diff --git a/README.md b/README.md
index 50ad8d0..5759a5e 100644
--- a/README.md
+++ b/README.md
@@ -53,8 +53,11 @@ it:
 
 `prain serve PLACEMENT MODEL_DIR -- VLLM_FLAGS` validates the placement file and starts
 `vllm serve` with prain's plugin. prain adds `--enforce-eager --moe-backend triton
---scheduling-policy priority`, and it takes `--max-num-seqs` and `--kv-cache-memory-bytes` from
-the file's `[serve]` table (it refuses them after `--`). The flags in `$FLAGS` above:
+--scheduling-policy priority --chat-template-content-format string`, and it takes
+`--max-num-seqs` and `--kv-cache-memory-bytes` from the file's `[serve]` table (it refuses them
+after `--`). vLLM would otherwise pick the `openai` content format for Gemma 4, whose template
+then prints a system prompt as a Python list (`[{'type': 'text', 'text': '…\n…'}]`, newlines
+escaped); a flag after `--` still overrides it. The flags in `$FLAGS` above:
 
 | flag | why |
 |---|---|
diff --git a/prain/cli.py b/prain/cli.py
index 8a9b891..1cc75b8 100644
--- a/prain/cli.py
+++ b/prain/cli.py
@@ -113,6 +113,9 @@ def serve(args):
                        "--scheduling-policy", "priority",
                        "--max-num-seqs", str(config.serve.max_requests),
                        "--kv-cache-memory-bytes", str(int(config.serve.kv_cache_gib * GIB)),
+                       # vLLM detects "openai" for Gemma 4, whose template then prints a system
+                       # prompt as a Python list; a flag in `extra` comes later and wins
+                       "--chat-template-content-format", "string",
                        *extra])
 
 
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 9285504..e717793 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -55,7 +55,8 @@ class Cli(TmpCase):
         execvp.assert_called_once_with("vllm", [
             "vllm", "serve", "/models/m", "--enforce-eager", "--moe-backend", "triton",
             "--scheduling-policy", "priority", "--max-num-seqs", "3",
-            "--kv-cache-memory-bytes", "1073741824", "--port", "8001"])
+            "--kv-cache-memory-bytes", "1073741824", "--chat-template-content-format", "string",
+            "--port", "8001"])
         with mock.patch("os.execvp") as execvp, self.assertRaisesRegex(
                 SystemExit, "--max-num-seqs=4 comes from the placement file"):
             main(["serve", config, "/models/m", "--", "--max-num-seqs=4"])