diff --git a/apps/web/app/editor/[projectId]/page.tsx b/apps/web/app/editor/[projectId]/page.tsx
index defcc94..1f1511d 100644
--- a/apps/web/app/editor/[projectId]/page.tsx
+++ b/apps/web/app/editor/[projectId]/page.tsx
@@ -1,8 +1,16 @@
+import { Suspense } from "react";
 import { EditorShell } from "../../../components/editor/EditorShell";
 import { aristotleFixture } from "../../../lib/fixtures/aristotle";
 
 // M1: every projectId resolves to the Aristotle fixture.
 // M4–M5 wire this to a real database lookup.
+//
+// Suspense boundary required because EditorShell uses useSearchParams (for
+// the M4 ?break=... demo of validation rules).
 export default async function EditorPage(_props: { params: Promise<{ projectId: string }> }) {
-  return <EditorShell data={aristotleFixture} />;
+  return (
+    <Suspense fallback={null}>
+      <EditorShell data={aristotleFixture} />
+    </Suspense>
+  );
 }
diff --git a/apps/web/components/diagram-canvas/DiagramCanvas.tsx b/apps/web/components/diagram-canvas/DiagramCanvas.tsx
index 4de96fe..ba37f62 100644
--- a/apps/web/components/diagram-canvas/DiagramCanvas.tsx
+++ b/apps/web/components/diagram-canvas/DiagramCanvas.tsx
@@ -35,6 +35,7 @@ import { NodeInspector } from "./NodeInspector";
 import { fixtureToFlow } from "./fixtureToFlow";
 import type { Density } from "../socrates/SocratesDock";
 import type { FixtureData, AssociationKind, BlockKind } from "../../lib/fixtures/aristotle";
+import type { ValidationIssue } from "../../lib/sysml/validate";
 
 export type DiagramVariant = "softened" | "formal" | "graph";
 
@@ -44,6 +45,7 @@ interface DiagramCanvasProps {
   variant?: DiagramVariant;
   focusBlockId: string | null;
   onSelect?: (id: string | null) => void;
+  issuesByElement?: Map<string, ValidationIssue[]>;
 }
 
 const nodeTypes = { sysmlBlock: BlockNode };
@@ -54,6 +56,13 @@ let nextEdgeId = 1000;
 function freshNodeId(): string { return `n${nextNodeId++}`; }
 function freshEdgeId(): string { return `e${nextEdgeId++}`; }
 
+function pickWorst(issues: ValidationIssue[]): "error" | "warning" | "soft" | undefined {
+  if (issues.some(i => i.severity === "error")) return "error";
+  if (issues.some(i => i.severity === "warning")) return "warning";
+  if (issues.some(i => i.severity === "soft")) return "soft";
+  return undefined;
+}
+
 export function DiagramCanvas(props: DiagramCanvasProps) {
   return (
     <ReactFlowProvider>
@@ -62,7 +71,7 @@ export function DiagramCanvas(props: DiagramCanvasProps) {
   );
 }
 
-function DiagramInner({ data, focusBlockId, onSelect }: DiagramCanvasProps) {
+function DiagramInner({ data, focusBlockId, onSelect, issuesByElement }: DiagramCanvasProps) {
   const initial = useMemo(() => fixtureToFlow(data), [data]);
   const [nodes, setNodes] = useState<Node<BlockNodeData>[]>(initial.nodes);
   const [edges, setEdges] = useState<Edge<SysmlEdgeData>[]>(initial.edges);
@@ -90,14 +99,20 @@ function DiagramInner({ data, focusBlockId, onSelect }: DiagramCanvasProps) {
   }, []);
 
   // Highlight focusBlockId from sibling components (e.g. narrative chip hover).
+  // Also surface validation severity onto each node so BlockNode can ring it.
   useEffect(() => {
     setNodes(ns =>
-      ns.map(n => ({
-        ...n,
-        selected: n.id === focusBlockId,
-      }))
+      ns.map(n => {
+        const issues = issuesByElement?.get(n.id) ?? [];
+        const severity = pickWorst(issues);
+        return {
+          ...n,
+          selected: n.id === focusBlockId,
+          data: { ...(n.data as BlockNodeData), issueSeverity: severity },
+        };
+      })
     );
-  }, [focusBlockId]);
+  }, [focusBlockId, issuesByElement]);
 
   const onNodeClick: NodeMouseHandler = useCallback(
     (_event, node) => {
diff --git a/apps/web/components/diagram-canvas/nodes/BlockNode.tsx b/apps/web/components/diagram-canvas/nodes/BlockNode.tsx
index b704583..7e21960 100644
--- a/apps/web/components/diagram-canvas/nodes/BlockNode.tsx
+++ b/apps/web/components/diagram-canvas/nodes/BlockNode.tsx
@@ -12,6 +12,8 @@ export interface BlockNodeData extends Record<string, unknown> {
   properties: string[];
   /** For constraint kinds, optional one-line expression. */
   expression?: string;
+  /** Highest severity of any validation issue anchored to this block. */
+  issueSeverity?: "error" | "warning" | "soft";
 }
 
 const STEREO: Record<string, string> = {
@@ -30,6 +32,7 @@ export function BlockNode({ data, selected }: NodeProps) {
     "sysml-node",
     `sysml-node-${kind}`,
     selected ? "sysml-node-selected" : "",
+    d.issueSeverity ? `sysml-node-issue-${d.issueSeverity}` : "",
   ]
     .filter(Boolean)
     .join(" ");
diff --git a/apps/web/components/editor/EditorShell.tsx b/apps/web/components/editor/EditorShell.tsx
index 48a2da8..4b872ab 100644
--- a/apps/web/components/editor/EditorShell.tsx
+++ b/apps/web/components/editor/EditorShell.tsx
@@ -1,19 +1,25 @@
 // The dual-canvas workspace shell.
 // Composes TopBar / SocratesDock / LeftRail / TextCanvas / DiagramCanvas / StatusBar.
-// Ported from docs/design-source/socrata/project/editor-shell.jsx (EditorShell).
+// M4 adds: SysML model derivation + validation, IssuesPanel, optional ?break=...
+// query param to demonstrate the validator surfacing rule violations.
 
