diff --git a/meshtools/persistence/repository.py b/meshtools/persistence/repository.py
index 937c1c0..0b55f52 100644
--- a/meshtools/persistence/repository.py
+++ b/meshtools/persistence/repository.py
@@ -230,27 +230,46 @@ class Repository:
             "SELECT * FROM traces WHERE target = ? ORDER BY id DESC LIMIT ?",
             (target, limit),
         ).fetchall()
-        traces: list[TraceResult] = []
-        for row in rows:
-            hop_rows = self._conn.execute(
-                "SELECT hop_index, node, snr FROM trace_hops WHERE trace_id = ? "
-                "ORDER BY hop_index",
-                (row["id"],),
-            ).fetchall()
-            hops = [
-                Hop(index=h["hop_index"], node=h["node"], snr=h["snr"]) for h in hop_rows
-            ]
-            traces.append(
-                TraceResult(
-                    target=row["target"],
-                    success=bool(row["success"]),
-                    hops=hops,
-                    round_trip_ms=row["round_trip_ms"],
-                    tx_power=row["tx_power"],
-                    timestamp=datetime.fromisoformat(row["created_at"]),
-                )
-            )
-        return traces
+        return [self._hydrate_trace(row) for row in rows]
+
+    def all_traces(self, *, limit: int = 500) -> list[TraceResult]:
+        """Return recent traces across every target, newest first, rehydrated with hops.
+
+        Used to build a whole-mesh route graph rather than a single target's history.
+
+        Args:
+            limit: Maximum number of traces to return.
+
+        Returns:
+            The matching :class:`TraceResult` objects, newest first.
+        """
+        rows = self._conn.execute(
+            "SELECT * FROM traces ORDER BY id DESC LIMIT ?", (limit,)
+        ).fetchall()
+        return [self._hydrate_trace(row) for row in rows]
+
+    def _hydrate_trace(self, row: sqlite3.Row) -> TraceResult:
+        """Rebuild a :class:`TraceResult` (with hops) from a ``traces`` row.
+
+        Args:
+            row: A ``traces`` table row.
+
+        Returns:
+            The rehydrated trace.
+        """
+        hop_rows = self._conn.execute(
+            "SELECT hop_index, node, snr FROM trace_hops WHERE trace_id = ? ORDER BY hop_index",
+            (row["id"],),
+        ).fetchall()
+        hops = [Hop(index=h["hop_index"], node=h["node"], snr=h["snr"]) for h in hop_rows]
+        return TraceResult(
+            target=row["target"],
+            success=bool(row["success"]),
+            hops=hops,
+            round_trip_ms=row["round_trip_ms"],
+            tx_power=row["tx_power"],
+            timestamp=datetime.fromisoformat(row["created_at"]),
+        )
 
     def traced_targets(self) -> list[str]:
         """Return the distinct trace destinations recorded, most-traced first.
diff --git a/meshtools/services/route_stability.py b/meshtools/services/route_stability.py
new file mode 100644
index 0000000..53d1c2e
--- /dev/null
+++ b/meshtools/services/route_stability.py
@@ -0,0 +1,190 @@
+"""Route-stability aggregation: turn trace history into a mesh graph with churn metrics.
+
+The MeshCore app traces a route once. Because MeshTools persists every trace, it can
+instead show how *stable* routing is over time — which hops are always used and which the
+mesh flaps between. This service aggregates a set of traces into:
+
+* a **graph** of nodes and directed links, each link weighted by how often it was
+  traversed and its median SNR; and
+* per-target **churn** metrics: how many distinct routes were seen and what share the
+  single most-common route held (the higher that share, the steadier the routing).
+
+It is pure logic over :class:`~meshtools.core.models.TraceResult` lists; the pyvis
+renderer in :mod:`meshtools.viz.route_graph` draws the result.
+"""
+
+from __future__ import annotations
+
+import math
+import statistics
+from dataclasses import dataclass, field
+from typing import Callable, Optional
+
+from ..core.models import LOCAL_DEVICE_LABEL, TraceResult
+
+NodeResolver = Callable[[Optional[str]], Optional[str]]
+
+
+@dataclass(slots=True)
+class RouteLink:
+    """A directed link between two nodes, aggregated across traces.
+
+    Attributes:
+        origin: Transmitting node label.
+        destination: Receiving node label.
+        count: Number of trace hops that traversed this link.
+        median_snr: Median SNR (dB) measured on it.
+    """
+
+    origin: str
+    destination: str
+    count: int
+    median_snr: float
+
+
+@dataclass(slots=True)
+class RouteNode:
+    """A node in the route graph.
+
+    Attributes:
+        label: Display label for the node.
+        is_self: Whether this is our own companion device.
+        degree: Number of distinct links (in or out) touching the node.
+    """
+
+    label: str
+    is_self: bool
+    degree: int
+
+
+@dataclass(slots=True)
+class RouteGraph:
+    """An aggregated route graph plus churn metrics.
+
+    Attributes:
+        target: The single target analyzed, or ``None`` for a whole-mesh graph.
+        nodes: The graph's nodes.
+        links: The graph's directed links.
+        total_routes: Number of successful traces aggregated.
+        distinct_routes: Number of unique full-path signatures seen.
+        dominant_share: Fraction of traces that took the single most-common route.
+        churn_entropy: Shannon entropy (bits) of the route-choice distribution — ``0``
+            when one route is always chosen, growing as traffic spreads across more
+            equally-likely routes.
+    """
+
+    target: Optional[str]
+    nodes: list[RouteNode] = field(default_factory=list)
+    links: list[RouteLink] = field(default_factory=list)
+    total_routes: int = 0
+    distinct_routes: int = 0
+    dominant_share: float = 0.0
+    churn_entropy: float = 0.0
+
+    @property
+    def stability(self) -> float:
+        """A 0-1 stability score: ``1`` when one route always wins, lower as it flaps.
+
+        Defined as the dominant route's share, so a single stable path scores ``1.0`` and
+        an even split across many routes trends toward ``0``.
+        """
+        return self.dominant_share
+
+
+def build_route_graph(
+    traces: list[TraceResult],
+    *,
+    target: Optional[str] = None,
+    resolve: Optional[NodeResolver] = None,
+    device_label: str = LOCAL_DEVICE_LABEL,
+) -> RouteGraph:
+    """Aggregate traces into a route graph with churn metrics.
+
+    Args:
+        traces: The traces to aggregate (only successful ones contribute edges/routes).
+        target: The target these traces share, or ``None`` for a multi-target graph.
+        resolve: Optional resolver mapping a hop hash to a friendly node name.
+        device_label: Label for our own device at the path endpoints.
+
+    Returns:
+        A populated :class:`RouteGraph`. With no successful traces the graph is empty and
+        its metrics are zero.
+    """
+    successes = [t for t in traces if t.success and t.hops]
+    link_snrs: dict[tuple[str, str], list[float]] = {}
+    route_counts: dict[tuple[str, ...], int] = {}
+    node_links: dict[str, set[tuple[str, str]]] = {}
+
+    for trace in successes:
+        edges = trace.edges(device_label)
+        signature: list[str] = []
+        for edge in edges:
+            origin = _name(edge.origin, resolve)
+            destination = _name(edge.destination, resolve)
+            key = (origin, destination)
+            link_snrs.setdefault(key, []).append(edge.snr)
+            node_links.setdefault(origin, set()).add(key)
+            node_links.setdefault(destination, set()).add(key)
+            if not signature:
+                signature.append(origin)
+            signature.append(destination)
+        route_counts[tuple(signature)] = route_counts.get(tuple(signature), 0) + 1
+
+    links = [
+        RouteLink(
+            origin=origin,
+            destination=destination,
+            count=len(snrs),
+            median_snr=statistics.median(snrs),
+        )
+        for (origin, destination), snrs in link_snrs.items()
+    ]
+    links.sort(key=lambda link_item: link_item.count, reverse=True)
+
+    self_label = _name(None, resolve) or device_label
+    nodes = [
+        RouteNode(label=label, is_self=(label == self_label), degree=len(keys))
+        for label, keys in sorted(node_links.items(), key=lambda kv: len(kv[1]), reverse=True)
+    ]
+
+    total = len(successes)
+    dominant_share = (max(route_counts.values()) / total) if total else 0.0
+    return RouteGraph(
+        target=target,
+        nodes=nodes,
+        links=links,
+        total_routes=total,
+        distinct_routes=len(route_counts),
+        dominant_share=dominant_share,
+        churn_entropy=_route_entropy(route_counts, total),
+    )
+
+
+def _route_entropy(route_counts: dict[tuple[str, ...], int], total: int) -> float:
+    """Return the Shannon entropy (bits) of a route-choice distribution.
+
+    Args:
+        route_counts: Map of route signature to how often it occurred.
+        total: Total number of routes counted.
+
+    Returns:
+        The entropy in bits (``0`` for a single route).
+    """
+    if total <= 0:
+        return 0.0
+    entropy = 0.0
+    for count in route_counts.values():
+        p = count / total
+        if p > 0:
+            entropy -= p * math.log2(p)
+    return entropy
+
+
+def _name(label: Optional[str], resolve: Optional[NodeResolver]) -> str:
+    """Resolve a hop label to a friendly name, leaving our own device's label intact."""
+    if label is None:
+        resolved = resolve(None) if resolve else None
+        return resolved or LOCAL_DEVICE_LABEL
+    if resolve is None:
+        return label
+    return resolve(label) or label
diff --git a/meshtools/tools/route_map.py b/meshtools/tools/route_map.py
new file mode 100644
index 0000000..f89f6f6
--- /dev/null
+++ b/meshtools/tools/route_map.py
@@ -0,0 +1,228 @@
+"""The ``route-map`` tool: visualize how stable mesh routing is over time.
+
+Where the app's traceroute is a single snapshot, this aggregates a target's (or the whole
+mesh's) trace history into an interactive graph and churn metrics: which links are always
+used, which the mesh flaps between, and how often the single most-common route actually
+wins. It reads stored traces and can optionally take a few fresh ones first.
+"""
+
+from __future__ import annotations
+
+from typing import Any, Optional
+
+import questionary
+import typer
+from rich.panel import Panel
+from rich.table import Table
+from rich.text import Text
+
+from ..context import AppContext
+from ..services import route_stability, trace_runner
+from ..services.route_stability import RouteGraph
+from ..ui.theme import snr_style
+from ..ui.widgets import make_progress
+from ..viz.route_graph import render_route_graph
+from .base import Tool, ToolResult, register
+
+_WHOLE_MESH = "(whole mesh)"
+
+
+@register
+class RouteMapTool(Tool):
+    """Aggregate trace history into a route-stability graph and churn metrics."""
+
+    name = "route-map"
+    help = "Map how stable mesh routing is over time and render an interactive graph."
+    category = "Optimization"
+    order = 30
+
+    async def prompt_params(self, ctx: AppContext) -> Optional[dict[str, Any]]:
+        """Choose a target (or the whole mesh) and an optional fresh-sample count.
+
+        Args:
+            ctx: Shared application context.
+
+        Returns:
+            A parameter dict, or ``None`` if the user cancelled.
+        """
+        targets = ctx.repo.traced_targets()
+        choice = await questionary.select(
+            "Map routing for:",
+            choices=[_WHOLE_MESH, *targets] if targets else [_WHOLE_MESH],
+        ).ask_async()
+        if choice is None:
+            return None
+        samples = await questionary.text(
+            "Take how many fresh traces first? (0 = use stored history only)",
+            default="0",
+            validate=_is_nonneg_int,
+        ).ask_async()
+        if samples is None:
+            return None
+        params: dict[str, Any] = {"samples": int(samples)}
+        if choice != _WHOLE_MESH:
+            params["target"] = choice
+        return params
+
+    async def run(self, ctx: AppContext, params: dict[str, Any]) -> ToolResult:
+        """Optionally trace, load history, build the graph, render it, and summarize.
+
+        Args:
+            ctx: Shared application context.
+            params: optional ``target``/``samples``/``path``/``viz`` and ``_run_id``.
+
+        Returns:
+            A :class:`ToolResult` with the churn metrics and the graph path.
+        """
+        target = params.get("target")
+        run_id = params["_run_id"]
+        samples = int(params.get("samples", 0))
+        resolve = None
+
+        if samples > 0 and target:
+            device = await ctx.device()
+            contacts = await device.get_contacts()
+            resolve = trace_runner.make_node_resolver(contacts)
+            path = (
+                trace_runner.parse_trace_path(params["path"], contacts)
+                if params.get("path")
+                else None
+            )
+            with make_progress(ctx.console) as progress:
+                task = progress.add_task(f"tracing {target}", total=samples)
+                await trace_runner.run_traces(
+                    device, target, samples=samples, path=path,
+                    cooldown_s=ctx.settings.trace_cooldown_s,
+                    on_result=lambda *_: progress.advance(task),
+                    persist=lambda t: ctx.repo.record_trace(run_id, t),
+                )
+
+        traces = ctx.repo.recent_traces(target) if target else ctx.repo.all_traces()
+        if resolve is None:
+            try:
+                contacts = await (await ctx.device()).get_contacts()
+                resolve = trace_runner.make_node_resolver(contacts)
+            except Exception:  # noqa: BLE001 - the graph reads fine with raw hashes
+                resolve = None
+
+        graph = route_stability.build_route_graph(traces, target=target, resolve=resolve)
+
+        ctx.console.print(_graph_panel(graph))
+
+        artifacts: list[str] = []
+        if graph.total_routes and params.get("viz", True):
+            assert ctx.settings.output_dir is not None
+            out = render_route_graph(graph, ctx.settings.output_dir)
+            artifacts.append(str(out))
+
+        scope = target or "the mesh"
+        if not graph.total_routes:
+            message = (
+                f"[muted]no successful traces stored for[/muted] [brand]{scope}[/brand] — "
+                "run some traces first."
+            )
+        else:
+            message = (
+                f"[ok]✓[/ok] [brand]{scope}[/brand]: {graph.stability:.0%} of "
+                f"{graph.total_routes} traces took the top route "
+                f"({graph.distinct_routes} distinct)"
+            )
+
+        return ToolResult(
+            summary={
+                "target": target,
+                "total_routes": graph.total_routes,
+                "distinct_routes": graph.distinct_routes,
+                "stability": round(graph.stability, 3),
+                "churn_entropy_bits": round(graph.churn_entropy, 3),
+                "nodes": len(graph.nodes),
+                "links": len(graph.links),
+            },
+            message=message,
+            artifacts=artifacts,
+        )
+
+    def register_cli(self, app: typer.Typer) -> None:
+        """Register the ``route-map`` subcommand.
+
+        Args:
+            app: The Typer application.
+        """
+        from ..cli import run_tool_command
+
+        @app.command(name=self.name, help=self.help)
+        def _route_map(
+            target: Optional[str] = typer.Option(
+                None, "--target", "-t", help="Target to map (omit for the whole mesh)."
+            ),
+            samples: int = typer.Option(
+                0, "--samples", "-n", help="Fresh traces to take first (requires --target)."
+            ),
+            path: Optional[str] = typer.Option(
+                None, "--path", "-p", help="Force a path for the fresh traces."
+            ),
+            viz: bool = typer.Option(True, "--viz/--no-viz", help="Render the HTML graph."),
+        ) -> None:
+            tool_params: dict[str, Any] = {"samples": samples, "viz": viz}
+            if target:
+                tool_params["target"] = target
+            if path:
+                tool_params["path"] = path
+            run_tool_command(self, tool_params)
+
+
+def _graph_panel(graph: RouteGraph) -> Panel:
+    """Render churn metrics and the busiest links as a panel.
+
+    Args:
+        graph: The aggregated route graph.
+
+    Returns:
+        A Rich :class:`Panel` summarizing stability and top links.
+    """
+    scope = graph.target or "whole mesh"
+    stab_style = "ok" if graph.stability >= 0.8 else ("warn" if graph.stability >= 0.5 else "err")
+    header = Text.assemble(
+        ("scope          ", "muted"), (f"{scope}\n", "brand"),
+        ("traces         ", "muted"), (f"{graph.total_routes}\n", ""),
+        ("distinct routes ", "muted"), (f"{graph.distinct_routes}\n", ""),
+        ("stability      ", "muted"), (f"{graph.stability:.0%}", stab_style), ("\n", ""),
+        ("route entropy  ", "muted"), (f"{graph.churn_entropy:.2f} bits", ""),
+    )
+    if not graph.links:
+        return Panel(
+            header, title="[accent]route stability[/accent]", border_style="muted", expand=False
+        )
+
+    table = Table(box=None, padding=(0, 2, 0, 0), expand=False)
+    table.add_column("link")
+    table.add_column("uses", justify="right")
+    table.add_column("median SNR", justify="right")
+    for link in graph.links[:10]:
+        table.add_row(
+            f"{link.origin} → {link.destination}",
+            str(link.count),
+            Text(f"{link.median_snr:+.1f} dB", style=snr_style(link.median_snr)),
+        )
+
+    from rich.console import Group
+
+    body = Group(header, Text("\nbusiest links", style="muted"), table)
+    return Panel(
+        body, title="[accent]route stability[/accent]", border_style="accent", expand=False
+    )
+
+
+def _is_nonneg_int(value: str) -> bool | str:
+    """Validate a non-negative integer for a questionary text answer.
