diff --git a/README.md b/README.md
index db164bd..9c5b102 100644
--- a/README.md
+++ b/README.md
@@ -301,9 +301,10 @@ RELEASE="$RELEASE" OTEL_URL="https://otel.example.com" \
 ```
 
 The receiver stores uploaded maps under `OTEL_SOURCEMAP_DIR` (default
-`./sourcemaps`, `/sourcemaps` in the Kubernetes deployment). Upload maps for
-each release before investigating its errors. The API never returns map
-contents or original source files.
+`./sourcemaps`, `/sourcemaps` in the Kubernetes deployment). Maps are reloaded on start, so keep the directory on persistent storage. See
+`instrumentation.md` (*Source maps → Requirements checklist*) for everything a
+project needs for stacks to resolve. `GET /api/sourcemaps` lists loaded maps.
+The API never returns map files themselves.
 
 `build.sh` passes the release into the Docker UI build. If `OTEL_URL` is set,
 it also builds the matching UI locally and uploads all generated JavaScript
@@ -362,6 +363,18 @@ already points at it. The UI's Trends tab (and live issue updates) appear
 automatically once the stats endpoints answer; on file-store deployments the
 tab hides itself. SPA routes (/issues, /traces, /logs, /metrics, /trends,
 /setup) are served client-side by wouter and survive the OIDC login redirect.
+Selection lives in the URL too, so every step can be reloaded, stepped with
+back/forward, and shared (each detail view has a link chip that copies its
+permalink):
+
+| Path | Shows |
+| --- | --- |
+| `/issues/<issueId>` | one issue with its traces |
+| `/issues/<issueId>/trace/<traceId>` | the same, with a trace selected |
+| `/patterns/<patternId>[/<issueId>]` | a pattern card open, optionally reviewing one member |
+| `/traces/<traceId>[/<spanId>]` | a trace, optionally with a span selected |
+| `/rum/session/<sessionId>` | a RUM session |
+| `/rum/errors[/<errorId>]` | JS errors, optionally one error expanded (`errorId` is a hash of type + message) |
 
 Currently, the history buffer size is hardcoded to 100 entries. This can be made configurable in future versions.
 
diff --git a/instrumentation.md b/instrumentation.md
index 37a00b4..a320e5a 100644
--- a/instrumentation.md
+++ b/instrumentation.md
@@ -150,9 +150,59 @@ For example, if the deployed JavaScript is available at
 `source=/assets/main-abc123.js`. The source-map filename on the build machine
 does not need to match the URL, but the `source` field must.
 
-Maps are parsed and retained server-side. Issue responses include a resolved
-stack when `service.release` and the generated bundle path match. Original
-source files and map contents are never returned by the API.
+Maps are parsed, written to `OTEL_SOURCEMAP_DIR`, and reloaded on receiver
+start. Issue responses include a resolved stack when `service.release` and the
+generated bundle path match. Uploaded maps are never returned, but when a map
+embeds `sourcesContent` the RUM "JS errors" view shows a few lines of original
+source around each mapped frame to signed-in users.
+
+### Requirements checklist
+
+A stack only resolves when every one of these holds. When it does not, check
+them in this order:
+
+1. **The build emits maps, with sources embedded.** Enable source maps in the
+   bundler (Vite `build.sourcemap: true`, webpack `devtool: "source-map"`).
+   Keep `sourcesContent` (the default) or the code view has nothing to show.
+2. **The browser reports a release.** Pass the release to `OtelRUM.init({ release })`
+   (or `data-release` on the script tag), injected at build time. An event
+   with no release, or `dev`, can never match an upload.
+3. **The uploaded release is byte-for-byte the same string.** Use one value for
+   both the bundle build and the upload (a git short SHA works).
+4. **`source` matches the path in the browser stack.** Everything before
+   `/assets/` and any `?query` is ignored, so `source=/assets/main-abc123.js`
+   matches `https://app.example.com/assets/main-abc123.js`. Content-hashed
+   names must match exactly, and chunks each need their own upload.
+5. **The map belongs to the deployed bundle.** Upload the maps from the very
+   build that produced the deployed files. A second local rebuild can differ
+   (dependencies, Node version) and silently mismatch. In Docker, copy the maps
+   out of the build stage (`docker build --target <stage>`, then `docker cp`)
+   instead of rebuilding. `build.sh` does this.
+6. **Every chunk is uploaded.** Loop over all `*.js.map` files and fail the
+   deploy when none are found, since an empty loop otherwise succeeds silently.
+7. **The receiver keeps its maps.** `OTEL_SOURCEMAP_DIR` must be on persistent
+   storage (a PVC in Kubernetes, not `emptyDir`). Maps written to an ephemeral
+   directory vanish on restart.
+8. **Maps are not published with the site.** Delete `*.map` from the served
+   image unless public maps are intended (see below).
+
+### Verifying a deploy
+
+List what the receiver has loaded (requires a signed-in session or token):
+
+```sh
+curl -H "Authorization: Bearer $TOKEN" https://otel.example.com/api/sourcemaps
+# [{"release":"2026.09.18-a1b2c3","source":"/assets/main-abc123.js"}, ...]
+```
+
+Then trigger a real uncaught error from application code, for example a
+button that runs `setTimeout(() => { throw new Error("source-map check") })`
+(this repository's RUM "JS errors" view has a *Generate test error* button),
+and confirm the card shows **Source-mapped** with the original file and line.
+
+Frame status in the stack explorer explains a miss: `no-release` (event has no
+release), `no-map` (nothing uploaded for that release and file), or `unmapped`
+(a map exists but has no mapping for that position).
 
 The Docker build removes `.map` files from production images by default. For
 development or staging, public maps can be enabled for Chrome DevTools:
@@ -170,6 +220,7 @@ production unless that is intentional.
 2. Trigger a controlled backend error span or ERROR log.
 3. Confirm both appear under Issues as separate frontend/backend issues.
 4. Confirm the issue sample contains the expected release.
-5. Confirm a mapped frontend stack shows original file and line information.
+5. Confirm a mapped frontend stack shows original file and line information
+   (see *Requirements checklist* under Source maps if it does not).
 6. Confirm trace IDs connect frontend requests to backend traces where headers
    were propagated.
diff --git a/otel-ui/src/components/IssueOccurrences.tsx b/otel-ui/src/components/IssueOccurrences.tsx
index 4ca73f2..314e323 100644
--- a/otel-ui/src/components/IssueOccurrences.tsx
+++ b/otel-ui/src/components/IssueOccurrences.tsx
@@ -1,4 +1,5 @@
 import { useMemo, useState } from "react"
+import { CopyRef } from "./common/CopyRef"
 import { MasterDetail } from "./MasterDetail"
 import { BackButton } from "./common/BackButton"
 import { SectionLabel } from "./common/DetailField"
@@ -26,15 +27,18 @@ interface Row {
 /**
  * Focused view of one issue: an overview of every trace it occurred in
  * (server-side summaries, paged), with the selected trace's waterfall and its
- * occurrences below. `initialTraceId` preselects the trace the user came from.
+ * occurrences below. The selected trace comes from the URL (`traceId`).
  */
 export const IssueOccurrences = ({
   issue,
-  initialTraceId,
+  traceId: routeTraceId,
+  onTraceChange,
   onBack,
 }: {
   issue: Issue
-  initialTraceId?: string
+  /** Trace named by the URL; the URL is the source of truth for the selection. */
+  traceId?: string
+  onTraceChange?: (traceId: string | null) => void
   onBack: () => void
 }) => {
   const { traces, untraced, total, complete, hasMore, loading, error, loadMore } = useIssueTraces(issue.id)
@@ -59,7 +63,12 @@ export const IssueOccurrences = ({
     )
   }, [traces, untraced, sort])
 
-  const [picked, setPicked] = useState<string | null>(initialTraceId ?? null)
+  const [pickedLocal, setPickedLocal] = useState<string | null>(null)
+  const picked = routeTraceId ?? pickedLocal
+  const setPicked = (key: string) => {
+    setPickedLocal(key)
+    onTraceChange?.(rows.find((r) => r.key === key)?.traceId ?? null)
+  }
   const selected = rows.find((r) => r.key === picked) ?? rows[0]
   const selectedTraceId = selected?.traceId
 
@@ -81,7 +90,19 @@ export const IssueOccurrences = ({
     <div className="flex flex-col gap-5">
       <StatHeader
         back={<BackButton label="All issues" onClick={onBack} />}
-        eyebrow={`service=${issue.service}`}
+        eyebrow={
+          <>
+            <span>service={issue.service}</span>
+            <CopyRef id={issue.id} path={`/issues/${encodeURIComponent(issue.id)}`} label="Copy issue link" />
+            {selectedTraceId && (
+              <CopyRef
+                id={selectedTraceId}
+                path={`/issues/${encodeURIComponent(issue.id)}/trace/${encodeURIComponent(selectedTraceId)}`}
+                label="Copy trace link"
+              />
+            )}
+          </>
+        }
         title={issue.title}
         stats={
           <>
diff --git a/otel-ui/src/components/IssuesVisualizer.tsx b/otel-ui/src/components/IssuesVisualizer.tsx
index 79e847e..51e6ba2 100644
--- a/otel-ui/src/components/IssuesVisualizer.tsx
+++ b/otel-ui/src/components/IssuesVisualizer.tsx
@@ -32,6 +32,8 @@ import {
   strongestCorrelate,
   type SeverityTone,
 } from "../lib/issue-utils"
+import { useSubRoute } from "../lib/routes"
+import { CopyRef } from "./common/CopyRef"
 import { IssueOccurrences } from "./IssueOccurrences"
 import { NeedleTestModal } from "./NeedleTestModal"
 import { FilterBar, type FilterChip } from "./FilterBar"
@@ -343,6 +345,7 @@ const IssueCard = ({
               <meta.Icon className="h-3 w-3 shrink-0" aria-hidden="true" />
               {meta.label}
             </span>
+            <CopyRef id={issue.id} path={`/issues/${encodeURIComponent(issue.id)}`} />
             <span className="text-[12.5px] text-ds-text-3">
               Detected {formatRelativeTime(issue.firstSeen / 1e6)} · last seen{" "}
               {formatRelativeTime(issue.lastSeen / 1e6)}
@@ -653,8 +656,10 @@ export const IssuesVisualizer = ({
 }: IssuesVisualizerProps) => {
   const { selectedServices, toggleService, resetFilters, searchQuery, setSearchQuery, uniqueServices } =
     useObservabilityData()
-  const [focus, setFocus] = useState<{ issueId: string; traceId?: string } | null>(null)
-  const focusIssue = (issue: Issue, traceId?: string) => setFocus({ issueId: issue.id, traceId })
+  // /issues/<issueId>[/trace/<traceId>] - the focused issue and trace live in the URL.
+  const { segments, go } = useSubRoute()
+  const focus = segments[0] ? { issueId: segments[0], traceId: segments[1] === "trace" ? segments[2] : undefined } : null
+  const focusIssue = (issue: Issue, traceId?: string) => go(traceId ? [issue.id, "trace", traceId] : [issue.id])
   const [statusFilter, setStatusFilter] = useState<StatusFilter>("all")
   const [kindFilter, setKindFilter] = useState<KindFilter>("all")
   const [needleModalOpen, setNeedleModalOpen] = useState(false)
@@ -819,11 +824,22 @@ export const IssuesVisualizer = ({
       <IssueOccurrences
         key={focus.issueId}
         issue={focusedIssue}
-        initialTraceId={focus.traceId}
-        onBack={() => setFocus(null)}
+        traceId={focus.traceId}
+        onTraceChange={(traceId) => go(traceId ? [focus.issueId, "trace", traceId] : [focus.issueId], { replace: true })}
+        onBack={() => go([])}
       />
     )
   }
+  if (focus) {
+    return (
+      <div className="flex flex-col items-center gap-3 py-20 text-center">
+        <p className="text-[14px] font-medium text-ds-text">Issue not found</p>
+        <p className="font-[family-name:var(--font-data)] text-[12px] text-ds-text-3">{focus.issueId}</p>
+        <p className="max-w-sm text-[12.5px] text-ds-text-3">It may have been evicted, or the link is from another environment.</p>
+        <button type="button" onClick={() => go([])} className="inline-flex h-9 items-center rounded-[9px] border border-ds-line-strong px-3 text-[12.5px] font-medium text-ds-text-2 hover:text-ds-text">All issues</button>
+      </div>
+    )
+  }
 
   return (
     <div className="flex flex-col gap-5">
diff --git a/otel-ui/src/components/PatternsVisualizer.tsx b/otel-ui/src/components/PatternsVisualizer.tsx
index 799a854..48b69bb 100644
--- a/otel-ui/src/components/PatternsVisualizer.tsx
+++ b/otel-ui/src/components/PatternsVisualizer.tsx
@@ -1,5 +1,7 @@
-import { useMemo, useState } from "react"
+import { useEffect, useMemo, useRef, useState } from "react"
 import { Bug, ChevronDown, ChevronUp, GitBranch, RefreshCw, ThumbsDown, ThumbsUp } from "lucide-react"
+import { useSubRoute } from "../lib/routes"
+import { CopyRef } from "./common/CopyRef"
 import { usePatterns } from "../hooks/usePatterns"
 import { formatRelativeTime } from "../lib/timeUtils"
 import { issueSeverityTone, issueStatusOf, type SeverityTone } from "../lib/issue-utils"
@@ -15,14 +17,18 @@ const mergeIssues = (members: Issue[]): Issue => {
   return { ...members[0], trend }
 }
 
-const PatternCard = ({ pattern, members, similar, review }: {
+const PatternCard = ({ pattern, members, similar, review, expanded, selected, onExpand, onSelect }: {
   pattern: IssuePattern
   members: Issue[]
+  expanded: boolean
+  /** Member issue whose similar-issue suggestions are open (from the URL). */
+  selected: string | null
+  onExpand: (open: boolean) => void
+  onSelect: (issueId: string | null) => void
   similar: (id: string) => Promise<SimilarIssue[]>
   review: (id: string, otherId: string, decision: "same" | "different") => Promise<void>
 }) => {
-  const [expanded, setExpanded] = useState(false)
-  const [selected, setSelected] = useState<string | null>(null)
+  const card = useRef<HTMLElement>(null)
   const [related, setRelated] = useState<Record<string, SimilarIssue[]>>({})
   const [busy, setBusy] = useState<string | null>(null)
 
@@ -32,14 +38,20 @@ const PatternCard = ({ pattern, members, similar, review }: {
   const meta = kindMeta[pattern.kind] ?? { label: pattern.kind, Icon: Bug }
   const title = [...members].sort((a, b) => b.count - a.count)[0]?.title ?? pattern.signature.split("\u0000")[0]
 
-  const select = async (issue: Issue) => {
-    if (selected === issue.id) { setSelected(null); return }
-    setSelected(issue.id)
-    if (!related[issue.id]) {
-      const suggestions = await similar(issue.id)
-      setRelated((prev) => ({ ...prev, [issue.id]: suggestions }))
-    }
-  }
+  // A deep link to /patterns/<id> scrolls its card into view once.
+  useEffect(() => {
+    if (expanded) card.current?.scrollIntoView({ block: "start" })
+    // eslint-disable-next-line react-hooks/exhaustive-deps -- only on first render of a linked card
+  }, [])
+
+  // Suggestions load whenever the URL selects a member issue.
+  useEffect(() => {
+    if (!selected || related[selected]) return
+    void similar(selected)
+      .then((suggestions) => setRelated((prev) => ({ ...prev, [selected]: suggestions })))
+      .catch(() => setRelated((prev) => ({ ...prev, [selected]: [] })))
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [selected])
 
   const submitReview = async (issue: Issue, item: SimilarIssue, decision: "same" | "different") => {
     const key = `${issue.id}:${item.issue.id}`
@@ -53,9 +65,9 @@ const PatternCard = ({ pattern, members, similar, review }: {
   const selectedIssue = members.find((m) => m.id === selected)
 
   return (
-    <article className={`flex flex-col gap-3 rounded-[14px] border border-ds-line border-l-[3px] ${toneLine[tone]} bg-ds-surface`}>
+    <article ref={card} className={`flex flex-col gap-3 rounded-[14px] border border-ds-line border-l-[3px] ${toneLine[tone]} bg-ds-surface`}>
       <div className="flex w-full flex-wrap items-start gap-4 px-5 pt-[18px] pb-1">
-        <button type="button" onClick={() => setExpanded((v) => !v)} aria-expanded={expanded} className="flex min-w-0 flex-1 flex-col gap-1.5 text-left">
+        <button type="button" onClick={() => onExpand(!expanded)} aria-expanded={expanded} className="flex min-w-0 flex-1 flex-col gap-1.5 text-left">
           <div className="flex flex-wrap items-center gap-2.5">
             <span className={`inline-flex h-[23px] items-center rounded-[6px] px-2 text-[10.5px] font-bold uppercase tracking-wider ${tonePill[tone]}`}>
               {openCount > 0 ? `${openCount} open` : "All closed"}
@@ -63,6 +75,7 @@ const PatternCard = ({ pattern, members, similar, review }: {
             <span className="inline-flex h-[23px] items-center gap-1.5 rounded-[6px] border border-ds-line-strong px-2 text-[10.5px] font-bold uppercase tracking-wider text-ds-text-2">
               <meta.Icon className="h-3 w-3" aria-hidden="true" />{meta.label}
             </span>
+            <CopyRef id={pattern.id} path={`/patterns/${encodeURIComponent(pattern.id)}`} label="Copy pattern link" />
             <span className="text-[12.5px] text-ds-text-3">last seen {formatRelativeTime(pattern.lastSeen / 1e6)}</span>
           </div>
           <h3 className="break-all font-[family-name:var(--font-display)] text-[17px] font-bold leading-snug text-ds-text">{title}</h3>
@@ -70,7 +83,7 @@ const PatternCard = ({ pattern, members, similar, review }: {
         </button>
         <button
           type="button"
-          onClick={() => setExpanded((v) => !v)}
+          onClick={() => onExpand(!expanded)}
           aria-expanded={expanded}
           aria-label={`${expanded ? "Collapse" : "Expand"} pattern ${title}`}
           className="inline-flex h-9 w-9 shrink-0 items-center justify-center rounded-[9px] text-ds-text-3 hover:bg-ds-surface-2 hover:text-ds-text-2"
@@ -96,7 +109,7 @@ const PatternCard = ({ pattern, members, similar, review }: {
               <button
                 key={issue.id}
                 type="button"
-                onClick={() => void select(issue)}
+                onClick={() => onSelect(selected === issue.id ? null : issue.id)}
                 aria-pressed={selected === issue.id}
                 className={`flex items-center gap-3 rounded-[9px] border bg-ds-surface px-2.5 py-1.5 text-left hover:border-ds-accent/40 ${selected === issue.id ? "border-ds-accent/50" : "border-ds-line"}`}
               >
@@ -135,6 +148,8 @@ const PatternCard = ({ pattern, members, similar, review }: {
 
 export function PatternsVisualizer() {
   const { patterns, issues, loading, error, refresh, similar, review } = usePatterns()
+  // /patterns/<patternId>[/<issueId>]: which card is open and which member is reviewed.