 "use client";
 
-import { useState } from "react";
+import { useMemo, useState } from "react";
+import { useSearchParams } from "next/navigation";
 import { TopBar } from "./TopBar";
 import { LeftRail } from "./LeftRail";
 import { CanvasHeader } from "./CanvasHeader";
 import { StatusBar } from "./StatusBar";
+import { IssuesPanel } from "./IssuesPanel";
 import { TextCanvas } from "../text-canvas/TextCanvas";
 import { DiagramCanvas, type DiagramVariant } from "../diagram-canvas/DiagramCanvas";
 import { SocratesDock, type Density, type SocratesPresence } from "../socrates/SocratesDock";
 import type { MarkupStyle } from "../text-canvas/Chip";
 import type { FixtureData } from "../../lib/fixtures/aristotle";
+import { fromFixture } from "../../lib/sysml/fromFixture";
+import { validate, type ValidationIssue } from "../../lib/sysml/validate";
+import { applyBreaks, BREAKS, type BreakName } from "../../lib/sysml/breaks";
 
 interface EditorShellProps {
   data: FixtureData;
@@ -31,6 +37,33 @@ export function EditorShell({
   presence = "default",
 }: EditorShellProps) {
   const [focusBlockId, setFocusBlockId] = useState<string | null>(null);
+  const searchParams = useSearchParams();
+
+  // Parse `?break=S1-dangling,M2-cycle&...` into a list of named corruptions.
+  const breaks = useMemo<BreakName[]>(() => {
+    const raw = searchParams?.get("break") ?? "";
+    if (!raw) return [];
+    return raw
+      .split(",")
+      .map(s => s.trim())
+      .filter((s): s is BreakName => s in BREAKS);
+  }, [searchParams]);
+
+  // Derive the SysML model from the fixture, optionally apply breaks, validate.
+  const { issues, issuesByElement } = useMemo(() => {
+    const baseModel = fromFixture(data);
+    const broken = breaks.length > 0 ? applyBreaks(baseModel, breaks) : baseModel;
+    const issues = validate(broken);
+    const issuesByElement = new Map<string, ValidationIssue[]>();
+    for (const i of issues) {
+      const key = anchorKey(i.anchor);
+      if (!key) continue;
+      const prev = issuesByElement.get(key) ?? [];
+      prev.push(i);
+      issuesByElement.set(key, prev);
+    }
+    return { issues, issuesByElement };
+  }, [data, breaks]);
 
   return (
     <div className={`shell shell-density-${density} shell-presence-${presence}`}>
@@ -38,7 +71,12 @@ export function EditorShell({
 
       <div className="shell-body">
         <SocratesDock thread={data.socratesThread} presence={presence} density={density} />
-        <LeftRail data={data} focusBlockId={focusBlockId} setFocusBlockId={setFocusBlockId} />
+        <LeftRail
+          data={data}
+          focusBlockId={focusBlockId}
+          setFocusBlockId={setFocusBlockId}
+          issuesByElement={issuesByElement}
+        />
 
         <main className="canvases">
           <section className="canvas canvas-text">
@@ -59,7 +97,9 @@ export function EditorShell({
           <section className="canvas canvas-diagram">
             <CanvasHeader
               title="Model"
-              subtitle="SysML · 6 blocks · 6 associations · 1 constraint"
+              subtitle={breaks.length > 0
+                ? `SysML · breaks active: ${breaks.join(", ")}`
+                : "SysML · 6 blocks · 6 associations · 1 constraint"}
               right={
                 <div className="canvas-actions">
                   <span className="canvas-mode-pill">Fit</span>
@@ -75,6 +115,7 @@ export function EditorShell({
                 variant={diagramStyle}
                 focusBlockId={focusBlockId}
                 onSelect={setFocusBlockId}
+                issuesByElement={issuesByElement}
               />
             </div>
           </section>
@@ -82,6 +123,22 @@ export function EditorShell({
       </div>
 
       <StatusBar data={data} />
+
+      <IssuesPanel issues={issues} onSelectAnchor={id => setFocusBlockId(id)} />
     </div>
   );
 }
+
+/** Map an issue's anchor to a single string key matching either a block id or
+ * a synthetic key (`assoc:a1`, `req:req-001`, `constraint:ferpa`). The UI uses
+ * block ids most often, so block anchors return the bare id. */
+function anchorKey(anchor: ValidationIssue["anchor"]): string | null {
+  switch (anchor.kind) {
+    case "block": return anchor.id;
+    case "association": return `assoc:${anchor.id}`;
+    case "constraint": return `constraint:${anchor.id}`;
+    case "requirement": return `req:${anchor.id}`;
+    case "property": return anchor.blockId;
+    case "model": return null;
+  }
+}
diff --git a/apps/web/components/editor/IssuesPanel.tsx b/apps/web/components/editor/IssuesPanel.tsx
new file mode 100644
index 0000000..bc2fbec
--- /dev/null
+++ b/apps/web/components/editor/IssuesPanel.tsx
@@ -0,0 +1,73 @@
+// Floating panel listing current validation issues — shown next to the
+// status bar. Click an issue to focus its anchored element across the rail
+// + diagram (via setFocusBlockId). Lets the user verify the validator is
+// actually firing on the broken-fixture demos.
+
+"use client";
+
+import { useState } from "react";
+import type { ValidationIssue } from "../../lib/sysml/validate";
+
+interface IssuesPanelProps {
+  issues: ValidationIssue[];
+  onSelectAnchor?: (id: string) => void;
+}
+
+const SEV_GLYPH: Record<ValidationIssue["severity"], string> = {
+  error: "●",
+  warning: "▲",
+  soft: "·",
+};
+
+export function IssuesPanel({ issues, onSelectAnchor }: IssuesPanelProps) {
+  const [collapsed, setCollapsed] = useState(false);
+
+  const grouped = {
+    error: issues.filter(i => i.severity === "error"),
+    warning: issues.filter(i => i.severity === "warning"),
+    soft: issues.filter(i => i.severity === "soft"),
+  };
+
+  if (issues.length === 0) {
+    return (
+      <div className={`issues-panel issues-panel-clean ${collapsed ? "issues-panel-collapsed" : ""}`}>
+        <button className="issues-panel-head" onClick={() => setCollapsed(c => !c)} type="button">
+          <span className="issues-panel-clean-glyph">✓</span>
+          <span className="issues-panel-title">Model is clean</span>
+        </button>
+      </div>
+    );
+  }
+
+  return (
+    <div className={`issues-panel ${collapsed ? "issues-panel-collapsed" : ""}`}>
+      <button className="issues-panel-head" onClick={() => setCollapsed(c => !c)} type="button">
+        <span className="issues-panel-counts">
+          {grouped.error.length > 0 && <span className="issues-count issues-count-error">● {grouped.error.length}</span>}
+          {grouped.warning.length > 0 && <span className="issues-count issues-count-warning">▲ {grouped.warning.length}</span>}
+          {grouped.soft.length > 0 && <span className="issues-count issues-count-soft">· {grouped.soft.length}</span>}
+        </span>
+        <span className="issues-panel-title">{issues.length} validation issue{issues.length === 1 ? "" : "s"}</span>
+        <span className="issues-panel-caret">{collapsed ? "▴" : "▾"}</span>
+      </button>
+
+      {!collapsed && (
+        <ul className="issues-panel-list">
+          {(["error", "warning", "soft"] as const).flatMap(sev =>
+            grouped[sev].map((i, idx) => (
+              <li
+                key={`${sev}-${idx}`}
+                className={`issues-item issues-item-${i.severity}`}
+                onClick={() => onSelectAnchor?.(i.anchor.kind === "property" ? i.anchor.blockId : (i.anchor as { id: string }).id ?? "")}
+              >
+                <span className="issues-item-sev">{SEV_GLYPH[i.severity]}</span>
+                <span className="issues-item-code">{i.code}</span>
+                <span className="issues-item-msg">{i.message}</span>
+              </li>
+            ))
+          )}
+        </ul>
+      )}
+    </div>
+  );
+}
diff --git a/apps/web/components/editor/LeftRail.tsx b/apps/web/components/editor/LeftRail.tsx
index c76d741..b905c50 100644
--- a/apps/web/components/editor/LeftRail.tsx
+++ b/apps/web/components/editor/LeftRail.tsx
@@ -6,14 +6,30 @@
 
