Release v0.51.277 — Release IS (stage-p3f — preserve context-window in usage indicator #3663) (#3683)

* fix(ui): preserve resolved context window in usage indicator (#3663, #3185, #3660)

Co-authored-by: Frank Song <franksong2702@gmail.com>

* docs(changelog): v0.51.277 — Release IS (stage-p3f)

---------

Co-authored-by: nesquena-hermes <[email protected]>
Co-authored-by: Frank Song <franksong2702@gmail.com>

CHANGELOG.md

@@ -3,6 +3,11 @@
 
 ## [Unreleased]
 
+## [v0.51.277] — 2026-06-05 — Release IS (stage-p3f — preserve context-window in usage indicator)
+
+### Fixed
+- **The context-window usage indicator no longer flips to the 128K/131.1k fallback after a turn completes.** A terminal or reload usage snapshot that arrived with missing/stale context metadata could replace already-resolved values; the indicator now merges usage snapshots field-by-field, preserving a known `context_length` (and other resolved usage fields) instead of overwriting them with nulls. (#3663, @franksong2702; fixes #3185, #3660)
+
 ## [v0.51.276] — 2026-06-05 — Release IR (stage-p3e — preserve manually-named session titles)
 
 ### Fixed

static/messages.js

@@ -2187,7 +2187,18 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
           // Stamp _ts on the last assistant message if it has no timestamp
           if(lastAsst&&!lastAsst._ts&&!lastAsst.timestamp) lastAsst._ts=Date.now()/1000;
           if(d.usage){
-            S.lastUsage=d.usage;_syncCtxIndicator(d.usage);
+            const _doneUsageFallback={...(S.lastUsage||{})};
+            if(S.session){
+              for(const _usageField of ['context_length','threshold_tokens','last_prompt_tokens']){
+                if(_doneUsageFallback[_usageField]==null&&S.session[_usageField]!=null){
+                  _doneUsageFallback[_usageField]=S.session[_usageField];
+                }
+              }
+            }
+            S.lastUsage=typeof _mergeUsageForCtxIndicator==='function'
+              ? _mergeUsageForCtxIndicator(d.usage,_doneUsageFallback)
+              : {..._doneUsageFallback,...d.usage};
+            _syncCtxIndicator(S.lastUsage);
             // #503 — compute per-turn cost delta and attach to last assistant message
             if(lastAsst){
               const prevIn=_prevIn;
@@ -2389,7 +2400,9 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
       const displaySid=currentSid;
       const message=String(d.message||'Context auto-compressed to continue the conversation').trim();
       if(d.usage&&typeof _syncCtxIndicator==='function'){
-        S.lastUsage={...(S.lastUsage||{}),...d.usage};
+        S.lastUsage=typeof _mergeUsageForCtxIndicator==='function'
+          ? _mergeUsageForCtxIndicator(d.usage,S.lastUsage||{})
+          : {...(S.lastUsage||{}),...d.usage};
         _syncCtxIndicator(S.lastUsage);
       }
       if(typeof setCompressionUi==='function'){
@@ -2425,7 +2438,9 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
         if((d.session_id||activeSid)!==activeSid) return;
         if(d.usage&&typeof _syncCtxIndicator==='function'){
           if(S.session&&S.session.session_id===activeSid){
-            S.lastUsage={...(S.lastUsage||{}),...d.usage};
+            S.lastUsage=typeof _mergeUsageForCtxIndicator==='function'
+              ? _mergeUsageForCtxIndicator(d.usage,S.lastUsage||{})
+              : {...(S.lastUsage||{}),...d.usage};
             _syncCtxIndicator(S.lastUsage);
           }
         }

