diff --git a/backend/services/fetchers/_store.py b/backend/services/fetchers/_store.py
index 81eacf1..1eeba77 100644
--- a/backend/services/fetchers/_store.py
+++ b/backend/services/fetchers/_store.py
@@ -265,10 +265,27 @@ def get_latest_data_subset(*keys: str) -> DashboardData:
 
 
 def get_latest_data_deepcopy_snapshot() -> DashboardData:
-    """Deep-copy the full dashboard for legacy /api/live-data consumers."""
-    with _data_lock:
-        items = list(latest_data.items())
-    return {key: copy.deepcopy(value) for key, value in items}
+    """Deep-copy the full dashboard for /api/health and legacy /api/live-data.
+
+    The per-value deepcopy runs OUTSIDE ``_data_lock`` so a large clone cannot
+    block fetcher writers (#375). The store contract is replace-don't-mutate,
+    but a writer that mutates a nested object in place (e.g. a live bridge
+    updating an entry that is also published in this store) can race the
+    deepcopy and raise ``RuntimeError: dictionary changed size during
+    iteration`` — surfacing a 500 on the health/live-data path. The racing
+    mutation window is tiny, so retry a few times rather than fail; a fresh
+    attempt almost always lands on a quiescent moment. Defense-in-depth on top
+    of fixing the offending writers, not a substitute for it.
+    """
+    attempts = 4
+    for attempt in range(attempts):
+        with _data_lock:
+            items = list(latest_data.items())
+        try:
+            return {key: copy.deepcopy(value) for key, value in items}
+        except RuntimeError:
+            if attempt == attempts - 1:
+                raise
 
 
 def get_latest_data_subset_refs(*keys: str) -> DashboardData:
diff --git a/backend/services/fetchers/sigint.py b/backend/services/fetchers/sigint.py
index f6a2c7a..185cbe0 100644
--- a/backend/services/fetchers/sigint.py
+++ b/backend/services/fetchers/sigint.py
@@ -21,12 +21,21 @@ def _merge_sigint_snapshot(
     because they include fresher region/channel metadata.
     """
 
-    merged = list(live_signals)
+    # Shallow-copy every entry so the published list owns its own dicts. The
+    # inputs alias objects that other threads keep mutating in place: live
+    # signals are the SIGINT bridge's own dicts (updated as packets arrive),
+    # and api_nodes are the same objects published under latest_data
+    # ["meshtastic_map_nodes"]. Publishing those references into
+    # latest_data["sigint"] lets a concurrent mutation race the lock-free
+    # deepcopy in get_latest_data_deepcopy_snapshot() (/api/health, /api/live-
+    # data) and raise "dictionary changed size during iteration". Copying
+    # honors the replace-don't-mutate contract in fetchers/_store.py.
+    merged = [dict(s) for s in live_signals]
     live_callsigns = {s["callsign"] for s in merged if s.get("source") == "meshtastic"}
     for node in api_nodes:
         if node.get("callsign") in live_callsigns:
             continue
-        merged.append(node)
+        merged.append(dict(node))
     merged.sort(key=lambda item: str(item.get("timestamp", "") or ""), reverse=True)
     return merged
 