 import { useState } from "react";
 import type { FixtureData } from "../../lib/fixtures/aristotle";
+import type { ValidationIssue, Severity } from "../../lib/sysml/validate";
 
 interface LeftRailProps {
   data: FixtureData;
   focusBlockId: string | null;
   setFocusBlockId: (id: string | null) => void;
+  /** Map keyed by element key (block id, `req:<id>`, `assoc:<id>`, …) → issues. */
+  issuesByElement?: Map<string, ValidationIssue[]>;
 }
 
-export function LeftRail({ data, focusBlockId, setFocusBlockId }: LeftRailProps) {
+function maxSeverityForKey(map: Map<string, ValidationIssue[]> | undefined, key: string): Severity | null {
+  const items = map?.get(key);
+  if (!items || items.length === 0) return null;
+  if (items.some(i => i.severity === "error")) return "error";
+  if (items.some(i => i.severity === "warning")) return "warning";
+  return "soft";
+}
+
+function IssueDot({ severity, title }: { severity: Severity | null; title?: string }) {
+  if (!severity) return null;
+  return <span className={`rail-issue-dot rail-issue-dot-${severity}`} title={title} />;
+}
+
+export function LeftRail({ data, focusBlockId, setFocusBlockId, issuesByElement }: LeftRailProps) {
   const [collapsed, setCollapsed] = useState(false);
   const [open, setOpen] = useState({ outline: true, model: true, requirements: true });
   const toggle = (k: keyof typeof open) => setOpen(s => ({ ...s, [k]: !s[k] }));
@@ -80,26 +96,31 @@ export function LeftRail({ data, focusBlockId, setFocusBlockId }: LeftRailProps)
         </button>
         {open.model && (
           <ul className="rail-list rail-blocks">
-            {data.blocks.map(b => (
-              <li
-                key={b.id}
-                className={`rail-block rail-${b.kind} ${focusBlockId === b.id ? "rail-block-active" : ""}`}
-                onMouseEnter={() => setFocusBlockId(b.id)}
-                onMouseLeave={() => setFocusBlockId(null)}
-              >
-                <span className="rail-block-glyph">
-                  {b.kind === "constraint" ? "{}" : b.kind === "actor" ? "◐" : "▢"}
-                </span>
-                <span className="rail-block-label">{b.label}</span>
-                <span
-                  className="rail-block-count"
-                  title={`${b.properties.length} ${b.properties.length === 1 ? "property" : "properties"}`}
+            {data.blocks.map(b => {
+              const sev = maxSeverityForKey(issuesByElement, b.id);
+              const tooltip = issuesByElement?.get(b.id)?.map(i => `${i.code}: ${i.message}`).join("\n");
+              return (
+                <li
+                  key={b.id}
+                  className={`rail-block rail-${b.kind} ${focusBlockId === b.id ? "rail-block-active" : ""}`}
+                  onMouseEnter={() => setFocusBlockId(b.id)}
+                  onMouseLeave={() => setFocusBlockId(null)}
                 >
-                  <span className="rail-block-count-glyph">·</span>
-                  {b.properties.length}
-                </span>
-              </li>
-            ))}
+                  <span className="rail-block-glyph">
+                    {b.kind === "constraint" ? "{}" : b.kind === "actor" ? "◐" : "▢"}
+                  </span>
+                  <span className="rail-block-label">{b.label}</span>
+                  <IssueDot severity={sev} title={tooltip} />
+                  <span
+                    className="rail-block-count"
+                    title={`${b.properties.length} ${b.properties.length === 1 ? "property" : "properties"}`}
+                  >
+                    <span className="rail-block-count-glyph">·</span>
+                    {b.properties.length}
+                  </span>
+                </li>
+              );
+            })}
           </ul>
         )}
       </div>
@@ -111,18 +132,22 @@ export function LeftRail({ data, focusBlockId, setFocusBlockId }: LeftRailProps)
         </button>
         {open.requirements && (
           <ul className="rail-list">
-            <li className="rail-req">
-              <span className="req-tag">REQ-001</span>
-              <span className="req-status req-traced" />
-            </li>
-            <li className="rail-req">
-              <span className="req-tag">REQ-002</span>
-              <span className="req-status req-traced" />
-            </li>
-            <li className="rail-req">
-              <span className="req-tag">REQ-003</span>
-              <span className="req-status req-untraced" />
-            </li>
+            {(["req-001", "req-002", "req-003"] as const).map(reqId => {
+              const tag = reqId.toUpperCase().replace("-", "-");
+              const sev = maxSeverityForKey(issuesByElement, `req:${reqId}`);
+              const tooltip = issuesByElement?.get(`req:${reqId}`)?.map(i => `${i.code}: ${i.message}`).join("\n");
+              const isUntraced = sev === "warning"; // T1 is the warning we surface here
+              return (
+                <li key={reqId} className="rail-req">
+                  <span className="req-tag">{tag.toUpperCase()}</span>
+                  {sev ? (
+                    <IssueDot severity={sev} title={tooltip} />
+                  ) : (
+                    <span className={`req-status ${isUntraced ? "req-untraced" : "req-traced"}`} />
+                  )}
+                </li>
+              );
+            })}
           </ul>
         )}
       </div>
diff --git a/apps/web/components/text-canvas/ChipView.tsx b/apps/web/components/text-canvas/ChipView.tsx
index 7d343d3..4ec09b8 100644
--- a/apps/web/components/text-canvas/ChipView.tsx
+++ b/apps/web/components/text-canvas/ChipView.tsx
@@ -30,7 +30,8 @@ export function ChipView({ node, selected, editor }: NodeViewProps) {
   const label = (node.attrs.label as string) ?? "untitled";
   const refId = (node.attrs.refId as string | null) ?? null;
   // Read the current markup style from the editor's storage; defaults to "color".
-  const markupStyle = ((editor.storage as Record<string, unknown>).markupStyle as MarkupStyle) ?? "color";
+  const markupStyle =
+    ((editor.storage as unknown as Record<string, unknown>).markupStyle as MarkupStyle | undefined) ?? "color";
 
