diff --git a/bin/gstack-developer-profile b/bin/gstack-developer-profile
index 1b0594630..a5b1ab771 100755
--- a/bin/gstack-developer-profile
+++ b/bin/gstack-developer-profile
@@ -55,7 +55,7 @@ do_migrate() {
 
   # Run migration in a temp file, then atomic rename.
   local TMPOUT
-  TMPOUT=$(mktemp "$GSTACK_HOME/developer-profile.json.XXXXXX.tmp")
+  TMPOUT=$(mktemp "$GSTACK_HOME/developer-profile.json.tmp.XXXXXX")
   trap 'rm -f "$TMPOUT"' EXIT
 
   cat "$LEGACY_FILE" | bun -e "
@@ -182,7 +182,7 @@ do_log_session() {
   ensure_profile
 
   local TMPOUT
-  TMPOUT=$(mktemp "$GSTACK_HOME/developer-profile.json.XXXXXX.tmp")
+  TMPOUT=$(mktemp "$GSTACK_HOME/developer-profile.json.tmp.XXXXXX")
   trap 'rm -f "$TMPOUT"' EXIT
 
   PROFILE_FILE_PATH="$PROFILE_FILE" RECORD_INPUT="$INPUT" TMPOUT_PATH="$TMPOUT" bun -e "
diff --git a/bin/gstack-paths b/bin/gstack-paths
index 1a7e07306..a2682aa44 100755
--- a/bin/gstack-paths
+++ b/bin/gstack-paths
@@ -56,6 +56,15 @@ else
   _tmp_root=".gstack/tmp"
 fi
 
+# Strip any trailing slash so consumers can safely concatenate "$TMP_ROOT/name"
+# without producing a double slash. On macOS $TMPDIR ends in `/` by default
+# (e.g. /var/folders/.../T/), which would otherwise yield paths like
+# `…/T//codex-err-…`. Normalizing at the source means every consumer benefits,
+# not just /codex.
+_tmp_root="${_tmp_root%/}"
+# A value of "/" collapses to "" above; restore it so TMP_ROOT is never empty.
+[ -z "$_tmp_root" ] && _tmp_root="/"
+
 # Best-effort mkdir; if it fails (read-only fs, permission denied), the caller
 # will discover that on their own write attempt. Don't fail the eval here.
 mkdir -p "$_tmp_root" 2>/dev/null || true
diff --git a/claude/SKILL.md.tmpl b/claude/SKILL.md.tmpl
index 94552cbe4..e109f21f7 100644
--- a/claude/SKILL.md.tmpl
+++ b/claude/SKILL.md.tmpl
@@ -95,8 +95,8 @@ Create temp files:
 
 ```bash
 PROMPT_FILE=$(mktemp /tmp/gstack-claude-prompt-XXXXXX)
-RESP_FILE=$(mktemp /tmp/gstack-claude-response-XXXXXX.json)
-ERR_FILE=$(mktemp /tmp/gstack-claude-error-XXXXXX.txt)
+RESP_FILE=$(mktemp /tmp/gstack-claude-response-XXXXXX)
+ERR_FILE=$(mktemp /tmp/gstack-claude-error-XXXXXX)
 ```
 
 Cleanup at the end of every mode:
@@ -151,7 +151,7 @@ Review the current branch diff with nested Claude in tool-less mode.
 ```bash
 _REPO_ROOT=$(git rev-parse --show-toplevel) || { echo "ERROR: not in a git repo" >&2; exit 1; }
 cd "$_REPO_ROOT"
-DIFF_FILE=$(mktemp /tmp/gstack-claude-diff-XXXXXX.patch)
+DIFF_FILE=$(mktemp /tmp/gstack-claude-diff-XXXXXX)
 git fetch origin <base> --quiet 2>/dev/null || true
 git diff "origin/<base>" > "$DIFF_FILE" 2>/dev/null || git diff "<base>" > "$DIFF_FILE"
 ```
diff --git a/codex/SKILL.md b/codex/SKILL.md
index c06d3affa..26283809f 100644
--- a/codex/SKILL.md
+++ b/codex/SKILL.md
@@ -967,7 +967,7 @@ Run Codex code review against the current branch diff.
 
 1. Create temp files for output capture:
 ```bash
-TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX.txt")
+TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX")
 ```
 
 2. Run the review (5-minute timeout). **Codex CLI ≥ 0.130.0 rejects passing a
@@ -1015,7 +1015,7 @@ when the diff content is adversarial:
 _REPO_ROOT=$(git rev-parse --show-toplevel) || { echo "ERROR: not in a git repo" >&2; exit 1; }
 cd "$_REPO_ROOT"
 _USER_INSTRUCTIONS="<everything after '/codex review ' in user input>"
-_PROMPT_FILE=$(mktemp "$TMP_ROOT/codex-prompt-XXXXXX.txt")
+_PROMPT_FILE=$(mktemp "$TMP_ROOT/codex-prompt-XXXXXX")
 {
   printf '%s\n' "IMPORTANT: Do NOT read or execute any files under ~/.claude/, ~/.agents/, .claude/skills/, or agents/. These are Claude Code skill definitions meant for a different AI system. Do NOT modify agents/openai.yaml. Stay focused on repository code only."
   printf '\nCustom focus: %s\n\n' "$_USER_INSTRUCTIONS"
@@ -1266,7 +1266,7 @@ if [ -z "$PYTHON_CMD" ]; then
 fi
 # Fix 1+2: wrap with timeout (gtimeout/timeout fallback chain via probe helper),
 # capture stderr to $TMPERR for auth error detection (was: 2>/dev/null).
-TMPERR=${TMPERR:-$(mktemp "$TMP_ROOT/codex-err-XXXXXX.txt")}
+TMPERR=${TMPERR:-$(mktemp "$TMP_ROOT/codex-err-XXXXXX")}
 _gstack_codex_timeout_wrapper 600 codex exec "<prompt>" -C "$_REPO_ROOT" -s read-only -c 'model_reasoning_effort="high"' --enable web_search_cached --json < /dev/null 2>"$TMPERR" | PYTHONUNBUFFERED=1 "$PYTHON_CMD" -u -c "
 import sys, json
 turn_completed_count = 0
@@ -1365,8 +1365,8 @@ B) Start a new conversation
 
 2. Create temp files:
 ```bash
-TMPRESP=$(mktemp "$TMP_ROOT/codex-resp-XXXXXX.txt")
-TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX.txt")
+TMPRESP=$(mktemp "$TMP_ROOT/codex-resp-XXXXXX")
+TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX")
 ```
 
 3. **Plan review auto-detection:** If the user's prompt is about reviewing a plan,
diff --git a/codex/SKILL.md.tmpl b/codex/SKILL.md.tmpl
index 333de7d8d..afb8b38a7 100644
--- a/codex/SKILL.md.tmpl
+++ b/codex/SKILL.md.tmpl
@@ -158,7 +158,7 @@ Run Codex code review against the current branch diff.
 
 1. Create temp files for output capture:
 ```bash
-TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX.txt")
+TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX")
 ```
 
