qa: JSON-mode stop-sequence guards against post-brace token runaway

Bench data shows residual JSON-mode token runaway after the
pointer-ID switch: ~4 cases out of 66 land UNGROUNDED 0/0 at
12-15s instead of ~2-4s normal. Pattern: Hermes-3-8B emits a
valid claim object, then keeps generating whitespace / blank
lines until max_tokens (512) exhausts. The truncated payload
won't parse and the lenient pre-parser returns no claims.

Concrete instances in the latest bench (2026-04-30T19-55-11Z):
  - tell me about the apollo program (3/3 samples runaway)
  - tell me about the python programming language (1/3)

Fix: pass `stop=["\n\n"]` to the chat completion in JSON mode so
vLLM cuts generation at the first blank line. Well-formed JSON-
mode output never legitimately contains a blank line — Hermes
emits one object on a single line (or with simple internal
newlines), never `\n\n`. The stop sequence is the runaway
signature itself.

Plumbing:
  - OpenAICompatibleClient.chat_completion: new `stop` kwarg,
    injects into request payload when non-empty
  - StubClient already absorbs **kwargs; no change needed
  - DEFAULT_POLICY (runner) + DEFAULT_QUERY_POLICY (query) gain
    `claim_lattice_json_stop_sequences = ["\n\n"]`. Folds into
    governance_policy_hash so changing the stop list invalidates
    prior cached records.
  - Both call sites in runner.py / query.py read the policy
    field and pass it only on JSON mode (pointer + quote modes
    don't need it).

Defensive measure: worst case the stop sequence never fires;
best case the apollo/python residuals recover and JSON's
strict-rate climbs further.

aborist/qa/client.py

@@ -101,6 +101,7 @@ class OpenAICompatibleClient:
         max_tokens: int = 512,
         top_p: float = 1.0,
         extra_body: dict | None = None,
+        stop: list[str] | None = None,
     ) -> str:
         import httpx
         import time as _time
@@ -115,6 +116,8 @@ class OpenAICompatibleClient:
             "max_tokens": max_tokens,
             "top_p": top_p,
         }
+        if stop:
+            payload["stop"] = list(stop)
         # extra_body merges into the payload root — vLLM accepts knobs
         # like {"guided_json": {...schema...}} or {"guided_grammar": "..."}.
         # Endpoints that don't recognize a key silently drop it.

aborist/qa/query.py

@@ -420,6 +420,7 @@ DEFAULT_QUERY_POLICY = {
         "Now answer the question on the next message."
     ),
     "claim_lattice_use_guided_json": True,
+    "claim_lattice_json_stop_sequences": ["\n\n"],
 }
 
 
@@ -1492,10 +1493,18 @@ def query(
         # time. Non-vLLM endpoints silently drop the field; the lenient
         # pre-parser in the verifier handles whatever drift remains.
         extra_body: dict | None = None
+        stop_seqs: list[str] | None = None
         if answer_mode == "claim_lattice" and policy.get(
             "claim_lattice_use_guided_json", True
         ):
             extra_body = {"guided_json": CLAIM_LATTICE_JSON_SCHEMA}
+        if answer_mode == "claim_lattice":
+            # JSON-mode token-runaway guard — see runner.py for the
+            # full rationale. Stops generation on a blank line so
+            # post-JSON whitespace spam doesn't blow max_tokens.
+            stop_seqs = list(policy.get(
+                "claim_lattice_json_stop_sequences", ["\n\n"]
+            ))
         t_phase = time.monotonic()
         raw_answer = chat_client.chat_completion(
             messages,
@@ -1504,6 +1513,7 @@ def query(
             max_tokens=policy["max_tokens"],
             top_p=policy.get("top_p", 1.0),
             extra_body=extra_body,
+            stop=stop_seqs,
         )
         llm_ms = _ms_since(t_phase)
 

aborist/qa/runner.py

@@ -252,6 +252,15 @@ DEFAULT_POLICY = {
         "characters. Now answer the question on the next message."
     ),
     "claim_lattice_use_guided_json": True,
+    # JSON-mode stop sequences. Hermes-3-8B sometimes spams whitespace
+    # / newlines after the closing brace on broad-descriptive shapes
+    # ("plot of X", "tell me about Y") — the response runs out the
+    # max_tokens budget and the lenient parser sees truncated JSON.
+    # Stopping on a blank line cuts the runaway. JSON-mode output
+    # never legitimately contains a blank line (single object, single
+    # line) so this is a safe filter. Folds into
+    # governance_policy_hash on change.
+    "claim_lattice_json_stop_sequences": ["\n\n"],
 }
 
 
@@ -500,10 +509,24 @@ def ask(
     # sampling time. Endpoints without guided-decoding silently drop the
     # field; the lenient pre-parser handles whatever drift remains.
     extra_body: dict | None = None
+    stop_seqs: list[str] | None = None
     if answer_mode == "claim_lattice" and policy.get(
         "claim_lattice_use_guided_json", True
     ):
         extra_body = {"guided_json": CLAIM_LATTICE_JSON_SCHEMA}
+    if answer_mode == "claim_lattice":
+        # JSON-mode token-runaway guard. On broad-descriptive /
+        # comparison questions Hermes-3-8B sometimes spams whitespace
+        # / newlines after the closing brace until max_tokens
+        # exhausts; the resulting truncated payload won't parse and
+        # the run lands UNGROUNDED 0/0 at 12-15s instead of 2-4s.
+        # Stopping on a blank line (\n\n) cuts the runaway —
+        # well-formed JSON-mode output never contains a blank line
+        # since the model emits a single object on one line (or
+        # with simple internal newlines).
+        stop_seqs = list(policy.get(
+            "claim_lattice_json_stop_sequences", ["\n\n"]
+        ))
     raw_answer = client.chat_completion(
         messages,
         model=model_id,
@@ -511,6 +534,7 @@ def ask(
         max_tokens=policy["max_tokens"],
         top_p=policy.get("top_p", 1.0),
         extra_body=extra_body,
+        stop=stop_seqs,
     )
     llm_ms = _ms_since(t_llm)