diff --git a/packages/app/src/pages/session.tsx b/packages/app/src/pages/session.tsx
index b2b897c62b..d6ef37c5fa 100644
--- a/packages/app/src/pages/session.tsx
+++ b/packages/app/src/pages/session.tsx
@@ -89,6 +89,7 @@ import { formatServerError, isSessionNotFoundError } from "@/utils/server-errors
 import { legacySessionHref, requireServerKey, sessionHref } from "@/utils/session-route"
 import { useUsageExceededDialogs } from "./session/usage-exceeded-dialogs"
 import { createSessionOwnership } from "./session/session-ownership"
+import { SessionRouteBoundary } from "./session/route-boundary"
 
 type FollowupItem = FollowupDraft & { id: string }
 type FollowupEdit = Pick<FollowupItem, "id" | "prompt" | "context">
@@ -146,34 +147,38 @@ export function SessionPage() {
 export function TargetSessionRoute() {
   const params = useParams<{ serverKey: string; id: string }>()
   return (
-    <Show when={`${params.serverKey}\0${params.id}`} keyed>
-      <SessionRouteErrorBoundary sessionID={params.id} serverKey={requireServerKey(params.serverKey)} padded>
-        <ResolvedTargetSessionRoute />
-      </SessionRouteErrorBoundary>
-    </Show>
+    <SessionRouteBoundary
+      serverKey={params.serverKey}
+      sessionID={params.id}
+      fallback={(error) => (
+        <SessionRouteFallback
+          error={error}
+          sessionID={params.id}
+          serverKey={requireServerKey(params.serverKey)}
+          padded
+        />
+      )}
+    >
+      <ResolvedTargetSessionRoute />
+    </SessionRouteBoundary>
   )
 }
 
-function SessionRouteErrorBoundary(
-  props: ParentProps<{ sessionID?: string; serverKey?: ServerConnection.Key; padded?: boolean }>,
-) {
+function SessionRouteFallback(props: {
+  error: unknown
+  sessionID?: string
+  serverKey?: ServerConnection.Key
+  padded?: boolean
+}) {
   const settings = useSettings()
   return (
-    <ErrorBoundary
-      fallback={(error) =>
-        settings.general.newLayoutDesigns() ? (
-          <SessionRouteFrame padded={props.padded}>
-            <SessionPanelFrame newLayout raised={!!props.sessionID}>
-              <SessionErrorFallback error={error} sessionID={props.sessionID} serverKey={props.serverKey} />
-            </SessionPanelFrame>
-          </SessionRouteFrame>
-        ) : (
-          <ErrorPage error={error} />
-        )
-      }
-    >
-      {props.children}
-    </ErrorBoundary>
+    <Show when={settings.general.newLayoutDesigns()} fallback={<ErrorPage error={props.error} />}>
+      <SessionRouteFrame padded={props.padded}>
+        <SessionPanelFrame newLayout raised={!!props.sessionID}>
+          <SessionErrorFallback error={props.error} sessionID={props.sessionID} serverKey={props.serverKey} />
+        </SessionPanelFrame>
+      </SessionRouteFrame>
+    </Show>
   )
 }
 
diff --git a/packages/app/src/pages/session/route-boundary.ts b/packages/app/src/pages/session/route-boundary.ts
new file mode 100644
index 0000000000..9df8d8e8c2
--- /dev/null
+++ b/packages/app/src/pages/session/route-boundary.ts
@@ -0,0 +1,37 @@
+import { ErrorBoundary, Show, createComponent, createEffect, on } from "solid-js"
+import type { JSX } from "solid-js"
+
+// Structural boundary for the target session route: decides when the route
+// subtree remounts and how route-level errors are scoped. Kept free of app
+// contexts (and JSX) so the remount semantics can be tested directly.
+//
+// Keyed by server only. Workspace-scoped state (notably TerminalProvider and
+// its PTY WebSockets) lives inside the route subtree, so switching session
+// tabs within the same workspace must not remount it; session changes are
+// handled reactively below (TargetSessionPage re-keys per workspace).
+export function SessionRouteBoundary(props: {
+  serverKey: string | undefined
+  sessionID: string | undefined
+  fallback: (error: unknown) => JSX.Element
+  children: JSX.Element
+}) {
+  return createComponent(Show, {
+    get when() {
+      return props.serverKey
+    },
+    keyed: true,
+    get children() {
+      return createComponent(ErrorBoundary, {
+        fallback: (error: unknown, reset: () => void) => {
+          // Without a per-session remount, a stale error (e.g. session not
+          // found) must clear when navigating to a different session.
+          createEffect(on(() => props.sessionID, reset, { defer: true }))
+          return props.fallback(error)
+        },
+        get children() {
+          return props.children
+        },
+      })
+    },
+  })
+}
diff --git a/packages/app/test-browser/session-route-boundary.test.ts b/packages/app/test-browser/session-route-boundary.test.ts
new file mode 100644
index 0000000000..13f35d43dc
--- /dev/null
+++ b/packages/app/test-browser/session-route-boundary.test.ts
@@ -0,0 +1,85 @@
+import { expect, test } from "bun:test"
+import { createComponent, createSignal, onCleanup } from "solid-js"
+import { render } from "solid-js/web"
+import { SessionRouteBoundary } from "@/pages/session/route-boundary"
+
+// Terminals (and other workspace-scoped state) live inside the route subtree,
+// so switching session tabs within the same server must not remount it.
+test("switching sessions on the same server does not remount the route subtree", () => {
+  const [server, setServer] = createSignal("srv_1")
+  const [session, setSession] = createSignal("ses_a")
+  let mounts = 0
+  let disposals = 0
+  const Probe = () => {
+    mounts += 1
+    onCleanup(() => {
+      disposals += 1
+    })
+    return null
+  }
+
+  const dispose = render(
+    () =>
+      createComponent(SessionRouteBoundary, {
+        get serverKey() {
+          return server()
+        },
+        get sessionID() {
+          return session()
+        },
+        fallback: () => null,
+        get children() {
+          return createComponent(Probe, {})
+        },
+      }),
+    document.createElement("div"),
+  )
+
+  const initialMounts = mounts
+  expect(initialMounts).toBeGreaterThan(0)
+
+  setSession("ses_b")
+  expect(mounts).toBe(initialMounts)
+  expect(disposals).toBe(0)
+
+  setServer("srv_2")
+  expect(mounts).toBeGreaterThan(initialMounts)
+  expect(disposals).toBeGreaterThan(0)
+
+  dispose()
+})
+
+// Without a per-session remount, the error boundary must clear a stale error
+// (e.g. session not found) when navigating to a different session.
+test("route error clears when navigating to a different session", () => {
+  const [session, setSession] = createSignal("ses_a")
+  const [broken, setBroken] = createSignal(true)
+  const Thrower = () => {
+    if (broken()) throw new Error(`Session not found: ${session()}`)
+    return "content"
+  }
+
+  const container = document.createElement("div")
+  const dispose = render(
+    () =>
+      createComponent(SessionRouteBoundary, {
+        serverKey: "srv_1",
+        get sessionID() {
+          return session()
+        },
+        fallback: () => "error-fallback",
+        get children() {
+          return createComponent(Thrower, {})
+        },
+      }),
+    container,
+  )
+
+  expect(container.textContent).toBe("error-fallback")
+
+  setBroken(false)
+  setSession("ses_b")
+  expect(container.textContent).toBe("content")
+
+  dispose()
+})