diff --git a/tilert/pd_vllm/pd_router.py b/tilert/pd_vllm/pd_router.py
index 87a61e5..7114a9d 100644
--- a/tilert/pd_vllm/pd_router.py
+++ b/tilert/pd_vllm/pd_router.py
@@ -97,6 +97,37 @@ def _thinking_enabled(body: dict) -> bool:
     return bool(ctk.get("enable_thinking", True))
 
 
+# Client fields that must not survive into the prefill request, which is
+# forwarded verbatim apart from the fields we set: stream_options contradicts
+# the stream=False we force (vLLM rejects the pair with a 400 during body
+# parsing), and max_completion_tokens takes precedence over max_tokens, so it
+# would override our max_tokens=1. Streaming clients send both.
+_PREFILL_DROP_FIELDS = ("stream_options", "max_completion_tokens")
+
+
+def build_prefill_body(path: str, body: dict, node: DecodeNode) -> dict:
+    """The vLLM request that prefills only and hands the KV state to ``node``.
+
+    Lives outside ``build_app`` so the rewrite can be exercised without a
+    router process, a vLLM instance or a decode node.
+    """
+    prefill_body = dict(body)
+    prefill_body["max_tokens"] = 1
+    prefill_body["stream"] = False
+    for field in _PREFILL_DROP_FIELDS:
+        prefill_body.pop(field, None)
+    if path.endswith("chat/completions"):
+        prefill_body["logprobs"] = True
+        prefill_body["top_logprobs"] = 1
+    else:
+        prefill_body["logprobs"] = 1
+    prefill_body["kv_transfer_params"] = {
+        "tilert_host": node.host,
+        "tilert_ctrl_port": node.ctrl_port,
+    }
+    return prefill_body
+
+
 class RouterCtx:
     """Immutable per-process context (tokenizer, parser factory, config)."""
 
@@ -133,18 +164,7 @@ def pool_status():
 
     # ── shared prefill step ──────────────────────────────────────────────
     def _prefill(path, body, node):
-        prefill_body = dict(body)
-        prefill_body["max_tokens"] = 1
-        prefill_body["stream"] = False
-        if path.endswith("chat/completions"):
-            prefill_body["logprobs"] = True
-            prefill_body["top_logprobs"] = 1
-        else:
-            prefill_body["logprobs"] = 1
-        prefill_body["kv_transfer_params"] = {
-            "tilert_host": node.host,
-            "tilert_ctrl_port": node.ctrl_port,
-        }
+        prefill_body = build_prefill_body(path, body, node)
         r = requests.post(f"{ctx.vllm_url}{path}", json=prefill_body, timeout=600)
         r.raise_for_status()
         return r.json()