static/sessions.js

@@ -1014,6 +1014,7 @@ async function loadSession(sid){
   if(_s&&typeof _syncCtxIndicator==='function'){
     const u=S.lastUsage||{};
     const _pick=(latest,stored,dflt=0)=>latest!=null?latest:(stored!=null?stored:dflt);
+    const _pickPositive=(latest,stored,dflt=0)=>Number(latest)>0?latest:(Number(stored)>0?stored:dflt);
     _syncCtxIndicator({
       input_tokens:      _pick(u.input_tokens,      _s.input_tokens),
       output_tokens:     _pick(u.output_tokens,     _s.output_tokens),
@@ -1021,7 +1022,7 @@ async function loadSession(sid){
       cache_read_tokens: _pick(u.cache_read_tokens, _s.cache_read_tokens),
       cache_write_tokens:_pick(u.cache_write_tokens,_s.cache_write_tokens),
       cache_hit_percent: _pick(u.cache_hit_percent, _s.cache_hit_percent, null),
-      context_length:    _pick(_s.context_length,    u.context_length),
+      context_length:    _pickPositive(u.context_length, _s.context_length),
       last_prompt_tokens:_pick(u.last_prompt_tokens,_s.last_prompt_tokens),
       threshold_tokens:  _pick(_s.threshold_tokens,  u.threshold_tokens),
     });
@@ -1451,7 +1452,8 @@ function _resolveSessionModelForDisplaySoon(sid){
       if(!model||!S.session||S.session.session_id!==sid) return;
       S.session.model=model;
       S.session.model_provider=provider||null;
-      S.session.context_length=data.session.context_length||0;
+      const resolvedContextLength=data.session.context_length||S.session.context_length||0;
+      S.session.context_length=resolvedContextLength;
       S.session.threshold_tokens=data.session.threshold_tokens||0;
       S.session.last_prompt_tokens=data.session.last_prompt_tokens||0;
       S.session._modelResolutionDeferred=false;
@@ -1466,7 +1468,7 @@ function _resolveSessionModelForDisplaySoon(sid){
           cache_read_tokens:_pick(u.cache_read_tokens,S.session.cache_read_tokens),
           cache_write_tokens:_pick(u.cache_write_tokens,S.session.cache_write_tokens),
           cache_hit_percent:_pick(u.cache_hit_percent,S.session.cache_hit_percent,null),
-          context_length:data.session.context_length||0,
+          context_length:resolvedContextLength||u.context_length||0,
           last_prompt_tokens:_pick(u.last_prompt_tokens,S.session.last_prompt_tokens),
           threshold_tokens:data.session.threshold_tokens||0,
         });

static/ui.js

@@ -2753,6 +2753,30 @@ function _syncMobileCtxDisplay(state){
   _setCtxCompressButton(compressBtn,state.compressText||'');
 }
 
+function _mergeUsageForCtxIndicator(latest, fallback){
+  const latestObj=(latest&&typeof latest==='object')?latest:{};
+  const fallbackObj=(fallback&&typeof fallback==='object')?fallback:{};
+  const merged={...latestObj};
+  for(const field of [
+    'input_tokens','output_tokens','estimated_cost',
+    'cache_read_tokens','cache_write_tokens','cache_hit_percent',
+    'turn_cache_hit_percent','duration_seconds','tps','gateway_routing',
+  ]){
+    if(merged[field]==null&&fallbackObj[field]!=null){
+      merged[field]=fallbackObj[field];
+    }
+  }
+  if(!(Number(latestObj.context_length)>0)&&Number(fallbackObj.context_length)>0){
+    merged.context_length=fallbackObj.context_length;
+  }
+  for(const field of ['threshold_tokens','last_prompt_tokens']){
+    if(latestObj[field]==null&&fallbackObj[field]!=null){
+      merged[field]=fallbackObj[field];
+    }
+  }
+  return merged;
+}
+
 // Context usage indicator in composer footer
 function _syncCtxIndicator(usage){
   const wrap=$('ctxIndicatorWrap');

tests/test_auto_compression_card.py

@@ -266,7 +266,7 @@ def test_auto_compression_done_sse_refreshes_context_indicator_usage():
     block = _compressed_listener_block()
 
     assert "if(d.usage&&typeof _syncCtxIndicator==='function')" in block
-    assert "S.lastUsage={...(S.lastUsage||{}),...d.usage};" in block
+    assert "_mergeUsageForCtxIndicator(d.usage,S.lastUsage||{})" in block
     assert "_syncCtxIndicator(S.lastUsage);" in block
     assert block.index("_syncCtxIndicator(S.lastUsage);") < block.index("setCompressionUi")
 

tests/test_issue1617_tps_message_header.py

@@ -53,7 +53,10 @@ def test_live_metering_usage_is_provisional_until_done():
     assert listener_end != -1, "apperror listener should follow metering listener"
     listener = MESSAGES_JS[listener_start:listener_end]
 
