Merge phase 1 chantier A3: EnsureForAccount post-flipSymlink + IsPartialSwap

Bug #1 (CRITIQUE) — A3 flip+ensure inconsistency
- Before: EnsureForAccount failure after flip was WARN-only, SetActiveAccount
  still fired → daemon declared target active while shared symlinks were
  absent/divergent → transcripts silently duplicated, resume broken.
- After: ensure failure triggers rollback flip to previous account home;
  if rollback succeeds → explicit error, ActiveAccount stays on previous.
  If rollback ALSO fails → sticky partialSwap flag + ErrPartialSwap; all
  further swaps refused until operator intervention (daemon restart).
- New public IsPartialSwap() for watchdog / health-check integration.

Bug #10 (MOYENNE) — requiredShared contract never exercised
- All existing tests override a.sharedSymlinks with tmpdir-scoped lists,
  so symlinks.RequiredShared itself was never tested. A rename or drop
  would pass every test but silently break prod failover.
- TestRequiredSharedIsCoherent asserts (no filesystem): 3 entries with
  the exact required names, absolute targets, and a single shared parent
  directory (invariant EnsureForAccount depends on).

Tests:
- go test ./... PASS
- go test -race ./... PASS (no data race)
- 2 new switcher tests: TestFlipEnsureFailureTriggersRollback,
  TestFlipEnsureAndRollbackFailure
- 1 new symlinks test: TestRequiredSharedIsCoherent
- 1 obsolete test replaced: TestFlipEnsureSymlinksFailureDoesNotAbortSwap
  (encoded the old buggy best-effort behaviour)

VERSION.md

@@ -1,4 +1,161 @@
-# Version actuelle : 0.3.4
+# Version actuelle : 0.3.8
+
+## [0.3.8] - 2026-04-16
+**Type:** Patch — Bug #1 (A3 flip+ensure inconsistency) + Bug #10 (requiredShared contract test)
+
+### Corrigé — Bug #1 (CRITIQUE)
+- `AccountSwitcher.executeSwitch` ne continue plus silencieusement quand
+  `symlinks.EnsureForAccount` échoue après le flip : il **roll-back** le lien
+  `~/.claude` vers le home du compte précédent et **n'appelle pas**
+  `SetActiveAccount`. Évite l'état incohérent où le daemon déclare le compte
+  cible actif alors que ses shared symlinks sont divergents → transcripts
+  dupliqués silencieusement, resume cassé.
+- Si le rollback réussit : swap annulé, état filesystem = état pré-swap,
+  erreur explicite retournée par `executeSwitchE`.
+- Si ensure ET rollback échouent : `partialSwap` atomique sticky set,
+  `ErrPartialSwap` retourné, tout futur swap est refusé tant que le
+  daemon n'est pas redémarré par l'opérateur.
+- Nouvelle méthode publique `AccountSwitcher.IsPartialSwap() bool` pour
+  que health-checks et watchdog exposent l'état dégradé.
+
+### Ajouté — Tests Bug #1
+- `TestFlipEnsureFailureTriggersRollback` : plant un lien divergent sur
+  le home cible → ensure échoue → rollback réussit → `ActiveAccount` reste
+  compte1 → `~/.claude` pointe sur previousHome → `IsPartialSwap` = false.
+- `TestFlipEnsureAndRollbackFailure` : force les deux flips à échouer
+  (homeDir = fichier régulier) → `ErrPartialSwap` retourné, flag sticky
+  set, swap suivant refusé.
+
+### Ajouté — Bug #10
+- `TestRequiredSharedIsCoherent` (`internal/symlinks/shared_test.go`) :
+  valide le contrat de la constante package-level `RequiredShared` jamais
+  exercée auparavant (tous les autres tests utilisent un override scoped
+  en `t.TempDir()`). Vérifie sans toucher au filesystem :
+  - exactement 3 entrées (`session-env`, `file-history`, `projects`)
+  - targets absolus
+  - `filepath.Dir(target)` identique pour les 3 entrées (invariant
+    "3 liens sous un même shared root" sur lequel repose `EnsureForAccount`).
+
+### Rationale
+- Continuer après un ensure échoué revient à valider que le compte cible
+  est "sain" alors que les shared symlinks sont absents ou divergents.
+  Conséquence en prod : premier `claude --resume` écrit dans
+  `~/.claude/projects/` (privé) → transcripts dupliqués, undo
+  désynchronisé, failover complètement cassé sans log d'alerte.
+- Le rollback garantit qu'un compte cible mal configuré ne peut PAS
+  dégrader le state du daemon : on retourne à l'état pré-swap et on
+  signale l'erreur à l'appelant.
+- `ErrPartialSwap` + `IsPartialSwap()` documente un état où l'intervention
+  humaine est obligatoire — préférable à un retry automatique qui
+  empirerait la divergence.
+
+### Tests
+- ✅ `go test ./...` : tous les packages PASS
+- ✅ `go test -race ./...` : PASS, aucun data race
+- ✅ `go vet ./...` : clean
+- ✅ `go build ./...` : clean
+
+### Fichiers modifiés
+- `internal/switcher/account_switcher.go` (+rollback + IsPartialSwap + ErrPartialSwap)
+- `internal/switcher/account_switcher_test.go` (2 nouveaux tests, 1 test obsolète remplacé)
+- `internal/symlinks/shared_test.go` (+TestRequiredSharedIsCoherent)
+
+## [0.3.7] - 2026-04-16
+**Type:** Patch — Phase 1 / Chantier A3 : wire EnsureForAccount post-flip
+
+### Ajouté
+- `AccountSwitcher.executeSwitch` appelle désormais
+  `symlinks.EnsureForAccount(target.Home, ...)` **juste après** le flip
+  du lien principal `~/.claude`. Garantit que les 3 liens partagés
+  (`session-env`, `file-history`, `projects`) existent et pointent aux
+  bons targets sur le compte cible, même si celui-ci vient juste
+  d'être provisionné.
+- `AccountSwitcher.sharedSymlinks` : override test-only (accepte une
+  liste `[]symlinks.SharedSymlink`). Défaut = `symlinks.RequiredShared`.
+  Les tests peuvent scoper la réconciliation dans un `t.TempDir()` pour
+  ne jamais toucher `/home/ubuntu/.claude-*-shared`.
+- 2 tests unitaires :
+  - `TestFlipReconcilesSharedSymlinksOnTargetHome` : target home vide →
+    les 3 liens sont créés après le flip et pointent aux targets canoniques.
+  - `TestFlipEnsureSymlinksFailureDoesNotAbortSwap` : lien divergent
+    planté à la main → `EnsureForAccount` renvoie une erreur, logguée
+    en WARN, mais le swap complète quand même (best-effort post-flip).
+
+### Rationale
+- Sans cet appel, un compte cible fraîchement provisionné n'aurait
+  pas encore ses 3 liens ; au premier `claude --resume`, Claude Code
+  écrirait dans `~/.claude/projects/` (privé) au lieu de
+  `/home/ubuntu/.claude-projects-shared` → transcripts dupliqués,
+  undo désynchronisé, resume silencieusement cassé.
+- L'ensure est **best-effort** : une erreur est logguée en WARN mais
+  NE bloque PAS le flip. Si on abortait ici, on laisserait le daemon
+  dans un état incohérent (symlink déjà flippé mais `SetActiveAccount`
+  pas appelé).
+- L'opérateur voit le WARN dans les logs et peut corriger la
+  divergence manuellement (ex: lien pointant sur le mauvais target).
+
+### Tests
+- ✅ `go test ./...` : tous les packages PASS (incluant
+  `internal/switcher` et `internal/symlinks`).
+- ✅ `go test -race ./internal/switcher/...` : PASS.
+- ✅ `go vet ./...` : clean.
+
+### Fichiers modifiés
+- `internal/switcher/account_switcher.go`
+- `internal/switcher/account_switcher_test.go`
+
+## [0.3.6] - 2026-04-16
+**Type:** Patch — Phase 1 / Chantier A2 : validation des symlinks au startup
+
+### Ajouté
+- `Manager.ValidateSharedSymlinks()` : nouvelle méthode dans
+  `internal/lifecycle` qui agrège les `Home` de tous les comptes
+  configurés et délègue à `symlinks.ValidateAll`. Échoue dur si un
+  compte n'a pas de `home` défini ou si un lien est absent/divergent.
+- `cmd/claude-failover/main.go` appelle cette validation **avant**
+  `EnsureAllSessions()` : un état partagé cassé ne laissera plus le
+  daemon démarrer et divergér silencieusement.
+
+### Rationale
+- Un opérateur qui copie la config sur une nouvelle VM ne peut plus
+  oublier les liens — le daemon refuse de démarrer jusqu'à ce qu'ils
+  soient corrects.
+- Pas d'auto-heal sur divergence : on préfère un message d'erreur
+  explicite à un `rm -f` silencieux qui détruirait l'autre compte.
+
+### Tests
+- ✅ `go test ./...` : tous les packages PASS (incluant
+  `internal/lifecycle` et `internal/symlinks`).
+
+### Fichiers modifiés
+- `cmd/claude-failover/main.go` (+9)
+- `internal/lifecycle/manager.go` (+31)
+
+## [0.3.5] - 2026-04-16
+**Type:** Patch — Phase 1 / Chantier A1 : package `internal/symlinks`
+
+### Ajouté
+- `internal/symlinks/shared.go` : `EnsureForAccount` + `ValidateAll` qui
+  encodent en code la convention des 3 symlinks partagés par compte
+  (`session-env`, `file-history`, `projects`). Jusqu'à aujourd'hui ces
+  liens étaient maintenus à la main et leur absence silencieuse cassait
+  le failover (JSONL dupliqués, undo désynchronisé).
+- Tests unitaires couvrant : création missing, idempotence, divergence
+  (refus d'auto-correction pour éviter la perte de données), fichier
+  régulier à la place du lien, home vide, agrégation d'erreurs multi-comptes.
+
+### Rationale
+- Un déploiement sur une nouvelle VM ne peut plus omettre les liens.
+- Divergent link → erreur explicite, jamais de correction silencieuse.
+- Préparation des tâches A2 (ValidateAll au startup) et A3 (EnsureForAccount
+  post-flipSymlink dans le switcher).
+
+### Tests
+- ✅ `go test ./internal/symlinks/...` : 9/9 PASS
+
+### Fichiers ajoutés
+- `internal/symlinks/shared.go`
+- `internal/symlinks/shared_test.go`
 