   const { focusBlockId, setFocusBlockId } = useChipFocus();
   const isFocused = (refId !== null && refId === focusBlockId) || selected;
diff --git a/apps/web/components/text-canvas/TextCanvas.tsx b/apps/web/components/text-canvas/TextCanvas.tsx
index 47c242b..68e2642 100644
--- a/apps/web/components/text-canvas/TextCanvas.tsx
+++ b/apps/web/components/text-canvas/TextCanvas.tsx
@@ -62,7 +62,7 @@ export function TextCanvas({ data, density, markupStyle, focusBlockId, setFocusB
   // Push the markup style into the editor so the ChipView NodeView can read it.
   useEffect(() => {
     if (!editor) return;
-    (editor.storage as Record<string, unknown>).markupStyle = markupStyle;
+    (editor.storage as unknown as Record<string, unknown>).markupStyle = markupStyle;
     // Force a re-render of all chip node views so they pick up the new style.
     editor.view.dispatch(editor.state.tr.setMeta("force-update", true));
   }, [editor, markupStyle]);
diff --git a/apps/web/lib/sysml/breaks.ts b/apps/web/lib/sysml/breaks.ts
new file mode 100644
index 0000000..b626cac
--- /dev/null
+++ b/apps/web/lib/sysml/breaks.ts
@@ -0,0 +1,147 @@
+// Known model corruptions for demonstrating the validator end-to-end.
+//
+// Each "break" returns a model variant that should fire one specific rule
+// (or a small cluster) without firing others. Used by the editor's `?break=...`
+// query param to make the validator's behavior visible.
+
+import type { SysMLModel } from "./model";
+
+export type BreakName =
+  | "S1-dangling"
+  | "S2-dangling-constraint"
+  | "S3-untraced-satisfier"
+  | "M2-cycle"
+  | "M3-multi-system"
+  | "M5-dup-property"
+  | "T1-untraced-req"
+  | "T2-unused-block";
+
+export const BREAKS: Record<BreakName, { label: string; description: string; apply: (m: SysMLModel) => SysMLModel }> = {
+  "S1-dangling": {
+    label: "S1 — dangling association endpoint",
+    description: "Add an association whose target block doesn't exist.",
+    apply(m) {
+      return {
+        ...m,
+        associations: [
+          ...m.associations,
+          { id: "a-bad", fromBlockId: "aristotle", toBlockId: "ghost-block", label: "haunts", kind: "association" },
+        ],
+      };
+    },
+  },
+
+  "S2-dangling-constraint": {
+    label: "S2 — constraint applies to unknown block",
+    description: "Add a constraint pointing at a block that doesn't exist.",
+    apply(m) {
+      return {
+        ...m,
+        constraints: [
+          ...m.constraints,
+          { id: "c-bad", label: "Phantom rule", expression: "{}", appliesTo: ["ghost-block"] },
+        ],
+      };
+    },
+  },
+
+  "S3-untraced-satisfier": {
+    label: "S3 — requirement satisfied by unknown block",
+    description: "Reroute REQ-001 to a non-existent block.",
+    apply(m) {
+      return {
+        ...m,
+        requirements: m.requirements.map(r =>
+          r.tag === "REQ-001" ? { ...r, relations: [{ kind: "satisfy" as const, blockId: "ghost-block" }] } : r
+        ),
+      };
+    },
+  },
+
+  "M2-cycle": {
+    label: "M2 — cyclic composition",
+    description: "Add a composition Assignment → Course (the inverse of the existing Course → Assignment).",
+    apply(m) {
+      return {
+        ...m,
+        associations: [
+          ...m.associations,
+          {
+            id: "a-cycle",
+            fromBlockId: "assignment",
+            toBlockId: "course",
+            label: "wraps",
+            kind: "composition",
+          },
+        ],
+      };
+    },
+  },
+
+  "M3-multi-system": {
+    label: "M3 — two System-of-Interest blocks",
+    description: "Mark both Aristotle and Course as kind=system.",
+    apply(m) {
+      return {
+        ...m,
+        blocks: m.blocks.map(b =>
+          b.id === "aristotle" || b.id === "course" ? { ...b, kind: "system" as const, stereotypes: ["system"] } : b
+        ),
+      };
+    },
+  },
+
+  "M5-dup-property": {
+    label: "M5 — duplicate property name in a block",
+    description: "Add a second `interaction_style` property to Aristotle.",
+    apply(m) {
+      return {
+        ...m,
+        blocks: m.blocks.map(b =>
+          b.id === "aristotle"
+            ? {
+                ...b,
+                properties: [
+                  ...b.properties,
+                  { id: "p-dup", name: "interaction_style", type: { kind: "string" as const }, multiplicity: "0..1" as const },
+                ],
+              }
+            : b
+        ),
+      };
+    },
+  },
+
+  "T1-untraced-req": {
+    label: "T1 — requirement with no satisfier",
+    description: "Already present in the fixture (REQ-003 has no satisfy relation).",
+    apply(m) {
+      // No mutation needed; the fixture already triggers T1 on REQ-003.
+      return m;
+    },
+  },
+
+  "T2-unused-block": {
+    label: "T2 — block with no incoming references",
+    description: "Add a Tutor block that nothing connects to.",
+    apply(m) {
+      return {
+        ...m,
+        blocks: [
+          ...m.blocks,
+          {
+            id: "tutor-orphan",
+            label: "TutorOrphan",
+            kind: "block",
+            stereotypes: ["block"],
+            properties: [],
+          },
+        ],
+      };
+    },
+  },
+};
+
+export function applyBreaks(model: SysMLModel, names: BreakName[]): SysMLModel {
+  return names.reduce((m, name) => BREAKS[name].apply(m), model);
+}
diff --git a/apps/web/lib/sysml/depgraph.ts b/apps/web/lib/sysml/depgraph.ts
new file mode 100644
index 0000000..1365c39
--- /dev/null
+++ b/apps/web/lib/sysml/depgraph.ts
@@ -0,0 +1,150 @@
+// Dependency graph over a SysML model.
+//
+// Nodes: every Block, Property, Constraint, Requirement.
+// Edges encode the structural and traceability relationships from
+// docs/sysml-modeling.md §8.
+//
+// Used by the UI for cross-surface highlighting now (M4) and by Socrates'
+// impact analysis later (M7).
+
+import type { SysMLModel } from "./model";
+
