fix(minimax): drop stale ≤204,800 cache entries for MiniMax-M3 (#36726)

M3 is 1M context, but pre-catalog builds resolved it via the generic
'minimax' catch-all (204,800) and persisted that to the context-length
cache. Step 1 of get_model_context_length returned the cached value
directly before reaching the 'minimax-m3' (1M) catalog entry, so users
who first probed M3 on an older build were stuck at 204K forever (e.g.
/new in the Telegram gateway showing 'Context: 204K tokens (detected)').

Mirror the existing Kimi/Codex stale-cache guards: when a cached entry
for a minimax-m3 slug is <= 204,800, drop it and re-resolve. M2.x slugs
(correctly 204,800) are untouched since they don't match the M3 name.

tests/agent/test_minimax_provider.py

@@ -4,8 +4,9 @@ from unittest.mock import patch
 
 
 class TestMinimaxContextLengths:
-    """Verify context length entries match official docs (204,800 for all models).
+    """Verify context length entries match official docs.
 
+    M2.x series is 204,800; M3 is 1M (max output 512K).
     Source: https://platform.minimax.io/docs/api-reference/text-anthropic-api
     """
 
@@ -15,11 +16,74 @@ class TestMinimaxContextLengths:
 
     def test_minimax_models_resolve_via_prefix(self):
         from agent.model_metadata import get_model_context_length
-        # All MiniMax models should resolve to 204,800 via the "minimax" prefix
+        # M2.x models resolve to 204,800 via the "minimax" catch-all
         for model in ("MiniMax-M2.7", "MiniMax-M2.5", "MiniMax-M2.1", "MiniMax-M2"):
             ctx = get_model_context_length(model, "")
             assert ctx == 204_800, f"{model} expected 204800, got {ctx}"
 
+    def test_minimax_m3_resolves_to_1m(self):
+        from agent.model_metadata import get_model_context_length
+        # M3 must beat the generic "minimax" catch-all (204,800) and resolve to
+        # a 1M-class context. The exact value depends on the source: our
+        # hardcoded catalog says 1,000,000; the OpenRouter catalog reports
+        # 1,048,576 (1024²). Either is correct — assert "≥ 1M, not 204,800".
+        for model in ("MiniMax-M3", "minimax/minimax-m3", "minimax-m3"):
+            ctx = get_model_context_length(model, "")
+            assert ctx >= 1_000_000, f"{model} expected 1M-class, got {ctx}"
+
+
+class TestMinimaxM3StaleCacheGuard:
+    """Pre-catalog builds resolved M3 via the generic 'minimax' catch-all
+    (204,800) and persisted it before the 'minimax-m3' (1M) catalog entry
+    existed.  The step-1 cache guard must drop that stale value and re-resolve
+    to 1M, while leaving correct M2.x entries (204,800) untouched.
+    """
+
+    def test_suggests_minimax_m3(self):
+        from agent.model_metadata import _model_name_suggests_minimax_m3
+        assert _model_name_suggests_minimax_m3("MiniMax-M3")
+        assert _model_name_suggests_minimax_m3("minimax/minimax-m3")
+        assert not _model_name_suggests_minimax_m3("MiniMax-M2.7")
+        assert not _model_name_suggests_minimax_m3("MiniMax-M2.5")
+
+    def test_stale_m3_cache_dropped_and_reresolves_to_1m(self, tmp_path, monkeypatch):
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+        import importlib
+        import agent.model_metadata as mm
+        importlib.reload(mm)
+        base = "https://api.minimaxi.com/anthropic"
+        mm.save_context_length("MiniMax-M3", base, 204_800)
+        ctx = mm.get_model_context_length(
+            "MiniMax-M3", base_url=base, api_key="", provider="minimax-cn"
+        )
+        assert ctx == 1_000_000
+
+    def test_correct_m3_cache_preserved(self, tmp_path, monkeypatch):
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+        import importlib
+        import agent.model_metadata as mm
+        importlib.reload(mm)
+        base = "https://api.minimaxi.com/anthropic"
+        mm.save_context_length("MiniMax-M3", base, 1_000_000)
+        ctx = mm.get_model_context_length(
+            "MiniMax-M3", base_url=base, api_key="", provider="minimax-cn"
+        )
+        assert ctx == 1_000_000
+
+    def test_m2_cache_not_clobbered(self, tmp_path, monkeypatch):
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+        import importlib
+        import agent.model_metadata as mm
+        importlib.reload(mm)
+        base = "https://api.minimaxi.com/anthropic"
+        # 204,800 is the CORRECT value for M2.x — guard must not touch it.
+        for slug in ("MiniMax-M2.7", "MiniMax-M2.5", "MiniMax-M2.1"):
+            mm.save_context_length(slug, base, 204_800)
+            ctx = mm.get_model_context_length(
+                slug, base_url=base, api_key="", provider="minimax-cn"
+            )
+            assert ctx == 204_800, f"{slug} should stay 204800, got {ctx}"
+
 
 
 class TestMinimaxThinkingSupport: