# Native CLI boundary proof — 2026-09-09

The installed Claude Code 2.1.266 binary was exercised against a deterministic
Anthropic-shaped API in a separate bwrap network namespace. The namespace had
no provider network route, operator home, real credential or inference service.
Only the CLI binary, system runtime and a temporary fixture workspace were mounted.
The executable receipt records the binary SHA-256 and exact adapter-generated argv.

## Findings and resulting change

With `--max-budget-usd 0.01`, the first fixture response reported 60,000 input
and 10 output tokens. The CLI estimated **USD 0.18015**, then exited with
`error_max_budget_usd`. Only one messages request reached the fixture. This
reproduces a stop after the response, not a pre-request maximum-liability check.
The rein terminal-accounting validator refused success and preserved known cost.
No amount in this experiment was billed by a provider.

The existing spend ledger remains necessary: it reserves declared per-run
liability and retains unknown outcomes. The experiment does not establish that
the declared liability is a valid bound on every possible native request. A
provider-facing request guard is the next HFACT-WP-0001-T01 implementation return;
raising the native threshold or accepting a newer binary cannot substitute for it.

The adapter also previously used `--allowedTools` with `acceptEdits`. Those flags
alone did not constrain the available tool inventory or repository-provided
customizations. Runs with native USD or turn controls now require CLI >=2.1.266
and use `dontAsk`, explicit builtin tools derived from registered rules, `--bare`,
empty settings sources, empty strict MCP configuration, an MCP deny rule, and no
session persistence. Legacy unconfigured sessions retain their existing behavior.
The worker prompt explicitly requires reading applicable AGENTS.md/CLAUDE.md;
these files are instructions, not permission to load executable configuration or
expand grants. The factory's protected runtime must be rebuilt and re-proven for
the new minimum; this source change installs or admits no runtime.

The real binary's bare mode exposed Bash, Read and Edit, a subset of the explicit
inventory. A synthetic tool sequence successfully ran permitted `git status` and
created a new file through Edit. An unapproved shell write was denied. A fixture
project containing a broad Bash permission override, a SessionStart hook and an
MCP command could activate none of them; its ambient CLAUDE.md was not auto-loaded.
The second messages request contained the expected allowed/denied tool results.

These tests prove the specified direct-tool and configuration boundaries. They
are not an arbitrary-code sandbox proof: permitted Git operations can invoke
repository-controlled helpers, and file edits can influence later commands.
Filesystem, credential and network confinement remain sandbox-owner controls.
To enforce spend against such indirect execution, the workload must not possess
a provider credential plus an unmetered route to the provider. A run-scoped,
owner-controlled request path must cover every forwarded request and retry.

## Reproduction and acceptance

`REIN_REAL_CLAUDE=1` enables `tests/test_native_cli_boundary.py`. The default binary
comes from `claude` on PATH; `REIN_CLAUDE_PROOF_BINARY` can select an exact installed
file. The fixture deliberately requires version 2.1.266; revalidate and record a
new receipt before claiming the same proof for another version. The adapter's
minimum-version gate does not itself prove arbitrary future binaries.

```text
REIN_REAL_CLAUDE=1 REIN_REAL_BWRAP=1 python -m pytest tests/ -q
```

The reproducible fixture is `tests/native_cli_fixture.py`; optional
`REIN_CLAUDE_PROOF_OUTPUT` writes bounded evidence without prompts, headers,
credentials or response bodies. The actual CLI checks are opt-in because ordinary
CI does not provision that binary. Deterministic native launch/accounting tests
are now included in both mandatory runtime and recovery gates. Paired receipts:
`docs/evidence/2026-09-09-native-cli-overrun.json` and
`docs/evidence/2026-09-09-native-cli-tools.json`.

No paid factory request, profile promotion, credential reader or production
rollout was authorized or performed. REINAH-WP-0003-T05/T06 and HFACT-WP-0001-T01/
T03/T04/T05 retain the request-admission, exact identity, protected placement and
natural queue/model execution returns.

## Upstream contract reviewed

The [CLI reference](https://code.claude.com/docs/en/cli-reference) distinguishes
available tools from auto-permission rules and describes the native stop flag.
The [headless guide](https://code.claude.com/docs/en/headless) documents bare-mode
configuration isolation and the API-key authentication path. The
[cost guide](https://code.claude.com/docs/en/costs) identifies CLI totals as local
estimates, not authoritative billing, and notes the residency-cost correction in
2.1.239. Reviewed 2026-09-09; the local executable findings above are narrower
than a claim about all provider billing or future CLI releases.