diff --git a/docs/superpowers/specs/2026-03-23-codex-app-compatibility-design.md b/docs/superpowers/specs/2026-03-23-codex-app-compatibility-design.md
index c3fecc04cc..fbd5e159f8 100644
--- a/docs/superpowers/specs/2026-03-23-codex-app-compatibility-design.md
+++ b/docs/superpowers/specs/2026-03-23-codex-app-compatibility-design.md
@@ -237,6 +237,18 @@ If a third skill needs the same detection pattern, extract it into a shared `ref
 4. Full lifecycle — detection → commit → finishing detection → correct behavior → cleanup
 5. **Sandbox fallback in Local thread** — Start a Codex App **Local thread** (workspace-write sandbox). Prompt: "Use the superpowers skill `using-git-worktrees` to set up an isolated workspace for implementing a small change." Pre-check: `git checkout -b test-sandbox-check` should fail with `Operation not permitted`. Expected: the skill detects `GIT_DIR == GIT_COMMON` (normal repo), attempts `git worktree add -b`, hits Seatbelt denial, falls back to Step 0 "already in workspace" behavior — runs setup, baseline tests, reports ready from current directory. Pass: agent recovers gracefully without cryptic error messages. Fail: agent prints raw Seatbelt error, retries, or gives up with confusing output.
 
+### Manual Test Results (2026-03-23)
+
+| Test | Result | Notes |
+|------|--------|-------|
+| 1. Detection in Worktree thread (workspace-write) | PASS | GIT_DIR != GIT_COMMON, BRANCH empty |
+| 2. Detection in Worktree thread (Full access) | PASS | Same detection; Full access allows `git checkout -b` |
+| 3. Finishing skill handoff format | PASS (after fix) | Initially FAILED — Step 1.5 was unreachable because Step 1 (test verification) halted first. Fixed by reordering: environment detection is now Step 1, test verification is Step 2. Path A skips tests entirely. |
+| 4. Full lifecycle | PASS | Detection → commit → handoff payload → no cleanup attempt |
+| 5. Sandbox fallback in Local thread | N/A | Local thread workspace-write sandbox does not block `git checkout -b`; Seatbelt restriction not triggered in current Codex App version |
+
+**Fix applied:** `c5d93ac` — moved environment detection before test verification in `finishing-a-development-branch/SKILL.md`. Step numbering changed: old Step 1.5 → new Step 1, old Step 1 → new Step 2, all downstream steps renumbered.
+
 ### Regression
 
 - Existing Claude Code skill-triggering tests still pass
diff --git a/skills/executing-plans/SKILL.md b/skills/executing-plans/SKILL.md
index e67f94c573..4c88352aee 100644
--- a/skills/executing-plans/SKILL.md
+++ b/skills/executing-plans/SKILL.md
@@ -65,6 +65,6 @@ After all tasks complete and verified:
 ## Integration
 
 **Required workflow skills:**
-- **superpowers:using-git-worktrees** - REQUIRED: Set up isolated workspace before starting
+- **superpowers:using-git-worktrees** - REQUIRED: Ensures isolated workspace (creates one or verifies existing)
 - **superpowers:writing-plans** - Creates the plan this skill executes
 - **superpowers:finishing-a-development-branch** - Complete development after all tasks
diff --git a/skills/finishing-a-development-branch/SKILL.md b/skills/finishing-a-development-branch/SKILL.md
index c308b43b4b..b8e3661af9 100644
--- a/skills/finishing-a-development-branch/SKILL.md
+++ b/skills/finishing-a-development-branch/SKILL.md
@@ -15,7 +15,55 @@ Guide completion of development work by presenting clear options and handling ch
 
 ## The Process
 
-### Step 1: Verify Tests
+### Step 1: Detect Environment
+
+Run this FIRST, before test verification:
+
+````bash
+GIT_DIR=$(cd "$(git rev-parse --git-dir)" 2>/dev/null && pwd -P)
+GIT_COMMON=$(cd "$(git rev-parse --git-common-dir)" 2>/dev/null && pwd -P)
+BRANCH=$(git branch --show-current)
+````
+
+**Path A — `GIT_DIR` differs from `GIT_COMMON` AND `BRANCH` is empty (externally managed worktree, detached HEAD):**
+
+This is a restricted sandbox environment (e.g., Codex App worktree thread). The test toolchain may not be available. Do NOT attempt test verification — skip directly to the handoff.
+
+First, ensure all work is staged and committed (`git add` + `git commit`). If staging/committing fails due to sandbox restrictions, note the uncommitted files in the handoff message.
+
+Then present this to the user (do NOT present the 4-option menu):
+
+```
+Implementation complete.
+Current HEAD: <full-commit-sha>
+
+This workspace is externally managed (detached HEAD).
+I cannot create branches, push, or open PRs from here.
+
+⚠ These commits are on a detached HEAD. If you do not create a branch,
+they may be lost when this workspace is cleaned up.
+
+If your host application provides these controls:
+- "Create branch" — to name a branch, then commit/push/PR
+- "Hand off to local" — to move changes to your local checkout
+
+Suggested branch name: <ticket-id/short-description>
+Suggested commit message: <summary-of-work>
+```
+
+Branch name: use ticket ID if available (e.g., `pri-823/codex-compat`), otherwise slugify the first 5 words of the plan title, otherwise omit. Avoid sensitive content in branch names.
+
+Skip to Step 6 (cleanup is a no-op — see guard below).
+
+**Path B — `GIT_DIR` differs from `GIT_COMMON` AND `BRANCH` exists (externally managed worktree, named branch):**
+
+Proceed to Step 2 (verify tests, then 4-option menu).
+
+**Path C — `GIT_DIR` equals `GIT_COMMON` (normal environment):**
+
+Proceed to Step 2 (verify tests, then 4-option menu).
+
+### Step 2: Verify Tests
 
 **Before presenting options, verify tests pass:**
 
@@ -33,11 +81,11 @@ Tests failing (<N> failures). Must fix before completing:
 Cannot proceed with merge/PR until tests pass.
 ```
 
-Stop. Don't proceed to Step 2.
+Stop. Don't proceed to Step 3.
 
-**If tests pass:** Continue to Step 2.
+**If tests pass:** Continue to Step 3.
 
-### Step 2: Determine Base Branch
+### Step 3: Determine Base Branch
 
 ```bash
 # Try common base branches
@@ -46,7 +94,7 @@ git merge-base HEAD main 2>/dev/null || git merge-base HEAD master 2>/dev/null
 
 Or ask: "This branch split from main - is that correct?"
 
-### Step 3: Present Options
+### Step 4: Present Options
 
 Present exactly these 4 options:
 
@@ -63,7 +111,7 @@ Which option?
 
 **Don't add explanation** - keep options concise.
 
-### Step 4: Execute Choice
+### Step 5: Execute Choice
 
 #### Option 1: Merge Locally
 
@@ -84,7 +132,7 @@ git merge <feature-branch>
 git branch -d <feature-branch>
 ```
 
-Then: Cleanup worktree (Step 5)
+Then: Cleanup worktree (Step 6)
 
 #### Option 2: Push and Create PR
 
@@ -103,7 +151,7 @@ EOF
 )"
 ```
 
-Then: Cleanup worktree (Step 5)
+Then: Cleanup worktree (Step 6)
 
 #### Option 3: Keep As-Is
 
@@ -131,11 +179,20 @@ git checkout <base-branch>
 git branch -D <feature-branch>
 ```
 
-Then: Cleanup worktree (Step 5)
+Then: Cleanup worktree (Step 6)
+
+### Step 6: Cleanup Worktree
+
+**First, check if worktree is externally managed:**
+
+````bash
+GIT_DIR=$(cd "$(git rev-parse --git-dir)" 2>/dev/null && pwd -P)
+GIT_COMMON=$(cd "$(git rev-parse --git-common-dir)" 2>/dev/null && pwd -P)
+````
 
-### Step 5: Cleanup Worktree
+If `GIT_DIR` differs from `GIT_COMMON`: skip worktree removal — the host environment owns this workspace.
 
-**For Options 1, 2, 4:**
+**Otherwise, for Options 1 and 4:**
 
 Check if in worktree:
 ```bash
diff --git a/skills/subagent-driven-development/SKILL.md b/skills/subagent-driven-development/SKILL.md
index 5150b18632..10a1c3cc61 100644
--- a/skills/subagent-driven-development/SKILL.md
+++ b/skills/subagent-driven-development/SKILL.md
@@ -265,7 +265,7 @@ Done!
 ## Integration
 
 **Required workflow skills:**
-- **superpowers:using-git-worktrees** - REQUIRED: Set up isolated workspace before starting
+- **superpowers:using-git-worktrees** - REQUIRED: Ensures isolated workspace (creates one or verifies existing)
 - **superpowers:writing-plans** - Creates the plan this skill executes
 - **superpowers:requesting-code-review** - Code review template for reviewer subagents
 - **superpowers:finishing-a-development-branch** - Complete development after all tasks
diff --git a/skills/using-git-worktrees/SKILL.md b/skills/using-git-worktrees/SKILL.md
index e153843cd1..99f2bdebf0 100644
--- a/skills/using-git-worktrees/SKILL.md
+++ b/skills/using-git-worktrees/SKILL.md
@@ -13,6 +13,30 @@ Git worktrees create isolated workspaces sharing the same repository, allowing w
 
 **Announce at start:** "I'm using the using-git-worktrees skill to set up an isolated workspace."
 
+## Step 0: Check if Already in an Isolated Workspace
+
+Before creating a worktree, check if one already exists:
+
+````bash
+GIT_DIR=$(cd "$(git rev-parse --git-dir)" 2>/dev/null && pwd -P)
+GIT_COMMON=$(cd "$(git rev-parse --git-common-dir)" 2>/dev/null && pwd -P)
+BRANCH=$(git branch --show-current)
+````
+
+**If `GIT_DIR` differs from `GIT_COMMON`:** You are already inside a linked worktree (created by the Codex App, Claude Code's Agent tool, a previous skill run, or the user). Do NOT create another worktree. Instead:
+
+1. Run project setup (auto-detect package manager as in "Run Project Setup" below)
+2. Verify clean baseline (run tests as in "Verify Clean Baseline" below)
+3. Report with branch state:
+   - On a branch: "Already in an isolated workspace at `<path>` on branch `<name>`. Tests passing. Ready to implement."
+   - Detached HEAD: "Already in an isolated workspace at `<path>` (detached HEAD, externally managed). Tests passing. Note: branch creation needed at finish time. Ready to implement."
+
+After reporting, STOP. Do not continue to Directory Selection or Creation Steps.
+
+**If `GIT_DIR` equals `GIT_COMMON`:** Proceed with the full worktree creation flow below.
+
+**Sandbox fallback:** If you proceed to Creation Steps but `git worktree add -b` fails with a permission error (e.g., "Operation not permitted"), treat this as a late-detected restricted environment. Fall back to the behavior above — run setup and baseline tests in the current directory, report accordingly, and STOP.
+
 ## Directory Selection Process
 
 Follow this priority order:
@@ -209,9 +233,9 @@ Ready to implement auth feature
 ## Integration
 
 **Called by:**
-- **brainstorming** (Phase 4) - REQUIRED when design is approved and implementation follows
-- **subagent-driven-development** - REQUIRED before executing any tasks
-- **executing-plans** - REQUIRED before executing any tasks
+- **brainstorming** - REQUIRED: Ensures isolated workspace (creates one or verifies existing)
+- **subagent-driven-development** - REQUIRED: Ensures isolated workspace (creates one or verifies existing)
+- **executing-plans** - REQUIRED: Ensures isolated workspace (creates one or verifies existing)
 - Any skill needing isolated workspace
 
