diff --git a/tests/tools/test_tts_command_providers.py b/tests/tools/test_tts_command_providers.py
index 5bfe64125e78..34972f2c2d28 100644
--- a/tests/tools/test_tts_command_providers.py
+++ b/tests/tools/test_tts_command_providers.py
@@ -363,6 +363,39 @@ class TestRenderCommandTtsTemplate:
 # ---------------------------------------------------------------------------
 
 class TestRunCommandTts:
+    def test_reads_process_output_in_large_chunks(self):
+        read_sizes: dict[str, list[int]] = {"stdout": [], "stderr": []}
+
+        class FakeStream:
+            def __init__(self, name: str, chunks: list[str]):
+                self.name = name
+                self.chunks = chunks
+
+            def read(self, size: int) -> str:
+                read_sizes[self.name].append(size)
+                if self.chunks:
+                    return self.chunks.pop(0)
+                return ""
+
+        class FakeProcess:
+            def __init__(self):
+                self.pid = 12345
+                self.returncode = 0
+                self.stdout = FakeStream("stdout", ["done"])
+                self.stderr = FakeStream("stderr", ["tick"])
+
+            def wait(self, timeout=None):
+                return self.returncode
+
+        with patch("tools.tts_tool.subprocess.Popen", return_value=FakeProcess()):
+            result = _run_command_tts("fake tts", timeout=0.25)
+
+        assert result.returncode == 0
+        assert result.stdout == "done"
+        assert result.stderr == "tick"
+        assert read_sizes["stdout"][0] == 65536
+        assert read_sizes["stderr"][0] == 65536
+
     def test_stderr_progress_extends_beyond_timeout(self, tmp_path):
         script = tmp_path / "progress_then_exit.py"
         script.write_text(
@@ -403,6 +436,7 @@ class TestRunCommandTts:
             )
 
         assert "starting tier 1" in (excinfo.value.stderr or "")
+        assert isinstance(excinfo.value.__cause__, subprocess.TimeoutExpired)
 
 
 # ---------------------------------------------------------------------------
diff --git a/tools/tts_tool.py b/tools/tts_tool.py
index 9a0c37d1393b..cb37b6b13d2a 100644
--- a/tools/tts_tool.py
+++ b/tools/tts_tool.py
@@ -1025,9 +1025,19 @@ def _run_command_tts(command: str, timeout: float) -> subprocess.CompletedProces
     open_streams = {"stdout", "stderr"}
 
     def read_stream(name: str, stream: Any) -> None:
+        encoding = getattr(stream, "encoding", None) or "utf-8"
         try:
             while True:
-                chunk = stream.read(1)
+                try:
+                    fd = stream.fileno()
+                except (AttributeError, OSError):
+                    fd = None
+
+                if fd is None:
+                    chunk = stream.read(65536)
+                else:
+                    data = os.read(fd, 65536)
+                    chunk = data.decode(encoding, errors="replace")
                 if not chunk:
                     break
                 output_queue.put((name, chunk))
@@ -1079,12 +1089,15 @@ def _run_command_tts(command: str, timeout: float) -> subprocess.CompletedProces
                 chunks[name].append(chunk)
         stdout = "".join(chunks["stdout"])
         stderr = "".join(chunks["stderr"])
-        raise subprocess.TimeoutExpired(
-            command,
-            timeout,
-            output=stdout,
-            stderr=stderr,
-        )
+        try:
+            raise subprocess.TimeoutExpired(command, timeout)
+        except subprocess.TimeoutExpired as exc:
+            raise subprocess.TimeoutExpired(
+                command,
+                timeout,
+                output=stdout,
+                stderr=stderr,
+            ) from exc
 
     stdout = "".join(chunks["stdout"])
     stderr = "".join(chunks["stderr"])