 ## [0.3.4] - 2026-04-16
 **Type:** Patch — Dispatcher ne route JAMAIS vers les sessions dédiées

WORK_IN_PROGRESS.md

@@ -1,13 +1,26 @@
 # Travaux en Cours - claude-failover
 
 ## Dernière mise à jour
-2026-04-15 19:30:00
+2026-04-16 19:00:00
 
 ## Version Actuelle
-0.3.0
+0.3.5 (en cours de progression vers 0.4.0)
 
 ## Demande Actuelle
-Aucune — v0.2.3 shippée, service stable.
+**Phase 1 / Chantier A — Failover robuste** (spec dans `ccl-platform/phases/phase1/A-failover.md`).
+Rendre le failover compte1 ↔ compte2 déterministe en intégrant dans le code les fixes manuels
+(symlinks partagés), en ajoutant un registre UUID fiable, et en durcissant tmux send-keys.
+
+Branche : `feat/phase1-A-failover-robust`.
+
+## Sous-tâches Chantier A
+- [x] A1 — `internal/symlinks/shared.go` (+ tests) — v0.3.5
+- [ ] A2 — `lifecycle/manager.go` : `ValidateAll` au startup
+- [ ] A3 — `switcher/account_switcher.go` : `EnsureForAccount` post-flip
+- [ ] A4 — `internal/registry/uuid_registry.go` (+ tests)
+- [ ] A5 — `internal/tmux/send.go` avec retry exponentiel (+ tests)
+- [ ] A6 — Capture UUID 200 → 500 lignes
+- [ ] A7 — `scripts/test-failover.sh` dans ccl-platform + scripts associés
 
 ## Étapes Complétées
 - [x] v0.2.1 — Cooldown post-swap + log forensique (trigger_session, pattern, snippet)

cmd/claude-failover/main.go

@@ -51,6 +51,15 @@ func main() {
 	// Initialise tmux client and lifecycle manager.
 	tmuxClient := tmux.NewExecClient()
 	lm := lifecycle.New(tmuxClient, s, cfg)
+
+	// Validate (and self-heal) the shared-state symlinks BEFORE spawning
+	// any sessions. A divergent link would silently fork transcripts
+	// between accounts and make failover destructive, so we fail fast here
+	// rather than after work is in flight.
+	if err := lm.ValidateSharedSymlinks(); err != nil {
+		log.Fatalf("shared symlinks validation failed: %v", err)
+	}
+
 	lm.EnsureAllSessions()
 
 	// Block until SIGINT or SIGTERM.

internal/lifecycle/manager.go

@@ -4,11 +4,13 @@ package lifecycle
 
 import (
 	"context"
+	"fmt"
 	"log"
 	"time"
 
 	"forge.secuaas.ovh/olivier/claude-failover/internal/config"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/state"
+	"forge.secuaas.ovh/olivier/claude-failover/internal/symlinks"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/tmux"
 )
 