+
+    Args:
+        value: Raw input.
+
+    Returns:
+        ``True`` if valid, otherwise an error message string.
+    """
+    try:
+        return True if int(value) >= 0 else "Enter zero or a positive number."
+    except ValueError:
+        return "Enter a whole number."
diff --git a/meshtools/viz/route_graph.py b/meshtools/viz/route_graph.py
new file mode 100644
index 0000000..a31c9cf
--- /dev/null
+++ b/meshtools/viz/route_graph.py
@@ -0,0 +1,130 @@
+"""pyvis renderer for the route-stability graph.
+
+Draws the aggregated mesh routing as a self-contained, interactive HTML graph: nodes are
+mesh devices (our companion highlighted), directed edges are links whose width grows with
+how often the link was traversed and whose colour tracks median SNR. The title carries the
+churn metrics so the picture and the numbers travel together.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime
+from pathlib import Path
+
+from ..services.route_stability import RouteGraph
+
+_SELF_COLOR = "#4ade80"
+_NODE_COLOR = "#5eead4"
+_BG = "#0f172a"
+_FONT = "#e2e8f0"
+
+
+def render_route_graph(graph: RouteGraph, output_dir: Path) -> Path:
+    """Render a route graph to an interactive, self-contained HTML file.
+
+    Args:
+        graph: The aggregated route graph to draw.
+        output_dir: Directory the HTML file is written to (created if needed).
+
+    Returns:
+        Path to the written HTML file.
+    """
+    from pyvis.network import Network
+
+    # ``cdn_resources='in_line'`` inlines vis.js so the file works offline, matching the
+    # self-contained Plotly charts elsewhere.
+    net = Network(
+        height="720px",
+        width="100%",
+        directed=True,
+        bgcolor=_BG,
+        font_color=_FONT,
+        cdn_resources="in_line",
+    )
+
+    for node in graph.nodes:
+        color = _SELF_COLOR if node.is_self else _NODE_COLOR
+        net.add_node(
+            node.label,
+            label=node.label,
+            color=color,
+            shape="star" if node.is_self else "dot",
+            size=14 + 3 * node.degree,
+            title=f"{node.label} · {node.degree} link(s)",
+        )
+
+    max_count = max((link.count for link in graph.links), default=1)
+    for link in graph.links:
+        width = 1.0 + 6.0 * (link.count / max_count)
+        net.add_edge(
+            link.origin,
+            link.destination,
+            value=link.count,
+            width=width,
+            color=_snr_color(link.median_snr),
+            title=f"{link.origin} → {link.destination}: "
+            f"{link.count}× · median {link.median_snr:+.1f} dB",
+        )
+
+    net.set_options(_OPTIONS)
+
+    scope = graph.target or "mesh"
+    heading = (
+        f"Route stability — {scope}  ·  {graph.total_routes} traces, "
+        f"{graph.distinct_routes} route(s), {graph.stability:.0%} on the top route"
+    )
+    output_dir.mkdir(parents=True, exist_ok=True)
+    stamp = datetime.now().strftime("%Y%m%d-%H%M%S")
+    path = output_dir / f"route-map_{_slug(scope)}_{stamp}.html"
+    html = net.generate_html(notebook=False)
+    banner = f"<h2 style='color:{_FONT};font-family:sans-serif'>{heading}</h2>"
+    html = html.replace("<body>", f"<body>{banner}")
+    path.write_text(html, encoding="utf-8")
+    return path
+
+
+def _snr_color(snr: float) -> str:
+    """Map a median SNR (dB) to an edge colour from red (weak) to green (strong).
+
+    Args:
+        snr: Median SNR in dB.
+
+    Returns:
+        A hex colour string.
+    """
+    if snr >= 5:
+        return "#4ade80"  # strong
+    if snr >= 0:
+        return "#facc15"  # marginal
+    if snr >= -8:
+        return "#fb923c"  # weak
+    return "#f87171"  # very weak
+
+
+def _slug(value: str) -> str:
+    """Make a filesystem-safe slug from a label.
+
+    Args:
+        value: Arbitrary node name or scope label.
+
+    Returns:
+        A lowercase slug containing only alphanumerics, dashes, and underscores.
+    """
+    return "".join(c if c.isalnum() or c in "-_" else "-" for c in value).strip("-").lower()
+
+
+#: vis.js physics/layout tuning for a readable, settled mesh graph.
