diff --git a/AGENTS.md b/AGENTS.md
index e45a59a45..d359e29bf 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -61,9 +61,10 @@ Its scope applies to the entire repo unless overridden by more specific `AGENTS.
 - For product-facing slices, ensure issue scope and acceptance criteria explicitly align with the current thesis (reduce maintenance overhead/capture friction and preserve review-first trust).
 
 ### Windows Git Reliability Fallback
-- If `git` resolves to Cygwin or produces signal/pipe-style failures, use `C:\Program Files\Git\cmd\git.exe` explicitly for repo operations.
+- Run `bash scripts/check-git-env.sh` at the start of a session to validate git resolution and index.lock state.
+- If `git` resolves to Cygwin or produces signal/pipe-style failures, use `C:\Program Files\Git\cmd\git.exe` explicitly for repo operations (or add `C:\Program Files\Git\cmd` to the front of `PATH`).
 - When running automated commits in the background terminal, ALWAYS append `--no-gpg-sign` and `--no-verify` to `git commit` to prevent hidden GPG pinentry prompts from freezing the process.
-- If a commit fails because `.git/index.lock` cannot be created, first check for active `git` processes; remove `.git/index.lock` only when no git process is running.
+- If a commit fails because `.git/index.lock` cannot be created, first check for active `git` processes; remove `.git/index.lock` only when no git process is running. The `check-git-env.sh` script automates this detection.
 - For stacked branches with small conflict surfaces, prefer `merge` over `rebase` when branch reconciliation starts stalling (for example long-running interactive/conflict loops). Resolve conflicts once, merge, and continue delivery.
 
 ### Small Mainline Exception
diff --git a/CLAUDE.md b/CLAUDE.md
index 31ce1f229..fbb2738be 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -150,6 +150,7 @@ When launching subagents with `isolation: "worktree"`, follow the protocol in `d
 
 ## Windows Notes
 
-- If `git` resolves to Cygwin or fails with signal errors, use `C:\Program Files\Git\cmd\git.exe` explicitly.
+- Run `bash scripts/check-git-env.sh` to validate git resolution and index.lock state before a work session.
+- If `git` resolves to Cygwin or fails with signal errors, use `C:\Program Files\Git\cmd\git.exe` explicitly (or add `C:\Program Files\Git\cmd` to the front of `PATH`).
 - Do not chain commands with `&&` in PowerShell; use `;` and check `$LASTEXITCODE`.
-- If `.git/index.lock` blocks commits, check for active git processes before removing it.
+- If `.git/index.lock` blocks commits, check for active git processes before removing it. The `check-git-env.sh` script automates this check.
diff --git a/scripts/check-git-env.sh b/scripts/check-git-env.sh
new file mode 100755
index 000000000..67d874e1d
--- /dev/null
+++ b/scripts/check-git-env.sh
@@ -0,0 +1,95 @@
+#!/usr/bin/env bash
+# scripts/check-git-env.sh
+# Validates the local Git environment on Windows for safe agent/local workflows.
+# Checks:
+#   1. git resolves to Git for Windows (not Cygwin/MSYS)
+#   2. No stale .git/index.lock without an active git process
+#
+# Usage:
+#   bash scripts/check-git-env.sh            # from repo root
+#   bash scripts/check-git-env.sh /path/to   # check a specific repo
+
+set -euo pipefail
+
+REPO_DIR="${1:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"
+EXIT_CODE=0
+
+# ---------------------------------------------------------------------------
+# 1. Git executable resolution
+# ---------------------------------------------------------------------------
+GIT_PATH="$(command -v git 2>/dev/null || true)"
+
+if [ -z "$GIT_PATH" ]; then
+    echo "[ERROR] git is not on PATH. Install Git for Windows from https://git-scm.com"
+    EXIT_CODE=1
+else
+    GIT_VERSION="$(git --version 2>/dev/null || true)"
+    echo "[INFO]  git path:    $GIT_PATH"
+    echo "[INFO]  git version: $GIT_VERSION"
+
+    # Detect Cygwin or MSYS2 (non-MinGW) git — path typically contains
+    # /cygdrive/ or /usr/bin/. Git for Windows (MinGW) resolves to /mingw64/bin/git.
+    case "$GIT_PATH" in
+        /cygdrive/*|/usr/bin/git)
+            echo "[WARN]  git appears to be Cygwin or MSYS2 (non-MinGW) git ($GIT_PATH)."
+            echo "        This can cause signal errors and path translation issues."
+            echo "        Fix: add 'C:\\Program Files\\Git\\cmd' to the FRONT of your PATH,"
+            echo "        or set alias git='\"C:\\Program Files\\Git\\cmd\\git.exe\"' in your shell profile."
+            EXIT_CODE=1
+            ;;
+    esac
+
+    if echo "$GIT_VERSION" | grep -qi "cygwin"; then
+        echo "[WARN]  git --version reports Cygwin: $GIT_VERSION"
+        echo "        Use Git for Windows instead. See guidance above."
+        EXIT_CODE=1
+    fi
+
+    if [ $EXIT_CODE -eq 0 ]; then
+        echo "[OK]    git resolves to a non-Cygwin executable."
+    fi
+fi
+
+# ---------------------------------------------------------------------------
+# 2. Stale .git/index.lock detection
+# ---------------------------------------------------------------------------
+# In worktrees, .git is a file pointing to the real git dir.
+# Use git rev-parse to find the actual git directory.
+# --absolute-git-dir (Git 2.13+) returns an absolute path, avoiding the bug
+# where a relative ".git" would resolve against CWD instead of REPO_DIR.
+GIT_DIR="$(git -C "$REPO_DIR" rev-parse --absolute-git-dir 2>/dev/null || echo "$REPO_DIR/.git")"
+LOCK_FILE="$GIT_DIR/index.lock"
+
+if [ -f "$LOCK_FILE" ]; then
+    echo ""
+    echo "[WARN]  Stale lock file found: $LOCK_FILE"
+
+    # Check for active git processes (Windows: tasklist; Unix: pgrep)
+    ACTIVE_GIT=""
+    if command -v tasklist &>/dev/null; then
+        ACTIVE_GIT="$(tasklist //FI "IMAGENAME eq git.exe" 2>/dev/null | grep -i "git.exe" || true)"
+    elif command -v pgrep &>/dev/null; then
+        ACTIVE_GIT="$(pgrep -a git 2>/dev/null || true)"
+    fi
+
+    if [ -n "$ACTIVE_GIT" ]; then
+        echo "[WARN]  Active git process(es) detected — lock may be legitimate:"
+        echo "$ACTIVE_GIT"
+        echo "        Wait for the process to finish, or kill it if stuck, then remove the lock."
+    else
+        echo "[WARN]  No active git processes found. The lock file is likely stale."
+        echo "        Safe to remove: rm \"$LOCK_FILE\""
+    fi
+    EXIT_CODE=1
+else
+    echo "[OK]    No .git/index.lock present."
+fi
+
+echo ""
+if [ $EXIT_CODE -eq 0 ]; then
+    echo "All checks passed."
+else
+    echo "One or more issues detected. See warnings above."
+fi
+
+exit $EXIT_CODE