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.
Frank Song