
+    name = "parent2",
+    srcs = ["Parent.scala"],
+    scala_toolchain_name = "test_zinc_3",
+    tags = ["manual"],
+    deps = [":child"],
+)
diff --git a/tests/scala/toolchain/Child.scala b/tests/scala/toolchain/Child.scala
new file mode 100644
index 00000000..2ea5779c
--- /dev/null
+++ b/tests/scala/toolchain/Child.scala
@@ -0,0 +1,3 @@
+object Child {
+    val greeting: String = "Hello, world!"
+}
diff --git a/tests/scala/toolchain/Parent.scala b/tests/scala/toolchain/Parent.scala
new file mode 100644
index 00000000..6791469d
--- /dev/null
+++ b/tests/scala/toolchain/Parent.scala
@@ -0,0 +1,5 @@
+object Parent {
+    def main(arguments: Array[String]): Unit = {
+        println(Child.greeting)
+    }
+}
diff --git a/tests/scala/toolchain/test b/tests/scala/toolchain/test
new file mode 100755
index 00000000..5d38464d
--- /dev/null
+++ b/tests/scala/toolchain/test
@@ -0,0 +1,18 @@
+#!/bin/bash -e
+. "$(dirname "$0")"/../../common.sh
+
+
+bazel_cquery_output="$(
+    bazel cquery 'deps(:parent1 + :parent2, 1)' --output graph --nograph:factored |&
+        sed -nr 's/^ *"\/\/scala\/toolchain:child \(([0-9a-f]*)\)"$/\1/p'
+)"
+
+if [ "$(echo "$bazel_cquery_output" | wc -l)" -ne 1 ]; then
+    echo 'Expected //scala/toolchain:child to be built only once'
+    exit 1
+fi
+
+if echo "$bazel_cquery_output" | grep '@rules_scala_annex_scala_toolchain//:scala-toolchain'; then
+    echo 'Expected //scala/toolchain:child to be built with the default configuration'
+    exit 1
+fi