peltier.io/docs/src/architecture.md

Architecture

Peltier is split into a trust layer (Rust, the agent cannot write it) and an untrusted proposer (the agent). The split is structural, not advisory: the guarantees hold even if the proposer is adversarial.

crates/            Trust layer — agent has NO write access (SPEC §10)
  bench-runner/    Interleaved A/B timing, bootstrap CIs, A/A calibration,
                   service-mode latency (coordinated-omission correct)
  diff-test/       Equivalence gates, corpus hash-pinning, per-target spec
  ledger/          Append-only SQLite attempt ledger (enforced by triggers)
  report/          ROI: speedup CI → cores → dollars, methodology inline
  explain/         Advisory post-verdict diagnosis of a ledger row (why it
                   won or lost) — never on the accept path
  policy/          Learned class-selection prior from the ledger (advisory)
  verdict/         The pipeline in one command: gates → bench → ledger row
  harnessd/        The one door the agent talks through (7 JSON ops)
agent/             Untrusted proposer (Claude Agent SDK, Python)
playbook/          Optimization classes 1–7, tried strictly cheapest-first
config/            accept.toml (thresholds), pricing.toml (ROI inputs)
targets/           Vendored OSS targets — the only agent-writable path
corpora/           Hash-pinned golden-replay inputs (read-only to agent)
results/           Calibration evidence, case studies, generated reports
.claude/skills/    The trust layer as a consumable Agent Skill — mirrored
                   byte-identically at .agents/skills/ (CI-enforced); see
                   "Consume it: the peltier skill"

The seven-tool boundary

The agent never runs a shell, never writes a file, never touches the ledger directly. It speaks to harnessd — the only door — through exactly seven JSON operations:

Tool	What it can do
read_profile	Read a target's ranked hotspots (read-only)
read_ledger	Read prior attempts — the anti-double-attempt memory
read_playbook	Read an optimization class's preconditions/procedure
read_target_source	Read a workspace file at the pinned commit (windowed)
propose_patch	Submit a unified diff — path-allowlisted, then git apply
run_verdict	Launch the gate+bench pipeline (detached)
read_verdict	Poll the append-only ledger for the result

Note what is absent: no write outside targets/<name>/workspace, no shell, no way to run the bench or write the ledger except through the verdict binary, which only records a pass after the full gated pipeline.

The data flow of one attempt

profile ─▶ agent hypothesizes ─▶ propose_patch ─▶ [path allowlist + git apply]
                                                          │
   ledger row ◀─ verdict ◀─ bench (interleaved A/B) ◀─ gates (tests, golden,
   (append-only)             vs pristine-rebuilt         fuzz, sanitizers)
                             baseline

The baseline is rebuilt from a pristine checkout every session — never from the agent's workspace — so a patch cannot poison its own comparison. The verdict is decided by the CI-lower-bound rule and written to an append-only ledger the agent cannot mutate.

The next chapters walk each stage: measurement, equivalence gates, the agent loop, and the isolation that makes the boundary load-bearing.