fix(compactor): shrink oversized tool_calls arguments so trim_for_context can fit a tool-only turn (#2949)

tests/test_compact_truncate_tool_call_args.py

@@ -0,0 +1,62 @@
+"""Issue #2947 — _truncate_message_to_token_budget must shrink oversized tool_calls
+arguments, not just text content.
+
+A tool-only assistant turn persists content=None with its whole payload in
+tool_calls[].function.arguments. The text-content truncation can't reach it, so
+trim_for_context's last-resort message shrink left the message over budget and the
+upstream call 400'd. This pins that oversized args are bounded (so the message
+fits) while id/type/function.name are preserved, and that small args / plain text
+are untouched.
+"""
+import json
+import sys
+from unittest.mock import MagicMock
+
+import pytest
+
+for mod in [
+    'sqlalchemy', 'sqlalchemy.orm', 'sqlalchemy.ext', 'sqlalchemy.ext.declarative',
+    'sqlalchemy.ext.hybrid', 'sqlalchemy.sql', 'sqlalchemy.sql.expression',
+    'src.database',
+    'core.models', 'core.database',
+]:
+    if mod not in sys.modules:
+        sys.modules[mod] = MagicMock()
+
+from src.context_compactor import _truncate_message_to_token_budget  # noqa: E402
+from src.model_context import estimate_tokens  # noqa: E402
+
+
+def _tool_msg(arg_len):
+    return {
+        "role": "assistant",
+        "content": None,
+        "tool_calls": [{
+            "id": "c1", "type": "function",
+            "function": {"name": "create_document", "arguments": "x" * arg_len},
+        }],
+    }
+
+
+def test_oversized_tool_call_args_are_truncated_to_fit_budget():
+    budget = 200
+    out = _truncate_message_to_token_budget(_tool_msg(40000), budget)
+    # The message now fits the budget (before the fix it stayed ~12k tokens).
+    assert estimate_tokens([out]) <= budget, estimate_tokens([out])
+    tc = out["tool_calls"][0]
+    # Structure preserved so tool/result pairing + provider validation still hold.
+    assert tc["id"] == "c1" and tc["type"] == "function"
+    assert tc["function"]["name"] == "create_document"
+    # Arguments remain valid JSON, just bounded.
+    parsed = json.loads(tc["function"]["arguments"])
+    assert parsed.get("_truncated_for_context") == 40000
+
+
+def test_small_tool_call_args_are_left_untouched():
+    out = _truncate_message_to_token_budget(_tool_msg(20), 500)
+    assert out["tool_calls"][0]["function"]["arguments"] == "x" * 20
+
+
+def test_plain_text_content_still_truncates():
+    out = _truncate_message_to_token_budget({"role": "user", "content": "y" * 40000}, 200)
+    assert len(out["content"]) < 2000  # truncated, not left at 40k