It is deliberately not the API that external tool owners should depend on. Crates that contribute ordinary function tools through extensions should use codex-tool-api, which owns only that small executable-tool seam.

Today this crate owns the host-facing tool models and helpers that no longer need to live in core/src/tools/spec.rs or core/src/client_common.rs:

aggregate host models such as ToolSpec, ConfiguredToolSpec, LoadableToolSpec, ResponsesApiNamespace, and ResponsesApiNamespaceTool
host config and discovery models used while assembling tool sets, including ToolsConfig, discoverable-tool models, and request-plugin-install helpers
host adapters such as schema sanitization, MCP/dynamic conversion, code-mode augmentation, and image-detail normalization

That extraction is the first step in a longer migration. The goal is not to move all of core/src/tools into this crate in one shot. Instead, the plan is to peel off reusable pieces in reviewable increments while keeping compatibility-sensitive orchestration in codex-core until the surrounding boundaries are ready.

Vision

Over time, this crate should hold host-side tool machinery that is shared by multiple consumers, for example:

host-visible aggregate tool models
tool-set planning and discovery helpers
MCP and dynamic-tool adaptation into Responses API shapes
code-mode compatibility shims that do not depend on codex-core
other narrowly scoped host utilities that multiple crates need

The corresponding non-goals are just as important:

do not become the extension-facing authoring API for executable tools
do not move codex-core orchestration here prematurely
do not pull Session / TurnContext / approval flow / runtime execution logic into this crate unless those dependencies have first been split into stable shared interfaces
do not turn this crate into a grab-bag for unrelated helper code

Migration approach

The expected migration shape is:

Keep ordinary contributed function-tool authoring in codex-tool-api.
Move host-side planning/adaptation helpers here when they no longer need to stay coupled to codex-core.
Leave compatibility-sensitive adapters in codex-core while downstream call sites are updated.
Only extract higher-level host infrastructure after the crate boundaries are clear and independently testable.

Crate conventions

This crate should start with stricter structure than core/src/tools so it stays easy to grow:

src/lib.rs should remain exports-only.
Business logic should live in named module files such as foo.rs.
Unit tests for foo.rs should live in a sibling foo_tests.rs.
The implementation file should wire tests with:

#[cfg(test)]
#[path = "foo_tests.rs"]
mod tests;

If this crate starts accumulating code that needs runtime state from codex-core, that is a sign to revisit the extraction boundary before adding more here.