diff --git a/README.md b/README.md
index bb8254a..6333c26 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,11 @@ COMPLEMENT_BASE_IMAGE=homeserver:latest go test -v ./tests
 
 TODO: consider checking in working builds so you can git clone and run. Git LFS for `libmatrix_sdk_ffi.so` given it's 60MB?
 
+#### Environment Variables
+
+- `COMPLEMENT_CRYPTO_TEST_CLIENTS` : "mixed", "rust" or "js"
+   Control which kinds of clients to make for tests. `rust` only tests rust clients. `js` only tests JS clients. `mixed` tests all 4 permutations.
+
 
 ### Test hitlist
 There is an exhaustive set of tests that this repository aims to exercise which are below:
@@ -92,3 +97,11 @@ cargo install uniffi-bindgen-go --path ./uniffi-bindgen-go/bindgen
     * Replace field names `Error` with `Error2` to fix `unknown field Error in struct literal`.
 - Sanity check compile `LIBRARY_PATH="$LIBRARY_PATH:/path/to/matrix-rust-sdk/target/debug" go test -c ./tests`
 
+
+### Github Action (TODO)
+
+Inputs:
+ - version/commit/branch of JS SDK
+ - version/commit/branch of Rust SDK
+ - version/commit/branch of synapse?
+ - Test only JS, only Rust, mixed.
diff --git a/tests/happy_test.go b/tests/happy_test.go
index a1e8364..23e7adc 100644
--- a/tests/happy_test.go
+++ b/tests/happy_test.go
@@ -20,19 +20,7 @@ import (
 // asserting that isEncrypted() returns true. This test may be expanded in the
 // future to assert things like "there is a ciphertext".
 func TestAliceBobEncryptionWorks(t *testing.T) {
-	// TODO: factor out so we can just call "matrix subtests"
-	t.Run("Rust x Rust", func(t *testing.T) {
-		testAliceBobEncryptionWorks(t, api.ClientTypeRust, api.ClientTypeRust)
-	})
-	t.Run("JS x JS", func(t *testing.T) {
-		testAliceBobEncryptionWorks(t, api.ClientTypeJS, api.ClientTypeJS)
-	})
-	t.Run("Rust x JS", func(t *testing.T) {
-		testAliceBobEncryptionWorks(t, api.ClientTypeRust, api.ClientTypeJS)
-	})
-	t.Run("JS x Rust", func(t *testing.T) {
-		testAliceBobEncryptionWorks(t, api.ClientTypeJS, api.ClientTypeRust)
-	})
+	ClientTypeMatrix(t, testAliceBobEncryptionWorks)
 }
 
 func testAliceBobEncryptionWorks(t *testing.T, clientTypeA, clientTypeB api.ClientType) {
diff --git a/tests/main_test.go b/tests/main_test.go
index 5362682..aef557c 100644
--- a/tests/main_test.go
+++ b/tests/main_test.go
@@ -1,6 +1,7 @@
 package tests
 
 import (
+	"os"
 	"sync"
 	"testing"
 
@@ -10,12 +11,28 @@ import (
 	"github.com/matrix-org/complement/must"
 )
 
+const (
+	TestClientsMixed    = "mixed"
+	TestClientsRustOnly = "rust"
+	TestClientsJSOnly   = "js"
+)
+
 var (
 	ssDeployment *deploy.SlidingSyncDeployment
 	ssMutex      *sync.Mutex
+	testClients  = TestClientsMixed
 )
 
 func TestMain(m *testing.M) {
+	ccTestClients := os.Getenv("COMPLEMENT_CRYPTO_TEST_CLIENTS")
+	switch ccTestClients {
+	case TestClientsRustOnly:
+		testClients = TestClientsRustOnly
+	case TestClientsJSOnly:
+		testClients = TestClientsJSOnly
+	default:
+		testClients = TestClientsMixed
+	}
 	ssMutex = &sync.Mutex{}
 	defer func() { // always teardown even if panicking
 		ssMutex.Lock()
@@ -38,6 +55,32 @@ func Deploy(t *testing.T) *deploy.SlidingSyncDeployment {
 	return ssDeployment
 }
 
+func ClientTypeMatrix(t *testing.T, subTest func(tt *testing.T, a, b api.ClientType)) {
+	switch testClients {
+	case TestClientsJSOnly:
+		t.Run("JS|JS", func(t *testing.T) {
+			subTest(t, api.ClientTypeJS, api.ClientTypeJS)
+		})
+	case TestClientsRustOnly:
+		t.Run("Rust|Rust", func(t *testing.T) {
+			subTest(t, api.ClientTypeRust, api.ClientTypeRust)
+		})
+	case TestClientsMixed:
+		t.Run("Rust|Rust", func(t *testing.T) {
+			subTest(t, api.ClientTypeRust, api.ClientTypeRust)
+		})
+		t.Run("Rust|JS", func(t *testing.T) {
+			subTest(t, api.ClientTypeRust, api.ClientTypeJS)
+		})
+		t.Run("JS|Rust", func(t *testing.T) {
+			subTest(t, api.ClientTypeJS, api.ClientTypeRust)
+		})
+		t.Run("JS|JS", func(t *testing.T) {
+			subTest(t, api.ClientTypeJS, api.ClientTypeJS)
+		})
+	}
+}
+
 func MustLoginClient(t *testing.T, clientType api.ClientType, opts api.ClientCreationOpts, ssURL string) api.Client {
 	switch clientType {
 	case api.ClientTypeRust: