diff --git a/client/src/utils/connectionsVisibility.test.ts b/client/src/utils/connectionsVisibility.test.ts
new file mode 100644
index 000000000..086e25b45
--- /dev/null
+++ b/client/src/utils/connectionsVisibility.test.ts
@@ -0,0 +1,113 @@
+import { describe, it, expect } from 'vitest'
+import {
+  parseStoredConnections, resolveEffectiveConnections, resolveVisibleConnectionIds,
+  toggleConnectionId, toggleAllConnections,
+} from './connectionsVisibility'
+
+describe('parseStoredConnections', () => {
+  it('returns null when nothing was ever stored', () => {
+    expect(parseStoredConnections(null)).toBeNull()
+  })
+
+  it('reads a legacy bare-array value as "only" mode', () => {
+    expect(parseStoredConnections('[1,2,3]')).toEqual({ mode: 'only', ids: [1, 2, 3] })
+  })
+
+  it('reads an empty legacy array as "only" mode with no ids', () => {
+    expect(parseStoredConnections('[]')).toEqual({ mode: 'only', ids: [] })
+  })
+
+  it('reads the tagged "only" object format', () => {
+    expect(parseStoredConnections('{"mode":"only","ids":[5]}')).toEqual({ mode: 'only', ids: [5] })
+  })
+
+  it('reads the tagged "all-except" object format', () => {
+    expect(parseStoredConnections('{"mode":"all-except","ids":[7,8]}')).toEqual({ mode: 'all-except', ids: [7, 8] })
+  })
+
+  it('returns null for invalid JSON', () => {
+    expect(parseStoredConnections('not json')).toBeNull()
+  })
+
+  it('returns null for a well-formed but unrecognized shape', () => {
+    expect(parseStoredConnections('{"foo":"bar"}')).toBeNull()
+    expect(parseStoredConnections('{"mode":"bogus","ids":[1]}')).toBeNull()
+    expect(parseStoredConnections('null')).toBeNull()
+    expect(parseStoredConnections('42')).toBeNull()
+  })
+})
+
+describe('resolveEffectiveConnections', () => {
+  it('returns the stored preference unchanged when present, regardless of the account default', () => {
+    const stored = { mode: 'only' as const, ids: [1] }
+    expect(resolveEffectiveConnections(stored, true)).toEqual(stored)
+    expect(resolveEffectiveConnections(stored, false)).toEqual(stored)
+  })
+
+  it('falls back to "all-except []" when nothing is stored and the account default is on', () => {
+    expect(resolveEffectiveConnections(null, true)).toEqual({ mode: 'all-except', ids: [] })
+  })
+
+  it('falls back to "only []" when nothing is stored and the account default is off', () => {
+    expect(resolveEffectiveConnections(null, false)).toEqual({ mode: 'only', ids: [] })
+  })
+})
+
+describe('resolveVisibleConnectionIds', () => {
+  it('"only" mode shows exactly the stored ids, independent of what is routable', () => {
+    expect(resolveVisibleConnectionIds({ mode: 'only', ids: [2, 4] }, [1, 2, 3, 4, 5])).toEqual([2, 4])
+  })
+
+  it('"all-except" mode shows every routable id except the stored ones', () => {
+    expect(resolveVisibleConnectionIds({ mode: 'all-except', ids: [3] }, [1, 2, 3, 4])).toEqual([1, 2, 4])
+  })
+
+  it('"all-except []" shows every routable id', () => {
+    expect(resolveVisibleConnectionIds({ mode: 'all-except', ids: [] }, [1, 2, 3])).toEqual([1, 2, 3])
+  })
+})
+
+describe('toggleConnectionId', () => {
+  it('materializes the account default before toggling when nothing is stored yet (default on)', () => {
+    // Default on -> effective mode is 'all-except []'; turning id 5 off adds it to the exceptions.
+    expect(toggleConnectionId(null, true, 5)).toEqual({ mode: 'all-except', ids: [5] })
+  })
+
+  it('materializes the account default before toggling when nothing is stored yet (default off)', () => {
+    // Default off -> effective mode is 'only []'; turning id 5 on adds it to the inclusion list.
+    expect(toggleConnectionId(null, false, 5)).toEqual({ mode: 'only', ids: [5] })
+  })
+
+  it('adds an id in "only" mode', () => {
+    expect(toggleConnectionId({ mode: 'only', ids: [1] }, false, 2)).toEqual({ mode: 'only', ids: [1, 2] })
+  })
+
+  it('removes an id already present in "only" mode', () => {
+    expect(toggleConnectionId({ mode: 'only', ids: [1, 2] }, false, 2)).toEqual({ mode: 'only', ids: [1] })
+  })
+
+  it('adds an id to the exception list in "all-except" mode (hides it)', () => {
+    expect(toggleConnectionId({ mode: 'all-except', ids: [] }, true, 9)).toEqual({ mode: 'all-except', ids: [9] })
+  })
+
+  it('removes an id from the exception list in "all-except" mode (re-shows it)', () => {
+    expect(toggleConnectionId({ mode: 'all-except', ids: [9] }, true, 9)).toEqual({ mode: 'all-except', ids: [] })
+  })
+})
+
+describe('toggleAllConnections', () => {
+  it('flips "only" to "all-except []"', () => {
+    expect(toggleAllConnections({ mode: 'only', ids: [1, 2] }, false)).toEqual({ mode: 'all-except', ids: [] })
+  })
+
+  it('flips "all-except" to "only []"', () => {
+    expect(toggleAllConnections({ mode: 'all-except', ids: [3] }, false)).toEqual({ mode: 'only', ids: [] })
+  })
+
+  it('materializes the account default first when nothing is stored yet, then flips it', () => {
+    // Default on -> effective is 'all-except []' -> flips to 'only []'.
