diff --git a/agent/model_metadata.py b/agent/model_metadata.py
index 4493eae5f1f8..444ad6525eae 100644
--- a/agent/model_metadata.py
+++ b/agent/model_metadata.py
@@ -1646,6 +1646,34 @@ def get_model_context_length(
     if config_context_length is not None and isinstance(config_context_length, int) and config_context_length > 0:
         return config_context_length
 
+    # 0a. MoA virtual provider — ``model`` is a preset name, not a real model,
+    # and ``base_url`` is the local virtual endpoint, so every probe below would
+    # miss and fall through to the 256K default. The aggregator is the acting
+    # model, so resolve the context window from the aggregator slot's real
+    # provider+model instead. References are advisory-only and never bound the
+    # acting context, so they're ignored here.
+    if (provider or "").strip().lower() == "moa":
+        try:
+            from hermes_cli.config import load_config
+            from hermes_cli.moa_config import resolve_moa_preset
+            from hermes_cli.runtime_provider import resolve_runtime_provider
+
+            preset = resolve_moa_preset(load_config().get("moa") or {}, model)
+            agg = preset.get("aggregator") or {}
+            agg_provider = str(agg.get("provider") or "").strip()
+            agg_model = str(agg.get("model") or "").strip()
+            if agg_model and agg_provider and agg_provider.lower() != "moa":
+                rt = resolve_runtime_provider(requested=agg_provider, target_model=agg_model)
+                return get_model_context_length(
+                    agg_model,
+                    base_url=rt.get("base_url", "") or "",
+                    api_key=rt.get("api_key", "") or "",
+                    provider=agg_provider,
+                )
+        except Exception:
+            logger.debug("MoA aggregator context-length resolution failed", exc_info=True)
+        # Fall through to the generic default if aggregator resolution failed.
+
     # 0b. custom_providers per-model override — check before any probe.
     # This closes the gap where /model switch and display paths used to fall
     # back to 128K despite the user having a per-model context_length set.
diff --git a/tests/agent/test_model_metadata.py b/tests/agent/test_model_metadata.py
index ecde355d0590..a18d9fb46182 100644
--- a/tests/agent/test_model_metadata.py
+++ b/tests/agent/test_model_metadata.py
@@ -1524,3 +1524,51 @@ def test_grok_4_not_clobbered(self, tmp_path, monkeypatch):
                 slug, base_url=base, api_key="", provider="xai"
             )
             assert ctx == 256_000, f"{slug} should stay 256000, got {ctx}"
+
+
+class TestMoAContextLength:
+    """MoA virtual provider resolves context from the aggregator slot, not 256K default."""
+
+    def _write_moa_config(self, home, aggregator):
+        import os
+        os.makedirs(home, exist_ok=True)
+        with open(os.path.join(home, "config.yaml"), "w") as f:
+            yaml.safe_dump(
+                {
+                    "moa": {
+                        "default_preset": "p",
+                        "presets": {
+                            "p": {
+                                "enabled": True,
+                                "reference_models": [
+                                    {"provider": "openrouter", "model": "openai/gpt-5.5"}
+                                ],
+                                "aggregator": aggregator,
+                            }
+                        },
+                    }
+                },
+                f,
+            )
+
+    def test_moa_resolves_from_aggregator(self, tmp_path, monkeypatch):
+        home = str(tmp_path / ".hermes")
+        monkeypatch.setenv("HERMES_HOME", home)
+        self._write_moa_config(home, {"provider": "openrouter", "model": "anthropic/claude-opus-4.8"})
+
+        # The MoA preset name + virtual base_url would otherwise fall through to
+        # the 256K default; instead it mirrors the aggregator's real window.
+        agg_ctx = get_model_context_length(
+            "anthropic/claude-opus-4.8", base_url="https://openrouter.ai/api/v1", provider="openrouter"
+        )
+        moa_ctx = get_model_context_length("p", base_url="http://127.0.0.1/v1", provider="moa")
+        assert moa_ctx == agg_ctx
+
+    def test_moa_config_override_still_wins(self, tmp_path, monkeypatch):
+        home = str(tmp_path / ".hermes")
+        monkeypatch.setenv("HERMES_HOME", home)
+        self._write_moa_config(home, {"provider": "openrouter", "model": "anthropic/claude-opus-4.8"})
+        ctx = get_model_context_length(
+            "p", base_url="http://127.0.0.1/v1", provider="moa", config_context_length=500_000
+        )
+        assert ctx == 500_000