## Why
After `#13440` and `#13445`, macOS Seatbelt policy generation was still
deriving filesystem and network behavior from the legacy `SandboxPolicy`
projection.
That projection loses explicit unreadable carveouts and conflates split
network decisions, so the generated Seatbelt policy could still be wider
than the split policy that Codex had already computed.
## What changed
- added Seatbelt entrypoints that accept `FileSystemSandboxPolicy` and
`NetworkSandboxPolicy` directly
- built read and write policy stanzas from access roots plus excluded
subpaths so explicit unreadable carveouts survive into the generated
Seatbelt policy
- switched network policy generation to consult `NetworkSandboxPolicy`
directly
- failed closed when managed-network or proxy-constrained sessions do
not yield usable loopback proxy endpoints
- updated the macOS callers and test helpers that now need to carry the
split policies explicitly
## Verification
- added regression coverage in `core/src/seatbelt.rs` for unreadable
carveouts under both full-disk and scoped-readable policies
- verified the current PR state with `just clippy`
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/13448).
* #13453
* #13452
* #13451
* #13449
* __->__ #13448
* #13445
* #13440
* #13439
---------
Co-authored-by: viyatb-oai <viyatb@openai.com>
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