diff --git a/.buildkite/downstream.yml b/.buildkite/downstream.yml
index e635ca5e..8d033d8d 100644
--- a/.buildkite/downstream.yml
+++ b/.buildkite/downstream.yml
@@ -1,5 +1,8 @@
 # Tests of the packages that build on GPUCompiler, uploaded by `launch.sh`.
 #
+# The Julia version and the test selection of every step can be changed with the `[julia X]`
+# and `[test_args ...]` commit message tags, which `launch.sh` exports for interpolation here.
+#
 # Each step's `if_changed` lists the files that can affect that package: everything in
 # `src/` and `ext/`, `Project.toml` and the CI configuration, except the other back-ends'
 # target files. The `if:` conditions implement the commit message tags documented in the
@@ -8,7 +11,7 @@ steps:
   - label: "CUDA.jl"
     plugins:
       - JuliaCI/julia#v1:
-          version: "1.13"
+          version: "${DOWNSTREAM_JULIA:-1.13}"
       - JuliaCI/julia-coverage#v1:
           codecov: true
     command: |
@@ -20,7 +23,7 @@ steps:
 
       julia -e 'println("+++ :julia: Running tests")
                 using Pkg
-                Pkg.test("CUDA"; coverage=true)'
+                Pkg.test("CUDA"; coverage=true, test_args=split(ENV["DOWNSTREAM_TEST_ARGS"]))'
     agents:
       queue: "cuda"
     if_changed:
@@ -42,7 +45,7 @@ steps:
   - label: "oneAPI.jl"
     plugins:
       - JuliaCI/julia#v1:
-          version: "1.13"
+          version: "${DOWNSTREAM_JULIA:-1.13}"
       - JuliaCI/julia-coverage#v1:
           codecov: true
     command: |
@@ -57,7 +60,7 @@ steps:
 
       julia -e 'println("+++ :julia: Running tests")
                 using Pkg
-                Pkg.test("oneAPI"; coverage=true)'
+                Pkg.test("oneAPI"; coverage=true, test_args=split(ENV["DOWNSTREAM_TEST_ARGS"]))'
     agents:
       queue: "oneapi"
     if_changed:
@@ -75,7 +78,7 @@ steps:
   - label: "AMDGPU.jl"
     plugins:
       - JuliaCI/julia#v1:
-          version: "1.13"
+          version: "${DOWNSTREAM_JULIA:-1.13}"
       - JuliaCI/julia-coverage#v1:
           codecov: true
     command: |
@@ -86,7 +89,7 @@ steps:
 
        julia -e 'println("+++ :julia: Running tests")
                  using Pkg
-                 Pkg.test("AMDGPU"; coverage=true)'
+                 Pkg.test("AMDGPU"; coverage=true, test_args=split(ENV["DOWNSTREAM_TEST_ARGS"]))'
     agents:
       queue: "rocm"
     if_changed:
@@ -104,7 +107,7 @@ steps:
   - label: "Metal.jl"
     plugins:
       - JuliaCI/julia#v1:
-          version: "1.13"
+          version: "${DOWNSTREAM_JULIA:-1.13}"
       - JuliaCI/julia-coverage#v1:
           codecov: true
     command: |
@@ -126,7 +129,7 @@ steps:
 
       julia -e 'println("+++ :julia: Running tests")
                 using Pkg
-                Pkg.test("Metal"; coverage=true)'
+                Pkg.test("Metal"; coverage=true, test_args=split(ENV["DOWNSTREAM_TEST_ARGS"]))'
     agents:
       queue: "metal"
     if_changed:
@@ -144,7 +147,7 @@ steps:
   - label: "OpenCL.jl"
     plugins:
       - JuliaCI/julia#v1:
-          version: "1.13"
+          version: "${DOWNSTREAM_JULIA:-1.13}"
       - JuliaCI/julia-coverage#v1:
           codecov: true
     command: |
@@ -156,7 +159,7 @@ steps:
 
        julia -e 'println("+++ :julia: Running tests")
                 using Pkg
-                Pkg.test("OpenCL"; coverage=true)'
+                Pkg.test("OpenCL"; coverage=true, test_args=split(ENV["DOWNSTREAM_TEST_ARGS"]))'
     agents:
       # XXX: not on "juliaecosystem" because those workers don't have libc-dev
       #      see JuliaGPU/OpenCL.jl#303
@@ -175,3 +178,5 @@ steps:
 
 env:
   JULIA_PKG_SERVER: "" # it often struggles with our large artifacts
+  # set by `launch.sh` from the `[test_args ...]` commit message tag
+  DOWNSTREAM_TEST_ARGS: "${DOWNSTREAM_TEST_ARGS:-}"
diff --git a/.buildkite/launch.sh b/.buildkite/launch.sh
index 26aab624..4cb944c5 100755
--- a/.buildkite/launch.sh
+++ b/.buildkite/launch.sh
@@ -15,5 +15,17 @@ elif [[ "${BUILDKITE_MESSAGE:-}" =~ \[(only\ |run\ all\]) ]]; then
     export BUILDKITE_AGENT_APPLY_IF_CHANGED=false
 fi
 
+# `[julia X]` and `[test_args ...]` select the Julia version and the tests (passed as
+# `test_args` to `Pkg.test`) of the downstream steps, e.g. `[julia nightly]` or
+# `[test_args core/codegen gpuarrays/broadcasting]`.
+if [[ "${BUILDKITE_MESSAGE:-}" =~ \[julia\ ([^]]+)\] ]]; then
+    export DOWNSTREAM_JULIA="${BASH_REMATCH[1]}"
+    echo "Testing downstream packages on Julia $DOWNSTREAM_JULIA"
+fi
+if [[ "${BUILDKITE_MESSAGE:-}" =~ \[test_args\ ([^]]+)\] ]]; then
+    export DOWNSTREAM_TEST_ARGS="${BASH_REMATCH[1]}"
+    echo "Running downstream tests: $DOWNSTREAM_TEST_ARGS"
+fi
+
 # fetch the base branch first, as a stale local ref would widen the diff
 buildkite-agent pipeline upload --fetch-diff-base .buildkite/downstream.yml
diff --git a/README.md b/README.md
index fc8988a2..f348fb55 100644
--- a/README.md
+++ b/README.md
@@ -47,6 +47,8 @@ last commit of a pull request:
 | `[skip amdgpu]`      | Do not test the listed back-ends, even when the changes affect them. |
 | `[run all]`          | Test all back-ends, regardless of the files changed.               |
 | `[skip tests]`       | Do not test any back-end.                                          |
+| `[julia nightly]`    | Test the back-ends on the given Julia version (default: the latest release), e.g. `nightly` or `1.10`. |
+| `[test_args core]`   | Only run the listed tests of each back-end, passed as `test_args` to `Pkg.test`, e.g. `[test_args core/codegen gpuarrays/broadcasting]`. |
 
 Back-ends are named `cuda`, `amdgpu`, `metal`, `oneapi` and `opencl` (all lowercase), or by
 their target: `ptx` for CUDA.jl, `gcn` for AMDGPU.jl, and `spirv` for both oneAPI.jl and