fix(compression): preflight display-seed must not overwrite real provider usage

The preflight rough-estimate seed used 'last >= 0' semantics, so any
provider that reports real prompt_tokens got its reading replaced by
the schema/reasoning-inflated rough estimate whenever the estimate was
larger. last_prompt_tokens feeds both the CLI context meter (cli.py)
and the post-response compression gate (conversation_loop.py), so one
seed made the bar jump to an inflated number AND could push the
real-usage gate over threshold on estimator noise.

Observed in a production CLI session on a 1M-token window with a
reasoning-heavy history: the status bar showed ~492K real provider
prompt tokens; the next turn's preflight estimated ~685K (rough
estimates can inflate 1.4-2.5x on reasoning-heavy sessions, #81481)
and seeded it into last_prompt_tokens; compression then fired at ~69%
of the window while real usage was ~49%.

Policy change: a real provider reading (>0) always wins. The seed now
only fills the 0 state ('no reading yet'), keeping the status bar live
for usage-less providers (#34282's motivation); -1 remains protected
as the post-compression sentinel (#36718).

Updates TestPreflightSentinelGuard to encode the new policy and adds a
regression test with the measured 492K/685K numbers.

agent/turn_context.py

@@ -1067,8 +1067,22 @@ def build_turn_context(
 
         if not _preflight_deferred:
             _last = _compressor.last_prompt_tokens
-            # Do NOT overwrite the -1 sentinel (#36718).
-            if _last >= 0 and _preflight_tokens > _last:
+            # The seed exists so the status bar shows current occupancy when a
+            # provider reports no usage (#34282's motivation). But
+            # ``last_prompt_tokens`` is also the post-response compression
+            # gate's "real tokens" input (conversation_loop) and the CLI
+            # context meter's source (cli.py). Overwriting a REAL provider
+            # reading with the rough preflight estimate makes the bar jump to
+            # an inflated number and can push the real-usage gate over the
+            # threshold on estimator noise — compression then fires at far
+            # below the user-configured threshold (observed: 492K real vs
+            # ~685K rough on a 1M window; reasoning-heavy sessions inflate
+            # the rough estimate 1.4-2.5x, see #81481).
+            #
+            # Policy: a real provider reading (>0) always wins. Seed only
+            # from the 0 state ("no reading yet"); -1 stays protected as the
+            # post-compression sentinel (#36718).
+            if _last == 0 and _preflight_tokens > _last:
                 _compressor.last_prompt_tokens = _preflight_tokens
 
         _compression_cooldown = getattr(

tests/agent/test_context_compressor.py

@@ -2463,9 +2463,9 @@ class TestPreflightSentinelGuard:
     """
 
     def _seed(self, last_prompt_tokens, preflight_tokens):
-        # Mirror the exact guard in agent/conversation_loop.py run_conversation.
+        # Mirror the exact guard in agent/turn_context.py build_turn_context.
         _last = last_prompt_tokens
-        if _last >= 0 and preflight_tokens > _last:
+        if _last == 0 and preflight_tokens > _last:
             return preflight_tokens  # would overwrite
         return last_prompt_tokens   # preserved
 
@@ -2475,11 +2475,21 @@ class TestPreflightSentinelGuard:
         result = self._seed(compressor.last_prompt_tokens, 250_000)
         assert result == -1
 
-    def test_real_value_still_revises_upward(self, compressor):
-        compressor.last_prompt_tokens = 10_000
+    def test_zero_state_still_seeded(self, compressor):
+        # 0 means "no reading yet" — the seed keeps the status bar live when
+        # providers report no usage.
+        compressor.last_prompt_tokens = 0
         result = self._seed(compressor.last_prompt_tokens, 50_000)
         assert result == 50_000
 
+    def test_real_provider_reading_wins_over_rough_estimate(self, compressor):
+        # Regression for the 492K-vs-685K display jump: a real provider
+        # reading must never be replaced by the schema/reasoning-inflated
+        # rough preflight estimate (#81481 class inflation).
+        compressor.last_prompt_tokens = 492_000
+        result = self._seed(compressor.last_prompt_tokens, 685_344)
+        assert result == 492_000
+
 
 
 class TestTurnPairPreservation: