fix(agent): clamp compression threshold below context_length to ensure compression can trigger (#14690)

When context_length equals MINIMUM_CONTEXT_LENGTH (64000), the max()
floor in threshold calculation pushed threshold_tokens to 100% of the
context window, making auto-compression impossible since the API would
error before tokens ever reached the threshold. Add a min() clamp after
the max() floor so that threshold never exceeds 95% of context_length,
giving 5% headroom for compression to trigger. Applied to both __init__
and update_model.

agent/context_compressor.py

@@ -318,6 +318,12 @@ class ContextCompressor(ContextEngine):
             int(context_length * self.threshold_percent),
             MINIMUM_CONTEXT_LENGTH,
         )
+        # Clamp: threshold must stay below context_length so compression can
+        # actually trigger before the API rejects the request.  The 95% cap
+        # gives 5% headroom — enough to catch the threshold in time.
+        self.threshold_tokens = min(
+            self.threshold_tokens, int(context_length * 0.95)
+        )
 
     def __init__(
         self,
@@ -358,6 +364,12 @@ class ContextCompressor(ContextEngine):
             int(self.context_length * threshold_percent),
             MINIMUM_CONTEXT_LENGTH,
         )
+        # Clamp: threshold must stay below context_length so compression can
+        # actually trigger before the API rejects the request.  The 95% cap
+        # gives 5% headroom — enough to catch the threshold in time.
+        self.threshold_tokens = min(
+            self.threshold_tokens, int(self.context_length * 0.95)
+        )
         self.compression_count = 0
 
         # Derive token budgets: ratio is relative to the threshold, not total context

tests/agent/test_context_compressor.py

@@ -4,6 +4,7 @@ import pytest
 from unittest.mock import patch, MagicMock
 
 from agent.context_compressor import ContextCompressor, SUMMARY_PREFIX
+from agent.model_metadata import MINIMUM_CONTEXT_LENGTH
 
 
 @pytest.fixture()
@@ -969,3 +970,55 @@ class TestTruncateToolCallArgsJson:
         parsed = _json.loads(shrunk)
         assert parsed["path"] == "~/.hermes/skills/shopping/browser-setup-notes.md"
         assert parsed["content"].endswith("...[truncated]")
+
+class TestThresholdClampAtMinimumContext:
+    """Regression test for #14690: when context_length == MINIMUM_CONTEXT_LENGTH,
+    the max() floor pushed threshold_tokens to 100% of the context window,
+    making auto-compression impossible (API errors before threshold is reached).
+    The fix clamps threshold_tokens to at most 95% of context_length."""
+
+    def test_init_threshold_below_context_length(self):
+        """__init__ must produce a threshold strictly below context_length."""
+        with patch(
+            "agent.context_compressor.get_model_context_length",
+            return_value=MINIMUM_CONTEXT_LENGTH,
+        ):
+            c = ContextCompressor(model="test", quiet_mode=True)
+        assert c.threshold_tokens < c.context_length
+        assert c.threshold_tokens == int(MINIMUM_CONTEXT_LENGTH * 0.95)
+
+    def test_should_compress_at_threshold(self):
+        """should_compress returns True at the threshold and False just below."""
+        with patch(
+            "agent.context_compressor.get_model_context_length",
+            return_value=MINIMUM_CONTEXT_LENGTH,
+        ):
+            c = ContextCompressor(model="test", quiet_mode=True)
+        assert c.should_compress(c.threshold_tokens - 1) is False
+        assert c.should_compress(c.threshold_tokens) is True
+
+    def test_update_model_threshold_below_context_length(self):
+        """update_model() must also clamp the threshold."""
+        with patch(
+            "agent.context_compressor.get_model_context_length",
+            return_value=200_000,
+        ):
+            c = ContextCompressor(model="test", quiet_mode=True)
+        c.update_model(
+            model="small/model",
+            context_length=MINIMUM_CONTEXT_LENGTH,
+        )
+        assert c.threshold_tokens < MINIMUM_CONTEXT_LENGTH
+        assert c.threshold_tokens == int(MINIMUM_CONTEXT_LENGTH * 0.95)
+        assert c.should_compress(c.threshold_tokens - 1) is False
+        assert c.should_compress(c.threshold_tokens) is True
+
+    def test_large_context_unaffected_by_clamp(self):
+        """For large models the 95% cap does not change the threshold."""
+        with patch(
+            "agent.context_compressor.get_model_context_length",
+            return_value=200_000,
+        ):
+            c = ContextCompressor(model="test", quiet_mode=True)
+        # 50% of 200K = 100K, 95% of 200K = 190K => min(100K, 190K) = 100K
+        assert c.threshold_tokens == 100_000