+export type DepNodeKind = "block" | "property" | "constraint" | "requirement";
+
+export interface DepNode {
+  id: string;          // unique within graph; for properties: `${blockId}.${propertyId}`
+  kind: DepNodeKind;
+  label: string;
+}
+
+export type DepEdgeKind =
+  | "contains"          // block → property
+  | "relates"           // block → block (per association.kind)
+  | "applies-to"        // constraint → block
+  | "satisfies"         // requirement → block
+  | "derives-from";     // requirement → requirement
+
+export interface DepEdge {
+  from: string;
+  to: string;
+  kind: DepEdgeKind;
+  /** For "relates" edges, the underlying association kind. */
+  associationKind?: string;
+}
+
+export interface DepGraph {
+  nodes: Map<string, DepNode>;
+  /** Adjacency list, indexed by `from`. */
+  out: Map<string, DepEdge[]>;
+  /** Reverse adjacency, indexed by `to`. Useful for "what depends on this?". */
+  in: Map<string, DepEdge[]>;
+}
+
+export function buildDepGraph(model: SysMLModel): DepGraph {
+  const nodes = new Map<string, DepNode>();
+  const out = new Map<string, DepEdge[]>();
+  const inEdges = new Map<string, DepEdge[]>();
+
+  function addNode(node: DepNode) {
+    nodes.set(node.id, node);
+    if (!out.has(node.id)) out.set(node.id, []);
+    if (!inEdges.has(node.id)) inEdges.set(node.id, []);
+  }
+
+  function addEdge(edge: DepEdge) {
+    const o = out.get(edge.from);
+    if (o) o.push(edge);
+    else out.set(edge.from, [edge]);
+    const i = inEdges.get(edge.to);
+    if (i) i.push(edge);
+    else inEdges.set(edge.to, [edge]);
+  }
+
+  for (const b of model.blocks) {
+    addNode({ id: b.id, kind: "block", label: b.label });
+    for (const p of b.properties) {
+      const pid = `${b.id}.${p.id}`;
+      addNode({ id: pid, kind: "property", label: p.name });
+      addEdge({ from: b.id, to: pid, kind: "contains" });
+    }
+  }
+
+  for (const a of model.associations) {
+    addEdge({
+      from: a.fromBlockId,
+      to: a.toBlockId,
+      kind: "relates",
+      associationKind: a.kind,
+    });
+  }
+
+  for (const c of model.constraints) {
+    addNode({ id: c.id, kind: "constraint", label: c.label });
+    for (const target of c.appliesTo) {
+      addEdge({ from: c.id, to: target, kind: "applies-to" });
+    }
+  }
+
+  for (const r of model.requirements) {
+    addNode({ id: r.id, kind: "requirement", label: r.tag });
+    for (const rel of r.relations) {
+      if (rel.kind === "satisfy") {
+        addEdge({ from: r.id, to: rel.blockId, kind: "satisfies" });
+      } else if (rel.kind === "derive") {
+        addEdge({ from: r.id, to: rel.fromReqId, kind: "derives-from" });
+      }
+    }
+  }
+
+  return { nodes, out, in: inEdges };
+}
+
+/**
+ * BFS the dep graph downstream from `seedId`, returning the closure (excluding
+ * the seed itself by default). Use for "if I change X, what's affected?".
+ */
+export function dependentsOf(graph: DepGraph, seedId: string): Set<string> {
+  const visited = new Set<string>();
+  const queue: string[] = [seedId];
+  while (queue.length) {
+    const cur = queue.shift()!;
+    const out = graph.out.get(cur) ?? [];
+    for (const edge of out) {
+      if (!visited.has(edge.to)) {
+        visited.add(edge.to);
+        queue.push(edge.to);
+      }
+    }
+    // Reverse direction too — things that depend ON this element should also
+    // appear in the closure (an Assumption linked to this block, etc., once
+    // those are added in M8).
+    const inEdges = graph.in.get(cur) ?? [];
+    for (const edge of inEdges) {
+      if (!visited.has(edge.from)) {
+        visited.add(edge.from);
+        queue.push(edge.from);
+      }
+    }
+  }
+  visited.delete(seedId);
+  return visited;
+}
+
+export function neighborhood(graph: DepGraph, seedId: string, radius = 2): Set<string> {
+  const visited = new Set<string>([seedId]);
+  let frontier = new Set<string>([seedId]);
+  for (let i = 0; i < radius; i++) {
+    const next = new Set<string>();
+    for (const id of frontier) {
+      for (const e of graph.out.get(id) ?? []) {
+        if (!visited.has(e.to)) { visited.add(e.to); next.add(e.to); }
+      }
+      for (const e of graph.in.get(id) ?? []) {
+        if (!visited.has(e.from)) { visited.add(e.from); next.add(e.from); }
+      }
+    }
+    frontier = next;
+  }
+  visited.delete(seedId);
+  return visited;
+}
diff --git a/apps/web/lib/sysml/fromFixture.ts b/apps/web/lib/sysml/fromFixture.ts
new file mode 100644
index 0000000..9bcd80b
--- /dev/null
+++ b/apps/web/lib/sysml/fromFixture.ts
@@ -0,0 +1,88 @@
+// Convert the loose Aristotle FixtureData (M1 prototype port) into a strict
+// SysMLModel that the validator and dep-graph can operate on.
+
+import type {
+  SysMLModel,
+  Block,
+  Association,
+  Constraint,
+  Requirement,
+  AssociationKind,
+} from "./model";
+import type { FixtureData } from "../fixtures/aristotle";
+
+export function fromFixture(data: FixtureData): SysMLModel {
+  const constraintBlocks = data.blocks.filter(b => b.kind === "constraint");
+  const constraints: Constraint[] = constraintBlocks.map(b => ({
+    id: b.id,
+    label: b.label,
+    expression: "{ tenancy = institutional }",
+    // The fixture infers "what does this constraint apply to?" from its
+    // outgoing/incoming `kind: "constraint"` associations.
+    appliesTo: data.associations
+      .filter(a => a.kind === "constraint" && (a.from === b.id || a.to === b.id))
+      .map(a => (a.from === b.id ? a.to : a.from)),
+  }));
+
+  const blocks: Block[] = data.blocks
+    .filter(b => b.kind !== "constraint")
+    .map(b => ({
+      id: b.id,
+      label: b.label,
+      kind: b.kind,
+      stereotypes: [b.kind],
+      properties: b.properties.map((name, i) => ({
+        id: `p${i + 1}`,
+        name,
+        type: { kind: "string" } as const,
+        multiplicity: "0..1" as const,
+      })),
+    }));
+
+  const associations: Association[] = data.associations
+    .filter(a => a.kind !== "constraint")
