Replayer: closed-loop inter-turn think-time mode

Add --inter-turn-think (env REPLAY_INTER_TURN_THINK_S): turn 1 fires on
session admission, each later turn a FIXED think-time after the previous
turn COMPLETES, ignoring absolute trace timestamps. Combined with
--max-inflight-sessions (env REPLAY_MAX_INFLIGHT) this is a stable N-user
closed loop, removing the open-loop "fire immediately because timestamp is
in the past" retrigger artifact. Needed for the dispatch-coupling
(wall-clock amplification) sweep.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

replayer/__main__.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 import argparse
 import asyncio
 import logging
+import os
 from pathlib import Path
 
 from .replay import ReplayConfig, replay_trace
@@ -19,9 +20,16 @@ def main() -> None:
     p.add_argument("--model", type=str, default="default", help="Model name for API")
     p.add_argument("--concurrency-limit", type=int, default=2000,
                    help="Max concurrent HTTP requests (safety limit)")
-    p.add_argument("--max-inflight-sessions", type=int, default=None,
+    _env_inflight = os.environ.get("REPLAY_MAX_INFLIGHT")
+    p.add_argument("--max-inflight-sessions", type=int,
+                   default=int(_env_inflight) if _env_inflight else None,
                    help="Cap on concurrent sessions (None = unlimited; "
-                        "trace-driven dispatch otherwise)")
+                        "trace-driven dispatch otherwise). Env: REPLAY_MAX_INFLIGHT")
+    _env_think = os.environ.get("REPLAY_INTER_TURN_THINK_S")
+    p.add_argument("--inter-turn-think", type=float,
+                   default=float(_env_think) if _env_think else None,
+                   help="Closed-loop think-time (s) after each turn completes; "
+                        "ignore absolute trace schedule. Env: REPLAY_INTER_TURN_THINK_S")
     p.add_argument("--request-timeout", type=float, default=600.0)
     p.add_argument("--request-limit", type=int, default=None,
                    help="Limit number of requests to replay")
@@ -42,6 +50,7 @@ def main() -> None:
         request_timeout_s=args.request_timeout,
         request_limit=args.request_limit,
         max_inflight_sessions=args.max_inflight_sessions,
+        inter_turn_think_s=args.inter_turn_think,
     )
 
     results = asyncio.run(replay_trace(config))

replayer/replay.py

@@ -60,6 +60,12 @@ class ReplayConfig:
     request_limit: int | None = None
     model_name: str = "default"
     max_inflight_sessions: int | None = None  # cap on concurrent sessions; None = unlimited
+    # Closed-loop think-time mode: if set, ignore absolute trace timestamps for
+    # subsequent turns — fire turn 1 on session admission, then each later turn a
+    # FIXED think-time after the previous turn COMPLETES. Combined with
+    # max_inflight_sessions=N this is a stable N-user closed-loop (no open-loop
+    # runaway), so it removes the "immediate retrigger under load" artifact.
+    inter_turn_think_s: float | None = None
 
 
 def _build_prompt_token_ids(req: TraceRequest) -> list[int]:
@@ -279,12 +285,19 @@ async def _run_session(
         await session_sem.acquire()
     realized_context: list[int] = []
     try:
-        for req in state.turns:
+        for turn_idx, req in enumerate(state.turns):
-            # Wait until this request's trace timestamp
+            if config.inter_turn_think_s is not None:
-            target_wall = (req.timestamp_s - earliest_ts)
+                # Closed-loop: turn 1 fires on admission; later turns wait a fixed
-            elapsed = time.perf_counter() - sweep_start
+                # think-time AFTER the previous turn completed (no absolute schedule,
-            if elapsed < target_wall:
+                # so no "fire immediately because timestamp is in the past").
-                await asyncio.sleep(target_wall - elapsed)
+                if turn_idx > 0:
+                    await asyncio.sleep(config.inter_turn_think_s)
+            else:
+                # Original: dispatch at the request's absolute trace timestamp.
+                target_wall = (req.timestamp_s - earliest_ts)
+                elapsed = time.perf_counter() - sweep_start
+                if elapsed < target_wall:
+                    await asyncio.sleep(target_wall - elapsed)
 
             token_ids = _apply_realized_prefix(
                 _build_prompt_token_ids(req),