[eric] agents: OSW_COMPACT_CEILING_TOKENS makes the mid-turn breaker drillable without a 180K turn

backend/apps/agents/manager/context_budget.py

@@ -7,6 +7,7 @@ Compaction here only MARKS (sets compacted_through_msg_id); it never mutates
 session.messages, the originals stay for the UI drawer and only the history sent to the SDK
 is trimmed downstream (see backend/CLAUDE.md: "compaction must actually trim, not just mark")."""
 
+import os
 from typing import Dict, Optional
 
 from typeguard import typechecked
@@ -17,13 +18,34 @@ from backend.apps.agents.manager.session.history_compaction import get_branch_me
 from backend.apps.agents.manager.streaming.state import TurnState
 
 
+@typechecked
+def compact_ceiling_tokens(session: AgentSession) -> int:
+    """The absolute ceiling, with a drill override.
+
+    The mid-turn breaker is otherwise only reachable by paying for a genuine 180K-token turn,
+    which in practice meant it was never drilled at all: three attempts to fire it cost real
+    money and still failed. `OSW_COMPACT_CEILING_TOKENS` lowers the bar so the same code path
+    can be exercised in seconds. Unset everywhere except a drill, and a junk value is ignored
+    rather than silently trusted."""
+    raw = os.environ.get("OSW_COMPACT_CEILING_TOKENS", "").strip()
+    if raw:
+        try:
+            override = int(raw)
+        except ValueError:
+            return session.compact_abs_ceiling_tokens
+        if override > 0:
+            return override
+    return session.compact_abs_ceiling_tokens
+
+
 @typechecked
 def compact_trigger_tokens(session: AgentSession) -> int:
     """The token count where compaction fires: the TIGHTER of the pct threshold and the
     absolute ceiling (on a 200K window the pct wins at 130K; on a 1M window the ceiling
     wins at 180K, not 650K)."""
     window = max(1, session.context_window)
-    abs_pct = min(1.0, session.compact_abs_ceiling_tokens / window)
+    ceiling = compact_ceiling_tokens(session)
+    abs_pct = min(1.0, ceiling / window)
     return int(window * min(session.compact_threshold_pct, abs_pct))
 
 

backend/tests/test_compact_ceiling_override.py

@@ -0,0 +1,52 @@
+"""The mid-turn context breaker has to be drillable, or it only ever gets reasoned about.
+
+Three attempts to fire it live (2026-08-22) failed: the empty-finish path is not inducible by
+prompt (the model always writes text), and the breaker itself needs a genuine 180K-token turn,
+which costs real money and died to a lane 401 twice. `OSW_COMPACT_CEILING_TOKENS` lowers the bar
+so the SAME code path runs in seconds. It must be inert unless deliberately set, and it must not
+trust a junk value.
+"""
+
+import os
+
+import pytest
+
+from backend.apps.agents.core.models import AgentSession
+from backend.apps.agents.manager.context_budget import compact_ceiling_tokens, compact_trigger_tokens
+
+
+def p_session(window: int = 1_000_000) -> AgentSession:
+    s = AgentSession(name="t", model="opus-5")
+    s.context_window = window
+    return s
+
+
+@pytest.fixture(autouse=True)
+def p_clean_env(monkeypatch):
+    monkeypatch.delenv("OSW_COMPACT_CEILING_TOKENS", raising=False)
+
+
+def test_unset_is_exactly_todays_behaviour():
+    s = p_session()
+    assert compact_ceiling_tokens(s) == s.compact_abs_ceiling_tokens == 180_000
+    assert compact_trigger_tokens(s) == 180_000
+
+
+def test_the_override_lowers_the_trigger(monkeypatch):
+    monkeypatch.setenv("OSW_COMPACT_CEILING_TOKENS", "20000")
+    assert compact_trigger_tokens(p_session()) == 20_000
+
+
+def test_a_junk_value_is_ignored_not_trusted(monkeypatch):
+    for junk in ("banana", "", "  ", "1e5", "0", "-5"):
+        monkeypatch.setenv("OSW_COMPACT_CEILING_TOKENS", junk)
+        assert compact_ceiling_tokens(p_session()) == 180_000, f"junk {junk!r} must not change the ceiling"
+
+
+def test_the_pct_threshold_still_wins_when_it_is_tighter(monkeypatch):
+    # On a 200K window the pct (0.65 -> 130K) is tighter than a 180K ceiling; an override must not
+    # be able to raise the trigger above what the percentage already allows.
+    s = p_session(window=200_000)
+    assert compact_trigger_tokens(s) == 130_000
+    monkeypatch.setenv("OSW_COMPACT_CEILING_TOKENS", "900000")
+    assert compact_trigger_tokens(s) == 130_000