fix(#4927): durable fallback for tool output/diff body on cold reload

In the transparent stream, terminal output and patch diffs intermittently
vanished after a cold/paginated reload. A settled tool row renders its body from
`snippet`, but the raw-assistant-envelope 'derived' rebuild only joined the
result by tool id via resultsByTid (role:tool messages); when that join missed
(id mismatch, recovery-rebuilt turn) the snippet was empty, and #4622's live
enrichment only helps when S.toolCalls still matches (empty on cold load).

Build a persistedSnippetByTid lookup from the durable session.tool_calls compact
summary (which carries a bounded result snippet per tid) and use it as a
fallback in all three derived-push sites, so the persisted record is the
reliable source and the live join becomes a fast-path, not the only path that
shows the body. Part of #4925.

static/ui.js

@@ -11808,6 +11808,21 @@ function renderMessages(options){
     // Without this step CLI-origin sessions reload with empty tool cards.
     const resultsByTid={};
     const fallbackToolSources=[];
+    // Durable fallback: the persisted compact summary (session.tool_calls, built
+    // by _extract_tool_calls_from_messages) carries a bounded result `snippet`
+    // keyed by tid. On a cold/paginated load where the role:tool result-message
+    // join below misses (id mismatch, recovery-rebuilt turn), use this so the
+    // terminal output / diff body still renders instead of vanishing (#4927).
+    const persistedSnippetByTid={};
+    try{
+      const persisted=(S.session&&Array.isArray(S.session.tool_calls))?S.session.tool_calls:[];
+      persisted.forEach(tc=>{
+        if(!tc||typeof tc!=='object') return;
+        const ptid=tc.tid||tc.id||tc.tool_call_id||tc.call_id||'';
+        const psnip=tc.snippet||tc.result||tc.output||tc.preview||'';
+        if(ptid&&psnip&&!persistedSnippetByTid[ptid]) persistedSnippetByTid[ptid]=String(psnip);
+      });
+    }catch(e){}
     S.messages.forEach((m,rawIdx)=>{
       if(!m) return;
       // OpenAI / Hermes CLI format: role=tool with tool_call_id
@@ -11873,7 +11888,7 @@ function renderMessages(options){
         try{ args=JSON.parse(fn.arguments||'{}'); }catch(e){}
         const tid=tc.id||tc.call_id||'';
         const patchSnippet=_cliPatchSnippetFromArgs(name,args);
-        const resultSnippet=resultsByTid[tid]||'';
+        const resultSnippet=resultsByTid[tid]||persistedSnippetByTid[tid]||'';
         let argsSnap=_toolArgsSnapshot(args);
         derived.push(copyLiveToolMetadata({
           name,
@@ -11900,7 +11915,7 @@ function renderMessages(options){
         }
         const tid=tc.tid||tc.id||tc.tool_call_id||tc.call_id||'';
         const patchSnippet=_cliPatchSnippetFromArgs(name,args);
-        const resultSnippet=resultsByTid[tid]||tc.snippet||tc.preview||'';
+        const resultSnippet=resultsByTid[tid]||tc.snippet||tc.preview||persistedSnippetByTid[tid]||'';
         const argsSnap=_toolArgsSnapshot(args);
         derived.push(copyLiveToolMetadata({
           name,

tests/test_issue4927_durable_tool_snippet.py

@@ -0,0 +1,62 @@
+"""Regression coverage for #4927 — terminal output / patch diffs must survive a
+cold reload in the transparent activity stream.
+
+A settled tool row renders its output/diff from `snippet`. There are two rebuild
+paths: the persisted compact summary (`session.tool_calls`, which HAS a bounded
+`snippet`) and the raw-assistant-envelope `derived` rebuild (which joins the
+result by tool id via `resultsByTid`). On a cold/paginated load the
+`resultsByTid` join can miss (id mismatch, recovery-rebuilt turn) — and #4622's
+live enrichment only helps when `S.toolCalls` still matches (empty on cold
+load). So the derived build must fall back to the persisted `session.tool_calls`
+snippet by tid, making the durable record the reliable source.
+
+This is a source-structure guard: the derived rebuild is inlined in
+`renderMessages`, so we assert the fallback wiring is present rather than
+extracting a standalone function.
+"""
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+UI_JS = (Path(__file__).parent.parent / "static" / "ui.js").read_text(encoding="utf-8")
+
+
+def _slice_derived_rebuild() -> str:
+    """Return the renderMessages fallback-rebuild region (resultsByTid block)."""
+    start = UI_JS.index("const resultsByTid={};")
+    # The region runs through the _partial_tool_calls derived push; bound it
+    # generously so both derived-push sites are included.
+    return UI_JS[start:start + 6000]
+
+
+def test_persisted_snippet_lookup_is_built_from_session_tool_calls():
+    """A tid->snippet lookup must be built from S.session.tool_calls."""
+    region = _slice_derived_rebuild()
+    assert "persistedSnippetByTid" in region, "no persisted-snippet fallback lookup"
+    # It must be sourced from the durable persisted summary.
+    assert re.search(r"S\.session\s*&&\s*Array\.isArray\(S\.session\.tool_calls\)", region), \
+        "persistedSnippetByTid must be built from S.session.tool_calls"
+    # It must key by tid and capture the snippet.
+    assert re.search(r"persistedSnippetByTid\[\s*ptid\s*\]\s*=", region), \
+        "persisted lookup must be keyed by tid"
+
+
+def test_derived_result_snippet_falls_back_to_persisted():
+    """The derived OpenAI-format build must use persistedSnippetByTid as a
+    fallback when the live result-message join (resultsByTid) misses."""
+    region = _slice_derived_rebuild()
+    # The primary OpenAI-format derived push.
+    assert "resultsByTid[tid]||persistedSnippetByTid[tid]" in region, (
+        "derived resultSnippet must fall back to the persisted snippet by tid "
+        "(#4927) so cold-load tool output/diffs don't vanish"
+    )
+
+
+def test_partial_tool_calls_path_also_falls_back():
+    """The _partial_tool_calls derived path must also use the persisted fallback."""
+    region = _slice_derived_rebuild()
+    assert "resultsByTid[tid]||tc.snippet||tc.preview||persistedSnippetByTid[tid]" in region, (
+        "the partial-tool-calls derived path must also fall back to the "
+        "persisted snippet (#4927)"
+    )