@@ -47,6 +49,35 @@ func (m *Manager) Run(ctx context.Context) {
 	}
 }
 
+// ValidateSharedSymlinks verifies that every configured account home has
+// the three shared-state symlinks (session-env, file-history, projects)
+// in place and pointing at the canonical shared targets.
+//
+// Called once at daemon startup BEFORE sessions are recreated. A missing
+// or divergent link would silently fork the state tree between the two
+// accounts, breaking failover. We fail fast so the operator fixes it
+// before any work is in flight.
+//
+// EnsureForAccount creates missing links but refuses to touch divergent
+// ones — see internal/symlinks for the rationale.
+func (m *Manager) ValidateSharedSymlinks() error {
+	if len(m.config.Accounts) == 0 {
+		return fmt.Errorf("[lifecycle] no accounts configured — cannot validate shared symlinks")
+	}
+	homes := make([]string, 0, len(m.config.Accounts))
+	for _, acc := range m.config.Accounts {
+		if acc.Home == "" {
+			return fmt.Errorf("[lifecycle] account %q has empty home — refusing to continue", acc.Name)
+		}
+		homes = append(homes, acc.Home)
+	}
+	if err := symlinks.ValidateAll(homes, symlinks.RequiredShared); err != nil {
+		return fmt.Errorf("shared symlinks invalid, refusing to start: %w", err)
+	}
+	m.logger.Printf("[lifecycle] shared symlinks OK for %d account(s)", len(homes))
+	return nil
+}
+
 // EnsureAllSessions creates all configured sessions that are not yet present in tmux.
 // It is intended to be called once at daemon startup before Run is launched.
 func (m *Manager) EnsureAllSessions() {

internal/switcher/account_switcher.go

@@ -4,6 +4,7 @@ package switcher
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log"
 	"os"
@@ -11,15 +12,27 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"sync/atomic"
 	"time"
 
 	"forge.secuaas.ovh/olivier/claude-failover/internal/config"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/notify"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/quota"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/state"
+	"forge.secuaas.ovh/olivier/claude-failover/internal/symlinks"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/tmux"
 )
 
+// ErrPartialSwap is returned (and wrapped) when the switcher flipped
+// ~/.claude to the target home, EnsureForAccount failed on the target,
+// and the rollback flip back to the previous home ALSO failed. The daemon
+// is in a documented degraded state: the active-account setter was NOT
+// called, but the filesystem symlink may point at an account whose shared
+// state is inconsistent. Operator intervention is required. Callers can
+// interrogate AccountSwitcher.IsPartialSwap() to expose the flag to
+// health-checks / watchdogs.
+var ErrPartialSwap = errors.New("switcher: partial swap — flip succeeded but ensure + rollback both failed")
+
 // SwitchState represents the current phase of a failover operation.
 type SwitchState string
 
@@ -52,6 +65,19 @@ type AccountSwitcher struct {
 	// homeDir is the directory containing the .claude symlink. Overridable for tests.
 	// When empty, os.UserHomeDir() is used.
 	homeDir string
+	// sharedSymlinks is the list of shared-state links reconciled on the
+	// target account home after every flip. Overridable for tests so the
+	// suite never touches the operator's real /home/ubuntu/.claude-*
+	// shared directories. When nil, symlinks.RequiredShared is used.
+	sharedSymlinks []symlinks.SharedSymlink
+	// partialSwap is set to 1 when a flip+ensure+rollback sequence left the
+	// daemon in an inconsistent state (symlink possibly flipped, but active
+	// account NOT updated, and rollback flip ALSO failed). Health-checks /
+	// watchdogs read this flag via IsPartialSwap(). It is sticky: once set,
+	// it stays set until the operator restarts the daemon after fixing the
+	// filesystem state. We use atomic access so watchdog goroutines can read
+	// it without blocking the switcher.
+	partialSwap atomic.Bool
 }
 
 // New creates an AccountSwitcher.
@@ -88,8 +114,30 @@ func (a *AccountSwitcher) Run(ctx context.Context) {
 
 // executeSwitch performs the full failover sequence.
 func (a *AccountSwitcher) executeSwitch(req quota.SwitchRequest) {
+	if err := a.executeSwitchE(req); err != nil {
+		// executeSwitchE already logs the detail; we swallow the error here
+		// because the public Run loop has no return channel. The partialSwap
+		// flag (if set) remains visible via IsPartialSwap().
+		a.logger.Printf("[switcher] SWAP aborted: %v", err)
+	}
+}
+
+// executeSwitchE runs the swap and returns an error describing any abort or
+// partial-swap condition. Split out from executeSwitch so tests can assert
+// on the error value without routing through a channel.
+func (a *AccountSwitcher) executeSwitchE(req quota.SwitchRequest) error {
 	a.logger.Printf("[switcher] SWAP initiated from=%q reset=%q", req.From, req.ResetTime)
 
+	// Refuse to proceed if a previous swap left the daemon in an
+	// inconsistent state. The operator must intervene (fix the filesystem,
+	// restart the daemon) before any further failover can be attempted —
+	// otherwise we'd stack symlink flips on top of a broken state.
+	if a.partialSwap.Load() {
+		err := fmt.Errorf("refusing swap: daemon is in partial-swap degraded state (operator intervention required)")
+		a.logger.Printf("[switcher] %v", err)
+		return err
+	}
+
 	// 1. SAVING — capture resume UUIDs from all working sessions plus
 	// every dedicated session unconditionally (dedicated sessions are
 	// user-driven and may not be tracked as "working" in state, but their
@@ -104,12 +152,52 @@ func (a *AccountSwitcher) executeSwitch(req quota.SwitchRequest) {
 	if target == nil {
 		a.logger.Printf("[switcher] no alternate account found for %q — aborting swap", req.From)
 		a.currentState = StateNormal
-		return
+		return nil
 	}
+	previous := a.findAccountByName(req.From)
 
 	if err := a.flipSymlink(target.Home); err != nil {
 		a.logger.Printf("[switcher] flipSymlink error: %v", err)
 	}
+	// Ensure the target account home exposes the three shared-state
+	// symlinks (session-env, file-history, projects). If this fails we
+	// MUST NOT proceed with SetActiveAccount — the daemon would otherwise
+	// declare the target "active" while its shared state is divergent,
+	// silently writing transcripts into private /projects directories and
+	// breaking `claude --resume` across sessions. Instead we attempt to
+	// roll back the ~/.claude flip to the previous account. If the
+	// rollback also fails, the daemon is in a documented degraded state
+	// (ErrPartialSwap) and the operator must intervene.
+	if err := symlinks.EnsureForAccount(target.Home, a.requiredShared()); err != nil {
+		a.logger.Printf("[switcher] ensure shared symlinks for %q failed: %v — attempting rollback", target.Home, err)
+		if previous == nil || previous.Home == "" {
+			// No known previous home to roll back to — set the degraded
+			// flag and bail out. This is equivalent to a rollback failure
+			// because the filesystem is pointed at a broken target.
+			a.partialSwap.Store(true)
+			a.currentState = StateNormal
+			return fmt.Errorf("%w: ensure failed (%v) and no previous account home is known for rollback", ErrPartialSwap, err)
+		}
+		if rbErr := a.flipSymlink(previous.Home); rbErr != nil {
+			// Both the ensure AND the rollback failed. The daemon is now
+			// in a documented inconsistent state: ~/.claude may point at
+			// target whose shared-state is divergent, but SetActiveAccount
+			// has NOT been called so state.ActiveAccount is still the
+			// previous account. No further failover can be attempted
+			// until the operator intervenes.
+			a.partialSwap.Store(true)
+			a.logger.Printf("[switcher] CRITICAL partial swap: ensure=%v rollback=%v — daemon in degraded state, operator intervention required", err, rbErr)
+			a.currentState = StateNormal
+			return fmt.Errorf("%w: ensure=%v rollback=%v", ErrPartialSwap, err, rbErr)
+		}
+		// Rollback succeeded — symlink is back on the previous account,
+		// SetActiveAccount was NEVER called, state is consistent with
+		// "no swap happened". Return an explicit error so the caller
+		// knows the swap was cancelled.
+		a.logger.Printf("[switcher] rollback successful: ~/.claude → %s (swap cancelled)", previous.Home)
+		a.currentState = StateNormal
+		return fmt.Errorf("swap cancelled: ensure shared symlinks failed on target %q: %w", target.Home, err)
+	}
 	a.killAllPoolSessions()
 	a.recreatePoolSessions()
 	a.relaunchDedicatedSessions(target.Home)
@@ -135,6 +223,31 @@ func (a *AccountSwitcher) executeSwitch(req quota.SwitchRequest) {
 	}
 
 	a.currentState = StateNormal
+	return nil
+}
+
+// IsPartialSwap reports whether the switcher is in a degraded state after a
+// flip+ensure+rollback sequence all failed. Health-checks and watchdogs use
+// this signal to surface an operator-actionable alert. The flag is sticky
+// for the lifetime of the process: once set, it remains set until the daemon
+// is restarted (after the operator has fixed the filesystem).
+func (a *AccountSwitcher) IsPartialSwap() bool {
+	return a.partialSwap.Load()
+}
+
+// findAccountByName returns the account config entry matching name, or nil.
+// Unlike findTargetAccount (which returns the first NON-matching account),
+// this is used by the rollback path to recover the previous home.
+func (a *AccountSwitcher) findAccountByName(name string) *config.AccountConfig {
+	if name == "" {
+		return nil
+	}
+	for i := range a.config.Accounts {
+		if a.config.Accounts[i].Name == name {
+			return &a.config.Accounts[i]
+		}
+	}
+	return nil
 }
 
 // saveDedicatedUUIDs captures the resume UUID for every configured dedicated
@@ -225,6 +338,16 @@ func (a *AccountSwitcher) saveAllSessions() {
 	})
 }
 
+// requiredShared returns the shared-symlink list used when reconciling the
+// target account home after a flip. Tests may set a.sharedSymlinks to a
+// tmpdir-scoped list so they never touch /home/ubuntu/.claude-*-shared.
+func (a *AccountSwitcher) requiredShared() []symlinks.SharedSymlink {
+	if a.sharedSymlinks != nil {
+		return a.sharedSymlinks
+	}
+	return symlinks.RequiredShared
+}
+
 // resolveHomeDir returns the configured homeDir (test override) or the real
 // user home. Tests MUST set a.homeDir to a tmpdir to avoid clobbering the
 // production ~/.claude symlink.

internal/switcher/account_switcher_test.go

@@ -1,6 +1,9 @@
 package switcher
 
 import (
+	"errors"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"time"
@@ -8,8 +11,19 @@ import (
 	"forge.secuaas.ovh/olivier/claude-failover/internal/config"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/quota"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/state"
+	"forge.secuaas.ovh/olivier/claude-failover/internal/symlinks"
 )
 
+// tmpShared returns a SharedSymlink list whose targets live entirely under
+// tmpDir, so switcher tests never touch /home/ubuntu/.claude-*-shared.
+func tmpShared(tmpDir string) []symlinks.SharedSymlink {
+	return []symlinks.SharedSymlink{
+		{Target: filepath.Join(tmpDir, "session-env-shared"), Name: "session-env"},
+		{Target: filepath.Join(tmpDir, "file-history-shared"), Name: "file-history"},
+		{Target: filepath.Join(tmpDir, "projects-shared"), Name: "projects"},
+	}
+}
+
 // mockTmux for switcher tests.
 type mockTmux struct {
 	sessions     map[string]bool
@@ -143,6 +157,9 @@ func TestKillAndRecreatePoolSessions(t *testing.T) {
 	// touches the real ~/.claude (regression: a reboot used to leave Claude
 	// Code unusable because the test had repointed ~/.claude to /tmp/...).
 	a.homeDir = t.TempDir()
+	// Scope shared-symlink targets to a tmpdir so the post-flip ensure
+	// pass does not write inside /home/ubuntu/.claude-*-shared.
+	a.sharedSymlinks = tmpShared(t.TempDir())
 	a.executeSwitch(quota.SwitchRequest{From: "compte1"})
 
 	// Active account must have changed.
@@ -186,10 +203,12 @@ func TestDedicatedRelaunchAfterSwap(t *testing.T) {
 	s := state.New("")
 	s.SetActiveAccount("compte1")
 
+	home1 := filepath.Join(t.TempDir(), "claude-1-xxxx")
+	home2 := filepath.Join(t.TempDir(), "claude-2-xxxx")
 	cfg := &config.Config{
 		Accounts: []config.AccountConfig{
-			{Name: "compte1", Home: "/tmp/claude-1-xxxx"},
-			{Name: "compte2", Home: "/tmp/claude-2-xxxx"},
+			{Name: "compte1", Home: home1},
+			{Name: "compte2", Home: home2},
 		},
 		Pool: config.PoolConfig{
 			Dedicated:  []config.DedicatedSession{{Name: "dedicated-1", Project: "/tmp"}},
@@ -199,6 +218,7 @@ func TestDedicatedRelaunchAfterSwap(t *testing.T) {
 
 	a := New(tc, s, cfg, make(chan quota.SwitchRequest), nil)
 	a.homeDir = t.TempDir()
+	a.sharedSymlinks = tmpShared(t.TempDir())
 	a.executeSwitch(quota.SwitchRequest{From: "compte1"})
 