diff --git a/backend/tests/test_live_data_snapshot_resilience.py b/backend/tests/test_live_data_snapshot_resilience.py
new file mode 100644
index 0000000..0e3d02d
--- /dev/null
+++ b/backend/tests/test_live_data_snapshot_resilience.py
@@ -0,0 +1,47 @@
+"""The full-store snapshot must survive a transient concurrent-mutation race.
+
+``get_latest_data_deepcopy_snapshot`` deep-copies each top-level layer outside
+the data lock. If a misbehaving writer mutates a nested object in place during
+the copy, ``copy.deepcopy`` raises ``RuntimeError: dictionary changed size
+during iteration``. The snapshot retries a few times (the mutation window is
+tiny) so /api/health and /api/live-data do not 500 on a transient race.
+"""
+
+import copy
+
+from services.fetchers import _store
+
+
+def test_snapshot_retries_then_succeeds(monkeypatch):
+    real_deepcopy = copy.deepcopy
+    calls = {"n": 0}
+
+    def flaky_deepcopy(value, *args, **kwargs):
+        calls["n"] += 1
+        # Fail only on the very first deepcopy call, then behave normally.
+        if calls["n"] == 1:
+            raise RuntimeError("dictionary changed size during iteration")
+        return real_deepcopy(value, *args, **kwargs)
+
+    monkeypatch.setattr(_store.copy, "deepcopy", flaky_deepcopy)
+
+    snapshot = _store.get_latest_data_deepcopy_snapshot()
+
+    assert isinstance(snapshot, dict)
+    assert calls["n"] >= 2  # it retried after the simulated race
+
+
+def test_snapshot_reraises_if_race_never_clears(monkeypatch):
+    def always_racing(value, *args, **kwargs):
+        raise RuntimeError("dictionary changed size during iteration")
+
+    monkeypatch.setattr(_store.copy, "deepcopy", always_racing)
+
+    # A persistent (non-transient) violation is a real bug — surface it rather
+    # than hang or return corrupt data.
+    raised = False
+    try:
+        _store.get_latest_data_deepcopy_snapshot()
+    except RuntimeError:
+        raised = True
+    assert raised
diff --git a/backend/tests/test_sigint_snapshot_isolation.py b/backend/tests/test_sigint_snapshot_isolation.py
new file mode 100644
index 0000000..1dc7105
--- /dev/null
+++ b/backend/tests/test_sigint_snapshot_isolation.py
@@ -0,0 +1,58 @@
+"""Regression test for SIGINT snapshot dict aliasing.
+
+``_merge_sigint_snapshot`` used to publish the *same* dict objects it received
+into ``latest_data["sigint"]``. Those inputs are owned and mutated in place by
+other threads (the SIGINT bridge updating live signals, and the
+``meshtastic_map_nodes`` layer), so a concurrent mutation could race the
+lock-free deepcopy in ``get_latest_data_deepcopy_snapshot`` (/api/health,
+/api/live-data) and raise ``dictionary changed size during iteration``.
+
+The merged snapshot must own copies of every entry.
+"""
+
+from services.fetchers.sigint import _merge_sigint_snapshot
+
+
+def test_merged_entries_are_copies_not_aliases():
+    live = [{"callsign": "LIVE1", "source": "meshtastic", "timestamp": "2"}]
+    api = [{"callsign": "MAP1", "source": "meshtastic", "from_api": True, "timestamp": "1"}]
+
+    merged = _merge_sigint_snapshot(live, api)
+
+    # No published entry may be the *same object* as an input the bridge or the
+    # meshtastic_map_nodes layer keeps mutating.
+    inputs = {id(live[0]), id(api[0])}
+    assert all(id(entry) not in inputs for entry in merged)
+
+
+def test_mutating_inputs_after_merge_does_not_affect_snapshot():
+    live = [{"callsign": "LIVE1", "source": "meshtastic", "timestamp": "2"}]
+    api = [{"callsign": "MAP1", "source": "meshtastic", "from_api": True, "timestamp": "1"}]
+
+    merged = _merge_sigint_snapshot(live, api)
+
+    # Simulate the bridge adding a key to a live signal after publication — this
+    # must not change the size of any dict reachable from the published list.
+    live[0]["region"] = "added-later"
+    api[0]["channel"] = "added-later"
+
+    assert all("region" not in entry for entry in merged)
+    assert all("channel" not in entry for entry in merged)
+
+
+def test_merge_preserves_data_and_dedup():
+    # Live meshtastic observation wins over the map node for the same callsign.
+    live = [{"callsign": "DUP", "source": "meshtastic", "timestamp": "5"}]
+    api = [
+        {"callsign": "DUP", "source": "meshtastic", "from_api": True, "timestamp": "1"},
+        {"callsign": "OTHER", "source": "meshtastic", "from_api": True, "timestamp": "1"},
+    ]
+
+    merged = _merge_sigint_snapshot(live, api)
+
+    callsigns = [m["callsign"] for m in merged]
+    assert callsigns.count("DUP") == 1
+    assert "OTHER" in callsigns
+    # The surviving DUP is the live one (no from_api flag).
+    dup = next(m for m in merged if m["callsign"] == "DUP")
+    assert not dup.get("from_api")