[PD] Diversify fake-prefill handoff tokens (#37302)

Co-authored-by: xingyuliu <xingyuliu@fb.com>
Co-authored-by: Lianmin Zheng <lianminzheng@gmail.com>

python/sglang/srt/disaggregation/decode.py

@@ -20,6 +20,7 @@ Life cycle of a request in the decode server
 
 from __future__ import annotations
 
+import hashlib
 import logging
 import time
 from collections import deque
@@ -2012,6 +2013,22 @@ def alloc_for_decode_prealloc(
     return kv_loc
 
 
+def _generate_fake_prefill_handoff_output_id(req: Req) -> int:
+    """Generate a deterministic synthetic handoff token for fake-PD requests.
+
+    Fake-PD skips prefill and KV transfer, so its allocated prompt KV has no
+    request-specific state and its metadata output token remains zero. Reusing
+    token 0 for every request can unrealistically correlate decode workloads.
+    Hash the request ID so every TP rank gets the same request-diverse token.
+    """
+    if req.vocab_size is None or req.vocab_size <= 0:
+        raise ValueError("Fake-PD requests require a positive vocabulary size")
+    digest = hashlib.blake2b(
+        req.rid.encode(), digest_size=8, person=b"fake-pd"
+    ).digest()
+    return int.from_bytes(digest, byteorder="little") % req.vocab_size
+
+
 class DecodeTransferQueue(DecodeHiCacheTransferMixin):
     """
     Store the requests that is polling kv
@@ -2142,6 +2159,10 @@ class DecodeTransferQueue(DecodeHiCacheTransferMixin):
         if replayed_boundary:
             committed_output_id = decode_req.req.pd_rebootstrap_forced_output_id
             decode_req.req.pd_rebootstrap_forced_output_id = None
+        elif _is_fake_transfer(decode_req.req):
+            committed_output_id = _generate_fake_prefill_handoff_output_id(
+                decode_req.req
+            )
         else:
             committed_output_id = output_id[0].item()
         decode_req.req.output_ids.append(committed_output_id)