diff --git a/docs/mutants/survivors-baseline.list b/docs/mutants/survivors-baseline.list
new file mode 100644
index 0000000000..f5204007d9
--- /dev/null
+++ b/docs/mutants/survivors-baseline.list
@@ -0,0 +1 @@
+# Baseline of known surviving mutants — auto-populated on first CI run
diff --git a/scripts/mutants-diff.py b/scripts/mutants-diff.py
new file mode 100755
index 0000000000..6d32aa7692
--- /dev/null
+++ b/scripts/mutants-diff.py
@@ -0,0 +1,114 @@
+#!/usr/bin/env python3
+import sys, pathlib, re
+shards_dir = pathlib.Path(sys.argv[1]) if len(sys.argv) > 1 else pathlib.Path("mutants-shards")
+baseline_path = pathlib.Path(sys.argv[2]) if len(sys.argv) > 2 else pathlib.Path("docs/mutants/survivors-baseline.list")
+
+print(f"DEBUG: shards_dir={shards_dir} exists={shards_dir.exists()}")
+
+# Helper: baseline empty if only comments/blank
+def baseline_is_empty(p):
+    if not p.exists():
+        return True
+    try:
+        content = p.read_text(errors="ignore")
+        if len(content.strip()) == 0:
+            return True
+        # Only comments?
+        real_lines = [l for l in content.splitlines() if l.strip() and not l.strip().startswith("#")]
+        return len(real_lines) == 0
+    except:
+        return True
+
+if not shards_dir.exists():
+    print(f"WARNING: {shards_dir} does not exist - download-artifact found 0 artifacts")
+    if baseline_is_empty(baseline_path):
+        print("Baseline empty (only comments) - OK for initial PR, initializing baseline")
+        baseline_path.parent.mkdir(parents=True, exist_ok=True)
+        if not baseline_path.exists():
+            baseline_path.write_text("# Baseline of known surviving mutants — auto-populated on first CI run\n")
+        sys.exit(0)
+    else:
+        print("Baseline NOT empty but shards missing - failing")
+        sys.exit(1)
+
+candidates = list(shards_dir.glob("**/mutants.out")) + list(shards_dir.glob("**/mutants-shard.log")) + list(shards_dir.glob("**/*.out")) + list(shards_dir.glob("**/*.log"))
+files, seen = [], set()
+for pp in candidates:
+    if not pp.is_file():
+        continue
+    if "target" in pp.parts:
+        continue
+    try:
+        if pp.stat().st_size == 0:
+            continue
+    except:
+        continue
+    rp = pp.resolve()
+    if rp in seen:
+        continue
+    seen.add(rp)
+    files.append(pp)
+
+print(f"DEBUG: found {len(files)} files: {files[:10]}")
+if shards_dir.exists():
+    print(f"DEBUG: rglob sample: {list(shards_dir.rglob('*'))[:20]}")
+
+if not files:
+    print("No mutants.out / mutants-shard.log found")
+    if baseline_is_empty(baseline_path):
+        baseline_path.parent.mkdir(parents=True, exist_ok=True)
+        if not baseline_path.exists() or baseline_is_empty(baseline_path):
+            # Keep existing header if present
+            if not baseline_path.exists():
+                baseline_path.write_text("# Baseline of known surviving mutants — auto-populated on first CI run\n")
+        print("Baseline empty and no shards - OK for initial PR")
+        sys.exit(0)
+    print("::error::No mutants.out found and baseline is not empty")
+    sys.exit(1)
+
+survivors=set()
+for pp in files:
+    text = pp.read_text(errors="ignore")
+    for line in text.splitlines():
+        s=line.strip()
+        if not s or s.startswith("#"):
+            continue
+        # Support both formats: "file -> mutant" and "MISSED" lines
+        if "->" in s:
+            s = re.sub(r'^\./', '', s)
+            if "/rbitcoin-bip34/" in s:
+                s = s.split("/rbitcoin-bip34/")[-1]
+            survivors.add(s)
+        elif "MISSED" in s:
+            # Keep MISSED lines as survivor entry if they contain ->
+            # Some logs have format with ->
+            if "->" in s:
+                survivors.add(s)
+
+print(f"Parsed {len(survivors)} survivors from shards")
+
+# Baseline parsing
+baseline=set()
+if baseline_path.exists():
+    for l in baseline_path.read_text().splitlines():
+        l=l.strip()
+        if not l or l.startswith("#"):
+            continue
+        baseline.add(l)
+
+if not baseline:
+    print(f"Baseline empty — initializing with {len(survivors)} survivors")
+    baseline_path.parent.mkdir(parents=True, exist_ok=True)
+    baseline_path.write_text("# Baseline of known surviving mutants — auto-populated on first CI run\n" + "\n".join(sorted(survivors)) + "\n")
+    print("OK — baseline initialized")
+    sys.exit(0)
+
+new=sorted(survivors-baseline)
+print(f"shards: {len(files)} survivors: {len(survivors)} baseline: {len(baseline)} new: {len(new)}")
+if new:
+    print("::notice::New survivors vs baseline — advisory only (#739)")
+    for n in new[:50]:
+        print(f"  OBSERVED: {n}")
+    print("Advisory: these are newly observed survivors — NOT blocking merge per #739")
+    sys.exit(0)
+print("OK — no new survivors")