diff --git a/agent/model_metadata.py b/agent/model_metadata.py
index 976dada88f4d8..a469bdb619e3e 100644
--- a/agent/model_metadata.py
+++ b/agent/model_metadata.py
@@ -1933,8 +1933,19 @@ def _query_local_context_length_uncached(model: str, base_url: str, api_key: str
             resp = client.get(f"{server_url}/v1/models/{model}")
             if resp.status_code == 200:
                 data = resp.json()
-                # vLLM returns max_model_len
-                ctx = data.get("max_model_len") or data.get("context_length") or data.get("max_tokens")
+                # Read genuine context-WINDOW fields only. `max_tokens` is
+                # excluded on purpose: on an Anthropic-style /v1/models/{id}
+                # passthrough it is the max *output* tokens (e.g. 128000 for a
+                # 1M-context model), NOT the context window — reading it here
+                # collapses the window and triggers premature auto-compaction.
+                # `max_input_tokens` IS the input/context ceiling, so it's kept.
+                ctx = (
+                    data.get("max_model_len")
+                    or data.get("context_length")
+                    or data.get("context_window")
+                    or data.get("max_input_tokens")
+                    or data.get("max_position_embeddings")
+                )
                 if ctx and isinstance(ctx, (int, float)):
                     return int(ctx)
 
@@ -1946,7 +1957,14 @@ def _query_local_context_length_uncached(model: str, base_url: str, api_key: str
                 models_list = data.get("data", [])
                 for m in models_list:
                     if _model_id_matches(m.get("id", ""), model):
-                        ctx = m.get("max_model_len") or m.get("context_length") or m.get("max_tokens")
+                        # Context-window fields only (see the max_tokens note above).
+                        ctx = (
+                            m.get("max_model_len")
+                            or m.get("context_length")
+                            or m.get("context_window")
+                            or m.get("max_input_tokens")
+                            or m.get("max_position_embeddings")
+                        )
                         if ctx and isinstance(ctx, (int, float)):
                             return int(ctx)
     except Exception:
diff --git a/tests/agent/test_model_metadata_local_ctx.py b/tests/agent/test_model_metadata_local_ctx.py
index 9bcbb0aecd9bb..b598e519ff572 100644
--- a/tests/agent/test_model_metadata_local_ctx.py
+++ b/tests/agent/test_model_metadata_local_ctx.py
@@ -190,6 +190,61 @@ def test_vllm_context_length_key(self):
 
         assert result == 32768
 
+    def test_anthropic_passthrough_reads_max_input_tokens_not_max_tokens(self):
+        """Regression: an Anthropic-style /v1/models/{id} passthrough returns
+        `max_input_tokens` (the context window) AND `max_tokens` (the max OUTPUT
+        tokens). The resolver must read max_input_tokens, never max_tokens —
+        reading max_tokens collapsed a 1M-context model to its 128K output cap
+        and triggered premature auto-compaction (~96K on a 1M model)."""
+        from agent.model_metadata import _query_local_context_length
+
+        # Shape mirrors the fleet's Claude proxy passthrough of GET
+        # /v1/models/claude-fable-5.
+        detail_resp = self._make_resp(200, {
+            "type": "model",
+            "id": "claude-fable-5",
+            "display_name": "Claude Fable 5",
+            "max_input_tokens": 1000000,   # context window
+            "max_tokens": 128000,          # max OUTPUT tokens — must be ignored here
+        })
+
+        client_mock = MagicMock()
+        client_mock.__enter__ = lambda s: client_mock
+        client_mock.__exit__ = MagicMock(return_value=False)
+        client_mock.post.return_value = self._make_resp(404, {})
+        client_mock.get.return_value = detail_resp
+
+        with patch("agent.model_metadata.detect_local_server_type", return_value="vllm"), \
+             patch("httpx.Client", return_value=client_mock):
+            result = _query_local_context_length("claude-fable-5", "http://localhost:18801/anthropic")
+
+        assert result == 1000000, (
+            f"must read max_input_tokens (context), not max_tokens (output); got {result}"
+        )
+
+    def test_models_list_ignores_max_tokens_output_cap(self):
+        """Same guard on the /v1/models LIST branch: max_tokens (output) must
+        not be mistaken for the context window."""
+        from agent.model_metadata import _query_local_context_length
+
+        detail_miss = self._make_resp(404, {})
+        list_resp = self._make_resp(200, {"data": [
+            {"id": "claude-fable-5", "max_input_tokens": 1000000, "max_tokens": 128000},
+        ]})
+
+        client_mock = MagicMock()
+        client_mock.__enter__ = lambda s: client_mock
+        client_mock.__exit__ = MagicMock(return_value=False)
+        client_mock.post.return_value = self._make_resp(404, {})
+        # first GET is /v1/models/{model} (miss), second is /v1/models (list)
+        client_mock.get.side_effect = [detail_miss, list_resp]
+
+        with patch("agent.model_metadata.detect_local_server_type", return_value="vllm"), \
+             patch("httpx.Client", return_value=client_mock):
+            result = _query_local_context_length("claude-fable-5", "http://localhost:18801/anthropic")
+
+        assert result == 1000000, f"list branch must ignore max_tokens output cap; got {result}"
+
 
 class TestQueryLocalContextLengthModelsList:
     """_query_local_context_length: falls back to /v1/models list."""