diff --git a/nanobot/session/manager.py b/nanobot/session/manager.py
index f0a6484..acb6d7f 100644
--- a/nanobot/session/manager.py
+++ b/nanobot/session/manager.py
@@ -43,23 +43,60 @@ class Session:
         self.messages.append(msg)
         self.updated_at = datetime.now()
 
+    @staticmethod
+    def _tool_call_ids(messages: list[dict[str, Any]]) -> set[str]:
+        ids: set[str] = set()
+        for message in messages:
+            if message.get("role") != "assistant":
+                continue
+            for tool_call in message.get("tool_calls") or []:
+                if not isinstance(tool_call, dict):
+                    continue
+                tool_id = tool_call.get("id")
+                if tool_id:
+                    ids.add(str(tool_id))
+        return ids
+
+    @classmethod
+    def _trim_orphan_tool_messages(cls, messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
+        """Drop the oldest prefix that contains tool results without matching tool_calls."""
+        trimmed = list(messages)
+        while trimmed:
+            tool_call_ids = cls._tool_call_ids(trimmed)
+            cut_to = None
+            for index, message in enumerate(trimmed):
+                if message.get("role") != "tool":
+                    continue
+                tool_call_id = message.get("tool_call_id")
+                if tool_call_id and str(tool_call_id) not in tool_call_ids:
+                    cut_to = index + 1
+                    break
+            if cut_to is None:
+                break
+            trimmed = trimmed[cut_to:]
+        return trimmed
+
     def get_history(self, max_messages: int = 500) -> list[dict[str, Any]]:
-        """Return unconsolidated messages for LLM input, aligned to a user turn."""
+        """Return unconsolidated messages for LLM input, aligned to a legal tool-call boundary."""
         unconsolidated = self.messages[self.last_consolidated:]
         sliced = unconsolidated[-max_messages:]
 
-        # Drop leading non-user messages to avoid orphaned tool_result blocks
-        for i, m in enumerate(sliced):
-            if m.get("role") == "user":
+        # Drop leading non-user messages to avoid starting mid-turn when possible.
+        for i, message in enumerate(sliced):
+            if message.get("role") == "user":
                 sliced = sliced[i:]
                 break
 
+        # Some providers reject orphan tool results if the matching assistant
+        # tool_calls message fell outside the fixed-size history window.
+        sliced = self._trim_orphan_tool_messages(sliced)
+
         out: list[dict[str, Any]] = []
-        for m in sliced:
-            entry: dict[str, Any] = {"role": m["role"], "content": m.get("content", "")}
-            for k in ("tool_calls", "tool_call_id", "name"):
-                if k in m:
-                    entry[k] = m[k]
+        for message in sliced:
+            entry: dict[str, Any] = {"role": message["role"], "content": message.get("content", "")}
+            for key in ("tool_calls", "tool_call_id", "name"):
+                if key in message:
+                    entry[key] = message[key]
             out.append(entry)
         return out
 
diff --git a/tests/test_session_manager_history.py b/tests/test_session_manager_history.py
new file mode 100644
index 0000000..a0effac
--- /dev/null
+++ b/tests/test_session_manager_history.py
@@ -0,0 +1,52 @@
+from nanobot.session.manager import Session
+
+
+def test_get_history_drops_orphan_tool_results_when_window_cuts_tool_calls():
+    session = Session(key="telegram:test")
+    session.messages.append({"role": "user", "content": "old turn"})
+
+    for i in range(20):
+        session.messages.append(
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [
+                    {"id": f"old_{i}_a", "type": "function", "function": {"name": "x", "arguments": "{}"}},
+                    {"id": f"old_{i}_b", "type": "function", "function": {"name": "y", "arguments": "{}"}},
+                ],
+            }
+        )
+        session.messages.append({"role": "tool", "tool_call_id": f"old_{i}_a", "name": "x", "content": "ok"})
+        session.messages.append({"role": "tool", "tool_call_id": f"old_{i}_b", "name": "y", "content": "ok"})
+
+    session.messages.append({"role": "user", "content": "problem turn"})
+    for i in range(25):
+        session.messages.append(
+            {
+                "role": "assistant",
+                "content": None,
+                "tool_calls": [
+                    {"id": f"cur_{i}_a", "type": "function", "function": {"name": "x", "arguments": "{}"}},
+                    {"id": f"cur_{i}_b", "type": "function", "function": {"name": "y", "arguments": "{}"}},
+                ],
+            }
+        )
+        session.messages.append({"role": "tool", "tool_call_id": f"cur_{i}_a", "name": "x", "content": "ok"})
+        session.messages.append({"role": "tool", "tool_call_id": f"cur_{i}_b", "name": "y", "content": "ok"})
+
+    session.messages.append({"role": "user", "content": "new telegram question"})
+
+    history = session.get_history(max_messages=100)
+    assistant_ids = {
+        tool_call["id"]
+        for message in history
+        if message.get("role") == "assistant"
+        for tool_call in (message.get("tool_calls") or [])
+    }
+    orphan_tool_ids = [
+        message.get("tool_call_id")
+        for message in history
+        if message.get("role") == "tool" and message.get("tool_call_id") not in assistant_ids
+    ]
+
+    assert orphan_tool_ids == []