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63c9788f50
`readSessionCost` read only the trailing 8 KiB of
`~/.claude/metrics/costs.jsonl` to "avoid scanning entire file".
That ceiling is the opposite-sign sibling of the double-count bug
fixed in the previous commit: once a session's most recent
cumulative row gets pushed past the 8 KiB window by newer rows
from other sessions, the bridge silently reports `totalCost: 0`,
`totalIn: 0`, `totalOut: 0` for that session — same false signal
to `ecc-context-monitor.js`, same wrong number injected into the
live model turn as `additionalContext`.
`cost-tracker.js` has no rotation policy, so on any non-trivial
workstation costs.jsonl grows past 8 KiB within minutes of normal
use. For users who keep multiple concurrent sessions, this means
the second-and-later sessions silently report zero almost
immediately.
Reproduced before this commit:
$ HOME=/tmp/eccc node -e '
const fs = require("fs");
const m = require("./scripts/hooks/ecc-metrics-bridge.js");
// S1 row at file start, then 200 rows of OTHER-session noise (~16 KiB).
// S1 is the row we want, but it sits past the 8 KiB tail.
const s1 = `{"session_id":"S1","estimated_cost_usd":0.5,"input_tokens":500,"output_tokens":250}`;
const other = `{"session_id":"OTHER","estimated_cost_usd":1,"input_tokens":100,"output_tokens":50}`;
fs.mkdirSync("/tmp/eccc/.claude/metrics", { recursive: true });
fs.writeFileSync("/tmp/eccc/.claude/metrics/costs.jsonl",
[s1, ...Array(200).fill(other)].join("\\n") + "\\n");
console.log(JSON.stringify(m.readSessionCost("S1")));'
{"totalCost":0,"totalIn":0,"totalOut":0}
Expected: `{"totalCost":0.5, "totalIn":500, "totalOut":250}` (the
S1 row that exists in the file).
Actual: zero — the row is past the 8 KiB tail.
Fix: drop the `fs.openSync` + bounded `fs.readSync` + position
arithmetic in favour of `fs.readFileSync(costsPath, 'utf8')` and
iterate every line. Each row is ~150 bytes; even 100k rows is
~15 MB and a single sync read on PreToolUse is in the low ms.
If file rotation lands in `cost-tracker.js` later, this scan
becomes proportionally cheaper.
After this commit the reproduction above returns
`{"totalCost":0.5, "totalIn":500, "totalOut":250}`.
Regression test in `tests/hooks/ecc-metrics-bridge.test.js`:
`readSessionCost finds session row beyond the old 8 KiB tail
boundary`. The test asserts the costs.jsonl fixture is > 8 KiB
before reading so any reintroduction of a bounded tail would
re-fail the test (i.e. the assertion is the contract, not the
specific number 8192).
Together with the previous commit, both directions of the
metrics-bridge cost-reporting bug are closed.