 2. Run the review (5-minute timeout). **Codex CLI ≥ 0.130.0 rejects passing a
@@ -206,7 +206,7 @@ when the diff content is adversarial:
 _REPO_ROOT=$(git rev-parse --show-toplevel) || { echo "ERROR: not in a git repo" >&2; exit 1; }
 cd "$_REPO_ROOT"
 _USER_INSTRUCTIONS="<everything after '/codex review ' in user input>"
-_PROMPT_FILE=$(mktemp "$TMP_ROOT/codex-prompt-XXXXXX.txt")
+_PROMPT_FILE=$(mktemp "$TMP_ROOT/codex-prompt-XXXXXX")
 {
   printf '%s\n' "IMPORTANT: Do NOT read or execute any files under ~/.claude/, ~/.agents/, .claude/skills/, or agents/. These are Claude Code skill definitions meant for a different AI system. Do NOT modify agents/openai.yaml. Stay focused on repository code only."
   printf '\nCustom focus: %s\n\n' "$_USER_INSTRUCTIONS"
@@ -335,7 +335,7 @@ if [ -z "$PYTHON_CMD" ]; then
 fi
 # Fix 1+2: wrap with timeout (gtimeout/timeout fallback chain via probe helper),
 # capture stderr to $TMPERR for auth error detection (was: 2>/dev/null).
-TMPERR=${TMPERR:-$(mktemp "$TMP_ROOT/codex-err-XXXXXX.txt")}
+TMPERR=${TMPERR:-$(mktemp "$TMP_ROOT/codex-err-XXXXXX")}
 _gstack_codex_timeout_wrapper 600 codex exec "<prompt>" -C "$_REPO_ROOT" -s read-only -c 'model_reasoning_effort="high"' --enable web_search_cached --json < /dev/null 2>"$TMPERR" | PYTHONUNBUFFERED=1 "$PYTHON_CMD" -u -c "
 import sys, json
 turn_completed_count = 0
@@ -434,8 +434,8 @@ B) Start a new conversation
 
 2. Create temp files:
 ```bash
-TMPRESP=$(mktemp "$TMP_ROOT/codex-resp-XXXXXX.txt")
-TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX.txt")
+TMPRESP=$(mktemp "$TMP_ROOT/codex-resp-XXXXXX")
+TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX")
 ```
 
 3. **Plan review auto-detection:** If the user's prompt is about reviewing a plan,
diff --git a/office-hours/SKILL.md b/office-hours/SKILL.md
index d81d89f2d..31d7736b2 100644
--- a/office-hours/SKILL.md
+++ b/office-hours/SKILL.md
@@ -1329,7 +1329,7 @@ If B: skip Phase 3.5 entirely. Remember that the second opinion did NOT run (aff
 2. **Write the assembled prompt to a temp file** (prevents shell injection from user-derived content):
 
 ```bash
-CODEX_PROMPT_FILE=$(mktemp /tmp/gstack-codex-oh-XXXXXXXX.txt)
+CODEX_PROMPT_FILE=$(mktemp /tmp/gstack-codex-oh-XXXXXXXX)
 ```
 
 Write the full prompt to this file. **Always start with the filesystem boundary:**
diff --git a/scripts/resolvers/review.ts b/scripts/resolvers/review.ts
index 7dccd8e50..b708c90a3 100644
--- a/scripts/resolvers/review.ts
+++ b/scripts/resolvers/review.ts
@@ -351,7 +351,7 @@ If B: skip Phase 3.5 entirely. Remember that the second opinion did NOT run (aff
 2. **Write the assembled prompt to a temp file** (prevents shell injection from user-derived content):
 
 \`\`\`bash
-CODEX_PROMPT_FILE=$(mktemp /tmp/gstack-codex-oh-XXXXXXXX.txt)
+CODEX_PROMPT_FILE=$(mktemp /tmp/gstack-codex-oh-XXXXXXXX)
 \`\`\`
 