+    .map(a => ({
+      id: a.id,
+      fromBlockId: a.from,
+      toBlockId: a.to,
+      label: a.label,
+      kind: mapAssocKind(a.kind),
+    }));
+
+  // Requirements come from the narrative; the fixture chips reference REQ-001..003
+  // but the requirements themselves aren't stored. Fabricate them here so the
+  // validator has something meaningful to operate on.
+  const requirements: Requirement[] = [
+    {
+      id: "req-001",
+      tag: "REQ-001",
+      text: "Must never output a complete solution to a graded problem.",
+      relations: [{ kind: "satisfy", blockId: "aristotle" }],
+    },
+    {
+      id: "req-002",
+      tag: "REQ-002",
+      text: "Response latency under 1.2s P50 to preserve flow.",
+      relations: [{ kind: "satisfy", blockId: "aristotle" }],
+    },
+    {
+      id: "req-003",
+      tag: "REQ-003",
+      text: "Operates within FERPA boundaries; coursework never leaves institutional tenancy.",
+      // Intentionally untraced in the fixture — see the rail's "untraced" dot
+      relations: [],
+    },
+  ];
+
+  return {
+    blocks,
+    associations,
+    constraints,
+    requirements,
+  };
+}
+
+function mapAssocKind(k: "association" | "composition" | "constraint"): AssociationKind {
+  if (k === "constraint") return "constraintApplies";
+  return k;
+}
diff --git a/apps/web/lib/sysml/model.ts b/apps/web/lib/sysml/model.ts
new file mode 100644
index 0000000..de3f49a
--- /dev/null
+++ b/apps/web/lib/sysml/model.ts
@@ -0,0 +1,75 @@
+// Canonical SysML metamodel types — adapted from phase-0/src/types.ts
+// (the Phase 0 validation that ran 10/10 against the corpus).
+//
+// See docs/sysml-modeling.md §6 for the design rationale and §3 for what we
+// deliberately don't model.
+
+export type BlockKind = "block" | "actor" | "constraint" | "system";
+
+export type PropertyType =
+  | { kind: "string" }
+  | { kind: "number" }
+  | { kind: "boolean" }
+  | { kind: "enum"; values: string[] };
+
+export type Multiplicity = "0..1" | "1" | "0..*" | "1..*";
+
+export interface Property {
+  id: string;
+  name: string;
+  type: PropertyType;
+  multiplicity: Multiplicity;
+  description?: string;
+}
+
+export interface Block {
+  id: string;
+  label: string;
+  kind: BlockKind;
+  stereotypes: string[];
+  properties: Property[];
+  description?: string;
+}
+
+export type AssociationKind =
+  | "association"
+  | "composition"
+  | "aggregation"
+  | "generalization"
+  | "constraintApplies";
+
+export interface Association {
+  id: string;
+  fromBlockId: string;
+  toBlockId: string;
+  label: string;
+  kind: AssociationKind;
+  multiplicity?: { from: Multiplicity; to: Multiplicity };
+}
+
+export interface Constraint {
+  id: string;
+  label: string;
+  expression: string;
+  appliesTo: string[];
+}
+
+export type RequirementRelation =
+  | { kind: "satisfy"; blockId: string }
+  | { kind: "derive"; fromReqId: string }
+  | { kind: "verify"; experimentId: string };
+
+export interface Requirement {
+  id: string;
+  tag: string;
+  text: string;
+  relations: RequirementRelation[];
+}
+
+export interface SysMLModel {
+  systemOfInterestId?: string;
+  blocks: Block[];
+  associations: Association[];
+  constraints: Constraint[];
+  requirements: Requirement[];
+}
diff --git a/apps/web/lib/sysml/validate.ts b/apps/web/lib/sysml/validate.ts
new file mode 100644
index 0000000..e258ab9
--- /dev/null
+++ b/apps/web/lib/sysml/validate.ts
@@ -0,0 +1,324 @@
+// Pure SysML model validator.
+//
+// 13 rules, grouped:
+//   - Structural (S1–S5) — referential integrity. Errors.
+//   - Semantic (M1–M5) — SysML metamodel rules. Mostly errors.
+//   - Traceability (T1–T2) — soft warnings on coverage.
+//
+// All rules are described in docs/sysml-modeling.md §7.
+// Pure function; safe to call repeatedly. Output is element-anchored so the
+// UI can dot/highlight the offending block, association, etc.
+
+import type {
+  SysMLModel,
+  Block,
+  Association,
+  Constraint,
+  Requirement,
+} from "./model";
+
+export type Severity = "error" | "warning" | "soft";
+
+export type IssueAnchor =
+  | { kind: "block"; id: string }
+  | { kind: "association"; id: string }
+  | { kind: "constraint"; id: string }
+  | { kind: "requirement"; id: string }
+  | { kind: "property"; blockId: string; propertyId: string }
+  | { kind: "model" };
+
+export interface ValidationIssue {
+  /** Stable rule code: S1, M2, T1, etc. */
+  code: string;
+  severity: Severity;
+  message: string;
+  anchor: IssueAnchor;
+}
+
+export function validate(model: SysMLModel): ValidationIssue[] {
+  const issues: ValidationIssue[] = [];
+
+  const blockIds = new Set(model.blocks.map(b => b.id));
+  const reqIds = new Set(model.requirements.map(r => r.id));
+
+  // Lookups
+  const blockById = new Map<string, Block>(model.blocks.map(b => [b.id, b]));
+
+  // ─── Structural ────────────────────────────────────────────────────────
+
+  // S1 — every association.fromBlockId / toBlockId resolves to a block
+  for (const a of model.associations) {
+    if (!blockIds.has(a.fromBlockId)) {
+      issues.push(s1IssueFor(a, "from", a.fromBlockId));
+    }
+    if (!blockIds.has(a.toBlockId)) {
+      issues.push(s1IssueFor(a, "to", a.toBlockId));
+    }
+  }
+
+  // S2 — every constraint.appliesTo entry resolves to a block
+  for (const c of model.constraints) {
+    for (const target of c.appliesTo) {
+      if (!blockIds.has(target)) {
+        issues.push({
+          code: "S2",
+          severity: "error",
+          message: `Constraint "${c.label}" applies to unknown block "${target}".`,
+          anchor: { kind: "constraint", id: c.id },
+        });
+      }
+    }
+  }
+
+  // S3 — every requirement satisfy.blockId resolves to a block
+  // S4 — every requirement derive.fromReqId resolves to a requirement
+  for (const r of model.requirements) {
+    for (const rel of r.relations) {
+      if (rel.kind === "satisfy" && !blockIds.has(rel.blockId)) {