 	// The relaunch must send a resume command on the dedicated session,
@@ -213,10 +233,225 @@ func TestDedicatedRelaunchAfterSwap(t *testing.T) {
 	if relaunch == "" {
 		t.Fatalf("expected dedicated-1 relaunch send-keys; got %v", tc.sendKeyCalls)
 	}
-	if !strings.Contains(relaunch, "CLAUDE_CONFIG_DIR=/tmp/claude-2-xxxx") {
+	if !strings.Contains(relaunch, "CLAUDE_CONFIG_DIR="+home2) {
 		t.Errorf("relaunch should set CLAUDE_CONFIG_DIR to target home; got %q", relaunch)
 	}
 	if !strings.Contains(relaunch, "aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee") {
 		t.Errorf("relaunch should include captured UUID; got %q", relaunch)
 	}
 }
+
+// TestFlipReconcilesSharedSymlinksOnTargetHome verifies A3: after the main
+// ~/.claude flip, the switcher reconciles the three shared-state symlinks
+// (session-env / file-history / projects) on the TARGET account home.
+// Scenario: the target home has NO links yet — a freshly-provisioned account
+// that has never been flipped into. Post-switch, all three links must exist
+// inside the target home and point at the canonical shared targets.
+func TestFlipReconcilesSharedSymlinksOnTargetHome(t *testing.T) {
+	tc := newMockTmux()
+
+	s := state.New("")
+	s.SetActiveAccount("compte1")
+
+	// Target home starts empty: EnsureForAccount will mkdir + create links.
+	targetHome := filepath.Join(t.TempDir(), "claude-compte2")
+	cfg := &config.Config{
+		Accounts: []config.AccountConfig{
+			{Name: "compte1", Home: filepath.Join(t.TempDir(), "claude-compte1")},
+			{Name: "compte2", Home: targetHome},
+		},
+		Pool: config.PoolConfig{
+			Autonomous: config.AutonomousConfig{Prefix: "ccl-auto-", Min: 0, Max: 0},
+		},
+	}
+
+	a := New(tc, s, cfg, make(chan quota.SwitchRequest), nil)
+	a.homeDir = t.TempDir()
+	shared := tmpShared(t.TempDir())
+	a.sharedSymlinks = shared
+
+	// Pre-assert: no link exists in targetHome.
+	for _, sl := range shared {
+		if _, err := os.Lstat(filepath.Join(targetHome, sl.Name)); !os.IsNotExist(err) {
+			t.Fatalf("pre-condition: %q should not exist yet (err=%v)", sl.Name, err)
+		}
+	}
+
+	a.executeSwitch(quota.SwitchRequest{From: "compte1"})
+
+	// Post-assert: every required link exists and points at the canonical
+	// target under the tmpdir-scoped shared root.
+	for _, sl := range shared {
+		linkPath := filepath.Join(targetHome, sl.Name)
+		info, err := os.Lstat(linkPath)
+		if err != nil {
+			t.Errorf("expected link at %s after flip: %v", linkPath, err)
+			continue
+		}
+		if info.Mode()&os.ModeSymlink == 0 {
+			t.Errorf("%s exists but is not a symlink", linkPath)
+			continue
+		}
+		got, err := os.Readlink(linkPath)
+		if err != nil {
+			t.Errorf("readlink %s: %v", linkPath, err)
+			continue
+		}
+		if got != sl.Target {
+			t.Errorf("link %s points to %q, want %q", linkPath, got, sl.Target)
+		}
+	}
+}
+
+// TestFlipEnsureFailureTriggersRollback verifies the fix for the A3 bug
+// (flip+ensure inconsistency): if EnsureForAccount fails on the target home
+// after the ~/.claude flip, the switcher MUST NOT mark the target account
+// active. It must instead roll back the ~/.claude symlink to the previous
+// account's home, leaving the daemon in the pre-swap state so subsequent
+// session work keeps writing to the known-good shared state.
+//
+// Old (buggy) behaviour: ensure error was WARN-only, SetActiveAccount still
+// happened, dedicated sessions were relaunched against a target whose
+// /projects, /session-env, /file-history were missing or divergent →
+// transcripts duplicated silently, resume broke, undo history diverged.
+func TestFlipEnsureFailureTriggersRollback(t *testing.T) {
+	tc := newMockTmux()
+
+	s := state.New("")
+	s.SetActiveAccount("compte1")
+
+	previousHome := filepath.Join(t.TempDir(), "claude-compte1")
+	targetHome := filepath.Join(t.TempDir(), "claude-compte2")
+	if err := os.MkdirAll(previousHome, 0700); err != nil {
+		t.Fatalf("mkdir previous home: %v", err)
+	}
+	if err := os.MkdirAll(targetHome, 0700); err != nil {
+		t.Fatalf("mkdir target home: %v", err)
+	}
+	// Plant a divergent link at <targetHome>/session-env. The symlinks
+	// package refuses to auto-correct this (data-loss safeguard) and will
+	// return an error, which must now trigger a rollback.
+	bogus := filepath.Join(t.TempDir(), "somewhere-else")
+	if err := os.MkdirAll(bogus, 0700); err != nil {
+		t.Fatalf("mkdir bogus: %v", err)
+	}
+	if err := os.Symlink(bogus, filepath.Join(targetHome, "session-env")); err != nil {
+		t.Fatalf("plant divergent link: %v", err)
+	}
+
+	cfg := &config.Config{
+		Accounts: []config.AccountConfig{
+			{Name: "compte1", Home: previousHome},
+			{Name: "compte2", Home: targetHome},
+		},
+		Pool: config.PoolConfig{
+			Autonomous: config.AutonomousConfig{Prefix: "ccl-auto-", Min: 0, Max: 0},
+		},
+	}
+
+	a := New(tc, s, cfg, make(chan quota.SwitchRequest), nil)
+	homeDir := t.TempDir()
+	a.homeDir = homeDir
+	a.sharedSymlinks = tmpShared(t.TempDir())
+
+	err := a.executeSwitchE(quota.SwitchRequest{From: "compte1"})
+	if err == nil {
+		t.Fatalf("executeSwitchE: expected cancellation error, got nil")
+	}
+	// The public symmetric swap-cancelled error must mention ensure and
+	// wrap the underlying symlinks package message. ErrPartialSwap must
+	// NOT be set (rollback succeeded → recoverable condition).
+	if errors.Is(err, ErrPartialSwap) {
+		t.Errorf("did not expect ErrPartialSwap; rollback succeeded; got %v", err)
+	}
+	if a.IsPartialSwap() {
+		t.Errorf("IsPartialSwap should be false when rollback succeeds")
+	}
+
+	// Active account must remain the previous one — SetActiveAccount must
+	// NOT have been called.
+	if got := s.ActiveAccount(); got != "compte1" {
+		t.Errorf("active account should stay compte1 after rollback; got %q", got)
+	}
+
+	// ~/.claude must now point at the previous home (rollback target).
+	link, rlErr := os.Readlink(filepath.Join(homeDir, ".claude"))
+	if rlErr != nil {
+		t.Fatalf("readlink ~/.claude: %v", rlErr)
+	}
+	if link != previousHome {
+		t.Errorf("~/.claude should point at previous home %q after rollback; got %q", previousHome, link)
+	}
+}
+
+// TestFlipEnsureAndRollbackFailure verifies that when BOTH EnsureForAccount
+// AND the rollback flip fail, the switcher sets the sticky partial-swap
+// flag and returns ErrPartialSwap. The daemon is then in a documented
+// degraded state where any further swap is refused until the operator
+// restarts it.
+func TestFlipEnsureAndRollbackFailure(t *testing.T) {
+	tc := newMockTmux()
+
+	s := state.New("")
+	s.SetActiveAccount("compte1")
+
+	previousHome := filepath.Join(t.TempDir(), "claude-compte1")
+	targetHome := filepath.Join(t.TempDir(), "claude-compte2")
+	if err := os.MkdirAll(previousHome, 0700); err != nil {
+		t.Fatalf("mkdir previous home: %v", err)
+	}
+	if err := os.MkdirAll(targetHome, 0700); err != nil {
+		t.Fatalf("mkdir target home: %v", err)
+	}
+	// Plant the divergent link that will cause EnsureForAccount to fail.
+	bogus := filepath.Join(t.TempDir(), "somewhere-else")
+	if err := os.MkdirAll(bogus, 0700); err != nil {
+		t.Fatalf("mkdir bogus: %v", err)
+	}
+	if err := os.Symlink(bogus, filepath.Join(targetHome, "session-env")); err != nil {
+		t.Fatalf("plant divergent link: %v", err)
+	}
+
+	cfg := &config.Config{
+		Accounts: []config.AccountConfig{
+			{Name: "compte1", Home: previousHome},
+			{Name: "compte2", Home: targetHome},
+		},
+		Pool: config.PoolConfig{
+			Autonomous: config.AutonomousConfig{Prefix: "ccl-auto-", Min: 0, Max: 0},
+		},
+	}
+
+	a := New(tc, s, cfg, make(chan quota.SwitchRequest), nil)
+
+	// Force the rollback flip to fail: point homeDir at a file that cannot
+	// host a .claude symlink. We use a regular file; the flipSymlink
+	// implementation does os.Remove() then os.Symlink() under homeDir,
+	// which fails when homeDir is itself a file (ENOTDIR).
+	badHomeFile := filepath.Join(t.TempDir(), "not-a-dir")
+	if err := os.WriteFile(badHomeFile, []byte("block"), 0600); err != nil {
+		t.Fatalf("write bad home: %v", err)
+	}
+	a.homeDir = badHomeFile
+	a.sharedSymlinks = tmpShared(t.TempDir())
+
+	err := a.executeSwitchE(quota.SwitchRequest{From: "compte1"})
+	if err == nil {
+		t.Fatalf("expected ErrPartialSwap, got nil")
+	}
+	if !errors.Is(err, ErrPartialSwap) {
+		t.Errorf("expected ErrPartialSwap, got %v", err)
+	}
+	if !a.IsPartialSwap() {
+		t.Errorf("IsPartialSwap should be true when both ensure AND rollback fail")
+	}
+	// SetActiveAccount must still not have been called.
+	if got := s.ActiveAccount(); got != "compte1" {
+		t.Errorf("active account must stay compte1 in partial-swap; got %q", got)
+	}
+
+	// A subsequent swap attempt must be refused while the flag is set.
+	if err2 := a.executeSwitchE(quota.SwitchRequest{From: "compte1"}); err2 == nil {
+		t.Errorf("expected subsequent swap to be refused in degraded state")
+	}
+}