 Write the full prompt to this file. **Always start with the filesystem boundary:**
diff --git a/test/regression-issue2091-bsd-mktemp.test.ts b/test/regression-issue2091-bsd-mktemp.test.ts
new file mode 100644
index 000000000..4b858dbdf
--- /dev/null
+++ b/test/regression-issue2091-bsd-mktemp.test.ts
@@ -0,0 +1,165 @@
+/**
+ * Regression tests for issue #2091 / #2370 — mktemp templates fail on macOS
+ * (BSD mktemp) and Alpine (busybox mktemp) when the X placeholder run is not
+ * the LAST thing in the template.
+ *
+ * Two compounding bugs, both in gstack:
+ *
+ *   1. Suffix after the placeholder. Skills used templates like
+ *      `mktemp "$TMP_ROOT/codex-err-XXXXXX.txt"`. GNU mktemp tolerates a suffix
+ *      after the X run; BSD mktemp (macOS) does NOT — it does not substitute the
+ *      X's at all, so call #1 creates a LITERAL `codex-err-XXXXXX.txt` (exit 0)
+ *      and a later call (a second /codex run, a stale leftover, or a concurrent
+ *      worktree) fails with `mkstemp failed: File exists` and aborts the review.
+ *      busybox mktemp (Alpine) rejects the template outright on the FIRST run.
+ *      Fixed by moving the placeholder to the END of every mktemp template.
+ *
+ *   2. Trailing slash in TMP_ROOT. `bin/gstack-paths` emitted TMP_ROOT straight
+ *      from $TMPDIR, which on macOS ends in `/` (e.g. /var/folders/.../T/),
+ *      producing a double-slash path (`…/T//codex-err-…`). Fixed by stripping
+ *      the trailing slash at the source so every consumer benefits, not just
+ *      /codex.
+ *
+ * Bug 1's tripwire is repo-wide: it sweeps EVERY .tmpl, every SKILL.md, and
+ * every scripts/resolvers/*.ts (the sources that feed generated skills), so a
+ * new skill can't re-introduce the suffix shape anywhere.
+ */
+import { describe, test, expect } from 'bun:test';
+import { spawnSync } from 'child_process';
+import * as fs from 'fs';
+import * as path from 'path';
+
+const ROOT = path.resolve(import.meta.dir, '..');
+const PATHS_BIN = path.join(ROOT, 'bin', 'gstack-paths');
+
+// ── Bug 1: BSD mktemp requires the X placeholder at the END of the template ──
+// Swept across all skills (templates AND generated output) plus the resolver
+// modules that feed generated sections, so neither a hand-edit nor a regen
+// drift can reopen the bug.
+
+/** Directories that are not gstack-authored skill/template sources. */
+const SKIP_DIRS = new Set([
+  'node_modules',
+  '.git',
+  'dist',
+  '.claude',
+  '.agents',
+  '.factory',
+  'fixtures', // test/fixtures — goldens snapshot generated output separately
+]);
+
+function collectScannedFiles(dir: string, out: string[]): void {
+  for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+    if (SKIP_DIRS.has(entry.name)) continue;
+    const full = path.join(dir, entry.name);
+    // Symlinks (e.g. connect-chrome → open-gstack-browser) would double-count
+    // or escape the tree; the link target is scanned via its real path.
+    if (entry.isSymbolicLink()) continue;
+    if (entry.isDirectory()) {
+      collectScannedFiles(full, out);
+    } else if (
+      entry.name === 'SKILL.md' ||
+      entry.name.endsWith('.tmpl') ||
+      (full.includes(path.join('scripts', 'resolvers')) && entry.name.endsWith('.ts'))
+    ) {
+      out.push(full);
+    }
+  }
+}
+
+/**
+ * Extract every mktemp template token on a line and return the ones whose
+ * X-run (4+ X's) is NOT the final character run. Tokens are whitespace-split
+ * words after `mktemp`, stripped of shell/TS quoting and closers, so
+ * `$(mktemp "$TMP_ROOT/codex-err-XXXXXX.txt")` yields the offending
+ * `$TMP_ROOT/codex-err-XXXXXX.txt`.
+ */
+function offendingTemplatesOnLine(line: string): string[] {
+  const idx = line.indexOf('mktemp');
+  if (idx === -1) return [];
+  const offenders: string[] = [];
+  for (const raw of line.slice(idx).split(/\s+/)) {
