Recompute responded_ids after orphan-tool drop for PR #5710

Before, the final sweep computed `responded_ids` once before pruning
orphan tools, so a tool message arriving before its assistant got
dropped as orphan but the later assistant still matched
`ids <= responded_ids` and kept its dangling `tool_calls` entry,
violating the chat-template invariant the sweep exists to enforce.

Split the sweep into two passes: drop orphan tools first, then
recompute `responded_ids` from the surviving tools and mark assts
whose tool_calls aren't all responded for strip.

studio/backend/core/inference/context_compaction.py

@@ -314,18 +314,26 @@ class SlidingWindowCompact(CompactStrategy):
                 else:
                     rewrite_strip_tool_calls.add(asst_idx)
 
-        # Final invariant sweep: drop any surviving tool message whose
-        # tool_call_id has no matching assistant ``tool_calls`` earlier
-        # in the kept output. This catches orphans pair_map could not
-        # link -- e.g. a tool that arrives after a user boundary and
-        # references an assistant that the boundary-clearing logic in
-        # ``_pair_linked_indices`` no longer treats as the pair root.
-        # Anchored tool messages (multimodal content) stay regardless,
-        # matching the existing leak-rather-than-violate-anchor rule.
-        # Also collect assistant indices whose surviving tool_calls
-        # entries lack a matching tool follow-up so we can strip the
-        # orphan ids from a copy below (same reason: OpenAI 400s on
-        # tool_calls without matching tool responses).
+        # Final invariant sweep, two passes so the asst-strip decision
+        # sees the post-orphan-drop tool set:
+        #   pass 1: drop tools whose tcid has no matching assistant
+        #     ``tool_calls`` earlier in the kept output (anchored tools
+        #     stay; same leak-rather-than-violate-anchor rule);
+        #   pass 2: recompute responded_ids from the surviving tools and
+        #     mark assts whose tool_calls aren't all responded for strip.
+        # Computing responded_ids before pass 1 would let a stale tcid
+        # of a just-dropped orphan tool satisfy `ids <= responded_ids`,
+        # leaving the asst with dangling tool_calls (OpenAI 400).
+        seen_ids: set[str] = set()
+        for i, m in enumerate(messages):
+            if i in dropped:
+                continue
+            if m.get("role") == "assistant":
+                seen_ids |= _assistant_tool_call_ids(m)
+            elif m.get("role") == "tool" and i not in anchor_idx:
+                tcid = m.get("tool_call_id")
+                if isinstance(tcid, str) and tcid and tcid not in seen_ids:
+                    dropped.add(i)
         responded_ids: set[str] = set()
         for i, m in enumerate(messages):
             if i in dropped:
@@ -334,19 +342,13 @@ class SlidingWindowCompact(CompactStrategy):
                 tcid = m.get("tool_call_id")
                 if isinstance(tcid, str) and tcid:
                     responded_ids.add(tcid)
-        seen_ids: set[str] = set()
         for i, m in enumerate(messages):
             if i in dropped:
                 continue
             if m.get("role") == "assistant":
                 ids = _assistant_tool_call_ids(m)
-                seen_ids |= ids
                 if ids and not (ids <= responded_ids):
                     rewrite_strip_tool_calls.add(i)
-            elif m.get("role") == "tool" and i not in anchor_idx:
-                tcid = m.get("tool_call_id")
-                if isinstance(tcid, str) and tcid and tcid not in seen_ids:
-                    dropped.add(i)
 
         out: list[dict] = []
         for i, m in enumerate(messages):

studio/backend/tests/test_context_compaction_edge.py

@@ -883,3 +883,27 @@ def test_partial_tool_drop_strips_orphan_tool_call_id():
     out = SlidingWindowCompact(keep_recent = 1).compact(msgs, budget_tokens = 200)
     asst_ids, tool_ids = _surviving_tool_ids(out)
     assert asst_ids == tool_ids, (asst_ids, tool_ids)
+
+
+def test_orphan_tool_before_asst_does_not_leave_dangling_tool_calls():
+    """Malformed input: a tool message arrives BEFORE the assistant
+    that references its id. The final sweep must drop the orphan tool
+    AND strip the now-dangling tool_call from the later assistant so
+    the chat template stays valid. Earlier behavior computed
+    responded_ids once before the orphan-tool drop, so the post-drop
+    asst still matched ids <= responded_ids and kept its tool_calls.
+    """
+    msgs = [
+        _msg("system", "sys"),
+        _msg("user", "first task"),
+        _msg("tool", content = "early", tool_call_id = "call_a"),
+        _msg(
+            "assistant",
+            content = "thinking",
+            tool_calls = [_tool_call("call_a")],
+        ),
+        _long("user", 50),
+    ]
+    out = SlidingWindowCompact(keep_recent = 5).compact(msgs, budget_tokens = 1000)
+    asst_ids, tool_ids = _surviving_tool_ids(out)
+    assert asst_ids == tool_ids, (asst_ids, tool_ids)