diff --git a/.planning/PROJECT.md b/.planning/PROJECT.md
index a783bd5..d38d36e 100644
--- a/.planning/PROJECT.md
+++ b/.planning/PROJECT.md
@@ -12,7 +12,7 @@ Secrets never enter the Claude Code environment. If a secret is accessible insid
 
 ### Validated
 
-(None yet — ship to validate)
+- [x] Default prompt/instructions injected so Claude knows how to use nix/comma to get dev tools — Validated in Phase 3: Sandbox-Aware Prompting
 
 ### Active
 
@@ -24,7 +24,7 @@ Secrets never enter the Claude Code environment. If a secret is accessible insid
 - [ ] Secret paths hidden: `~/.ssh`, `~/.gnupg`, `~/.config/gcloud`, `~/.aws`, Tailscale state, age keys — none of these visible inside the sandbox
 - [ ] Minimal PATH: Nix store paths only — coreutils, git, curl, jq, ripgrep, fd, nix, comma
 - [ ] Claude can self-install tools via `nix shell` or `, <tool>` (comma) inside the sandbox
-- [ ] Default prompt/instructions injected so Claude knows how to use nix/comma to get dev tools
+- [x] Default prompt/instructions injected so Claude knows how to use nix/comma to get dev tools — Validated in Phase 3
 - [ ] Works on endurance (NixOS desktop)
 
 ### Out of Scope
@@ -77,4 +77,4 @@ This document evolves at phase transitions and milestone boundaries.
 4. Update Context with current state
 
 ---
-*Last updated: 2026-04-09 after initialization*
+*Last updated: 2026-04-09 after Phase 3 completion*