+    const token = raw.replace(/^["'(`]+|[)"'`;,\\}]+$/g, '');
+    if (!/X{4,}/.test(token)) continue;
+    if (!/X{4,}$/.test(token)) offenders.push(token);
+  }
+  return offenders;
+}
+
+describe('#2091/#2370 bug 1: every mktemp template is BSD-safe (X placeholder at end)', () => {
+  const files: string[] = [];
+  collectScannedFiles(ROOT, files);
+
+  test('scan sweep finds the known mktemp call sites (not vacuous)', () => {
+    // Guards against the walker silently matching nothing after a refactor.
+    const withMktemp = files.filter((f) => fs.readFileSync(f, 'utf-8').includes('mktemp'));
+    expect(withMktemp.length).toBeGreaterThanOrEqual(5);
+    expect(withMktemp).toContain(path.join(ROOT, 'codex', 'SKILL.md.tmpl'));
+    expect(withMktemp).toContain(path.join(ROOT, 'codex', 'SKILL.md'));
+    expect(withMktemp).toContain(path.join(ROOT, 'scripts', 'resolvers', 'review.ts'));
+  });
+
+  test('no .tmpl, SKILL.md, or resolver carries a suffix after the X-run', () => {
+    const violations: string[] = [];
+    for (const file of files) {
+      const lines = fs.readFileSync(file, 'utf-8').split('\n');
+      lines.forEach((line, i) => {
+        for (const token of offendingTemplatesOnLine(line)) {
+          violations.push(`${path.relative(ROOT, file)}:${i + 1} — ${token}`);
+        }
+      });
+    }
+    expect(violations).toEqual([]);
+  });
+
+  test('the offender-detector itself still detects the original bug shapes', () => {
+    // Self-test so a regex tweak can't quietly blind the tripwire.
+    expect(offendingTemplatesOnLine('TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX.txt")')).toEqual([
+      '$TMP_ROOT/codex-err-XXXXXX.txt',
+    ]);
+    expect(
+      offendingTemplatesOnLine('TMPOUT=$(mktemp "$GSTACK_HOME/developer-profile.json.XXXXXX.tmp")'),
+    ).toEqual(['$GSTACK_HOME/developer-profile.json.XXXXXX.tmp']);
+    expect(offendingTemplatesOnLine('RESP_FILE=$(mktemp /tmp/gstack-claude-response-XXXXXX.json)')).toEqual([
+      '/tmp/gstack-claude-response-XXXXXX.json',
+    ]);
+    // Fixed shapes pass.
+    expect(offendingTemplatesOnLine('TMPERR=$(mktemp "$TMP_ROOT/codex-err-XXXXXX")')).toEqual([]);
+    expect(offendingTemplatesOnLine('ALL_JSONL=$(mktemp -t autoplan-tasks.XXXXXXXX)')).toEqual([]);
+  });
+});
+
+// ── Bug 2: gstack-paths normalizes TMP_ROOT (no trailing slash) ──────────────
+// Mirrors the invocation contract used by test/gstack-paths.test.ts: the helper
+// is always sourced from a bash block, so we run it via `bash`.
+function tmpRoot(env: Record<string, string | undefined>): string {
+  const result = spawnSync('bash', [PATHS_BIN], {
+    env: { PATH: process.env.PATH, USERPROFILE: '', ...env } as Record<string, string>,
+    encoding: 'utf-8',
+  });
+  if (result.status !== 0) {
+    throw new Error(`gstack-paths failed (status ${result.status}): ${result.stderr}`);
+  }
+  for (const line of result.stdout.split('\n')) {
+    if (line.startsWith('TMP_ROOT=')) return line.slice('TMP_ROOT='.length);
+  }
+  throw new Error('gstack-paths did not emit TMP_ROOT');
+}
+
+describe('#2091 bug 2: gstack-paths strips the trailing slash from TMP_ROOT', () => {
+  test('macOS-style TMPDIR with trailing slash → trailing slash stripped', () => {
+    expect(tmpRoot({ TMPDIR: '/var/folders/ab/T/', HOME: '/tmp/h' })).toBe('/var/folders/ab/T');
+  });
+
+  test('TMP (Windows/container fallback) with trailing slash is also normalized', () => {
+    expect(tmpRoot({ TMP: '/tmp/y/', HOME: '/tmp/h' })).toBe('/tmp/y');
+  });
+
+  test('a path without a trailing slash is left unchanged', () => {
+    expect(tmpRoot({ TMPDIR: '/tmp/x', HOME: '/tmp/h' })).toBe('/tmp/x');
+  });
+
+  test('a bare "/" does not collapse to empty', () => {
+    expect(tmpRoot({ TMPDIR: '/', HOME: '/tmp/h' })).toBe('/');
+  });
+});