diff --git a/cli/BUILD b/cli/BUILD
index ee701a58..ec0c7700 100644
--- a/cli/BUILD
+++ b/cli/BUILD
@@ -120,6 +120,12 @@ kt_jvm_test(
     runtime_deps = [":cli-test-lib"],
 )
 
+kt_jvm_test(
+    name = "ProcessStdinHangRegressionTest",
+    test_class = "com.bazel_diff.process.ProcessStdinHangRegressionTest",
+    runtime_deps = [":cli-test-lib"],
+)
+
 kt_jvm_test(
     name = "E2ETest",
     timeout = "long",
diff --git a/cli/src/main/kotlin/com/bazel_diff/process/Process.kt b/cli/src/main/kotlin/com/bazel_diff/process/Process.kt
index 499e1e4b..26c2866f 100644
--- a/cli/src/main/kotlin/com/bazel_diff/process/Process.kt
+++ b/cli/src/main/kotlin/com/bazel_diff/process/Process.kt
@@ -57,6 +57,12 @@ suspend fun process(
           }
           .start()
 
+  // Close the subprocess's stdin so reads from it see EOF immediately.
+  // Without this, subprocesses that read stdin on startup (e.g. aspect CLI's
+  // interactive first-run path, #256) block forever waiting for input we
+  // never send, and waitFor() then blocks forever too.
+  process.outputStream.close()
+
   // Handles async consumptions before the blocking output handling.
   if (stdout is Redirect.Consume) {
     process.inputStream.lineFlow(stdout.consumer)
diff --git a/cli/src/test/kotlin/com/bazel_diff/process/ProcessStdinHangRegressionTest.kt b/cli/src/test/kotlin/com/bazel_diff/process/ProcessStdinHangRegressionTest.kt
new file mode 100644
index 00000000..98358811
--- /dev/null
+++ b/cli/src/test/kotlin/com/bazel_diff/process/ProcessStdinHangRegressionTest.kt
@@ -0,0 +1,43 @@
+package com.bazel_diff.process
+
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.runBlocking
+import kotlinx.coroutines.withTimeoutOrNull
+import org.junit.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
+
+/**
+ * Regression test for https://github.com/Tinder/bazel-diff/issues/256
+ * ("bug: bazel-diff freezes when aspect CLI is installed").
+ *
+ * Before the fix, [process] started subprocesses via `ProcessBuilder.start()`
+ * without redirecting or closing stdin. Java defaults stdin to `Redirect.PIPE`,
+ * so the subprocess received an open, never-closed stdin pipe. Any subprocess
+ * that reads from stdin (the aspect CLI's interactive first-run path, in #256)
+ * blocked indefinitely on `read()`, and `process.waitFor()` then blocked too —
+ * matching the `FUTEX_WAIT` strace the original reporter captured.
+ *
+ * The fix closes `process.outputStream` immediately after `start()` so the
+ * subprocess sees EOF on stdin and exits.
+ */
+@OptIn(ExperimentalCoroutinesApi::class)
+class ProcessStdinHangRegressionTest {
+
+  @Test
+  fun `process does not hang when subprocess reads from stdin`() = runBlocking {
+    // `cat` with no args reads from stdin until EOF. Before the fix this hung
+    // forever; after the fix `cat` sees EOF immediately and exits 0.
+    val result =
+        withTimeoutOrNull(timeMillis = 5_000) {
+          process(
+              "cat",
+              stdout = Redirect.CAPTURE,
+              stderr = Redirect.SILENT,
+          )
+        }
+
+    assertNotNull(result, "process() deadlocked — subprocess stdin was not closed (regression of #256).")
+    assertEquals(0, result.resultCode)
+  }
+}