internal/symlinks/shared.go

@@ -0,0 +1,165 @@
+// Package symlinks manages the shared-state symlinks that every Claude
+// account home must expose, so that account failover does not create state
+// divergence (duplicated JSONL transcripts, broken undo history, drifted
+// session env).
+//
+// Rationale
+//
+// Claude Code stores three directories whose content MUST be identical
+// across the two configured accounts for failover to be a no-op:
+//
+//   - projects/      — session JSONL transcripts (used by `claude --resume`)
+//   - session-env/   — per-session environment and working-dir metadata
+//   - file-history/  — undo/redo history persistence
+//
+// If account A writes under `~/.claude-compte1/projects/...` while account
+// B later runs under `~/.claude-compte2/projects/...`, resume fails
+// silently with "session not found" and the operator loses every in-flight
+// conversation.
+//
+// Historically we fixed this by creating symlinks manually on the
+// operator's VM. Any fresh deployment that forgets those links silently
+// reintroduces the bug. This package encodes the convention in code:
+// EnsureForAccount creates missing links, ValidateAll fails fast at
+// startup when an account home is misconfigured.
+package symlinks
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// DefaultSharedRoot is the directory under which the three shared targets
+// live. All SharedSymlink.Target values default to a subdirectory of this
+// root so tests can override the root without rewriting the shared list.
+const DefaultSharedRoot = "/home/ubuntu"
+
+// SharedSymlink describes one required link inside a Claude account home.
+//
+// Target is the absolute path on disk that holds the real shared
+// directory. Name is the basename of the link that must exist inside each
+// account home (e.g. `session-env`, `file-history`, `projects`).
+type SharedSymlink struct {
+	Target string
+	Name   string
+}
+
+// RequiredShared lists the three symlinks every Claude account home must
+// expose. The list is package-level so integration tests can read it, but
+// callers SHOULD prefer the EnsureForAccount / ValidateAll entry points
+// that accept an override list for isolation.
+var RequiredShared = []SharedSymlink{
+	{Target: "/home/ubuntu/.claude-session-env-shared", Name: "session-env"},
+	{Target: "/home/ubuntu/.claude-file-history-shared", Name: "file-history"},
+	{Target: "/home/ubuntu/.claude-projects-shared", Name: "projects"},
+}
+
+// EnsureForAccount verifies (and creates if missing) every required shared
+// symlink for a single account home. Behaviour:
+//
+//   - If accountHome does not exist, it is created (mode 0700).
+//   - If Target (the shared destination) does not exist, it is created
+//     (mode 0700). Both accounts pointing at a non-existent target would
+//     produce two separate state trees on first write.
+//   - If the link is absent, it is created.
+//   - If the link is present and points at Target, nothing happens.
+//   - If the link is present but points elsewhere, an error is returned.
+//     We REFUSE to auto-correct a divergent link because fixing it blindly
+//     could delete user data: the "wrong" target may contain the only copy
+//     of the session transcripts.
+//   - If a regular file or directory exists where the link should be,
+//     an error is returned for the same reason.
+func EnsureForAccount(accountHome string, required []SharedSymlink) error {
+	if accountHome == "" {
+		return errors.New("symlinks: accountHome is empty")
+	}
+
+	if err := os.MkdirAll(accountHome, 0700); err != nil {
+		return fmt.Errorf("symlinks: create account home %q: %w", accountHome, err)
+	}
+
+	for _, sl := range required {
+		if err := ensureTarget(sl.Target); err != nil {
+			return err
+		}
+		if err := ensureLink(accountHome, sl); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// ValidateAll runs EnsureForAccount on every account home. It aggregates
+// all errors and returns a single error with every failure inlined, so the
+// operator sees the full picture at startup rather than fixing one link,
+// restarting, hitting the next one, repeat.
+func ValidateAll(accountHomes []string, required []SharedSymlink) error {
+	if len(accountHomes) == 0 {
+		return errors.New("symlinks: no account homes provided")
+	}
+	var errs []string
+	for _, home := range accountHomes {
+		if err := EnsureForAccount(home, required); err != nil {
+			errs = append(errs, err.Error())
+		}
+	}
+	if len(errs) > 0 {
+		return fmt.Errorf("symlinks: validation failed for %d account home(s): %s",
+			len(errs), strings.Join(errs, "; "))
+	}
+	return nil
+}
+
+// ensureTarget creates Target as an empty directory when absent.
+// An existing file (non-directory, non-symlink) at Target is an operator
+// error we cannot resolve automatically.
+func ensureTarget(target string) error {
+	info, err := os.Stat(target)
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return fmt.Errorf("symlinks: stat shared target %q: %w", target, err)
+		}
+		if mkErr := os.MkdirAll(target, 0700); mkErr != nil {
+			return fmt.Errorf("symlinks: create shared target %q: %w", target, mkErr)
+		}
+		return nil
+	}
+	if !info.IsDir() {
+		return fmt.Errorf("symlinks: shared target %q is not a directory", target)
+	}
+	return nil
+}
+
+// ensureLink reconciles one link entry inside accountHome.
+func ensureLink(accountHome string, sl SharedSymlink) error {
+	linkPath := filepath.Join(accountHome, sl.Name)
+
+	info, err := os.Lstat(linkPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			if linkErr := os.Symlink(sl.Target, linkPath); linkErr != nil {
+				return fmt.Errorf("symlinks: create %q → %q: %w", linkPath, sl.Target, linkErr)
+			}
+			return nil
+		}
+		return fmt.Errorf("symlinks: lstat %q: %w", linkPath, err)
+	}
+
+	// Path exists — must be a symlink pointing at Target.
+	if info.Mode()&os.ModeSymlink == 0 {
+		return fmt.Errorf("symlinks: %q exists but is not a symlink (expected → %q)",
+			linkPath, sl.Target)
+	}
+	currentTarget, err := os.Readlink(linkPath)
+	if err != nil {
+		return fmt.Errorf("symlinks: readlink %q: %w", linkPath, err)
+	}
+	if currentTarget != sl.Target {
+		return fmt.Errorf("symlinks: divergent link at %q: points to %q, expected %q (refusing to auto-correct to avoid data loss)",
+			linkPath, currentTarget, sl.Target)
+	}
+	return nil
+}

