fix: handle Anthropic long-context tier 429 by reducing to 200k

Anthropic returns HTTP 429 'Extra usage is required for long context
requests' when a Claude Max subscription doesn't include the 1M context
tier. This is NOT a transient rate limit — retrying won't help.

Detect this specific error before the generic rate-limit handler and:
1. Reduce context_length from 1M to 200k (the standard tier)
2. Trigger context compression to fit
3. Retry with the reduced context

The reduction is session-scoped (not persisted) so it auto-recovers
if the user later enables extra usage on their subscription.

Fixes: Sonnet 4.6 instant rate limits on Claude Max without extra usage

tests/test_long_context_tier_429.py

@@ -0,0 +1,163 @@
+"""Tests for Anthropic long-context tier 429 handling.
+
+When Claude Max users without "extra usage" hit the 1M context tier,
+Anthropic returns HTTP 429 "Extra usage is required for long context
+requests."  This is NOT a transient rate limit — the agent should
+reduce context_length to 200k and compress instead of retrying.
+"""
+
+import pytest
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+
+# ---------------------------------------------------------------------------
+# Detection logic
+# ---------------------------------------------------------------------------
+
+
+class TestLongContextTierDetection:
+    """Verify the detection heuristic matches the Anthropic error."""
+
+    @staticmethod
+    def _is_long_context_tier_error(status_code, error_msg):
+        error_msg = error_msg.lower()
+        return (
+            status_code == 429
+            and "extra usage" in error_msg
+            and "long context" in error_msg
+        )
+
+    def test_matches_anthropic_error(self):
+        assert self._is_long_context_tier_error(
+            429,
+            "Extra usage is required for long context requests.",
+        )
+
+    def test_matches_lowercase(self):
+        assert self._is_long_context_tier_error(
+            429,
+            "extra usage is required for long context requests.",
+        )
+
+    def test_rejects_normal_429(self):
+        assert not self._is_long_context_tier_error(
+            429,
+            "Rate limit exceeded. Please retry after 30 seconds.",
+        )
+
+    def test_rejects_wrong_status(self):
+        assert not self._is_long_context_tier_error(
+            400,
+            "Extra usage is required for long context requests.",
+        )
+
+    def test_rejects_partial_match(self):
+        """Both 'extra usage' AND 'long context' must be present."""
+        assert not self._is_long_context_tier_error(
+            429, "extra usage required"
+        )
+        assert not self._is_long_context_tier_error(
+            429, "long context requests not supported"
+        )
+
+
+# ---------------------------------------------------------------------------
+# Context reduction
+# ---------------------------------------------------------------------------
+
+
+class TestContextReduction:
+    """When the long-context tier error fires, context_length should
+    drop to 200k and the reduced flag should be set correctly."""
+
+    def _make_compressor(self, context_length=1_000_000, threshold_percent=0.5):
+        c = SimpleNamespace(
+            context_length=context_length,
+            threshold_percent=threshold_percent,
+            threshold_tokens=int(context_length * threshold_percent),
+            _context_probed=False,
+            _context_probe_persistable=False,
+        )
+        return c
+
+    def test_reduces_1m_to_200k(self):
+        comp = self._make_compressor(1_000_000)
+        reduced_ctx = 200_000
+
+        if comp.context_length > reduced_ctx:
+            comp.context_length = reduced_ctx
+            comp.threshold_tokens = int(reduced_ctx * comp.threshold_percent)
+            comp._context_probed = True
+            comp._context_probe_persistable = False
+
+        assert comp.context_length == 200_000
+        assert comp.threshold_tokens == 100_000
+        assert comp._context_probed is True
+        # Must NOT persist — subscription tier, not model capability
+        assert comp._context_probe_persistable is False
+
+    def test_no_reduction_when_already_200k(self):
+        comp = self._make_compressor(200_000)
+        reduced_ctx = 200_000
+
+        original = comp.context_length
+        if comp.context_length > reduced_ctx:
+            comp.context_length = reduced_ctx
+
+        assert comp.context_length == original  # unchanged
+
+    def test_no_reduction_when_below_200k(self):
+        comp = self._make_compressor(128_000)
+        reduced_ctx = 200_000
+
+        original = comp.context_length
+        if comp.context_length > reduced_ctx:
+            comp.context_length = reduced_ctx
+
+        assert comp.context_length == original  # unchanged
+
+
+# ---------------------------------------------------------------------------
+# Integration: agent error handler path
+# ---------------------------------------------------------------------------
+
+
+class TestAgentErrorPath:
+    """Verify the long-context 429 doesn't hit the generic rate-limit
+    or client-error handlers."""
+
+    def test_long_context_429_not_treated_as_rate_limit(self):
+        """The error should be intercepted before the generic
+        is_rate_limited check fires a fallback switch."""
+        error_msg = "extra usage is required for long context requests."
+        status_code = 429
+
+        # The long-context check fires first
+        _is_long_context_tier_error = (
+            status_code == 429
+            and "extra usage" in error_msg
+            and "long context" in error_msg
+        )
+        assert _is_long_context_tier_error
+
+        # So we never reach the generic rate-limit path
+        # (in the real code, `break` exits the retry loop)
+
+    def test_normal_429_still_treated_as_rate_limit(self):
+        """A normal 429 should NOT match the long-context check."""
+        error_msg = "rate limit exceeded"
+        status_code = 429
+
+        _is_long_context_tier_error = (
+            status_code == 429
+            and "extra usage" in error_msg
+            and "long context" in error_msg
+        )
+        assert not _is_long_context_tier_error
+
+        is_rate_limited = (
+            status_code == 429
+            or "rate limit" in error_msg
+        )
+        assert is_rate_limited