diff --git a/docs/storybook-inventory.md b/docs/storybook-inventory.md
index 0d729eec7..dc695b5c9 100644
--- a/docs/storybook-inventory.md
+++ b/docs/storybook-inventory.md
@@ -10,7 +10,7 @@ buyer-facing control you can click before changing product CSS.
 | `AnalysisRun/CutoffKnownBody` | Read the cutoff-known sentence, then compare it with the live body below. | `--color-accent-border`, `--space-panel-block`, `--radius-panel`, `CutoffKnownBody` |
 | `Analysis/LineageEntityPicker` | Choose which corp to reconstruct, then click Request a lineage reconstruction. | `--space-control-gap`, `--size-control-min`, `--radius-control`, `LineageEntityPicker` |
 | `Admin/AdminPanel` | Change the tenant brand name, then verify the saved or failed state before leaving settings. | `--surface`, `--border`, `--space-panel-block`, `AdminPanel` |
-| `Lineage/LineageDag` | Open the current branch node; compare empty, grouped/forked, ungrouped, and long-title states before changing graph CSS. | `--surface`, `--border`, `LineageDag` |
+| `Lineage/LineageDag` | Open the current branch node; compare empty, grouped/forked, mobile-scroll, ungrouped, and long-title states before changing graph CSS. On narrow viewports, swipe the named viewport or focus it and use arrow keys to inspect the full lineage. | `--surface`, `--border`, `--color-focus-border`, `--size-control-min`, `LineageDag` |
 | `Chrome/PopupCloseButton` | Close the evidence panel or post popup. | `--space-close-inset`, `--font-size-close`, `PopupCloseButton` |
 | `Reports/LeftoverPairList` | Read residual R, observed Y, expected E, map rank, and distance after IRT main effects, then open the named post. | `--color-chip-border`, `LeftoverPairList` |
 
diff --git a/frontend/src/App.css b/frontend/src/App.css
index b88ee953a..3888c83b2 100644
--- a/frontend/src/App.css
+++ b/frontend/src/App.css
@@ -534,6 +534,15 @@
   stroke-width: 1.5;
 }
 
+/* The enlarged hit target sits under the visible mark. Author CSS beats the
+   fill="transparent" presentation attribute, so keep it invisible here or it
+   paints as a second opaque disc (WCAG 2.5.8 touch-target sizing must not
+   change what the reader sees). */
+.lineage-dag-node circle.lineage-dag-hit {
+  fill: transparent;
+  stroke: none;
+}
+
 .lineage-dag-branch circle {
   fill: var(--badge-actor-organization-bg);
   stroke: var(--color-accent-orange);
@@ -552,12 +561,14 @@
   outline: none;
 }
 
-.lineage-dag-node:focus circle,
-.lineage-dag-node:hover circle {
+.lineage-dag-node:focus circle:not(.lineage-dag-hit),
+.lineage-dag-node:hover circle:not(.lineage-dag-hit) {
   stroke-width: 2.5;
 }
 
