Allow reusing Xcode.app's per-workspace DerivedData (avoid duplicate builds); static XCODEBUILDMCP_DERIVED_DATA_PATH can't map per workspace

[davidpasztor]

Version: 2.7.0

Problem

For derived data the MCP currently offers two modes, neither of which lets agent builds share Xcode.app's DerivedData per workspace:

1. Default: per-workspace isolation (Isolate DerivedData per workspace to avoid conflicts with concurrent sessions #340), but under the MCP's own tree (~/Library/Developer/XcodeBuildMCP/workspaces/<workspace>/DerivedData). This is separate from Xcode.app's ~/Library/Developer/Xcode/DerivedData/<Workspace>-<hash>, so a project built both in the GUI and via the MCP is built and stored twice — double the disk and no incremental reuse across the two.
2. XCODEBUILDMCP_DERIVED_DATA_PATH: a single static path applied to every call (resolveEffectiveDerivedDataPath returns it verbatim). With multiple workspaces/worktrees, they all collide in one folder → build-product clobbering and full rebuilds.

Request

A mode to resolve derived data to the existing Xcode.app DerivedData folder for each workspace (e.g. matching WorkspacePath in each folder's info.plist), and/or per-workspace configurability instead of a single static override.

This would let a repo built in both Xcode and the MCP share one DerivedData per workspace — no duplication, true incremental reuse between interactive and agent builds.

Workaround

I currently achieve this with a client-side PreToolUse hook that rewrites each build/test call's derivedDataPath to the matching GUI folder (looked up via WorkspacePath in each folder's info.plist), falling back to a deterministic per-workspace path when no GUI folder exists yet. Happy to share it if useful.

Related

• Isolate DerivedData per workspace to avoid conflicts with concurrent sessions #340 (per-workspace DerivedData isolation) — implemented isolation, but into the MCP's own tree rather than Xcode.app's.
• [Feature]: Expand ~ (tilde) in derived data path passed via config #283 (tilde expansion in config'd derived data path).