peltier.io/results/phase2/case-study-agent-loop.md

# Phase 2 case study — the unattended agent loop

**Result: the Claude Agent SDK loop runs the full profile → hypothesize →
patch → gated-verdict cycle with no human in the loop, on two targets,
with ZERO false accepts — and produced its first verified auto-accepted
win: +10.7% median on tokei, 95% CI [+8.4%, +13.1%] (221 MB / 4360-file
pinned workload, single rayon thread, pinned core), surviving a
10,000-input differential fuzz and ASan/LSan on top of the pipeline's
gates. The agent twice refused to fabricate a verdict it could not read
back, verifying ledger integrity instead — exactly the
anti-reward-hacking behavior SPEC §10 is designed to produce.**

## What runs

- `agent/hotpath_agent/loop.py` — Claude Agent SDK (`claude-agent-sdk`,
  model `claude-fable-5`) driving a standalone stdio MCP server
  (`mcp_server.py`) that exposes the seven harness tools. The agent has
  no built-in file/shell/web tools in its allowlist.
- `harnessd` gained an async `run_verdict` (launches the multi-minute
  build+bench pipeline detached, returns immediately) and a pollable
  `read_verdict` that reads the append-only ledger row once written.
- `read_target_source` gained an `offset`/`limit` line window so 100 KB+
  source files can be paged instead of overflowing one response.

## The attempts (all in the ledger, `results/ledger.sqlite`)

| run_id | class chosen | patch | verdict | speedup CI |
|---|---|---|---|---|
| phase2-comrak-001 | 3 alloc-churn | syntect buffer `with_capacity` | rejected-bench | [0.971, 1.007] |
| phase2-comrak-002 | 5 algorithmic | gate dead `VecDeque` build in text postprocess | rejected-bench | [0.982, 1.026] |
| phase2-comrak-003 | 6 SIMD | hoist `^`-in-brackets check out of `find_special_char` predicate | rejected-bench | [0.970, 1.007] |
| phase2-tokei-001 | 5 algorithmic | first-byte gate table for per-byte token matchers in `perform_multi_line_analysis` | **accepted** | **[1.084, 1.131]** |

### phase2-tokei-001 — the first auto-accepted win, and its audit

Running behind the OS boundary (`scripts/agent-isolated.sh`, mountns
mode), the agent read the new cache-sim profile, saw `parse_lines` at
55.9% of instructions / 62% of branches / 44% of branch mispredicts,
re-entered class 5 with a hypothesis distinct from every ledger entry
(the hypothesis-granular `read_ledger` shipped for exactly this), and
proposed a 256-entry first-byte gate: most bytes of most lines can't
start any quote/comment token, so five per-byte token-matcher loops
collapse to one table load. `git apply` refused its first diff (bad
hunk counts); it recomputed and resubmitted — the harness, not the
agent, owns what reaches the tree.

Pipeline verdict: accepted at +10.7% median, 95% CI [+8.4%, +13.1%]
vs the pristine-rebuilt baseline (which already banks the Phase 0
wins, so this is marginal, not cumulative, speedup). 21 turns,
$4.61.

The pipeline's auto-gates cover upstream tests + golden replay +
bench; fuzz and sanitizers are per-attempt manual in this phase, so
the 100% Phase 2 audit ran them by hand before counting the win:

- **Differential fuzz** (`scripts/diff-fuzz-tokei.py`): 10,000 mutated
  inputs (quote/comment/backslash-focused mutations over 400 corpus
  seeds), pristine vs candidate, batched over 200 process pairs —
  **0 divergences** after canonicalizing JSON output order.
- **ASan + LSan** (nightly, patched tree): clean over the full 221 MB
  corpus and every kept fuzz batch.
- Mechanical re-check (`scripts/audit-attempt.py`): CI lower bound
  1.084 ≥ 1.02 bar; patch paths inside the workspace; hypothesis
  matches the patch; safe code only — correct auto-accept tier.

**Audit finding worth keeping: raw-byte differential comparison
overcounts.** The first fuzz runs reported 3/200 batch divergences —
but on different batches across two identical-seed runs, and none
reproducible. Cause: tokei's parallel directory walker
(`ignore::WalkBuilder::build_parallel`, its own thread pool,
unaffected by `RAYON_NUM_THREADS`) makes same-name report ordering
timing-dependent in both binaries — benign, count-identical, present
in the pristine baseline. The fuzzer now canonicalizes (sorts) JSON
before comparing and stores both raw outputs on any divergence.
Differential gates must compare semantics, not bytes, wherever the
target itself is legitimately nondeterministic in presentation.