-.lineage-dag-node[aria-current="true"] circle {
+/* :not(.lineage-dag-hit) never matches the hit target, so the current
+   ring cannot paint a hollow disc around it regardless of source order. */
+.lineage-dag-node[aria-current="true"] circle:not(.lineage-dag-hit) {
   stroke-width: 3;
   stroke: var(--text-h);
 }
diff --git a/frontend/src/LineageDag.css b/frontend/src/LineageDag.css
new file mode 100644
index 000000000..4a2b3f596
--- /dev/null
+++ b/frontend/src/LineageDag.css
@@ -0,0 +1,27 @@
+.lineage-dag-scroll-hint {
+  display: none;
+  margin: 0 0 var(--space-control-gap);
+  color: var(--text);
+  font-size: 0.8rem;
+}
+
+.lineage-dag-viewport {
+  max-width: 100%;
+  overflow-x: auto;
+  overflow-y: hidden;
+  overscroll-behavior-inline: contain;
+  scrollbar-gutter: stable;
+  -webkit-overflow-scrolling: touch;
+}
+
+.lineage-dag-viewport:focus-visible {
+  outline: 2px solid var(--color-focus-border);
+  outline-offset: 2px;
+  border-radius: var(--radius-control);
+}
+
+@media (max-width: 768px) {
+  .lineage-dag-scroll-hint {
+    display: block;
+  }
+}
diff --git a/frontend/src/LineageDag.responsive.test.tsx b/frontend/src/LineageDag.responsive.test.tsx
new file mode 100644
index 000000000..4ac272cb0
--- /dev/null
+++ b/frontend/src/LineageDag.responsive.test.tsx
@@ -0,0 +1,34 @@
+import { render, screen } from "@testing-library/react";
+import { describe, expect, it, vi } from "vitest";
+import { LineageDag } from "./LineageDag";
+import type { LineageGraph } from "./api";
+
+const wideGraph: LineageGraph = {
+  nodes: [
+    { id: "a1", group: "A-100", label: "Initial scope", occurred_at: "2026-01-01T00:00:00Z", is_root: true, is_branch_point: false },
+    { id: "a2", group: "A-100", label: "Terms follow-up", occurred_at: "2026-01-05T00:00:00Z", is_root: false, is_branch_point: false },
+    { id: "a3", group: "A-100", label: "Delivery question", occurred_at: "2026-01-06T00:00:00Z", is_root: false, is_branch_point: false },
+    { id: "a4", group: "A-100", label: "Delivery confirmed", occurred_at: "2026-01-11T00:00:00Z", is_root: false, is_branch_point: false },
+  ],
+  edges: [
+    { source: "a1", target: "a2", fused_score: 0.83 },
+    { source: "a2", target: "a3", fused_score: 0.87 },
+    { source: "a3", target: "a4", fused_score: 0.9 },
+  ],
+};
+
+describe("LineageDag responsive viewport", () => {
+  it("keeps an intrinsically wide DAG inside a named keyboard-focusable viewport", () => {
+    render(<LineageDag graph={wideGraph} onSelectPost={vi.fn()} />);
+
+    expect(screen.getByText("Swipe or use arrow keys to inspect the full lineage.")).toBeInTheDocument();
+
+    const viewport = screen.getByRole("region", { name: "A-100 lineage viewport" });
+    expect(viewport).toHaveAttribute("tabindex", "0");
+    expect(viewport).toHaveClass("lineage-dag-viewport");
+
+    const svg = screen.getByRole("img", { name: "A-100 lineage" });
+    expect(viewport).toContainElement(svg);
+    expect(Number(svg.getAttribute("width"))).toBeGreaterThan(320);
+  });
+});
diff --git a/frontend/src/LineageDag.stories.tsx b/frontend/src/LineageDag.stories.tsx
index a0f5101ac..f41fcbd1d 100644
--- a/frontend/src/LineageDag.stories.tsx
+++ b/frontend/src/LineageDag.stories.tsx
@@ -37,6 +37,36 @@ export const SingleBranch: Story = {
   },
 };
 
+// Mirrors the live Figma mobile authority: a 322px viewport contains an
+// intrinsically wider lineage canvas and gives the buyer explicit scroll help.
+export const MobileScrollable: Story = {
+  decorators: [
+    (Story) => (
+      <div style={{ width: 322 }}>
+        <Story />
+      </div>
+    ),
+  ],
+  args: {
+    graph: {
+      nodes: [
+        { id: "m1", group: "DEMO-PROJECT", label: "Initial scope", occurred_at: "2026-01-01T00:00:00Z", is_root: true, is_branch_point: false },
+        { id: "m2", group: "DEMO-PROJECT", label: "Terms follow-up", occurred_at: "2026-01-05T00:00:00Z", is_root: false, is_branch_point: true },
+        { id: "m3", group: "DEMO-PROJECT", label: "Revised quotation", occurred_at: "2026-01-09T00:00:00Z", is_root: false, is_branch_point: false },
+        { id: "m4", group: "DEMO-PROJECT", label: "Delivery question", occurred_at: "2026-01-06T00:00:00Z", is_root: false, is_branch_point: false },
+        { id: "m5", group: "DEMO-PROJECT", label: "Delivery confirmed", occurred_at: "2026-01-11T00:00:00Z", is_root: false, is_branch_point: false },
+      ],
+      edges: [
+        { source: "m1", target: "m2", fused_score: 0.83 },
+        { source: "m2", target: "m3", fused_score: 0.94 },
+        { source: "m2", target: "m4", fused_score: 0.87 },
+        { source: "m4", target: "m5", fused_score: 0.9 },
+      ],
+    } satisfies LineageGraph,
+    currentPostId: "m5",
+  },
+};
+
 // The multi-branch, git-branch-style case the Ask Agent answer view relies on:
 // several independent lineage threads rendered as separate figures, plus one
 // thread with an actual fork (a branch point with two children).
diff --git a/frontend/src/LineageDag.test.tsx b/frontend/src/LineageDag.test.tsx
index 8f1001a9a..aa3723cf7 100644
--- a/frontend/src/LineageDag.test.tsx
+++ b/frontend/src/LineageDag.test.tsx
@@ -1,9 +1,55 @@
 import { fireEvent, render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
 import { describe, expect, it, vi } from "vitest";
 import { LineageDag } from "./LineageDag";
 import type { LineageGraph } from "./api";
 