+  const { segments, go } = useSubRoute()
   const [query, setQuery] = useState("")
 
   const visible = useMemo(() => {
@@ -170,6 +185,12 @@ export function PatternsVisualizer() {
 
       {error && <p className="text-[12px] text-ds-critical">Failed to load patterns: {String(error)}</p>}
 
+      {segments[0] && !patterns.some((p) => p.id === segments[0]) && patterns.length > 0 && (
+        <p className="text-[12px] text-ds-warn">
+          Pattern <span className="font-[family-name:var(--font-data)]">{segments[0]}</span> not found (it may have been merged or evicted).
+        </p>
+      )}
+
       {visible.length === 0 ? (
         <div className="flex flex-col items-center justify-center gap-2 rounded-[14px] border border-dashed border-ds-line py-16 text-center">
           <GitBranch className="h-8 w-8 text-ds-text-3/60" aria-hidden="true" />
@@ -184,6 +205,10 @@ export function PatternsVisualizer() {
               members={pattern.issueIds.map((id) => byId.get(id)).filter((i): i is Issue => Boolean(i))}
               similar={similar}
               review={review}
+              expanded={segments[0] === pattern.id}
+              selected={segments[0] === pattern.id ? (segments[1] ?? null) : null}
+              onExpand={(open) => go(open ? [pattern.id] : [])}
+              onSelect={(issueId) => go(issueId ? [pattern.id, issueId] : [pattern.id])}
             />
           ))}
         </div>
diff --git a/otel-ui/src/components/RumJsErrors.tsx b/otel-ui/src/components/RumJsErrors.tsx
index 71ccc38..e1677ca 100644
--- a/otel-ui/src/components/RumJsErrors.tsx
+++ b/otel-ui/src/components/RumJsErrors.tsx
@@ -1,5 +1,6 @@
-import { useEffect, useMemo, useState } from "react"
+import { useEffect, useMemo, useRef, useState } from "react"
 import { Bug, ChevronDown, ChevronUp } from "lucide-react"
+import { CopyRef } from "./common/CopyRef"
 import { triggerTestError } from "../lib/test-error"
 import { authFetch } from "../lib/auth"
 import { formatRelativeTime } from "../lib/timeUtils"
@@ -8,6 +9,8 @@ import type { RumErrorEvent, RumSession } from "../lib/rum-utils"
 
 interface JsErrorGroup {
   key: string
+  /** Stable short id derived from the error's type and message, used in URLs. */
+  id: string
   message: string
   exceptionType?: string
   count: number
@@ -17,6 +20,12 @@ interface JsErrorGroup {
   sample?: RumErrorEvent
 }
 
+const hashId = (text: string): string => {
+  let h = 0x811c9dc5
+  for (let i = 0; i < text.length; i++) h = Math.imul(h ^ text.charCodeAt(i), 0x01000193)
+  return (h >>> 0).toString(16).padStart(8, "0")
+}
+
 const groupJsErrors = (sessions: RumSession[]): JsErrorGroup[] => {
   const groups = new Map<string, JsErrorGroup>()
   for (const session of sessions) {
@@ -25,7 +34,7 @@ const groupJsErrors = (sessions: RumSession[]): JsErrorGroup[] => {
       const key = `${err.exceptionType ?? ""}|${err.message}`
       let g = groups.get(key)
       if (!g) {
-        g = { key, message: err.message, exceptionType: err.exceptionType, count: 0, sessionIds: new Set(), lastSeen: 0 }
+        g = { key, id: hashId(key), message: err.message, exceptionType: err.exceptionType, count: 0, sessionIds: new Set(), lastSeen: 0 }
         groups.set(key, g)
       }
       g.count += 1
@@ -73,8 +82,12 @@ const useResolvedStacks = (groups: JsErrorGroup[]) => {
   return { resolved, frames }
 }
 
-const JsErrorCard = ({ group, resolvedStack, frames }: { group: JsErrorGroup; resolvedStack?: string; frames: StackFrame[] }) => {
-  const [expanded, setExpanded] = useState(false)
+const JsErrorCard = ({ group, resolvedStack, frames, expanded, onToggle }: { group: JsErrorGroup; resolvedStack?: string; frames: StackFrame[]; expanded: boolean; onToggle: () => void }) => {
+  const card = useRef<HTMLElement>(null)
+  useEffect(() => {
+    if (expanded) card.current?.scrollIntoView({ block: "start" })
+    // eslint-disable-next-line react-hooks/exhaustive-deps -- only when a link opens this card on first render
+  }, [])
   const [showRaw, setShowRaw] = useState(false)
   const raw = group.sample?.stacktrace
   const mapped = Boolean(raw && resolvedStack && resolvedStack !== raw)
@@ -82,8 +95,8 @@ const JsErrorCard = ({ group, resolvedStack, frames }: { group: JsErrorGroup; re
   const explorable = frames.length > 0 && frames.some((f) => f.status === "mapped")
 