Run 003 exercised the two shipped fixes: the agent **paged through a
2,400-line file with `read_target_source` windows** (no shell reach this
time) and **polled the async `read_verdict` cleanly** to a verbatim
verdict. Across all three attempts the agent chose the cheapest untried
class each time (3 → 5 → 6, skipping 4/7 whose profile preconditions the
callgrind data didn't support), and **0 of 3 were false accepts**.

The agent's class selection was correct each time: it read the ledger,
saw which classes were already attempted, and picked the cheapest
untried class whose profile preconditions were met. Both patches went
through `propose_patch` (path-allowlist + `git apply`) and the full
gate+bench pipeline. Both were correctly rejected — real mechanisms,
too small to clear the 2% CI-lower-bound bar. **No false accepts.**

## Two findings the loop surfaced about itself

1. **`run_verdict` outlived the MCP transport (attempt 001).** The
   build+bench pipeline takes minutes; the MCP tool-call cap is 60 s. The
   agent's calls timed out — and rather than retry blindly or invent a
   number, it checked `read_ledger`, confirmed exactly one row was
   written (dedup on run_id held), and reported the verdict as
   *unreadable, not fabricated*, asking a human to read the ledger. Fix
   shipped: async `run_verdict` + pollable `read_verdict`. Attempt 002
   then read its verdict back cleanly.

2. **The SDK tool-restriction is not the trust boundary in this
   environment.** The agent reached for a built-in `Bash`/`Monitor` tool
   to read a source file too large for the (then whole-file-only)
   `read_target_source`. Investigation showed that a nested Claude Code
   CLI running as root under a parent session does not reliably gate
   built-ins: `--dangerously-skip-permissions` is refused under root, and
   a `can_use_tool` deny-by-default callback is shadowed by settings
   inherited from the parent session (and broke legitimate MCP tool calls
   with "Stream closed"). **This is expected, not a regression:** SPEC §10
   specifies the boundary as OS-level process/user isolation and no-net
   containers — "enforced by filesystem permissions and a separate-process
   harness, not by prompt." The SDK allow/deny layer is defense in depth.
   Mitigation shipped: windowed `read_target_source` removes the agent's
   reason to reach for a shell; the load-bearing guarantees hold
   regardless (see below).

## What holds even when the tool layer leaks

- **Writes to the target are only mediated through `propose_patch`** —
  path-allowlist (relative paths, no `..`, no `.git`) then `git apply`
  rooted in the target workspace. A patch touching `config/` or `crates/`
  is refused before any apply. Verified against a malicious
  accept-threshold diff in Phase 1.
- **The ledger is append-only** — UPDATE/DELETE are refused by SQLite
  triggers, and `accepted` rows are written only by the `verdict` binary
  after the gated pipeline. An agent cannot forge a passing verdict
  through the harness.
- **The gate+bench pipeline is unbypassable through the harness** —
  pristine-rebuilt baseline, interleaved A/B, CI-lower-bound rule.

The residual hole in THIS container: an agent with host shell could, in
principle, `sqlite3 INSERT` a forged row (triggers block mutation, not
insertion) or invoke the verdict binary directly. That is precisely the
attack SPEC §10's process/user isolation exists to prevent, and it is the
**Phase 2 exit gap to close on real infrastructure** (run the agent
process as a separate unprivileged user with no shell and the trust layer
owned by another uid — the `bench-metal`/`docs/infra.md` setup).

## Phase 2 exit criteria (SPEC §5)

- Unattended profile→verdict loop on one target: **met** (comrak).
- ≥1 auto-accepted win: **not yet** — both attempts were honest
  rejections. The loop is proven; a win needs either a richer profile
  (perf `stat` cache-miss data to unlock class-4 preconditions) or more
  attempts against fresh hypotheses.
- Zero false accepts across audited attempts: **met so far** (2/2
  audited, both correctly rejected).
- The separate-uid isolation for the tool boundary is the remaining
  hardening item, tracked to `docs/infra.md`.