fix(agent): separate continuation fragments so joined text does not glue

truncated_response_parts were joined with no separator at both the
ceiling exit and the success path, so a fragment ending mid-word ran
straight into the next one (#78577). insert a newline only when the
previous fragment ends non-whitespace and the next starts
non-whitespace, so existing separators are not doubled.

agent/conversation_loop.py

@@ -182,6 +182,16 @@ _API_CALL_MODULES = frozenset({
 })
 
 
+def _join_truncated_parts(parts: List[str]) -> str:
+    """Join continuation fragments, adding a newline where two would glue together (#78577)."""
+    joined = ""
+    for part in parts:
+        if joined and not joined[-1].isspace() and part and not part[0].isspace():
+            joined += "\n"
+        joined += part
+    return joined
+
+
 def _apply_active_turn_redirect(agent: Any, messages: List[Dict[str, Any]], text: str) -> None:
     """Append a provider-safe checkpoint and correction to the live turn.
 
@@ -3291,7 +3301,7 @@ def run_conversation(
                                 _retry.restart_with_length_continuation = True
                                 break
 
-                            partial_response = agent._strip_think_blocks("".join(truncated_response_parts)).strip()
+                            partial_response = agent._strip_think_blocks(_join_truncated_parts(truncated_response_parts)).strip()
                             if partial_response:
                                 agent._vprint(
                                     f"{agent.log_prefix}⚠️  Response still truncated "
@@ -7199,7 +7209,7 @@ def run_conversation(
                 codex_ack_continuations = 0
 
                 if truncated_response_parts:
-                    final_response = "".join(truncated_response_parts) + final_response
+                    final_response = _join_truncated_parts([*truncated_response_parts, final_response])
                     truncated_response_parts = []
                     length_continue_retries = 0
                     # The continuation recovered, so the fragments stay in the transcript.

tests/run_agent/test_continuation_ceiling_wedge.py

@@ -229,3 +229,24 @@ class TestContinuationCeilingWedge:
         )
         assert "second turn partial" in result2["final_response"]
         assert "and the rest." in result2["final_response"]
+
+
+class TestTruncatedPartJoining:
+    """#78577 — parts joined with no separator glued text together."""
+
+    def test_glued_parts_get_a_newline(self):
+        from agent.conversation_loop import _join_truncated_parts
+        assert _join_truncated_parts(
+            ["Edited index.html", "Review the 5 changes"]
+        ) == "Edited index.html\nReview the 5 changes"
+
+    def test_existing_whitespace_is_not_doubled(self):
+        from agent.conversation_loop import _join_truncated_parts
+        assert _join_truncated_parts(["line one\n", "line two"]) == "line one\nline two"
+        assert _join_truncated_parts(["word", " next"]) == "word next"
+
+    def test_degenerate_inputs(self):
+        from agent.conversation_loop import _join_truncated_parts
+        assert _join_truncated_parts([]) == ""
+        assert _join_truncated_parts(["only"]) == "only"
+        assert _join_truncated_parts(["a", "", "b"]) == "a\nb"