   return (
-    <article className="flex flex-col gap-3 rounded-[14px] border border-ds-line border-l-[3px] border-l-ds-critical bg-ds-surface">
-      <button type="button" onClick={() => setExpanded((v) => !v)} aria-expanded={expanded} className="flex items-start gap-4 px-5 pt-4 text-left">
+    <article ref={card} className="flex flex-col gap-3 rounded-[14px] border border-ds-line border-l-[3px] border-l-ds-critical bg-ds-surface">
+      <button type="button" onClick={onToggle} aria-expanded={expanded} className="flex items-start gap-4 px-5 pt-4 text-left">
         <div className="flex min-w-0 flex-1 flex-col gap-1.5">
           <div className="flex flex-wrap items-center gap-2.5">
             {group.exceptionType && (
@@ -92,6 +105,7 @@ const JsErrorCard = ({ group, resolvedStack, frames }: { group: JsErrorGroup; re
             {mapped && (
               <span className="inline-flex h-[23px] items-center rounded-[6px] border border-ds-ok/30 px-2 text-[10.5px] font-bold uppercase tracking-wider text-ds-ok">Source-mapped</span>
             )}
+            <CopyRef id={group.id} path={`/rum/errors/${group.id}`} label="Copy error link" />
             <span className="text-[12.5px] text-ds-text-3">last seen {formatRelativeTime(group.lastSeen)}</span>
           </div>
           <h3 className="break-all font-[family-name:var(--font-display)] text-[16px] font-bold leading-snug text-ds-text">{group.message}</h3>
@@ -142,7 +156,7 @@ const TestErrorButton = () => (
   </button>
 )
 
-export const RumJsErrors = ({ sessions }: { sessions: RumSession[] }) => {
+export const RumJsErrors = ({ sessions, openId, onOpen }: { sessions: RumSession[]; openId: string | null; onOpen: (id: string | null) => void }) => {
   const groups = useMemo(() => groupJsErrors(sessions), [sessions])
   const { resolved, frames } = useResolvedStacks(groups)
 
@@ -155,5 +169,5 @@ export const RumJsErrors = ({ sessions }: { sessions: RumSession[] }) => {
       </div>
     )
   }
-  return <div className="flex flex-col gap-3"><TestErrorButton />{groups.map((g) => <JsErrorCard key={g.key} group={g} resolvedStack={resolved[g.key]} frames={frames[g.key] ?? []} />)}</div>
+  return <div className="flex flex-col gap-3"><TestErrorButton />{groups.map((g) => <JsErrorCard key={g.key} group={g} resolvedStack={resolved[g.key]} frames={frames[g.key] ?? []} expanded={openId === g.id} onToggle={() => onOpen(openId === g.id ? null : g.id)} />)}</div>
 }
diff --git a/otel-ui/src/components/RumSessionsVisualizer.tsx b/otel-ui/src/components/RumSessionsVisualizer.tsx
index 2cbf99a..36ee69d 100644
--- a/otel-ui/src/components/RumSessionsVisualizer.tsx
+++ b/otel-ui/src/components/RumSessionsVisualizer.tsx
@@ -24,6 +24,8 @@ import {
   type RumTimelineEntry,
   type VitalRating,
 } from "../lib/rum-utils"
+import { useSubRoute } from "../lib/routes"
+import { CopyRef } from "./common/CopyRef"
 import { RumJsErrors } from "./RumJsErrors"
 import { FilterBar, type FilterChip } from "./FilterBar"
 
@@ -266,6 +268,7 @@ const SessionDetail = ({
             {session.environment ? ` · ${session.environment}` : ""}
           </span>
         </div>
+        <CopyRef id={session.id} path={`/rum/session/${encodeURIComponent(session.id)}`} label="Copy session link" />
         <button
           type="button"
           onClick={copySessionId}
@@ -395,8 +398,12 @@ export const RumSessionsVisualizer = () => {
   const [deviceFilter, setDeviceFilter] = useState<DeviceFilter>("all")
   const [errorsOnly, setErrorsOnly] = useState(false)
   const [sortMode, setSortMode] = useState<SortMode>("worst")
-  const [selectedSessionId, setSelectedSessionId] = useState<string | null>(null)
-  const [jsFocus, setJsFocus] = useState(false)
+  // /rum, /rum/session/<sessionId>, /rum/errors[/<errorId>]
+  const { segments, go } = useSubRoute()
+  const jsFocus = segments[0] === "errors"
+  const selectedSessionId = segments[0] === "session" ? (segments[1] ?? null) : null
+  const setJsFocus = (on: boolean) => go(on ? ["errors"] : [])
+  const setSelectedSessionId = (id: string) => go(["session", id])
 
   const sessions = useMemo(() => buildRumSessions(logs), [logs])
   const backend = useMemo(() => backendRequestsFromTraces(allTraces), [allTraces])
@@ -532,7 +539,7 @@ export const RumSessionsVisualizer = () => {
