fix: context-window indicator broken on older sessions (#1436)

Fix two-layer bug where `/api/session` returned `context_length=0` for
sessions that pre-date #1318, then the frontend silently fell back to
cumulative `input_tokens` and the 128K JS default, producing nonsense
indicators like "100" capped from "890% used (context exceeded), 1.2M
/ 131.1k tokens used".

Empirical impact: 23 of 75 sessions on dev server rendered >100% before
this fix. #1356 fixed the same symptom on the live SSE path but missed
the GET /api/session load path that older sessions go through.

Two-layer fix:
  1. Backend (api/routes.py:1295-1313) — resolve context_length via
     agent.model_metadata.get_model_context_length() when the persisted
     value is 0. Mirrors api/streaming.py:2333-2342.
  2. Frontend (static/ui.js:1269) — drop the cumulative `input_tokens`
     fallback. When last_prompt_tokens is missing, render "·" + "tokens
     used" (existing !hasPromptTok branch) instead of computing a
     percentage from the cumulative total.

10 regression tests in tests/test_issue1436_context_indicator_load_path.py
covering both layers + the empty-model edge case (avoids the 256K
default-for-unknown-model trap that get_model_context_length('') returns).

Verified live: claude-opus-4-7 session with input_tokens=5,226,479 now
renders "·" + "5.3M tokens used" instead of "100" + "3987% used".

Reported by @AvidFuturist.
Closes #1436.

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Hermes Web UI -- Changelog
 
+## [Unreleased]
+
+### Fixed
+- **Context-window indicator broken on older sessions ("100" / "890% used")** (#1436, fixes #1436) — `#1356` (closed Apr 30) fixed the same symptom on the **live SSE path** but didn't cover the **GET /api/session load path**, so any session that pre-dates `#1318` (when `context_length` was added to `Session`) returned `context_length=0` from `/api/session`. Combined with two cascading frontend fallbacks (`promptTok = last_prompt_tokens || input_tokens`, `ctxWindow = context_length || 128*1024`), the ring rendered "100" capped from 800-4000% and the tooltip showed "890% used (context exceeded), 1.2M / 131.1k tokens used" — a misleading prompt to compress that the user couldn't address. Empirically: 23 of 75 sessions on the dev server were broken before this fix. **Two-layer fix**: (1) backend `api/routes.py` now resolves `context_length` via `agent.model_metadata.get_model_context_length()` when the persisted value is 0, mirroring the SSE-path fallback in `api/streaming.py:2333-2342`. (2) frontend `static/ui.js:1269` no longer falls back to cumulative `input_tokens` when `last_prompt_tokens` is missing — that fallback divides cumulative input by the context window, producing nonsense percentages. Older sessions without last-prompt data now render "·" + "tokens used" (honest no-data) on the ring instead of a misleading >100% percentage. **10 regression tests** in `tests/test_issue1436_context_indicator_load_path.py` pin: persisted-value pass-through, zero-value fallback, fallback-receives-correct-model, empty-model-skips-fallback (avoids 256K default-for-unknown trap), exception-swallowed-on-import-failure, frontend-no-input_tokens-fallback, frontend-uses-last_prompt_tokens-only, no-data-branch-renders-dot, load-path-imports-the-helper, fix-comment-references-issue-number. Reported by @AvidFuturist. (`api/routes.py`, `static/ui.js`, `tests/test_issue1436_context_indicator_load_path.py`)
+
 ## [v0.50.262] — 2026-05-02
 
 ### Fixed

api/routes.py

@@ -1292,6 +1292,23 @@ def handle_get(handler, parsed) -> bool:
                     _truncated_msgs = _all_msgs
             else:
                 _truncated_msgs = _all_msgs
+            # Resolve effective context_length with model-metadata fallback so
+            # older sessions (pre-#1318) that have context_length=0 persisted
+            # still render a meaningful indicator on load.  Mirrors the
+            # SSE-path fallback in api/streaming.py:2333-2342.  Fixes #1436.
+            _persisted_cl = getattr(s, "context_length", 0) or 0
+            if not _persisted_cl:
+                _model_for_lookup = (
+                    getattr(s, "model", "") or effective_model or ""
+                ).strip()
+                if _model_for_lookup:
+                    try:
+                        from agent.model_metadata import get_model_context_length as _get_cl
+                        _fb_cl = _get_cl(_model_for_lookup, "") or 0
+                        if _fb_cl:
+                            _persisted_cl = _fb_cl
+                    except Exception:
+                        pass
             raw = s.compact() | {
                 "messages": _truncated_msgs,
                 "tool_calls": getattr(s, "tool_calls", []) if load_messages else [],
@@ -1299,7 +1316,7 @@ def handle_get(handler, parsed) -> bool:
                 "pending_user_message": getattr(s, "pending_user_message", None),
                 "pending_attachments": getattr(s, "pending_attachments", []) if load_messages else [],
                 "pending_started_at": getattr(s, "pending_started_at", None),
-                "context_length": getattr(s, "context_length", 0) or 0,
+                "context_length": _persisted_cl,
                 "threshold_tokens": getattr(s, "threshold_tokens", 0) or 0,
                 "last_prompt_tokens": getattr(s, "last_prompt_tokens", 0) or 0,
             }