+        issues.push({
+          code: "S3",
+          severity: "error",
+          message: `Requirement ${r.tag} is satisfied by unknown block "${rel.blockId}".`,
+          anchor: { kind: "requirement", id: r.id },
+        });
+      }
+      if (rel.kind === "derive" && !reqIds.has(rel.fromReqId)) {
+        issues.push({
+          code: "S4",
+          severity: "error",
+          message: `Requirement ${r.tag} derives from unknown requirement "${rel.fromReqId}".`,
+          anchor: { kind: "requirement", id: r.id },
+        });
+      }
+    }
+  }
+
+  // S5 — block ids unique; requirement tags unique; association ids unique
+  uniquenessIssues(model.blocks, b => b.id, "S5", "block id", b => ({ kind: "block", id: b.id })).forEach(i => issues.push(i));
+  uniquenessIssues(model.associations, a => a.id, "S5", "association id", a => ({ kind: "association", id: a.id })).forEach(i => issues.push(i));
+  uniquenessIssues(model.constraints, c => c.id, "S5", "constraint id", c => ({ kind: "constraint", id: c.id })).forEach(i => issues.push(i));
+  uniquenessIssues(model.requirements, r => r.tag, "S5", "requirement tag", r => ({ kind: "requirement", id: r.id })).forEach(i => issues.push(i));
+
+  // ─── Semantic (SysML metamodel) ────────────────────────────────────────
+
+  // M1 — generalization is acyclic
+  cycles(
+    model.blocks,
+    block => model.associations.filter(a => a.kind === "generalization" && a.fromBlockId === block.id).map(a => a.toBlockId),
+    blockById
+  ).forEach(cycle =>
+    issues.push({
+      code: "M1",
+      severity: "error",
+      message: `Cyclic generalization: ${cycle.join(" → ")} → ${cycle[0]}`,
+      anchor: { kind: "block", id: cycle[0]! },
+    })
+  );
+
+  // M2 — composition is acyclic
+  cycles(
+    model.blocks,
+    block => model.associations.filter(a => a.kind === "composition" && a.fromBlockId === block.id).map(a => a.toBlockId),
+    blockById
+  ).forEach(cycle =>
+    issues.push({
+      code: "M2",
+      severity: "error",
+      message: `Cyclic composition: ${cycle.join(" → ")} → ${cycle[0]}`,
+      anchor: { kind: "block", id: cycle[0]! },
+    })
+  );
+
+  // M3 — exactly zero or one block has kind "system"
+  const systems = model.blocks.filter(b => b.kind === "system");
+  if (systems.length > 1) {
+    for (const s of systems) {
+      issues.push({
+        code: "M3",
+        severity: "error",
+        message: `Multiple System-of-Interest blocks: ${systems.map(x => x.label).join(", ")}.`,
+        anchor: { kind: "block", id: s.id },
+      });
+    }
+  }
+
+  // M4 — constraint blocks may not participate in composition
+  for (const a of model.associations) {
+    if (a.kind !== "composition") continue;
+    const from = blockById.get(a.fromBlockId);
+    const to = blockById.get(a.toBlockId);
+    if ((from && from.kind === "constraint") || (to && to.kind === "constraint")) {
+      issues.push({
+        code: "M4",
+        severity: "error",
+        message: `Composition involves a constraint block (${from?.kind === "constraint" ? from.label : to?.label}).`,
+        anchor: { kind: "association", id: a.id },
+      });
+    }
+  }
+
+  // M5 — property names unique within a block
+  for (const b of model.blocks) {
+    const seen = new Map<string, number>();
+    for (const p of b.properties) {
+      seen.set(p.name, (seen.get(p.name) ?? 0) + 1);
+    }
+    for (const [name, count] of seen) {
+      if (count > 1) {
+        issues.push({
+          code: "M5",
+          severity: "error",
+          message: `Block "${b.label}" has duplicate property "${name}" (×${count}).`,
+          anchor: { kind: "block", id: b.id },
+        });
+      }
+    }
+  }
+
+  // ─── Traceability ──────────────────────────────────────────────────────
+
+  // T1 — every requirement has at least one satisfy relation
+  for (const r of model.requirements) {
+    const hasSatisfier = r.relations.some(rel => rel.kind === "satisfy");
+    if (!hasSatisfier) {
+      issues.push({
+        code: "T1",
+        severity: "warning",
+        message: `Requirement ${r.tag} has no satisfying block.`,
+        anchor: { kind: "requirement", id: r.id },
+      });
+    }
+  }
+
+  // T2 — every block is referenced by ≥1 association OR satisfies ≥1 requirement
+  // (constraint blocks are exempt — they don't need to be referenced via association)
+  const referencedBlockIds = new Set<string>();
+  for (const a of model.associations) {
+    referencedBlockIds.add(a.fromBlockId);
+    referencedBlockIds.add(a.toBlockId);
+  }
+  for (const r of model.requirements) {
+    for (const rel of r.relations) {
+      if (rel.kind === "satisfy") referencedBlockIds.add(rel.blockId);
+    }
+  }
+  for (const c of model.constraints) {
+    for (const target of c.appliesTo) referencedBlockIds.add(target);
+  }
+  for (const b of model.blocks) {
+    if (b.kind === "constraint") continue;
+    if (!referencedBlockIds.has(b.id)) {
+      issues.push({
+        code: "T2",
+        severity: "soft",
+        message: `Block "${b.label}" is not referenced by any association, constraint, or requirement.`,
+        anchor: { kind: "block", id: b.id },
+      });
+    }
+  }
+
+  return issues;
+}
+
+// ─── Helpers ─────────────────────────────────────────────────────────────
+
+function s1IssueFor(a: Association, side: "from" | "to", missingId: string): ValidationIssue {
+  return {
+    code: "S1",
+    severity: "error",
+    message: `Association "${a.label || a.id}" ${side} unknown block "${missingId}".`,
+    anchor: { kind: "association", id: a.id },
+  };
+}
+
+function uniquenessIssues<T>(
+  items: T[],
+  keyOf: (item: T) => string,
+  code: string,
+  what: string,
+  anchorOf: (item: T) => IssueAnchor
+): ValidationIssue[] {
+  const counts = new Map<string, number>();
+  for (const it of items) counts.set(keyOf(it), (counts.get(keyOf(it)) ?? 0) + 1);
+  const issues: ValidationIssue[] = [];
+  for (const it of items) {