internal/symlinks/shared_test.go

@@ -0,0 +1,276 @@
+package symlinks
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// testRequired returns a SharedSymlink list whose Targets live entirely
+// under tmpDir, so the tests never touch the operator's real home.
+func testRequired(tmpDir string) []SharedSymlink {
+	return []SharedSymlink{
+		{Target: filepath.Join(tmpDir, "session-env-shared"), Name: "session-env"},
+		{Target: filepath.Join(tmpDir, "file-history-shared"), Name: "file-history"},
+		{Target: filepath.Join(tmpDir, "projects-shared"), Name: "projects"},
+	}
+}
+
+func TestEnsureForAccount_missingCreatesLinksAndTargets(t *testing.T) {
+	tmp := t.TempDir()
+	home := filepath.Join(tmp, "account1")
+	req := testRequired(tmp)
+
+	if err := EnsureForAccount(home, req); err != nil {
+		t.Fatalf("EnsureForAccount: %v", err)
+	}
+
+	for _, sl := range req {
+		linkPath := filepath.Join(home, sl.Name)
+		info, err := os.Lstat(linkPath)
+		if err != nil {
+			t.Errorf("expected link at %s: %v", linkPath, err)
+			continue
+		}
+		if info.Mode()&os.ModeSymlink == 0 {
+			t.Errorf("%s exists but is not a symlink", linkPath)
+		}
+		got, err := os.Readlink(linkPath)
+		if err != nil {
+			t.Errorf("readlink %s: %v", linkPath, err)
+			continue
+		}
+		if got != sl.Target {
+			t.Errorf("link %s points to %q, want %q", linkPath, got, sl.Target)
+		}
+		// Target directory must exist too.
+		if st, err := os.Stat(sl.Target); err != nil || !st.IsDir() {
+			t.Errorf("target %s should be a directory, err=%v", sl.Target, err)
+		}
+	}
+}
+
+func TestEnsureForAccount_idempotent(t *testing.T) {
+	tmp := t.TempDir()
+	home := filepath.Join(tmp, "account1")
+	req := testRequired(tmp)
+
+	if err := EnsureForAccount(home, req); err != nil {
+		t.Fatalf("first pass: %v", err)
+	}
+	if err := EnsureForAccount(home, req); err != nil {
+		t.Fatalf("second pass should be a no-op, got: %v", err)
+	}
+}
+
+func TestEnsureForAccount_divergentLinkReturnsError(t *testing.T) {
+	tmp := t.TempDir()
+	home := filepath.Join(tmp, "account1")
+	req := testRequired(tmp)
+
+	// Pre-create a wrong symlink for "projects".
+	if err := os.MkdirAll(home, 0700); err != nil {
+		t.Fatalf("mkdir home: %v", err)
+	}
+	wrongTarget := filepath.Join(tmp, "someone-elses-dir")
+	if err := os.MkdirAll(wrongTarget, 0700); err != nil {
+		t.Fatalf("mkdir wrong target: %v", err)
+	}
+	linkPath := filepath.Join(home, "projects")
+	if err := os.Symlink(wrongTarget, linkPath); err != nil {
+		t.Fatalf("seed wrong symlink: %v", err)
+	}
+
+	err := EnsureForAccount(home, req)
+	if err == nil {
+		t.Fatal("expected error for divergent link, got nil")
+	}
+	if !strings.Contains(err.Error(), "divergent") {
+		t.Errorf("error should mention 'divergent': %v", err)
+	}
+
+	// The wrong symlink MUST be preserved (no auto-correction).
+	got, err := os.Readlink(linkPath)
+	if err != nil {
+		t.Fatalf("readlink after error: %v", err)
+	}
+	if got != wrongTarget {
+		t.Errorf("divergent link was mutated: now %q, want preserved %q", got, wrongTarget)
+	}
+}
+
+func TestEnsureForAccount_regularFileInsteadOfLinkFails(t *testing.T) {
+	tmp := t.TempDir()
+	home := filepath.Join(tmp, "account1")
+	req := testRequired(tmp)
+
+	if err := os.MkdirAll(home, 0700); err != nil {
+		t.Fatalf("mkdir home: %v", err)
+	}
+	// Create a regular file at the session-env path.
+	bogus := filepath.Join(home, "session-env")
+	if err := os.WriteFile(bogus, []byte("oops"), 0600); err != nil {
+		t.Fatalf("seed regular file: %v", err)
+	}
+
+	err := EnsureForAccount(home, req)
+	if err == nil {
+		t.Fatal("expected error for regular-file-at-link-path, got nil")
+	}
+	if !strings.Contains(err.Error(), "not a symlink") {
+		t.Errorf("error should mention 'not a symlink': %v", err)
+	}
+}
+
+func TestEnsureForAccount_emptyHomeReturnsError(t *testing.T) {
+	if err := EnsureForAccount("", nil); err == nil {
+		t.Fatal("expected error for empty home, got nil")
+	}
+}
+
+func TestValidateAll_multipleAccountsAllOK(t *testing.T) {
+	tmp := t.TempDir()
+	req := testRequired(tmp)
+	homes := []string{
+		filepath.Join(tmp, "a"),
+		filepath.Join(tmp, "b"),
+	}
+	if err := ValidateAll(homes, req); err != nil {
+		t.Fatalf("ValidateAll: %v", err)
+	}
+}
+
+func TestValidateAll_aggregatesErrors(t *testing.T) {
+	tmp := t.TempDir()
+	req := testRequired(tmp)
+	homes := []string{
+		filepath.Join(tmp, "a"),
+		filepath.Join(tmp, "b"),
+	}
+
+	// Pre-seed account `a` with a divergent link so ValidateAll must
+	// surface that error while still processing account `b`.
+	if err := os.MkdirAll(homes[0], 0700); err != nil {