           value={String(aggregates.totalJsErrors)}
           detail={`${aggregates.distinctJsErrorMessages} distinct messages`}
           tone={aggregates.totalJsErrors > 0 ? "critical" : "ok"}
-          onClick={() => setJsFocus((v) => !v)}
+          onClick={() => setJsFocus(!jsFocus)}
           active={jsFocus}
         />
         <StatTile
@@ -550,7 +557,7 @@ export const RumSessionsVisualizer = () => {
       </div>
 
       {jsFocus ? (
-        <RumJsErrors sessions={filtered} />
+        <RumJsErrors sessions={filtered} openId={segments[1] ?? null} onOpen={(id) => go(id ? ["errors", id] : ["errors"])} />
       ) : sorted.length === 0 ? (
         <div className="flex flex-1 flex-col items-center justify-center gap-2 rounded-[14px] border border-ds-line bg-ds-surface py-16 text-center">
           <AlertOctagon className="h-6 w-6 text-ds-text-3" aria-hidden="true" />
diff --git a/otel-ui/src/components/common/CopyRef.tsx b/otel-ui/src/components/common/CopyRef.tsx
new file mode 100644
index 0000000..7a359dc
--- /dev/null
+++ b/otel-ui/src/components/common/CopyRef.tsx
@@ -0,0 +1,33 @@
+import { useState } from "react"
+import { Check, Link2 } from "lucide-react"
+import { shortId } from "../../lib/routes"
+
+/**
+ * Shows a short id and copies the full permalink (origin + `path`) on click,
+ * so any issue, pattern, session or error can be referenced in a message.
+ * The full id is in the tooltip and in the copied URL.
+ */
+export const CopyRef = ({ id, path, label = "Copy link" }: { id: string; path: string; label?: string }) => {
+  const [copied, setCopied] = useState(false)
+  return (
+    <button
+      type="button"
+      onClick={(e) => {
+        e.stopPropagation()
+        void navigator.clipboard
+          .writeText(window.location.origin + path)
+          .then(() => {
+            setCopied(true)
+            setTimeout(() => setCopied(false), 1200)
+          })
+          .catch(() => undefined)
+      }}
+      title={`${label}: ${id}`}
+      aria-label={`${label} for ${id}`}
+      className="inline-flex h-[23px] shrink-0 items-center gap-1 rounded-[6px] border border-ds-line px-1.5 font-[family-name:var(--font-data)] text-[10.5px] text-ds-text-3 hover:border-ds-line-strong hover:text-ds-text-2"
+    >
+      {copied ? <Check className="h-3 w-3 text-ds-ok" aria-hidden="true" /> : <Link2 className="h-3 w-3" aria-hidden="true" />}
+      {copied ? "copied" : shortId(id)}
+    </button>
+  )
+}
diff --git a/otel-ui/src/components/common/Stat.tsx b/otel-ui/src/components/common/Stat.tsx
index ddba3ff..c805531 100644
--- a/otel-ui/src/components/common/Stat.tsx
+++ b/otel-ui/src/components/common/Stat.tsx
@@ -16,7 +16,7 @@ export const StatHeader = ({
   back,
   stats,
 }: {
-  eyebrow?: string
+  eyebrow?: React.ReactNode
   title: string
   back?: React.ReactNode
   stats: React.ReactNode
@@ -24,7 +24,7 @@ export const StatHeader = ({
   <div className="flex flex-wrap items-end gap-x-7 gap-y-3">
     {back}
     <div className="min-w-0 flex-1 basis-[280px]">
-      {eyebrow && <p className="font-[family-name:var(--font-data)] text-[12px] text-ds-text-3">{eyebrow}</p>}
+      {eyebrow && <div className="flex flex-wrap items-center gap-2 font-[family-name:var(--font-data)] text-[12px] text-ds-text-3">{eyebrow}</div>}
       <h2 className="break-all font-[family-name:var(--font-display)] text-[22px] font-bold leading-tight text-ds-text">
         {title}
       </h2>
diff --git a/otel-ui/src/contexts/ObservabilityContextProvider.tsx b/otel-ui/src/contexts/ObservabilityContextProvider.tsx
index fb05ab3..4ca4671 100644
--- a/otel-ui/src/contexts/ObservabilityContextProvider.tsx
+++ b/otel-ui/src/contexts/ObservabilityContextProvider.tsx
@@ -2,7 +2,7 @@
 
 import { useState, useEffect, useRef, useMemo, useCallback, type ReactNode } from "react"
 import { useLocation } from "wouter"
-import { tabForPath, pathForTab } from "../lib/routes"
+import { tabForPath, pathForTab, splitPath, joinPath } from "../lib/routes"
 import type {
   MetricData,
   OtelHistoryMessage,
@@ -150,11 +150,31 @@ export function ObservabilityProvider({
   const setActiveTabWithRoute = useCallback(
     (tab: string) => {
       setActiveTab(tab)
+      // Already inside this tab (possibly on a deep link): keep the sub-route.
