diff --git a/src/agent_loop.py b/src/agent_loop.py
index cb2d33b8c..93943f98d 100644
--- a/src/agent_loop.py
+++ b/src/agent_loop.py
@@ -24193,11 +24193,14 @@ async def stream_agent_loop(
                 budget_is_explicit,
                 hard_max=hard_max,
             )
-            if candidate_context <= 8192:
+            if 0 < candidate_context <= 8192:
                 # Small local servers often tokenize chat wrappers and tool
                 # results much more generously than our rough estimator. Keep
                 # substantial headroom for those wrappers and generation so a
                 # follow-up tool round cannot exceed the server's n_ctx.
+                # 0 means the window is unknown, which already gets the
+                # conservative default budget; clamping it as well cut every
+                # request to 1200 tokens and dropped open documents (#6678).
                 effective_budget = min(
                     effective_budget,
                     max(1200, int(candidate_context * 0.40)),
diff --git a/tests/test_agent_unknown_context_budget.py b/tests/test_agent_unknown_context_budget.py
new file mode 100644
index 000000000..a9b660cac
--- /dev/null
+++ b/tests/test_agent_unknown_context_budget.py
@@ -0,0 +1,71 @@
+"""An unknown context window must not be budgeted like a tiny local server (#6678).
+
+``budget_context_for_model`` returns 0 when the endpoint does not prove its
+window, and ``compute_input_token_budget`` answers 0 with the conservative
+default budget. The agent loop's small-server clamp (``<= 8192``) also caught
+0 and cut the request to 1200 tokens, so an open document or earlier context
+of a few thousand tokens was trimmed away before the model saw it.
+"""
+import inspect
+import json
+
+import pytest
+
+DONE = 'data: [DONE]\n\n'
+MARKER = 'NIGHTJAR-7'
+# ~3000 estimated tokens: over the old 1200 clamp, under the 6000 default.
+CONTEXT = ''.join(
+    f'## Project-{n}\nOwner: team {n}. Status: active. Notes: routine maintenance '
+    'and quarterly review scheduled for every workstream in this registry.\n\n'
+    for n in range(60)
+) + f'## Project-Alpha\nAlias: {MARKER}\n'
+
+
+def _event(payload):
+    return 'data: ' + json.dumps(payload) + '\n\n'
+
+
+async def _first_request(monkeypatch, tmp_path, context_window):
+    import src.agent_loop as al
+    from tests.test_agent_runtime_context import _patch_fake_skills
+    _patch_fake_skills(monkeypatch)
+    monkeypatch.setattr('src.tool_index.get_tool_index', lambda: None)
+    monkeypatch.setattr('src.model_context.budget_context_for_model',
+                        lambda *args, **kwargs: context_window)
+    seen = []
+
+    async def provider(candidates, messages, **kwargs):
+        # Like llm_core: the per-candidate factory shapes and trims the request.
+        url, model, headers = candidates[0][:3]
+        request = kwargs['candidate_request_factory'](0, url, model, headers)
+        if inspect.isawaitable(request):
+            request = await request
+        seen.append((request or {}).get('messages', messages))
+        yield _event({'delta': 'ok'})
+        yield DONE
+
+    monkeypatch.setattr(al, 'stream_llm_with_fallback', provider)
+    history = [
+        {'role': 'user', 'content': 'Keep this registry for later:\n\n' + CONTEXT},
+        {'role': 'assistant', 'content': 'Saved.'},
+        {'role': 'user', 'content': 'What is the alias of Project-Alpha?'},
+    ]
+    async for _ in al.stream_agent_loop('http://127.0.0.1:9/v1', 'model', history,
+                                        max_rounds=1, workspace=str(tmp_path)):
+        pass
+    assert seen, 'the provider was never called'
+    return json.dumps(seen[0])
+
+
+@pytest.mark.asyncio
+async def test_unknown_context_window_keeps_earlier_context(monkeypatch, tmp_path):
+    request = await _first_request(monkeypatch, tmp_path, 0)
+    assert MARKER in request
+
+
+@pytest.mark.asyncio
+async def test_known_small_window_is_still_clamped(monkeypatch, tmp_path):
+    # The clamp exists for small local servers whose window is known (n_ctx
+    # 2048 here); it must keep protecting them.
+    request = await _first_request(monkeypatch, tmp_path, 2048)
+    assert MARKER not in request