fix(context): trim tool exchanges atomically

raven/context_engine/history_trimmer.py

@@ -167,6 +167,46 @@ def _first_droppable(ids: list[int], protected_ids: set[int]) -> int | None:
                 return pos
         return 0 if ids else None
 
+    @staticmethod
+    def _tool_exchange_ids(messages: list[dict[str, Any]], mid: int) -> set[int]:
+        """Return every message that must be dropped with ``mid``.
+
+        One assistant message may request several tools.  Its call message and
+        every corresponding result therefore form one structural unit: keeping
+        only part of that unit produces an invalid provider request.
+        """
+        message = messages[mid]
+        parent_idx: int | None = None
+        if message.get("role") == "assistant" and message.get("tool_calls"):
+            parent_idx = mid
+        elif message.get("role") == "tool" and message.get("tool_call_id"):
+            call_id = str(message["tool_call_id"])
+            for idx, candidate in enumerate(messages):
+                if candidate.get("role") != "assistant":
+                    continue
+                if any(
+                    isinstance(tool_call, dict) and str(tool_call.get("id", "")) == call_id
+                    for tool_call in candidate.get("tool_calls") or []
+                ):
+                    parent_idx = idx
+                    break
+
+        if parent_idx is None:
+            return {mid}
+
+        call_ids = {
+            str(tool_call["id"])
+            for tool_call in messages[parent_idx].get("tool_calls") or []
+            if isinstance(tool_call, dict) and tool_call.get("id")
+        }
+        exchange = {parent_idx}
+        exchange.update(
+            idx
+            for idx, candidate in enumerate(messages)
+            if candidate.get("role") == "tool" and str(candidate.get("tool_call_id", "")) in call_ids
+        )
+        return exchange
+
     # ------------------------------------------------------------------
     # Budget-driven trimming
     # ------------------------------------------------------------------
@@ -205,8 +245,10 @@ def trim(
             drop_idx = self._first_droppable(trimmed_ids, protected_ids)
             if drop_idx is None:
                 break
-            dropped = trimmed_ids.pop(drop_idx)
-            warnings.append(f"dropped message {dropped} to fit budget")
+            dropped = trimmed_ids[drop_idx]
+            dropped_group = self._tool_exchange_ids(session_messages, dropped)
+            trimmed_ids = [mid for mid in trimmed_ids if mid not in dropped_group]
+            warnings.extend(f"dropped message {mid} to fit budget" for mid in sorted(dropped_group))
             history = self.history_from_ids(session_messages, trimmed_ids)
             messages = build_messages(history)
             estimated, source = estimate_prompt_tokens_chain(

tests/test_history_trimmer.py

@@ -3,6 +3,15 @@
 from raven.context_engine.history_trimmer import HistoryTrimmer
 
 
+class _WeightedProvider:
+    def estimate_prompt_tokens(self, messages, tools, model):
+        del tools, model
+        total = 0
+        for message in messages:
+            total += 100 if message.get("tool_calls") else 1
+        return total, "test"
+
+
 def test_history_from_ids_preserves_reasoning_fields():
     messages = [
         {"role": "user", "content": "hi"},
@@ -28,3 +37,42 @@ def test_history_from_ids_drops_non_provider_keys():
     history = HistoryTrimmer.history_from_ids(messages, [0])
 
     assert history == [{"role": "user", "content": "hi"}]
+
+
+def test_budget_trim_drops_tool_call_and_result_as_one_group():
+    messages = [
+        {"role": "user", "content": "keep me"},
+        {
+            "role": "assistant",
+            "content": "calling a tool",
+            "tool_calls": [
+                {
+                    "id": "call_1|fc_1",
+                    "type": "function",
+                    "function": {"name": "message", "arguments": "{}"},
+                }
+            ],
+        },
+        {
+            "role": "tool",
+            "tool_call_id": "call_1|fc_1",
+            "name": "message",
+            "content": "sent",
+        },
+    ]
+    trimmer = HistoryTrimmer(_WeightedProvider(), "test-model", lambda: [], 10)
+
+    built, outcome = trimmer.trim(
+        session_messages=messages,
+        ids=[0, 1, 2],
+        protected_ids={0},
+        reserved_output=0,
+        build_messages=lambda history: [
+            {"role": "system", "content": "system"},
+            *history,
+            {"role": "user", "content": "current"},
+        ],
+    )
+
+    assert outcome.included_ids == [0]
+    assert HistoryTrimmer.structural_errors(built) == []