diff --git a/Tests/CLITests/Utilities/CLITest.swift b/Tests/CLITests/Utilities/CLITest.swift
index 58adf6cc..7bbe63cf 100644
--- a/Tests/CLITests/Utilities/CLITest.swift
+++ b/Tests/CLITests/Utilities/CLITest.swift
@@ -165,23 +165,41 @@ class CLITest {
         }
 
         let inputPipe = Pipe()
-        let outputPipe = Pipe()
-        let errorPipe = Pipe()
         process.standardInput = inputPipe
-        process.standardOutput = outputPipe
-        process.standardError = errorPipe
 
         let outputData: Data
         let errorData: Data
         do {
+            // Redirect stdout/stderr to temp files so the child process never
+            // blocks on `write()` when one stream fills the kernel pipe buffer
+            // before the parent drains it (issue #1456).
+            let tempDir = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+            try FileManager.default.createDirectory(at: tempDir, withIntermediateDirectories: true)
+            defer {
+                try? FileManager.default.removeItem(at: tempDir)
+            }
+
+            let stdoutURL = tempDir.appendingPathComponent("stdout")
+            let stderrURL = tempDir.appendingPathComponent("stderr")
+            FileManager.default.createFile(atPath: stdoutURL.path, contents: nil)
+            FileManager.default.createFile(atPath: stderrURL.path, contents: nil)
+
+            let stdoutHandle = try FileHandle(forWritingTo: stdoutURL)
+            defer { try? stdoutHandle.close() }
+            let stderrHandle = try FileHandle(forWritingTo: stderrURL)
+            defer { try? stderrHandle.close() }
+            process.standardOutput = stdoutHandle
+            process.standardError = stderrHandle
+
             try process.run()
             if let data = stdin {
                 inputPipe.fileHandleForWriting.write(data)
             }
             inputPipe.fileHandleForWriting.closeFile()
-            outputData = outputPipe.fileHandleForReading.readDataToEndOfFile()
-            errorData = errorPipe.fileHandleForReading.readDataToEndOfFile()
             process.waitUntilExit()
+
+            outputData = try Data(contentsOf: stdoutURL)
+            errorData = try Data(contentsOf: stderrURL)
         } catch {
             throw CLIError.executionFailed("Failed to run CLI: \(error)")
         }