diff --git a/.changeset/1520-mktemp-suffix-randomization.md b/.changeset/1520-mktemp-suffix-randomization.md
new file mode 100644
index 000000000..d3137c715
--- /dev/null
+++ b/.changeset/1520-mktemp-suffix-randomization.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1550
+---
+**Workflow temp files now randomize correctly on BSD/macOS** — several workflows called `mktemp` with templates where `XXXXXX` was followed by a `.json`/`.md` suffix (e.g. `gsd-worktree-wave-XXXXXX.json`, `gsd-pr-body.XXXXXX.md`). BSD/macOS `mktemp` only substitutes `XXXXXX` when it is the final path component, so those templates returned a literal, non-randomized path, letting concurrent workflow runs collide on the same temp manifest/body file (one run overwriting or consuming another's). The fix creates a suffixless temp then renames to add the extension — portable across BSD + GNU. Affected: `execute-phase`, `quick`, `spec-phase`, `ship`, `profile-user`. (#1520)
diff --git a/gsd-core/workflows/execute-phase.md b/gsd-core/workflows/execute-phase.md
index 99416e5de..155fccfc8 100644
--- a/gsd-core/workflows/execute-phase.md
+++ b/gsd-core/workflows/execute-phase.md
@@ -580,7 +580,7 @@ increases monotonically across waves. `{status}` is `complete` (success),
    DISPATCH_TS=$(date -u +"%Y-%m-%dT%H:%M:%SZ")
    EXPECTED_BRANCH=$(git rev-parse --abbrev-ref HEAD)
    if [ "${USE_WORKTREES_FOR_PLAN:-true}" != "false" ] && [ -z "${WAVE_WORKTREE_MANIFEST:-}" ]; then
-     WAVE_WORKTREE_MANIFEST=$(mktemp "${TMPDIR:-/tmp}/gsd-worktree-wave-XXXXXX.json")
+     M=$(mktemp "${TMPDIR:-/tmp}/gsd-worktree-wave-XXXXXX") && mv "$M" "$M.json" && WAVE_WORKTREE_MANIFEST="$M.json" || exit 1  # XXXXXX must be path-final on BSD/macOS (#1520)
      # Persist the dispatch-time orchestrator worktree root so wave-cleanup can pin back to the
      # orchestrator's OWN worktree — NOT `git worktree list`'s first entry (always the main
      # checkout), which pins a non-primary (per-phase lane) orchestrator off its branch (#630).
diff --git a/gsd-core/workflows/profile-user.md b/gsd-core/workflows/profile-user.md
index 3ef88d7eb..7d723cf1b 100644
--- a/gsd-core/workflows/profile-user.md
+++ b/gsd-core/workflows/profile-user.md
@@ -218,7 +218,9 @@ Collect all answers into an answers JSON object mapping dimension keys to select
 
 **Save answers to temp file:**
 ```bash
-ANSWERS_PATH=$(mktemp /tmp/gsd-profile-answers-XXXXXX.json)
+# BSD/macOS mktemp only randomizes XXXXXX when it is the final path component, so make a
+# suffixless temp then append the extension — portable across BSD + GNU (#1520).
+ANSWERS_PATH=$(mktemp "${TMPDIR:-/tmp}/gsd-profile-answers-XXXXXX") && mv "$ANSWERS_PATH" "${ANSWERS_PATH}.json" && ANSWERS_PATH="${ANSWERS_PATH}.json" || exit 1
 ```
 
 Write the answers JSON to `$ANSWERS_PATH`.
@@ -232,7 +234,9 @@ Parse the analysis JSON from the result.
 
 Save analysis JSON to a temp file:
 ```bash
-ANALYSIS_PATH=$(mktemp /tmp/gsd-profile-analysis-XXXXXX.json)
+# BSD/macOS mktemp only randomizes XXXXXX when it is the final path component, so make a
+# suffixless temp then append the extension — portable across BSD + GNU (#1520).
+ANALYSIS_PATH=$(mktemp "${TMPDIR:-/tmp}/gsd-profile-analysis-XXXXXX") && mv "$ANALYSIS_PATH" "${ANALYSIS_PATH}.json" && ANALYSIS_PATH="${ANALYSIS_PATH}.json" || exit 1
 ```
 
 Write the analysis JSON to `$ANALYSIS_PATH`.
diff --git a/gsd-core/workflows/quick.md b/gsd-core/workflows/quick.md
index a4e7ec3e0..cc6b03614 100644
--- a/gsd-core/workflows/quick.md
+++ b/gsd-core/workflows/quick.md
@@ -675,7 +675,9 @@ Capture current HEAD before spawning (used for worktree branch check):
 ```bash
 EXPECTED_BASE=$(git rev-parse HEAD)
 if [ "${USE_WORKTREES:-true}" != "false" ]; then
-  QUICK_WORKTREE_MANIFEST=$(mktemp "${TMPDIR:-/tmp}/gsd-quick-worktree-XXXXXX.json")
+  # BSD/macOS mktemp only randomizes XXXXXX when it is the final path component, so make a
+  # suffixless temp then append the extension — portable across BSD + GNU (#1520).
+  QUICK_WORKTREE_MANIFEST=$(mktemp "${TMPDIR:-/tmp}/gsd-quick-worktree-XXXXXX") && mv "$QUICK_WORKTREE_MANIFEST" "${QUICK_WORKTREE_MANIFEST}.json" && QUICK_WORKTREE_MANIFEST="${QUICK_WORKTREE_MANIFEST}.json" || exit 1
   printf '{"worktrees":[]}\n' > "$QUICK_WORKTREE_MANIFEST"
   export QUICK_WORKTREE_MANIFEST
 fi
diff --git a/gsd-core/workflows/ship.md b/gsd-core/workflows/ship.md
index ff966789f..845bf4189 100644
--- a/gsd-core/workflows/ship.md
+++ b/gsd-core/workflows/ship.md
@@ -280,7 +280,9 @@ Use the exact key order `skill=`, `fallback=`, `exempt=`, `missing=` so downstre
 Create the PR using the generated body. Write the body to a temp file first so large generated PRD sections do not hit shell argument limits:
 
 ```bash
-PR_BODY_FILE=$(mktemp "${TMPDIR:-/tmp}/gsd-pr-body.XXXXXX.md")
+# BSD/macOS mktemp only randomizes XXXXXX when it is the final path component, so make a
+# suffixless temp then append the extension — portable across BSD + GNU (#1520).
+PR_BODY_FILE=$(mktemp "${TMPDIR:-/tmp}/gsd-pr-body-XXXXXX") && mv "$PR_BODY_FILE" "${PR_BODY_FILE}.md" && PR_BODY_FILE="${PR_BODY_FILE}.md" || exit 1
 trap 'rm -f "${PR_BODY_FILE:-}"' EXIT
 printf '%s\n' "${PR_BODY}" > "${PR_BODY_FILE}"
 
diff --git a/gsd-core/workflows/spec-phase.md b/gsd-core/workflows/spec-phase.md
index 06c876d53..119fbf53c 100644
--- a/gsd-core/workflows/spec-phase.md
+++ b/gsd-core/workflows/spec-phase.md
@@ -235,7 +235,9 @@ fi
 # canonical coverage compute. Populate the heredoc from the SPEC's Requirements — one object
 # per requirement: {"id","text","shapes"?}. This is the load-bearing step: an empty file makes
 # the probe a no-op, so the guard below fails loud rather than silently skipping (RR-04).
-REQS_JSON=$(mktemp "${TMPDIR:-/tmp}/edge-probe-reqs-XXXXXX.json")
+# BSD/macOS mktemp only randomizes XXXXXX when it is the final path component, so make a
+# suffixless temp then append the extension — portable across BSD + GNU (#1520).
+REQS_JSON=$(mktemp "${TMPDIR:-/tmp}/edge-probe-reqs-XXXXXX") && mv "$REQS_JSON" "${REQS_JSON}.json" && REQS_JSON="${REQS_JSON}.json" || exit 1
 cat > "$REQS_JSON" <<'JSON'
 [
   { "id": "R1", "text": "<replace: requirement text from the SPEC>" }
diff --git a/tests/fix-1520-workflow-mktemp-suffix-final.test.cjs b/tests/fix-1520-workflow-mktemp-suffix-final.test.cjs
new file mode 100644
index 000000000..1e904a5f4
--- /dev/null
+++ b/tests/fix-1520-workflow-mktemp-suffix-final.test.cjs
@@ -0,0 +1,78 @@
+// allow-test-rule: source-text-is-the-product (#1520)
+// Workflow .md text IS what the runtime loads and the agent executes, so
+// asserting on its shell invocations tests the deployed contract directly.
+//
+// Repo-wide regression guard for #1520: NO workflow .md may invoke `mktemp`
+// with a template whose `XXXXXX` run is followed by a filename suffix
+// (e.g. `…-XXXXXX.json`, `…-XXXXXX.md`). BSD/macOS `mktemp` only substitutes
+// the `X` run when it is the FINAL path component; a trailing suffix yields a
+// literal, non-randomized path, so concurrent workflow runs collide on the same
+// temp file (one run overwriting or consuming another's). The portable fix is
+// `mktemp …-XXXXXX` (suffix-less) then `mv` to add the extension.
+//
+// This is a copy-paste-prone shell idiom — the same defect first shipped across
+// five workflows before #1520 — so a prose guard is the right lock-out, mirroring
+// the bug-637 hardcoded-$HOME workflow scan.
+
+'use strict';
+
+const { describe, test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const WORKFLOWS_DIR = path.join(__dirname, '..', 'gsd-core', 'workflows');
+
+// Match `mktemp <token>` where, within the single whitespace-delimited template
+// token, a maximal run of 3+ `X` is immediately followed by a filename
+// character (`.`, alnum, `-`, `_`) — i.e. a suffix the BSD/macOS substitution
+// can't reach.
+//   - `\s+`            requires an argument (bare `mktemp` is fine — path-final
+//                      is N/A — and prose like "mktemp only randomizes XXXXXX"
+//                      is excluded because the X-run is in a later token).
+//   - `["']?\S*?`      walks within the one quoted/unquoted template token.
+//   - `X{3,}(?!X)`     anchors on the WHOLE X-run (so `XXXXXX)` does not match
+//                      via a sub-run leaving a trailing `X`).
+//   - `[.A-Za-z0-9_-]` the offending suffix char. A legitimate path-final form
+//                      ends the token with `"`, `'`, whitespace, or `)`, none of
+//                      which are in this class.
+const SUFFIXED_MKTEMP_TEMPLATE = /mktemp\s+["']?\S*?X{3,}(?!X)[.A-Za-z0-9_-]/;
+
+function collectWorkflowMarkdown(dir) {
+  const out = [];
+  for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) {
+      out.push(...collectWorkflowMarkdown(full));
+    } else if (entry.isFile() && entry.name.endsWith('.md')) {
+      out.push(full);
+    }
+  }
+  return out;
+}
+
+describe('#1520: workflow mktemp templates keep XXXXXX path-final', () => {
+  test('no gsd-core/workflows/**/*.md calls mktemp with a suffix after the XXXXXX run', () => {
+    const files = collectWorkflowMarkdown(WORKFLOWS_DIR);
+    assert.ok(files.length > 0, 'expected workflow markdown files to exist');
+
+    const offenders = [];
+    for (const file of files) {
+      const lines = fs.readFileSync(file, 'utf8').split(/\r?\n/);
+      lines.forEach((line, i) => {
+        if (SUFFIXED_MKTEMP_TEMPLATE.test(line)) {
+          offenders.push(`${path.relative(WORKFLOWS_DIR, file)}:${i + 1}: ${line.trim()}`);
+        }
+      });
+    }
+
+    assert.deepStrictEqual(
+      offenders,
+      [],
+      'Workflow mktemp templates must keep XXXXXX as the final path component ' +
+        '(create suffix-less, then `mv` to add the extension) so BSD/macOS ' +
+        'randomizes the path. Offenders:\n' +
+        offenders.join('\n'),
+    );
+  });
+});
diff --git a/tests/workflow-size-baseline.json b/tests/workflow-size-baseline.json
index 1ed65362b..6e785faa5 100644
--- a/tests/workflow-size-baseline.json
+++ b/tests/workflow-size-baseline.json
@@ -24,7 +24,7 @@
   "docs-update.md": 55662,
   "edit-phase.md": 12883,
   "eval-review.md": 9923,
-  "execute-phase.md": 93426,
+  "execute-phase.md": 93517,
   "execute-plan.md": 32611,
   "explore.md": 10497,
   "extract-learnings.md": 12849,
@@ -56,9 +56,9 @@
   "plan-review-convergence.md": 23468,
   "plant-seed.md": 11741,
   "pr-branch.md": 15919,
-  "profile-user.md": 20650,
+  "profile-user.md": 21202,
   "progress.md": 30555,
-  "quick.md": 48830,
+  "quick.md": 49139,
   "reapply-patches.md": 20393,
   "remove-phase.md": 8469,
   "remove-workspace.md": 7507,
@@ -70,10 +70,10 @@
   "settings-advanced.md": 39666,
   "settings-integrations.md": 15848,
   "settings.md": 33413,
-  "ship.md": 24388,
+  "ship.md": 24647,
   "sketch-wrap-up.md": 14223,
   "sketch.md": 19960,
-  "spec-phase.md": 31503,
+  "spec-phase.md": 31752,
   "spike-wrap-up.md": 15092,
   "spike.md": 24517,
   "stats.md": 6718,