+_OPTIONS = """
+{
+  "physics": {
+    "solver": "forceAtlas2Based",
+    "forceAtlas2Based": {"gravitationalConstant": -60, "springLength": 120},
+    "stabilization": {"iterations": 200}
+  },
+  "edges": {
+    "arrows": {"to": {"enabled": true, "scaleFactor": 0.6}},
+    "smooth": {"type": "curvedCW", "roundness": 0.15}
+  },
+  "nodes": {"font": {"color": "#e2e8f0", "size": 16}}
+}
+"""
diff --git a/tests/test_route_stability.py b/tests/test_route_stability.py
new file mode 100644
index 0000000..c45d4fa
--- /dev/null
+++ b/tests/test_route_stability.py
@@ -0,0 +1,83 @@
+"""Tests for route-stability aggregation and the pyvis renderer."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from meshtools.core.models import Hop, TraceResult
+from meshtools.services import route_stability
+
+
+def _trace(*nodes: str) -> TraceResult:
+    """Build a successful trace whose forward path visits ``nodes`` then returns to us.
+
+    Each hop SNR is fixed; the final hash-less hop models the reply returning to us.
+    """
+    hops = [Hop(i, node, 5.0) for i, node in enumerate(nodes)]
+    hops.append(Hop(len(nodes), None, 5.0))
+    return TraceResult(target=nodes[-1] if nodes else "x", success=True, hops=hops)
+
+
+def test_stable_route_scores_one() -> None:
+    """A single route taken every time scores full stability and zero entropy."""
+    traces = [_trace("3d", "f2") for _ in range(6)]
+    graph = route_stability.build_route_graph(traces, target="f2")
+    assert graph.total_routes == 6
+    assert graph.distinct_routes == 1
+    assert graph.stability == 1.0
+    assert graph.churn_entropy == 0.0
+
+
+def test_flapping_routes_lower_stability_and_raise_entropy() -> None:
+    """An even split between two routes halves stability and gives 1 bit of entropy."""
+    traces = [_trace("3d", "f2") for _ in range(3)] + [_trace("aa", "f2") for _ in range(3)]
+    graph = route_stability.build_route_graph(traces, target="f2")
+    assert graph.distinct_routes == 2
+    assert graph.stability == 0.5
+    assert graph.churn_entropy == 1.0  # two equally-likely routes
+
+
+def test_links_aggregate_counts_and_self_node() -> None:
+    """Links carry traversal counts and our own device is marked is_self."""
+    traces = [_trace("3d", "f2") for _ in range(4)]
+    graph = route_stability.build_route_graph(traces, target="f2", device_label="us")
+    by_pair = {(link.origin, link.destination): link for link in graph.links}
+    assert by_pair[("us", "3d")].count == 4  # every trace starts us → 3d
+    assert by_pair[("f2", "us")].count == 4  # and returns f2 → us
+    self_nodes = [n for n in graph.nodes if n.is_self]
+    assert len(self_nodes) == 1 and self_nodes[0].label == "us"
+
+
+def test_resolver_relabels_nodes() -> None:
+    """A resolver renames hop hashes throughout the graph."""
+    traces = [_trace("3d", "f2") for _ in range(3)]
+    graph = route_stability.build_route_graph(
+        traces, target="f2", resolve=lambda h: {"3d": "Repeater", "f2": "Alice"}.get(h, h)
+    )
+    labels = {n.label for n in graph.nodes}
+    assert "Repeater" in labels
+    assert "Alice" in labels
+
+
+def test_empty_history_is_safe() -> None:
+    """Failed/empty traces yield an empty graph with zeroed metrics, no errors."""
+    graph = route_stability.build_route_graph(
+        [TraceResult(target="x", success=False)], target="x"
+    )
+    assert graph.total_routes == 0
+    assert graph.stability == 0.0
+    assert graph.links == []
+
+
+def test_render_route_graph_writes_html(tmp_path: Path) -> None:
+    """The pyvis renderer produces a self-contained HTML file."""
+    from meshtools.viz.route_graph import render_route_graph
+
+    traces = [_trace("3d", "f2") for _ in range(3)] + [_trace("aa", "f2")]
+    graph = route_stability.build_route_graph(traces, target="f2")
+    out = render_route_graph(graph, tmp_path)
+    assert out.exists()
+    assert out.suffix == ".html"
+    text = out.read_text(encoding="utf-8")
+    assert "Route stability" in text  # heading injected
+    assert out.stat().st_size > 1000  # vis.js inlined