 **Pairs with:**
diff --git a/tests/codex-app-compat/test-environment-detection.sh b/tests/codex-app-compat/test-environment-detection.sh
new file mode 100755
index 0000000000..daee039817
--- /dev/null
+++ b/tests/codex-app-compat/test-environment-detection.sh
@@ -0,0 +1,94 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Test environment detection logic from PRI-823
+# Tests the git-dir vs git-common-dir comparison used by
+# using-git-worktrees Step 0 and finishing-a-development-branch Step 1.5
+
+PASS=0
+FAIL=0
+TEMP_DIR=$(mktemp -d)
+trap 'rm -rf "$TEMP_DIR"' EXIT
+
+log_pass() { echo "  PASS: $1"; PASS=$((PASS + 1)); }
+log_fail() { echo "  FAIL: $1"; FAIL=$((FAIL + 1)); }
+
+# Helper: run detection and return "linked" or "normal"
+detect_worktree() {
+  local git_dir git_common
+  git_dir=$(cd "$(git rev-parse --git-dir)" 2>/dev/null && pwd -P)
+  git_common=$(cd "$(git rev-parse --git-common-dir)" 2>/dev/null && pwd -P)
+  if [ "$git_dir" != "$git_common" ]; then
+    echo "linked"
+  else
+    echo "normal"
+  fi
+}
+
+echo "=== Test 1: Normal repo detection ==="
+cd "$TEMP_DIR"
+git init test-repo > /dev/null 2>&1
+cd test-repo
+git commit --allow-empty -m "init" > /dev/null 2>&1
+result=$(detect_worktree)
+if [ "$result" = "normal" ]; then
+  log_pass "Normal repo detected as normal"
+else
+  log_fail "Normal repo detected as '$result' (expected 'normal')"
+fi
+
+echo "=== Test 2: Linked worktree detection ==="
+git worktree add "$TEMP_DIR/test-wt" -b test-branch > /dev/null 2>&1
+cd "$TEMP_DIR/test-wt"
+result=$(detect_worktree)
+if [ "$result" = "linked" ]; then
+  log_pass "Linked worktree detected as linked"
+else
+  log_fail "Linked worktree detected as '$result' (expected 'linked')"
+fi
+
+echo "=== Test 3: Detached HEAD detection ==="
+git checkout --detach HEAD > /dev/null 2>&1
+branch=$(git branch --show-current)
+if [ -z "$branch" ]; then
+  log_pass "Detached HEAD: branch is empty"
+else
+  log_fail "Detached HEAD: branch is '$branch' (expected empty)"
+fi
+
+echo "=== Test 4: Linked worktree + detached HEAD (Codex App simulation) ==="
+result=$(detect_worktree)
+branch=$(git branch --show-current)
+if [ "$result" = "linked" ] && [ -z "$branch" ]; then
+  log_pass "Codex App simulation: linked + detached HEAD"
+else
+  log_fail "Codex App simulation: result='$result', branch='$branch'"
+fi
+
+echo "=== Test 5: Cleanup guard — linked worktree should NOT remove ==="
+cd "$TEMP_DIR/test-wt"
+result=$(detect_worktree)
+if [ "$result" = "linked" ]; then
+  log_pass "Cleanup guard: linked worktree correctly detected (would skip removal)"
+else
+  log_fail "Cleanup guard: expected 'linked', got '$result'"
+fi
+
+echo "=== Test 6: Cleanup guard — main repo SHOULD remove ==="
+cd "$TEMP_DIR/test-repo"
+result=$(detect_worktree)
+if [ "$result" = "normal" ]; then
+  log_pass "Cleanup guard: main repo correctly detected (would proceed with removal)"
+else
+  log_fail "Cleanup guard: expected 'normal', got '$result'"
+fi
+
+# Cleanup worktree before temp dir removal
+cd "$TEMP_DIR/test-repo"
+git worktree remove "$TEMP_DIR/test-wt" > /dev/null 2>&1 || true
+
+echo ""
+echo "=== Results: $PASS passed, $FAIL failed ==="
+if [ "$FAIL" -gt 0 ]; then
+  exit 1
+fi