+const graph = {
+  nodes: [
+    {
+      id: "rec-001",
+      group: "A-100",
+      label: "Initial site visit and project scope discussion",
+      occurred_at: "2026-01-01T00:00:00",
+      is_root: true,
+      is_branch_point: false,
+    },
+    {
+      id: "rec-002",
+      group: "A-100",
+      label: "Pricing renegotiation follow-up",
+      occurred_at: "2026-01-06T00:00:00",
+      is_root: false,
+      is_branch_point: false,
+    },
+  ],
+  edges: [{ source: "rec-001", target: "rec-002", fused_score: 0.8 }],
+};
+
 describe("LineageDag", () => {
+  it("gives every node mark a 24x24px-minimum transparent hit target ahead of the visible mark", () => {
+    render(<LineageDag graph={graph} onSelectPost={() => undefined} />);
+    const button = screen.getByRole("button", { name: "Open post: Initial site visit and project scope discussion" });
+    const circles = button.querySelectorAll("circle");
+    expect(circles).toHaveLength(2);
+
+    // The hit circle must come first so the visible mark still paints on top of it.
+    const [hit, visible] = circles;
+    expect(hit.getAttribute("fill")).toBe("transparent");
+    expect(hit.style.pointerEvents).toBe("all");
+    // r=12 -> 24px diameter, matching --size-control-min (tokens.css) at the
+    // DAG's ~1 SVG-user-unit-per-px scale -- the WCAG 2.5.8 AA minimum.
+    expect(Number(hit.getAttribute("r"))).toBeGreaterThanOrEqual(12);
+    expect(Number(visible.getAttribute("r"))).toBeLessThan(Number(hit.getAttribute("r")));
+  });
+
+  it("still opens the post when the enlarged hit target is clicked", async () => {
+    const onSelectPost = vi.fn();
+    render(<LineageDag graph={graph} onSelectPost={onSelectPost} />);
+    await userEvent.click(screen.getByRole("button", { name: "Open post: Pricing renegotiation follow-up" }));
+    expect(onSelectPost).toHaveBeenCalledWith("rec-002");
+  });
+
   it("shows an empty-state message instead of an empty graph", () => {
     render(<LineageDag graph={{ nodes: [], edges: [] }} onSelectPost={vi.fn()} />);
     expect(screen.getByText("No reconstructed lineage yet. Rebuild after seeding posts.")).toBeInTheDocument();
diff --git a/frontend/src/LineageDag.tsx b/frontend/src/LineageDag.tsx
index f26574d8e..b707f5249 100644
--- a/frontend/src/LineageDag.tsx
+++ b/frontend/src/LineageDag.tsx
@@ -1,11 +1,17 @@
 import type { LineageGraph } from "./api";
 import { t, tf } from "./i18n";
 import { layoutLineageDag } from "./lineageLayout";
+import "./LineageDag.css";
 
 function truncateLabel(label: string): string {
   return label.length > 34 ? `${label.slice(0, 33)}…` : label;
 }
 