+      if (tabForPath(window.location.pathname) === tab) return
       const path = pathForTab(tab)
-      if (path && path !== window.location.pathname) navigate(path)
+      if (path) navigate(path)
     },
     [navigate],
   )
+
+  // Trace selection <-> /traces/<traceId>[/<spanId>]. The URL is adopted when
+  // it names a trace; a bare /traces never clears the selection, because other
+  // views set the trace first and switch tabs second.
+  useEffect(() => {
+    const { base, rest } = splitPath(location)
+    if (base !== "/traces" || !rest[0]) return
+    setActiveTraceId((prev) => (prev === rest[0] ? prev : rest[0]))
+    setActiveSpanId((prev) => (prev === (rest[1] ?? null) ? prev : (rest[1] ?? null)))
+    // eslint-disable-next-line react-hooks/exhaustive-deps -- only follow URL changes
+  }, [location])
+
+  useEffect(() => {
+    if (activeTab !== "tab1" || !activeTraceId) return
+    const target = joinPath("/traces", activeSpanId ? [activeTraceId, activeSpanId] : [activeTraceId])
+    if (target !== window.location.pathname) navigate(target)
+    // eslint-disable-next-line react-hooks/exhaustive-deps -- location is read imperatively
+  }, [activeTab, activeTraceId, activeSpanId, navigate])
   const [filtersExpanded, setFiltersExpanded] = useState<boolean>(false)
   // Trace ids we already have spans for, so selecting a trace that fell
   // outside the initial history window (or arrived before this session
diff --git a/otel-ui/src/lib/routes.ts b/otel-ui/src/lib/routes.ts
index ccaa917..264780a 100644
--- a/otel-ui/src/lib/routes.ts
+++ b/otel-ui/src/lib/routes.ts
@@ -1,3 +1,6 @@
+import { useCallback, useMemo } from "react"
+import { useLocation } from "wouter"
+
 // Route ↔ tab mapping. Tabs live in the URL (wouter) so deep links survive
 // the OIDC login redirect at the ingress: users.k6n.net sends you back to
 // exactly the view you requested, and URLs are shareable/bookmarkable.
@@ -12,14 +15,28 @@ export const TAB_PATHS: Record<string, string> = {
   "/setup": "tab4",
   "/trends": "tab5",
   "/search": "tab6",
-  "/frontend": "tab7",
   "/rum": "tab7",
+  "/frontend": "tab7",
 }
 
 
-/** Tab value for a location pathname, or null when the path isn't a tab. */
+/** Splits "/issues/abc/trace/def" into base "/issues" and rest ["abc","trace","def"]. */
+export function splitPath(path: string): { base: string; rest: string[] } {
+  const parts = path.split("/").filter(Boolean)
+  return { base: "/" + (parts[0] ?? ""), rest: parts.slice(1).map((p) => decodeURIComponent(p)) }
+}
+
+/** Builds a path below a tab base, e.g. joinPath("/issues", ["abc", "trace", "def"]). */
+export function joinPath(base: string, rest: string[]): string {
+  return [base, ...rest.map((p) => encodeURIComponent(p))].join("/")
+}
+
+/**
+ * Tab value for a location pathname, or null when the path isn't a tab.
+ * Deeper paths ("/issues/<id>/trace/<id>") belong to their first segment's tab.
+ */
 export function tabForPath(path: string): string | null {
-  return TAB_PATHS[path] ?? null
+  return TAB_PATHS[splitPath(path).base] ?? null
 }
 
 /** Canonical path for a tab value, or null when the tab has no route. */
@@ -29,3 +46,28 @@ export function pathForTab(tab: string): string | null {
   }
   return null
 }
+
+/**
+ * Sub-route below the current tab: `segments` are the path parts after the
+ * tab base and `go` rewrites them, so selection state (which issue, which
+ * trace, ...) lives in the URL and survives reloads, back/forward and sharing.
+ */
+export function useSubRoute(): {
+  segments: string[]
+  go: (segments: string[], options?: { replace?: boolean }) => void
+} {
+  const [location, navigate] = useLocation()
+  const { base, rest } = splitPath(location)
+  const key = rest.join("/")
+  // Stable identity per URL so effects can depend on `segments` directly.
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  const segments = useMemo(() => rest, [key])
+  const go = useCallback(
+    (next: string[], options?: { replace?: boolean }) => navigate(joinPath(base, next), options),
+    [base, navigate],
+  )
+  return { segments, go }
+}
+
+/** Short reference for showing an id to humans (full id stays in the URL). */
+export const shortId = (id: string) => (id.length > 10 ? id.slice(0, 8) : id)