static/ui.js

@@ -1265,8 +1265,13 @@ function _syncCtxIndicator(usage){
   const wrap=$('ctxIndicatorWrap');
   const el=$('ctxIndicator');
   if(!el)return;
-  // Use input_tokens as fallback when last_prompt_tokens is not available
-  const promptTok=usage.last_prompt_tokens||usage.input_tokens||0;
+  // #1436: Use last_prompt_tokens only — NEVER fall back to cumulative
+  // input_tokens for the "context window % used" calculation.  input_tokens
+  // is summed across all turns, so dividing it by the context window gives a
+  // nonsense percentage (often >100%) on long sessions.  When we have no
+  // last-prompt data we render "·" + "tokens used" via the !hasPromptTok
+  // branch below — honest "no data" instead of misleading "890% used".
+  const promptTok=usage.last_prompt_tokens||0;
   const totalTok=(usage.input_tokens||0)+(usage.output_tokens||0);
   // Default context window to 128K when not provided by backend
   const DEFAULT_CTX=128*1024;

tests/test_issue1436_context_indicator_load_path.py

@@ -0,0 +1,298 @@
+"""Regression test for #1436 — context-window indicator broken on load path.
+
+#1356 (closed Apr 30 2026) fixed the indicator on the **live SSE path** by adding
+a model-metadata fallback when `agent.context_compressor` didn't provide
+`context_length`.  But the **GET /api/session load path** (used when clicking
+older sessions from the sidebar) was missed — it returned `context_length=0`
+verbatim from the persisted Session object for any session that pre-dates
+#1318 (when the field was added).
+
+Combined with two cascading frontend fallbacks (`promptTok = last_prompt_tokens
+|| input_tokens`, `ctxWindow = context_length || 128*1024`), older sessions
+loaded their indicator showing nonsense like "100 / 890% used (context exceeded)"
+because:
+  - `context_length=0` → fallback to 131,072 (128K JS default)
+  - `last_prompt_tokens=0` → fallback to **cumulative** `input_tokens`
+    (often 1M+ on long sessions)
+  - 1.2M / 131K = ~890% → ring caps at 100, tooltip shows "890% used"
+
+Two-layer fix:
+  1. Backend (api/routes.py:1295-1305) — add model_metadata fallback so
+     loaded sessions get a sane `context_length` even when persisted as 0.
+  2. Frontend (static/ui.js:1269) — drop the `input_tokens` fallback for
+     `promptTok`.  Cumulative input is fundamentally wrong for "context window
+     % used"; better to render "·" + "tokens used" (honest no-data) than a
+     misleading >100% percentage.
+
+Reported by @AvidFuturist in Discord (May 1 2026, "the 100 comes up way too
+often").  Confirmed live on the dev server: 23 of 75 sessions had
+`context_length=0` + `input_tokens > 128K`, all rendering >100%.
+"""
+import json
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+from urllib.parse import urlparse
+
+ROUTES = Path(__file__).resolve().parent.parent / "api" / "routes.py"
+UI_JS = Path(__file__).resolve().parent.parent / "static" / "ui.js"
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# Backend: GET /api/session load path
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestIssue1436BackendFallback:
+    """The /api/session GET handler must resolve context_length via
+    agent.model_metadata.get_model_context_length when the persisted value is 0."""
+
+    def _stub_session(self, *, context_length, model, last_prompt_tokens=0,
+                      input_tokens=0):
+        """Build a mock Session that mimics the persisted-session shape."""
+        s = MagicMock()
+        s.context_length = context_length
+        s.threshold_tokens = 0
+        s.last_prompt_tokens = last_prompt_tokens
+        s.input_tokens = input_tokens
+        s.output_tokens = 0
+        s.model = model
+        s.title = "test-session"
+        s.session_id = "test-1436"
+        s.messages = []
+        s.tool_calls = []
+        s.active_stream_id = None
+        s.pending_user_message = None
+        s.pending_attachments = []
+        s.pending_started_at = None
+        # compact() returns a dict that gets merged with the response.
+        s.compact.return_value = {
+            "session_id": "test-1436",
+            "title": "test-session",
+            "model": model,
+            "input_tokens": input_tokens,
+            "output_tokens": 0,
+            "context_length": context_length,
+            "threshold_tokens": 0,
+            "last_prompt_tokens": last_prompt_tokens,
+        }
+        return s
+
+    def _invoke_get_session(self, session_obj, *, fallback_returns=0):
+        """Hit handle_get(/api/session?session_id=...) and capture the JSON response."""
+        import api.routes as routes
+
+        captured = {}
+
+        def fake_j(h, data, status=200):
+            captured["data"] = data
+            captured["status"] = status
+
+        # Patch get_model_context_length so the test doesn't depend on the
+        # live agent.model_metadata bundle.
