diff --git a/agent/turn_context.py b/agent/turn_context.py
index 8f333870cf..e664034db7 100644
--- a/agent/turn_context.py
+++ b/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(
diff --git a/tests/agent/test_context_compressor.py b/tests/agent/test_context_compressor.py
index 2b822a10cd..ae87008e04 100644
--- a/tests/agent/test_context_compressor.py
+++ b/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: