fix(webui): split merge-into-tail compaction so reply renders as its own bubble (#29824)

The compressor has a "double-collision" fallback path: when the
chosen ``summary_role`` collides with the first tail message AND
the flipped role would collide with the last head message, it can't
emit a standalone summary turn (consecutive same-role messages
break Anthropic and friends). It instead prepends the summary +
end-of-summary marker to the first tail message's content via
``_merge_summary_into_tail``.

With the matching anchor from the previous commit, that first tail
message is now usually the user's previously-visible assistant
reply — so the persisted assistant turn ends up shaped as
``[CONTEXT COMPACTION ...] ... --- END OF CONTEXT SUMMARY --- ...
THE ACTUAL REPLY``. Without splitting it, the session viewer
renders one big "Context handoff" bubble and the reply text is
buried inside the metadata blob — which is exactly the
"can't see the last reply" experience #29824 reports, just one
layer deeper.

Added ``splitCompactionContent`` that detects the merge marker
(kept in sync with ``--- END OF CONTEXT SUMMARY — respond to the
message below, not the summary above ---`` in
``agent/context_compressor.py``) and ``MessageBubble`` now
recurses on the two halves: the prefix half renders as the muted
"Context handoff" row, the remainder half renders with the
original assistant styling. Pure (non-merged) summary messages
hit the no-remainder branch and still render as a single
"Context handoff" row, preserving the original behaviour.

web/src/pages/SessionsPage.tsx

@@ -157,22 +157,50 @@ function ToolCallBlock({
 // detect them here and downgrade them to a muted, clearly-labelled
 // "Context handoff" row.
 //
-// Keep these prefixes in sync with ``SUMMARY_PREFIX`` and
-// ``LEGACY_SUMMARY_PREFIX`` in ``agent/context_compressor.py``.
+// Keep these prefixes (and the END marker below) in sync with
+// ``SUMMARY_PREFIX`` / ``LEGACY_SUMMARY_PREFIX`` and the
+// merge-into-tail marker in ``agent/context_compressor.py``.
 const COMPACTION_PREFIXES = [
   "[CONTEXT COMPACTION — REFERENCE ONLY]",
   "[CONTEXT COMPACTION - REFERENCE ONLY]",
   "[CONTEXT SUMMARY]:",
 ] as const;
 
-function isCompactionMessage(msg: SessionMessage): boolean {
-  if (msg.role !== "user" && msg.role !== "assistant") return false;
-  const content = msg.content;
-  if (typeof content !== "string") return false;
-  const head = content.trimStart();
-  return COMPACTION_PREFIXES.some((p) => head.startsWith(p));
+// Marker the compressor inserts between a merged summary and the
+// original tail message content. When the summary role would collide
+// with both head and tail roles (e.g. head ends with ``user`` and tail
+// starts with ``assistant``), the compressor merges the summary as a
+// prefix on the first tail message instead of inserting a standalone
+// row. We split on this marker so the WebUI still shows the original
+// assistant reply as its own readable bubble — otherwise the merged
+// row reads as a single opaque "Context compaction" block and the
+// user can't see the reply (#29824).
+const COMPACTION_END_MARKER =
+  "--- END OF CONTEXT SUMMARY — respond to the message below, not the summary above ---";
+
+interface CompactionSplit {
+  /** Summary text (header + body, without the end marker). */
+  summary: string;
+  /** Original message content that came after the end marker. */
+  remainder: string;
 }
 
+function splitCompactionContent(content: string): CompactionSplit | null {
+  const head = content.trimStart();
+  if (!COMPACTION_PREFIXES.some((p) => head.startsWith(p))) return null;
+  const markerIdx = content.indexOf(COMPACTION_END_MARKER);
+  if (markerIdx < 0) {
+    return { summary: content, remainder: "" };
+  }
+  return {
+    summary: content.slice(0, markerIdx),
+    remainder: content
+      .slice(markerIdx + COMPACTION_END_MARKER.length)
+      .replace(/^\s+/, ""),
+  };
+}
+
+
 function MessageBubble({
   msg,
   highlight,
@@ -216,7 +244,42 @@ function MessageBubble({
     },
   };
 
-  const isCompaction = isCompactionMessage(msg);
+  // When a compaction handoff is merged into the front of the first
+  // tail message (the compressor's double-collision path —
+  // ``_merge_summary_into_tail`` in ``agent/context_compressor.py``),
+  // the message we received is ``[CONTEXT COMPACTION ...] + END_MARKER
+  // + <original assistant reply>``. We split it back into two visual
+  // rows here so the operator's actual answer survives as a readable
+  // bubble next to the (clearly-labelled) handoff metadata (#29824).
+  const compactionSplit =
+    typeof msg.content === "string"
+      ? splitCompactionContent(msg.content)
+      : null;
+
+  if (compactionSplit && compactionSplit.remainder) {
+    return (
+      <>
+        <MessageBubble
+          msg={{ ...msg, content: compactionSplit.summary }}
+          highlight={highlight}
+        />
+        <MessageBubble
+          msg={{
+            ...msg,
+            content: compactionSplit.remainder,
+            // The remainder is the original assistant reply that the
+            // compressor pre-pended the summary to — render with the
+            // normal assistant styling, NOT the muted handoff style.
+            // ``isCompactionMessage`` returns false on this stripped
+            // content because it no longer starts with the prefix.
+          }}
+          highlight={highlight}
+        />
+      </>
+    );
+  }
+
+  const isCompaction = compactionSplit !== null;
   const style = isCompaction
     ? ROLE_STYLES.compaction
     : ROLE_STYLES[msg.role] ?? ROLE_STYLES.system;