diff --git a/run_agent.py b/run_agent.py
index 7543d1d0c..e98863f5e 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -3872,6 +3872,7 @@ class AIAgent:
                         'token limit', 'too many tokens', 'reduce the length',
                         'exceeds the limit', 'context window',
                         'request entity too large',  # OpenRouter/Nous 413 safety net
+                        'prompt is too long',  # Anthropic: "prompt is too long: N tokens > M maximum"
                     ])
                     
                     if is_context_length_error:
@@ -4256,6 +4257,7 @@ class AIAgent:
                     
                     messages.append(assistant_msg)
                     
+                    _msg_count_before_tools = len(messages)
                     self._execute_tool_calls(assistant_message, messages, effective_task_id, api_call_count)
 
                     # Refund the iteration if the ONLY tool(s) called were
@@ -4265,7 +4267,20 @@ class AIAgent:
                     if _tc_names == {"execute_code"}:
                         self.iteration_budget.refund()
                     
-                    if self.compression_enabled and self.context_compressor.should_compress():
+                    # Estimate next prompt size using real token counts from the
+                    # last API response + rough estimate of newly appended tool
+                    # results.  This catches cases where tool results push the
+                    # context past the limit that last_prompt_tokens alone misses
+                    # (e.g. large file reads, web extractions).
+                    _compressor = self.context_compressor
+                    _new_tool_msgs = messages[_msg_count_before_tools:]
+                    _new_chars = sum(len(str(m.get("content", "") or "")) for m in _new_tool_msgs)
+                    _estimated_next_prompt = (
+                        _compressor.last_prompt_tokens
+                        + _compressor.last_completion_tokens
+                        + _new_chars // 3  # conservative: JSON-heavy tool results ≈ 3 chars/token
+                    )
+                    if self.compression_enabled and _compressor.should_compress(_estimated_next_prompt):
                         messages, active_system_prompt = self._compress_context(
                             messages, system_message,
                             approx_tokens=self.context_compressor.last_prompt_tokens,
diff --git a/tests/test_413_compression.py b/tests/test_413_compression.py
index 62fee8b8e..1736bbde5 100644
--- a/tests/test_413_compression.py
+++ b/tests/test_413_compression.py
@@ -396,3 +396,73 @@ class TestPreflightCompression:
             result = agent.run_conversation("hello", conversation_history=big_history)
 
         mock_compress.assert_not_called()
+
+
+class TestToolResultPreflightCompression:
+    """Compression should trigger when tool results push context past the threshold."""
+
+    def test_large_tool_results_trigger_compression(self, agent):
+        """When tool results push estimated tokens past threshold, compress before next call."""
+        agent.compression_enabled = True
+        agent.context_compressor.context_length = 200_000
+        agent.context_compressor.threshold_tokens = 140_000
+        agent.context_compressor.last_prompt_tokens = 130_000
+        agent.context_compressor.last_completion_tokens = 5_000
+
+        tc = SimpleNamespace(
+            id="tc1", type="function",
+            function=SimpleNamespace(name="web_search", arguments='{"query":"test"}'),
+        )
+        tool_resp = _mock_response(
+            content=None, finish_reason="stop", tool_calls=[tc],
+            usage={"prompt_tokens": 130_000, "completion_tokens": 5_000, "total_tokens": 135_000},
+        )
+        ok_resp = _mock_response(
+            content="Done after compression", finish_reason="stop",
+            usage={"prompt_tokens": 50_000, "completion_tokens": 100, "total_tokens": 50_100},
+        )
+        agent.client.chat.completions.create.side_effect = [tool_resp, ok_resp]
+        large_result = "x" * 100_000
+
+        with (
+            patch("run_agent.handle_function_call", return_value=large_result),
+            patch.object(agent, "_compress_context") as mock_compress,
+            patch.object(agent, "_persist_session"),
+            patch.object(agent, "_save_trajectory"),
+            patch.object(agent, "_cleanup_task_resources"),
+        ):
+            mock_compress.return_value = (
+                [{"role": "user", "content": "hello"}], "compressed prompt",
+            )
+            result = agent.run_conversation("hello")
+
+        mock_compress.assert_called_once()
+        assert result["completed"] is True
+
+    def test_anthropic_prompt_too_long_safety_net(self, agent):
+        """Anthropic 'prompt is too long' error triggers compression as safety net."""
+        err_400 = Exception(
+            "Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', "
+            "'message': 'prompt is too long: 233153 tokens > 200000 maximum'}}"
+        )
+        err_400.status_code = 400
+        ok_resp = _mock_response(content="Recovered", finish_reason="stop")
+        agent.client.chat.completions.create.side_effect = [err_400, ok_resp]
+        prefill = [
+            {"role": "user", "content": "previous"},
+            {"role": "assistant", "content": "answer"},
+        ]
+
+        with (
+            patch.object(agent, "_compress_context") as mock_compress,
+            patch.object(agent, "_persist_session"),
+            patch.object(agent, "_save_trajectory"),
+            patch.object(agent, "_cleanup_task_resources"),
+        ):
+            mock_compress.return_value = (
+                [{"role": "user", "content": "hello"}], "compressed",
+            )
+            result = agent.run_conversation("hello", conversation_history=prefill)
+
+        mock_compress.assert_called_once()
+        assert result["completed"] is True