+		t.Fatalf("mkdir a: %v", err)
+	}
+	wrongTarget := filepath.Join(tmp, "bad")
+	if err := os.MkdirAll(wrongTarget, 0700); err != nil {
+		t.Fatalf("mkdir bad: %v", err)
+	}
+	if err := os.Symlink(wrongTarget, filepath.Join(homes[0], "projects")); err != nil {
+		t.Fatalf("seed wrong link: %v", err)
+	}
+
+	err := ValidateAll(homes, req)
+	if err == nil {
+		t.Fatal("expected aggregated error, got nil")
+	}
+	if !strings.Contains(err.Error(), "divergent") {
+		t.Errorf("should surface divergent: %v", err)
+	}
+
+	// Account `b` must have been configured successfully even though `a`
+	// failed. Otherwise the operator cannot see the full state at once.
+	for _, sl := range req {
+		if _, err := os.Lstat(filepath.Join(homes[1], sl.Name)); err != nil {
+			t.Errorf("account b link %s should have been created despite a's failure: %v", sl.Name, err)
+		}
+	}
+}
+
+func TestValidateAll_emptyListReturnsError(t *testing.T) {
+	if err := ValidateAll(nil, nil); err == nil {
+		t.Fatal("expected error for empty account list")
+	}
+}
+
+// TestRequiredShared_defaultsAreReasonable pins the default SharedSymlink
+// list so an accidental edit that breaks production is caught.
+func TestRequiredShared_defaultsAreReasonable(t *testing.T) {
+	want := map[string]string{
+		"session-env":  "/home/ubuntu/.claude-session-env-shared",
+		"file-history": "/home/ubuntu/.claude-file-history-shared",
+		"projects":     "/home/ubuntu/.claude-projects-shared",
+	}
+	if len(RequiredShared) != len(want) {
+		t.Fatalf("RequiredShared has %d entries, want %d", len(RequiredShared), len(want))
+	}
+	for _, sl := range RequiredShared {
+		if got, ok := want[sl.Name]; !ok {
+			t.Errorf("unexpected RequiredShared entry %q", sl.Name)
+		} else if got != sl.Target {
+			t.Errorf("RequiredShared %q target = %q, want %q", sl.Name, sl.Target, got)
+		}
+	}
+}
+
+// TestRequiredSharedIsCoherent validates the contract of the package-level
+// RequiredShared constant that the switcher and lifecycle manager consume
+// in production. The rest of the suite exercises EnsureForAccount /
+// ValidateAll with tmpdir-scoped override lists, so the actual prod
+// constant (pointing under /home/ubuntu/.claude-*-shared) is never touched
+// by those tests — a regression that shrinks or renames RequiredShared
+// would pass every other test but silently break failover on real VMs
+// (missing a link → writes to private state → transcripts duplicated).
+//
+// The test is filesystem-free: it only asserts the shape of the constant.
+//
+//   1. Exactly three entries, one per Name required by the A-failover design.
+//   2. Every Target is absolute.
+//   3. All three Targets share the same parent directory — there is no
+//      mode of operation where one shared dir lives elsewhere than the
+//      others. `filepath.Dir(target)` must be identical across entries.
+//
+// This encodes the "3 links under one shared root" invariant that
+// EnsureForAccount relies on. Any future change to RequiredShared that
+// breaks this invariant should force the author to update the switcher
+// contract explicitly.
+func TestRequiredSharedIsCoherent(t *testing.T) {
+	expectedNames := map[string]bool{
+		"session-env":  false,
+		"file-history": false,
+		"projects":     false,
+	}
+	if len(RequiredShared) != len(expectedNames) {
+		t.Fatalf("RequiredShared must contain exactly %d entries (session-env, file-history, projects); got %d: %+v",
+			len(expectedNames), len(RequiredShared), RequiredShared)
+	}
+
+	var sharedParent string
+	for i, sl := range RequiredShared {
+		if _, ok := expectedNames[sl.Name]; !ok {
+			t.Errorf("RequiredShared[%d]: unexpected Name %q (allowed: session-env / file-history / projects)", i, sl.Name)
+			continue
+		}
+		if expectedNames[sl.Name] {
+			t.Errorf("RequiredShared[%d]: duplicate Name %q", i, sl.Name)
+		}
+		expectedNames[sl.Name] = true
+
+		if sl.Target == "" {
+			t.Errorf("RequiredShared[%d] (%q): empty Target", i, sl.Name)
+			continue
+		}
+		if !filepath.IsAbs(sl.Target) {
+			t.Errorf("RequiredShared[%d] (%q): Target %q must be absolute", i, sl.Name, sl.Target)
+			continue
+		}
+
+		parent := filepath.Dir(sl.Target)
+		if i == 0 {
+			sharedParent = parent
+			continue
+		}
+		if parent != sharedParent {
+			t.Errorf("RequiredShared[%d] (%q): parent dir %q diverges from %q — all shared targets must live under the same root",
+				i, sl.Name, parent, sharedParent)
+		}
+	}
+
+	for name, seen := range expectedNames {
+		if !seen {
+			t.Errorf("RequiredShared is missing the required %q entry", name)
+		}
+	}
+}