diff --git a/renderers/client.py b/renderers/client.py
index fd13e6d61..d663f82cf 100644
--- a/renderers/client.py
+++ b/renderers/client.py
@@ -7,6 +7,7 @@
 from __future__ import annotations
 
 import asyncio
+import base64
 import json
 import logging
 import math
@@ -14,6 +15,7 @@
 from typing import Any, cast
 
 import httpx
+import numpy as np
 from openai import AsyncOpenAI
 
 from renderers.base import (
@@ -134,9 +136,26 @@ def parse_generate_response(raw: bytes) -> dict[str, Any]:
     return payload
 
 
+def _parse_packed_completion_logprobs(
+    packed: Mapping[str, Any], completion_ids: list[int]
+) -> list[float]:
+    """prime-rl's packed form: base64 float32 ``{data, shape, dtype}``, one per token."""
+    values = np.frombuffer(base64.b64decode(packed["data"]), dtype=packed["dtype"])
+    valid = np.isfinite(values) & (values > VLLM_LOGPROB_SENTINEL)
+    if len(values) != len(completion_ids) or not valid.all():
+        raise MalformedGenerateResponseError(
+            f"Engine response choice.completion_logprobs: {len(values)} values for "
+            f"{len(completion_ids)} completion tokens, or a non-finite or missing (-9999) logprob."
+        )
+    return values.astype(np.float64).tolist()
+
+
 def _parse_completion_logprobs(
     choice: Mapping[str, Any], completion_ids: list[int]
 ) -> list[float]:
+    packed = choice.get("completion_logprobs")
+    if packed is not None:
+        return _parse_packed_completion_logprobs(packed, completion_ids)
     raw_logprobs = choice.get("logprobs")
     if not isinstance(raw_logprobs, Mapping):
         raise MalformedGenerateResponseError(
@@ -376,7 +395,8 @@ def _prepare():
 
     routed_experts = choice.get("routed_experts")
     # vLLM's native kept-set sampling masks (``--return-sampling-mask``):
-    # one list of surviving vocab ids per completion token.
+    # one list of surviving vocab ids per completion token, or (prime-rl's
+    # server) packed CSR ``{"ids", "counts"}`` base64 int32 arrays.
     sampling_mask = choice.get("sampling_mask")
 
     # /inference/v1/generate returns finish_reason in {"stop","length",...} —
diff --git a/tests/test_client.py b/tests/test_client.py
index 16fdafef6..d9fc53724 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -336,6 +336,33 @@ def test_generate_preserves_zero_completion_logprob():
     assert result["completion_logprobs"] == [0.0, -0.2]
 
 
+def test_generate_reads_packed_completion_logprobs():
+    client = _FakeClient()
+    client.choice.pop("logprobs")
+    values = np.array([-0.1, 0.0], dtype=np.float32)
+    client.choice["completion_logprobs"] = {
+        "data": base64.b64encode(values.tobytes()).decode(),
+        "shape": [2],
+        "dtype": "float32",
+    }
+
+    result = _run_generate(client)
+
+    assert result["completion_logprobs"] == values.tolist()
+
+    client.choice["completion_logprobs"]["data"] = base64.b64encode(
+        np.array([-0.1, -9999.0], dtype=np.float32).tobytes()
+    ).decode()
+    with pytest.raises(MalformedGenerateResponseError, match="missing"):
+        _run_generate(client)
+
+    client.choice["completion_logprobs"]["data"] = base64.b64encode(
+        np.array([-0.1, np.inf], dtype=np.float32).tobytes()
+    ).decode()
+    with pytest.raises(MalformedGenerateResponseError, match="non-finite"):
+        _run_generate(client)
+
+
 class _MalformedToolRenderer(_FakeRenderer):
     """Returns only a malformed tool-call attempt — finish_reason must stay "stop"."""