## Why
When Codex uses a remote `ExecutorFileSystem`, every `get_metadata` call
is an exec-server round trip. Upward discovery currently pays those
round trips serially in two latency-sensitive places:
- session startup, while locating the configured project root before
loading `AGENTS.md`; and
- Git-root discovery, which runs before per-turn Git diff enrichment.
The goal is to remove the serial ancestor dependency without adding a
new filesystem RPC, JSON-RPC batch method, Git executable dependency, or
cache.
## Example
Assume this layout, with `.git` as the configured project-root marker:
```text
/workspace/repo/.git
/workspace/repo/AGENTS.md
/workspace/repo/crates/core/ <- cwd
```
The marker probes have this required precedence:
```text
1. /workspace/repo/crates/core/.git
2. /workspace/repo/crates/.git
3. /workspace/repo/.git
4. /workspace/.git
5. /.git
```
Previously, probe 2 was not sent until probe 1 returned, and probe 3 was
not sent until probe 2 returned. With this change, the client lazily
keeps up to eight ordinary `fs/getMetadata` requests in flight, but
consumes their results in the order above. Codex must still learn that
probes 1 and 2 are absent before accepting probe 3, so the nearest root
always wins. Once probe 3 succeeds, the client has its answer and stops
awaiting probes 4 and 5. Requests that were already sent may still
finish on the worker.
For the marker phase alone, with a 50 ms client-to-worker round trip and
fast local metadata calls, finding the root at probe 3 changes from
roughly three serialized round trips (150 ms) to one round trip plus
worker processing. The later `AGENTS.md` candidate phase remains
separate and ordered.
Only after `/workspace/repo` is selected does `AGENTS.md` discovery
check instruction candidates, in root-to-cwd order:
```text
/workspace/repo/AGENTS.override.md
/workspace/repo/AGENTS.md
/workspace/repo/crates/AGENTS.override.md
/workspace/repo/crates/AGENTS.md
/workspace/repo/crates/core/AGENTS.override.md
/workspace/repo/crates/core/AGENTS.md
```
The first configured candidate found in each directory wins. These
checks remain ordered and no instruction candidate above
`/workspace/repo` is issued. Git-root discovery uses the same bounded
lookup with only `.git` as the marker.
## What changed
- Added a client-side find-up helper that generates `ancestor x marker`
probes lazily, nearest directory first and configured marker order
within each directory.
- Uses an ordered concurrency window of eight scalar metadata requests.
This bounds executor load while preserving nearest-root and marker
precedence.
- Reuses the helper for both configured project-root discovery and
remote Git-root discovery.
- Keeps Git ancestor and marker construction in `AbsolutePathBuf`,
converting only each complete `.git` probe to `PathUri`. This preserves
native paths that require an opaque URI fallback, such as Windows
namespace paths.
- Preserves existing error behavior: `AGENTS.md` discovery propagates
non-`NotFound` metadata errors, while Git discovery treats a failed
marker probe as absent and continues upward.
- Reads each discovered `AGENTS.md` directly instead of statting it a
second time.
No filesystem trait or exec-server protocol method is added. An empty
`project_root_markers` list performs no ancestor-marker I/O and checks
instruction candidates only in `cwd`. This change also deliberately does
not cache roots across turns.
## Symlinks
Upward traversal remains **lexical**. The helper does not canonicalize
`cwd`; it appends marker names to the supplied path and walks that
path's textual parents. The filesystem performs the actual metadata/read
operation, and the current local and exec-server implementations follow
live symlink targets.
For example:
```text
/tmp/pkg -> /workspace/repo/packages/pkg
cwd = /tmp/pkg/src
actual Git marker = /workspace/repo/.git
```
The lexical probes are `/tmp/pkg/src/.git`, `/tmp/pkg/.git`,
`/tmp/.git`, and `/.git`. They do not jump from `/tmp/pkg` to the
target's parent `/workspace/repo`, so this spelling of `cwd` does not
discover `/workspace/repo/.git`. That is the existing behavior and is
unchanged by this PR.
Conversely, if `/tmp/repo -> /workspace/repo`, then probing
`/tmp/repo/.git` follows the directory symlink and finds
`/workspace/repo/.git`; the reported root remains the lexical path
`/tmp/repo`. A live symlink used directly as `.git`, another configured
marker, or `AGENTS.md` is also followed. A symlinked `AGENTS.md` is
loaded when its target is a regular file, while a broken symlink behaves
as `NotFound`.
## Why
`PathUri::from_abs_path` can fail for absolute paths that do not have a
normal `file:` URI representation, forcing filesystem call sites to
handle a conversion error even though the original path can be preserved
losslessly.
## What
Make `from_abs_path` infallible and migrate its callers. Unrepresentable
paths use `file:///%00/bad/path/<base64>`, encoding Unix bytes or
Windows UTF-16LE; `to_abs_path` validates and decodes that fallback. The
leading encoded null reserves a namespace that cannot collide with a
real Unix or Windows path, and fallback URIs remain opaque to lexical
path operations.
## Validation
Added path-URI coverage for Unix null and non-UTF-8 paths, Windows
device/verbatim and non-Unicode paths, serialization, malformed
fallbacks, opaque lexical operations, invalid native payloads, and
literal `/bad/path` collision resistance.
## Why
We're moving exec-server to use PathUri for its internal path
representations.
## What
Move `ExecutorFileSystem` APIs to use `PathUri` instead of
`AbsolutePathBuf`. Future changes will convert higher-level parts of
exec-server.
Codex forces `core.fsmonitor=false` on internal Git commands so a
repository cannot select an executable fsmonitor helper. This also
disables Git's built-in daemon for `status`, `diff`, and `ls-files`,
turning those worktree reads into full scans in large repositories.
Read the raw effective `core.fsmonitor` value and preserve it only when
Git interprets it as true and advertises built-in daemon support through
`git version --build-options`. Query uncommon boolean spellings back
through Git using the exact effective value. Unset, false, helper paths,
malformed values, probe failures, and unsupported Git builds continue to
force `core.fsmonitor=false`.
Centralize this policy in `git-utils` while keeping process execution in
the existing local and workspace-command adapters. Probe once per
worktree workflow and reuse the result for its Git commands, including
the TUI `/diff` path. Metadata-only commands and repository discovery
remain disabled without probing. Each probe and requested Git process
keeps its own existing timeout, and the decision is not cached because
layered and conditional Git configuration can change while Codex runs.
---------
Co-authored-by: Chris Bookholt <bookholt@openai.com>