+    expect(toggleAllConnections(null, true)).toEqual({ mode: 'only', ids: [] })
+    // Default off -> effective is 'only []' -> flips to 'all-except []'.
+    expect(toggleAllConnections(null, false)).toEqual({ mode: 'all-except', ids: [] })
+  })
+})
diff --git a/client/src/utils/connectionsVisibility.ts b/client/src/utils/connectionsVisibility.ts
new file mode 100644
index 000000000..edacd326d
--- /dev/null
+++ b/client/src/utils/connectionsVisibility.ts
@@ -0,0 +1,89 @@
+// Per-trip booking-route visibility, persisted in localStorage under
+// `trek:visible-connections:<tripId>` (see useTripPlanner.ts). Two modes:
+//   'only'        — nothing shown except `ids`
+//   'all-except'  — everything routable shown except `ids`
+// The account-wide "always show booking routes" setting only decides which
+// mode a trip starts in the first time it's ever touched (see
+// resolveEffectiveConnections) — once a trip has a stored preference, that
+// preference always wins, regardless of the account setting's current value.
+
+export type ConnectionsMode = 'only' | 'all-except'
+
+export interface StoredConnections {
+  mode: ConnectionsMode
+  ids: number[]
+}
+
+function isNumberArray(v: unknown): v is number[] {
+  return Array.isArray(v) && v.every(n => typeof n === 'number')
+}
+
+/**
+ * Parses the raw localStorage value for a trip's route-visibility
+ * preference. Returns null when nothing has ever been stored (the caller
+ * should fall back to the account-wide default in that case — see
+ * resolveEffectiveConnections). Accepts the legacy bare-array format (from
+ * before the 'all-except' mode existed) as an 'only' mode for backward
+ * compatibility with values already sitting in users' browsers.
+ */
+export function parseStoredConnections(raw: string | null): StoredConnections | null {
+  if (raw === null) return null
+  let parsed: unknown
+  try {
+    parsed = JSON.parse(raw)
+  } catch {
+    return null
+  }
+  if (isNumberArray(parsed)) return { mode: 'only', ids: parsed }
+  if (
+    parsed && typeof parsed === 'object' && !Array.isArray(parsed) &&
+    ((parsed as { mode?: unknown }).mode === 'only' || (parsed as { mode?: unknown }).mode === 'all-except') &&
+    isNumberArray((parsed as { ids?: unknown }).ids)
+  ) {
+    const obj = parsed as { mode: ConnectionsMode; ids: number[] }
+    return { mode: obj.mode, ids: obj.ids }
+  }
+  return null
+}
+
+/**
+ * The mode/ids actually in effect for a trip: the stored preference if the
+ * trip has one, otherwise the account-wide default (all routes shown, or
+ * none) as an ephemeral, unwritten fallback.
+ */
+export function resolveEffectiveConnections(stored: StoredConnections | null, alwaysShowRoutesDefault: boolean): StoredConnections {
+  if (stored) return stored
+  return alwaysShowRoutesDefault ? { mode: 'all-except', ids: [] } : { mode: 'only', ids: [] }
+}
+
+/** The concrete set of reservation ids visible right now. */
+export function resolveVisibleConnectionIds(effective: StoredConnections, routableIds: number[]): number[] {
+  if (effective.mode === 'only') return effective.ids
+  const excluded = new Set(effective.ids)
+  return routableIds.filter(id => !excluded.has(id))
+}
+
+/**
+ * Applies one reservation's toggle to the current stored preference,
+ * materializing the account-default fallback first if the trip has no
+ * stored preference yet. Toggling just flips the id's membership in `ids` —
+ * under 'only' that turns it on/off directly; under 'all-except' it
+ * removes/re-adds it from the exception list.
+ */
+export function toggleConnectionId(stored: StoredConnections | null, alwaysShowRoutesDefault: boolean, id: number): StoredConnections {
+  const base = resolveEffectiveConnections(stored, alwaysShowRoutesDefault)
+  const ids = base.ids.includes(id) ? base.ids.filter(x => x !== id) : [...base.ids, id]
+  return { mode: base.mode, ids }
+}
+
+/**
+ * Flips the whole trip between "show everything" and "show nothing" (the
+ * toolbar's bulk show-all/hide-all button), materializing the account
+ * default first if needed. Always starts the new mode with an empty
+ * ids list — an explicit bulk action discards whatever per-leg overrides
+ * applied to the mode it's leaving.
+ */
+export function toggleAllConnections(stored: StoredConnections | null, alwaysShowRoutesDefault: boolean): StoredConnections {
+  const base = resolveEffectiveConnections(stored, alwaysShowRoutesDefault)
+  return base.mode === 'all-except' ? { mode: 'only', ids: [] } : { mode: 'all-except', ids: [] }
+}