-    assert "S.lastUsage={...(S.lastUsage||{}),...d.usage}" in listener, (
+    assert "_mergeUsageForCtxIndicator(d.usage,S.lastUsage||{})" in listener, (
+        "live usage should merge through the context indicator usage guard"
+    )
+    assert ": {...(S.lastUsage||{}),...d.usage}" in listener, (
         "live usage should update the transient usage cache for the indicator"
     )
     assert "_syncCtxIndicator(S.lastUsage)" in listener, (

tests/test_issue3660_context_window_stale.py

@@ -0,0 +1,64 @@
+"""Regression coverage for #3660/#3185 stale context-window indicators.
+
+The live SSE metering path can know the real model context window while the
+terminal `done` snapshot or cold session snapshot omits that field. The UI must
+not replace a known 1M window with the JavaScript 128K fallback on settlement or
+history reload.
+"""
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[1]
+MESSAGES_JS = (ROOT / "static" / "messages.js").read_text(encoding="utf-8")
+SESSIONS_JS = (ROOT / "static" / "sessions.js").read_text(encoding="utf-8")
+UI_JS = (ROOT / "static" / "ui.js").read_text(encoding="utf-8")
+
+
+def _listener_block(event_name: str, next_event_name: str) -> str:
+    start = MESSAGES_JS.find(f"source.addEventListener('{event_name}'")
+    assert start != -1, f"{event_name} listener not found"
+    end = MESSAGES_JS.find(f"source.addEventListener('{next_event_name}'", start)
+    assert end != -1, f"{next_event_name} listener after {event_name} not found"
+    return MESSAGES_JS[start:end]
+
+
+def test_done_event_preserves_last_known_context_window():
+    block = _listener_block("done", "stream_end")
+
+    assert "S.lastUsage=d.usage;_syncCtxIndicator(d.usage);" not in block
+    assert "const _doneUsageFallback={...(S.lastUsage||{})};" in block
+    assert "_doneUsageFallback[_usageField]=S.session[_usageField];" in block
+    assert "_mergeUsageForCtxIndicator(d.usage,_doneUsageFallback)" in block
+    assert "_syncCtxIndicator(S.lastUsage);" in block
+
+
+def test_usage_merge_helper_keeps_context_when_latest_omits_it():
+    start = UI_JS.find("function _mergeUsageForCtxIndicator")
+    assert start != -1, "context usage merge helper not found"
+    end = UI_JS.find("// Context usage indicator in composer footer", start)
+    assert end != -1, "context usage merge helper end marker not found"
+    block = UI_JS[start:end]
+
+    assert "const merged={...latestObj};" in block
+    assert "'input_tokens','output_tokens','estimated_cost'," in block
+    assert "if(!(Number(latestObj.context_length)>0)&&Number(fallbackObj.context_length)>0)" in block
+    assert "merged.context_length=fallbackObj.context_length;" in block
+    assert "threshold_tokens" in block
+    assert "last_prompt_tokens" in block
+
+
+def test_session_load_prefers_positive_last_usage_context_over_stale_snapshot():
+    assert "const _pickPositive=(latest,stored,dflt=0)=>Number(latest)>0?latest:(Number(stored)>0?stored:dflt);" in SESSIONS_JS
+    assert "context_length:    _pickPositive(u.context_length, _s.context_length)," in SESSIONS_JS
+
+
+def test_deferred_model_resolve_does_not_zero_out_existing_context_window():
+    block_start = SESSIONS_JS.find("function _resolveSessionModelForDisplaySoon")
+    assert block_start != -1, "deferred model resolver not found"
+    block_end = SESSIONS_JS.find("// Tracks whether the current session has older messages", block_start)
+    assert block_end != -1, "deferred model resolver end marker not found"
+    block = SESSIONS_JS[block_start:block_end]
+
+    assert "const resolvedContextLength=data.session.context_length||S.session.context_length||0;" in block
+    assert "S.session.context_length=resolvedContextLength;" in block
+    assert "context_length:resolvedContextLength||u.context_length||0," in block

tests/test_session_metadata_fast_path.py

@@ -54,7 +54,9 @@ def test_deferred_model_resolution_refreshes_context_metadata():
     )
     assert "S.session.threshold_tokens" in block
     assert "_syncCtxIndicator" in block
-    assert "context_length:data.session.context_length||0" in block.replace(" ", "")
+    compact = block.replace(" ", "")
+    assert "constresolvedContextLength=data.session.context_length||S.session.context_length||0;" in compact
+    assert "context_length:resolvedContextLength||u.context_length||0" in compact
 
 
 def test_boot_does_not_block_session_restore_on_model_catalog():