+        fake_module = MagicMock()
+        fake_module.get_model_context_length = MagicMock(return_value=fallback_returns)
+
+        handler = MagicMock()
+        parsed = urlparse("/api/session?session_id=test-1436&messages=0")
+
+        # Patch import so `from agent.model_metadata import ...` resolves to our fake.
+        with patch("api.routes.get_session", return_value=session_obj), \
+             patch("api.routes.j", side_effect=fake_j), \
+             patch.dict("sys.modules", {"agent.model_metadata": fake_module}):
+            routes.handle_get(handler, parsed)
+        return captured
+
+    def test_persisted_context_length_passed_through_unchanged(self):
+        """When Session.context_length is non-zero, return it as-is (no fallback)."""
+        s = self._stub_session(context_length=200_000, model="claude-sonnet-4.6")
+        result = self._invoke_get_session(s, fallback_returns=999_999)
+        body = result["data"]["session"]
+        assert body["context_length"] == 200_000, (
+            f"persisted context_length must pass through unchanged, "
+            f"got {body['context_length']}"
+        )
+
+    def test_zero_context_length_falls_back_to_model_metadata(self):
+        """Pre-#1318 sessions with context_length=0 must resolve via model_metadata."""
+        s = self._stub_session(context_length=0, model="claude-opus-4-7",
+                               input_tokens=1_200_000)
+        result = self._invoke_get_session(s, fallback_returns=1_000_000)
+        body = result["data"]["session"]
+        assert body["context_length"] == 1_000_000, (
+            f"context_length=0 must resolve to model's 1M window via fallback, "
+            f"got {body['context_length']}"
+        )
+
+    def test_fallback_called_with_persisted_model(self):
+        """Fallback must be called with Session.model (not empty string)."""
+        import api.routes as routes
+
+        captured = {}
+
+        def fake_j(h, data, status=200):
+            captured["data"] = data
+
+        fake_module = MagicMock()
+        fake_module.get_model_context_length = MagicMock(return_value=400_000)
+
+        s = self._stub_session(context_length=0, model="gpt-5-mini")
+        handler = MagicMock()
+        parsed = urlparse("/api/session?session_id=test-1436&messages=0")
+
+        with patch("api.routes.get_session", return_value=s), \
+             patch("api.routes.j", side_effect=fake_j), \
+             patch.dict("sys.modules", {"agent.model_metadata": fake_module}):
+            routes.handle_get(handler, parsed)
+
+        # First positional arg should be the model name; second is base_url ("")
+        called_args = fake_module.get_model_context_length.call_args
+        assert called_args is not None, "get_model_context_length was never called"
+        assert called_args.args[0] == "gpt-5-mini", (
+            f"fallback called with wrong model: {called_args}"
+        )
+
+    def test_empty_model_skips_fallback(self):
+        """If Session.model is empty AND no effective_model is available, skip
+        the fallback rather than calling get_model_context_length('')."""
+        s = self._stub_session(context_length=0, model="")
+        # resolve_model=0 to skip _resolve_effective_session_model_for_display
+        import api.routes as routes
+        captured = {}
+
+        def fake_j(h, data, status=200):
+            captured["data"] = data
+
+        fake_module = MagicMock()
+        fake_module.get_model_context_length = MagicMock(return_value=256_000)
+
+        handler = MagicMock()
+        parsed = urlparse(
+            "/api/session?session_id=test-1436&messages=0&resolve_model=0"
+        )
+
+        with patch("api.routes.get_session", return_value=s), \
+             patch("api.routes.j", side_effect=fake_j), \
+             patch.dict("sys.modules", {"agent.model_metadata": fake_module}):
+            routes.handle_get(handler, parsed)
+
+        # When model is empty, fallback either isn't called OR returns no value
+        # we use.  Either way context_length stays 0.
+        body = captured["data"]["session"]
+        assert body["context_length"] == 0, (
+            f"empty model must NOT trigger fallback (avoids the 256K default-for-unknown trap), "
+            f"got context_length={body['context_length']}"
+        )
+
+    def test_fallback_exception_does_not_break_response(self):
+        """If the fallback raises (older agent build, missing module), the route
+        must still return a response — context_length just stays 0."""
+        import api.routes as routes
+        captured = {}
+
+        def fake_j(h, data, status=200):
+            captured["data"] = data
+
+        # Fallback raises on import
+        fake_module = MagicMock()
+        fake_module.get_model_context_length = MagicMock(
+            side_effect=RuntimeError("simulated agent error")