+    const k = keyOf(it);
+    if ((counts.get(k) ?? 0) > 1) {
+      issues.push({
+        code,
+        severity: "error",
+        message: `Duplicate ${what} "${k}".`,
+        anchor: anchorOf(it),
+      });
+    }
+  }
+  return issues;
+}
+
+/**
+ * Find all simple cycles in a directed graph, returning each cycle as an
+ * array of node ids. Uses DFS with a recursion stack (Tarjan-style).
+ * Each cycle is reported once, starting from its lexicographically smallest
+ * node (so duplicates from different start positions collapse).
+ */
+function cycles<T extends { id: string }>(
+  nodes: T[],
+  outgoing: (node: T) => string[],
+  byId: Map<string, T>
+): string[][] {
+  const result: string[][] = [];
+  const seenCycles = new Set<string>();
+
+  function dfs(currentId: string, path: string[], onPath: Set<string>) {
+    const node = byId.get(currentId);
+    if (!node) return;
+    for (const nextId of outgoing(node)) {
+      if (onPath.has(nextId)) {
+        // Found a cycle from nextId back to itself through the path
+        const startIdx = path.indexOf(nextId);
+        if (startIdx >= 0) {
+          const cycle = path.slice(startIdx);
+          const key = canonicalCycleKey(cycle);
+          if (!seenCycles.has(key)) {
+            seenCycles.add(key);
+            result.push(cycle);
+          }
+        }
+        continue;
+      }
+      onPath.add(nextId);
+      path.push(nextId);
+      dfs(nextId, path, onPath);
+      path.pop();
+      onPath.delete(nextId);
+    }
+  }
+
+  for (const n of nodes) {
+    dfs(n.id, [n.id], new Set([n.id]));
+  }
+  return result;
+}
+
+function canonicalCycleKey(cycle: string[]): string {
+  // Rotate so the smallest id is first; then join.
+  let minIdx = 0;
+  for (let i = 1; i < cycle.length; i++) {
+    if (cycle[i]! < cycle[minIdx]!) minIdx = i;
+  }
+  return [...cycle.slice(minIdx), ...cycle.slice(0, minIdx)].join("→");
+}
+
+// ─── Convenience accessors used by the UI ────────────────────────────────
+
+export function severityRank(s: Severity): number {
+  return s === "error" ? 3 : s === "warning" ? 2 : 1;
+}
+
+export function maxSeverity(issues: ValidationIssue[]): Severity | null {
+  let best: Severity | null = null;
+  for (const i of issues) {
+    if (best === null || severityRank(i.severity) > severityRank(best)) best = i.severity;
+  }
+  return best;
+}
diff --git a/apps/web/styles/base.css b/apps/web/styles/base.css
index b43b4a2..6ed7db7 100644
--- a/apps/web/styles/base.css
+++ b/apps/web/styles/base.css
@@ -923,3 +923,123 @@ button { font-family: inherit; }
 .tiptap .ProseMirror-selectednode .chip {
   box-shadow: 0 0 0 1.5px var(--accent);
 }
+
+/* ─── Issues panel (M4) ─── */
+.issues-panel {
+  position: fixed;
+  bottom: 32px;
+  right: 16px;
+  width: 380px;
+  max-height: 50vh;
+  background: var(--surface);
+  border: 1px solid var(--border-strong);
+  border-radius: 6px;
+  box-shadow: 0 8px 24px var(--shadow-strong);
+  display: flex;
+  flex-direction: column;
+  z-index: 50;
+  font-family: var(--font-body);
+  overflow: hidden;
+}
+.issues-panel-clean {
+  width: auto;
+  min-width: 200px;
+}
+.issues-panel-clean .issues-panel-head { color: var(--ok-strong); }
+.issues-panel-clean-glyph {
+  color: var(--ok);
+  font-size: 14px;
+  font-weight: 600;
+}
+.issues-panel-collapsed {
+  max-height: none;
+}
+.issues-panel-head {
+  display: flex; align-items: center; gap: 10px;
+  padding: 8px 12px;
+  background: transparent;
+  border: none;
+  border-bottom: 1px solid var(--border);
+  cursor: pointer;
+  font-family: inherit;
+  color: var(--fg);
+  width: 100%;
+  text-align: left;
+}
+.issues-panel-collapsed .issues-panel-head { border-bottom: none; }
+.issues-panel-head:hover { background: var(--surface-2); }
+.issues-panel-title {
+  font-family: var(--font-display);
+  font-weight: 500;
+  font-size: 13px;
+  flex: 1;
+}
+.issues-panel-counts { display: flex; gap: 8px; }
+.issues-count {
+  font-family: var(--font-mono);
+  font-size: 11px;
+  font-weight: 500;
+}
+.issues-count-error { color: var(--warn-strong); }
+.issues-count-warning { color: var(--accent-strong); }
+.issues-count-soft { color: var(--muted); }
+.issues-panel-caret { font-size: 9px; color: var(--muted); }
+
+.issues-panel-list {
+  list-style: none;
+  margin: 0;
+  padding: 4px;
+  overflow-y: auto;
+  flex: 1;
+}
+.issues-item {
+  display: grid;
+  grid-template-columns: 16px auto 1fr;
+  gap: 8px;
+  padding: 6px 10px;
+  border-radius: 4px;
+  cursor: pointer;
+  font-size: 12.5px;
+  line-height: 1.35;
+}
+.issues-item:hover { background: var(--surface-2); }
+.issues-item-error .issues-item-sev { color: var(--warn-strong); }
+.issues-item-warning .issues-item-sev { color: var(--accent); }
+.issues-item-soft .issues-item-sev { color: var(--muted); }
+.issues-item-sev { font-size: 11px; line-height: 1.6; text-align: center; }
+.issues-item-code {
+  font-family: var(--font-mono);
+  font-size: 10.5px;
+  color: var(--muted-strong);
+  background: var(--surface-2);
+  padding: 1px 5px;
+  border-radius: 3px;
+  align-self: start;
+  margin-top: 1px;
+}
+.issues-item-msg { color: var(--prose); }
+
+/* Issue dots in the rail */
+.rail-issue-dot {
+  width: 6px; height: 6px;
+  border-radius: 50%;
+  margin-left: auto;
+  flex-shrink: 0;
+  margin-right: 4px;
+}
+.rail-issue-dot-error { background: var(--warn-strong); box-shadow: 0 0 0 2px var(--warn-soft); }
+.rail-issue-dot-warning { background: var(--accent); box-shadow: 0 0 0 2px var(--accent-soft); }
+.rail-issue-dot-soft { background: var(--muted); }
+
+/* Diagram node issue ring */
+.sysml-node-issue-error {
+  border-color: var(--warn-strong) !important;
+  box-shadow: 0 0 0 2px var(--warn-soft), 0 2px 4px var(--shadow);
+}
+.sysml-node-issue-warning {
+  border-color: var(--accent) !important;
+  box-shadow: 0 0 0 2px var(--accent-soft), 0 2px 4px var(--shadow);
+}
+.sysml-node-issue-soft {
+  border-style: dotted !important;
+}