+// Mirrors --size-control-min (24px, styles/tokens.css). One SVG user unit is
+// ~1px here (see lineageLayout ROW_H/COL_W/PAD), so this radius gives the
+// visible 7px node mark a 24x24px minimum hit area without CSS scale-up.
+const NODE_HIT_RADIUS = 12;
+
 /** Render the authorized lineage projection and let the buyer open a post. */
 export function LineageDag({
   graph,
@@ -34,67 +40,83 @@ export function LineageDag({
                 edges: group.edges.length,
               })}
             </figcaption>
-            <svg
-              viewBox={`0 0 ${group.width} ${group.height}`}
-              width="100%"
-              height={Math.max(120, group.height)}
-              role="img"
-              aria-label={tf("{group} lineage", { group: group.heading })}
+            <p className="lineage-dag-scroll-hint">
+              {t("Swipe or use arrow keys to inspect the full lineage.")}
+            </p>
+            <div
+              className="lineage-dag-viewport"
+              role="region"
+              tabIndex={0}
+              aria-label={tf("{group} lineage viewport", { group: group.heading })}
             >
-              {group.edges.map((edge) => {
-                const from = byId[edge.source];
-                const to = byId[edge.target];
-                const midX = (from.x + to.x) / 2;
-                return (
-                  <path
-                    key={`${edge.source}-${edge.target}`}
-                    className="lineage-dag-edge"
-                    d={`M ${from.x} ${from.y} C ${midX} ${from.y}, ${midX} ${to.y}, ${to.x} ${to.y}`}
-                  >
-                    <title>
-                      {tf("{from} follows {to} ({score})", {
-                        from: from.label,
-                        to: to.label,
-                        score: edge.fused_score.toFixed(2),
-                      })}
-                    </title>
-                  </path>
-                );
-              })}
-              {group.nodes.map((node) => {
-                const kind = node.is_branch_point ? "branch" : node.is_root ? "root" : "node";
-                const isCurrent = node.id === currentPostId;
-                return (
-                  <g
-                    key={node.id}
-                    className={`lineage-dag-node lineage-dag-${kind}`}
-                    transform={`translate(${node.x}, ${node.y})`}
-                    role="button"
-                    tabIndex={0}
-                    aria-label={tf("Open post: {label}", { label: node.label })}
-                    aria-current={isCurrent ? "true" : undefined}
-                    onClick={() => onSelectPost(node.id)}
-                    onKeyDown={(event) => {
-                      if (event.key === "Enter" || event.key === " ") {
-                        event.preventDefault();
-                        onSelectPost(node.id);
-                      }
-                    }}
-                  >
-                    <circle r={7} />
-                    <text x={12} y={4}>
-                      {truncateLabel(node.label)}
-                    </text>
-                    <title>
-                      {tf("{label} — {date}", {
-                        label: node.label,
-                        date: node.occurred_at.slice(0, 10),
-                      })}
-                    </title>
-                  </g>
-                );
-              })}
-            </svg>
+              <svg
+                viewBox={`0 0 ${group.width} ${group.height}`}
+                width={group.width}
+                height={Math.max(120, group.height)}
+                role="img"
+                aria-label={tf("{group} lineage", { group: group.heading })}
+              >
+                {group.edges.map((edge) => {
+                  const from = byId[edge.source];
+                  const to = byId[edge.target];
+                  const midX = (from.x + to.x) / 2;
+                  return (
+                    <path
+                      key={`${edge.source}-${edge.target}`}
+                      className="lineage-dag-edge"
+                      d={`M ${from.x} ${from.y} C ${midX} ${from.y}, ${midX} ${to.y}, ${to.x} ${to.y}`}
+                    >
+                      <title>
+                        {tf("{from} follows {to} ({score})", {
+                          from: from.label,
+                          to: to.label,
+                          score: edge.fused_score.toFixed(2),
+                        })}
+                      </title>
+                    </path>
+                  );
+                })}
+                {group.nodes.map((node) => {
+                  const kind = node.is_branch_point ? "branch" : node.is_root ? "root" : "node";
+                  const isCurrent = node.id === currentPostId;
+                  return (
+                    <g
+                      key={node.id}
+                      className={`lineage-dag-node lineage-dag-${kind}`}
+                      transform={`translate(${node.x}, ${node.y})`}
+                      role="button"
+                      tabIndex={0}
+                      aria-label={tf("Open post: {label}", { label: node.label })}
+                      aria-current={isCurrent ? "true" : undefined}
+                      onClick={() => onSelectPost(node.id)}
+                      onKeyDown={(event) => {
+                        if (event.key === "Enter" || event.key === " ") {
+                          event.preventDefault();
+                          onSelectPost(node.id);
+                        }
+                      }}
+                    >
+                      <circle
+                        className="lineage-dag-hit"
+                        r={NODE_HIT_RADIUS}
+                        fill="transparent"
+                        style={{ pointerEvents: "all" }}
+                      />
+                      <circle r={7} />
+                      <text x={12} y={4}>
+                        {truncateLabel(node.label)}
+                      </text>
+                      <title>
+                        {tf("{label} — {date}", {
+                          label: node.label,
+                          date: node.occurred_at.slice(0, 10),
+                        })}
+                      </title>
+                    </g>
+                  );
+                })}
+              </svg>
+            </div>
           </figure>
         );
       })}