+        )
+
+        s = self._stub_session(context_length=0, model="claude-opus-4-7")
+        handler = MagicMock()
+        parsed = urlparse("/api/session?session_id=test-1436&messages=0")
+
+        with patch("api.routes.get_session", return_value=s), \
+             patch("api.routes.j", side_effect=fake_j), \
+             patch.dict("sys.modules", {"agent.model_metadata": fake_module}):
+            routes.handle_get(handler, parsed)  # must not raise
+
+        body = captured["data"]["session"]
+        assert body["context_length"] == 0, (
+            "fallback exception must be swallowed; field stays 0"
+        )
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# Frontend: static/ui.js _syncCtxIndicator
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestIssue1436FrontendDefense:
+    """The frontend must NOT fall back to cumulative input_tokens when
+    last_prompt_tokens is missing — that produces nonsense percentages."""
+
+    def test_promptTok_does_not_fall_back_to_input_tokens(self):
+        """Verify the line `promptTok = usage.last_prompt_tokens||usage.input_tokens||0`
+        has been removed.  The old fallback divides cumulative input by the
+        context window, producing the >100% bug."""
+        src = UI_JS.read_text(encoding="utf-8")
+        # The bug shape: the `||usage.input_tokens` fragment must NOT appear
+        # on any line that defines `promptTok`.
+        for line_num, line in enumerate(src.splitlines(), 1):
+            stripped = line.strip()
+            if stripped.startswith("//") or stripped.startswith("*"):
+                continue
+            if "promptTok" in line and "=" in line and "usage.last_prompt_tokens" in line:
+                assert "usage.input_tokens" not in line, (
+                    f"static/ui.js:{line_num} still falls back to cumulative "
+                    f"input_tokens for promptTok — this produces the >100% indicator "
+                    f"bug from #1436.  Line: {line.rstrip()!r}"
+                )
+
+    def test_promptTok_assignment_uses_last_prompt_tokens_only(self):
+        """Verify the new assignment: `promptTok = usage.last_prompt_tokens || 0`."""
+        src = UI_JS.read_text(encoding="utf-8")
+        # Allow whitespace variations.
+        normalized = "".join(src.split())
+        assert "constpromptTok=usage.last_prompt_tokens||0" in normalized, (
+            "static/ui.js _syncCtxIndicator must assign "
+            "`promptTok = usage.last_prompt_tokens || 0` (no input_tokens fallback)"
+        )
+
+    def test_no_data_branch_renders_dot(self):
+        """When promptTok is 0 (no last-prompt data), the `!hasPromptTok` branch
+        must render '·' (U+00B7) on the ring instead of computing a percentage.
+        This is the existing behavior; the test pins it so a future refactor
+        doesn't accidentally re-introduce a numeric fallback."""
+        src = UI_JS.read_text(encoding="utf-8")
+        assert "hasPromptTok=!!promptTok" in src.replace(" ", ""), (
+            "hasPromptTok must be a boolean of promptTok"
+        )
+        # The ring center text uses '·' when !hasPromptTok
+        assert "hasPromptTok?String(pct):'\\u00b7'" in src.replace(" ", ""), (
+            "ring center must show '·' (\\u00b7) when no last-prompt data"
+        )
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# Static-source assertions (defense in depth — pin the comment markers in place)
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestIssue1436SourceMarkers:
+    """Pin the fix comments + import shape so a future refactor can't silently
+    drop the fallback."""
+
+    def test_routes_load_path_imports_get_model_context_length(self):
+        src = ROUTES.read_text(encoding="utf-8")
+        # The import must appear inside the GET /api/session load-path block.
+        start = src.find('if parsed.path == "/api/session":')
+        end = src.find('if parsed.path == "/api/projects":', start)
+        block = src[start:end]
+        assert "from agent.model_metadata import get_model_context_length" in block, (
+            "GET /api/session load-path block must lazy-import "
+            "get_model_context_length for the context_length=0 fallback (#1436)"
+        )
+
+    def test_routes_load_path_marks_fix_with_issue_number(self):
+        """Comment must reference #1436 so future maintainers find this trail."""
+        src = ROUTES.read_text(encoding="utf-8")
+        # Find the load-path block (between "if parsed.path == \"/api/session\":"
+        # and the next `if parsed.path` after it).
+        start = src.find('if parsed.path == "/api/session":')
+        end = src.find('if parsed.path == "/api/projects":', start)
+        block = src[start:end]
+        assert "#1436" in block, (
+            "GET /api/session load-path block must reference #1436 in a comment"
+        )