diff --git a/server/internal/daemon/execenv/codex_home.go b/server/internal/daemon/execenv/codex_home.go
index 32bc787357..f8e40611ec 100644
--- a/server/internal/daemon/execenv/codex_home.go
+++ b/server/internal/daemon/execenv/codex_home.go
@@ -1,6 +1,7 @@
 package execenv
 
 import (
+	"encoding/hex"
 	"fmt"
 	"io"
 	"log/slog"
@@ -210,17 +211,25 @@ func codexSessionStoreDir(sharedHome, key string) string {
 	return filepath.Join(sharedHome, codexSessionStoreRoot, key)
 }
 
-// codexSessionStoreNamespace isolates one profile-daemon's session stores from
-// another's when several run on the same machine sharing one ~/.codex (profiles
-// get separate daemon state but the same Codex home). Each daemon writes under,
-// and only ever reclaims, its own namespace, so a staging daemon's GC can never
-// delete a production task's live store and vice versa. Empty profile ->
-// "default". See CodexSessionStorePath / PruneCodexSessionStores (MUL-4424).
+// codexSessionStoreNamespace maps a daemon's profile to the directory segment
+// that isolates its session stores from another profile-daemon's when several
+// run on the same machine sharing one ~/.codex (profiles get separate daemon
+// state but the same Codex home). Each daemon writes under, and only ever
+// reclaims, its own namespace, so a staging daemon's GC can never delete a
+// production task's live store and vice versa.
+//
+// The map MUST be injective: distinct profiles are distinct daemons and must
+// never share a namespace. A lossy "drop unsafe characters" scheme is not — the
+// CLI treats "" and "default" as separate daemons, and would sanitize
+// "staging.prod" and "stagingprod" to the same segment. So the empty (default)
+// profile gets a reserved bare literal, and every named profile is hex-encoded
+// (bijective, filesystem-safe) under a "p_" prefix a bare literal can never
+// collide with (MUL-4424).
 func codexSessionStoreNamespace(profile string) string {
-	if ns := sanitizeCodexPathSegment(profile); ns != "" {
-		return ns
+	if profile == "" {
+		return "default"
 	}
-	return "default"
+	return "p_" + hex.EncodeToString([]byte(profile))
 }
 
 // codexSessionStoreKey builds the per-(profile, agent, issue) key for a task's
diff --git a/server/internal/daemon/execenv/codex_home_sessions_test.go b/server/internal/daemon/execenv/codex_home_sessions_test.go
index 43df80c77c..2bdf196349 100644
--- a/server/internal/daemon/execenv/codex_home_sessions_test.go
+++ b/server/internal/daemon/execenv/codex_home_sessions_test.go
@@ -596,6 +596,61 @@ func TestPruneCodexSessionStores_IsolatesProfiles(t *testing.T) {
 	assertAbsent(t, stagingStore)
 }
 
+// TestCodexSessionStoreNamespaceInjective guards the profile->namespace map
+// against the collisions Elon flagged: the CLI treats these profile pairs as
+// distinct daemons, so they must never share a store namespace (a lossy
+// character-dropping scheme merged them). MUL-4424.
+func TestCodexSessionStoreNamespaceInjective(t *testing.T) {
+	t.Parallel()
+	pairs := [][2]string{
+		{"", "default"},                 // empty (default) vs a profile literally named "default"
+		{"staging.prod", "stagingprod"}, // punctuation must not be dropped into a collision
+		{"a-b", "a_b"},
+		{"prod", "Prod"},
+		{"p_default", "default"}, // an encoded-looking name must not alias another
+	}
+	for _, p := range pairs {
+		if a, b := codexSessionStoreNamespace(p[0]), codexSessionStoreNamespace(p[1]); a == b {
+			t.Errorf("profiles %q and %q collide in namespace %q", p[0], p[1], a)
+		}
+	}
+	// Deterministic for a fixed profile.
+	if codexSessionStoreNamespace("staging") != codexSessionStoreNamespace("staging") {
+		t.Error("namespace must be deterministic for a fixed profile")
+	}
+}
+
+// TestPruneCodexSessionStores_NoCrossProfileCollision proves the injective
+// namespace holds end-to-end: profiles the CLI treats as distinct never reclaim
+// each other's stores, including the "" vs "default" and punctuation cases that
+// a lossy sanitizer collapsed (Elon's round-7 repro). MUL-4424.
+func TestPruneCodexSessionStores_NoCrossProfileCollision(t *testing.T) {
+	for _, pair := range [][2]string{{"", "default"}, {"staging.prod", "stagingprod"}} {
+		home := t.TempDir()
+		t.Setenv("CODEX_HOME", home)
+		a, b := pair[0], pair[1]
+		storeA := codexSessionStoreDir(home, codexSessionStoreKey(a, "agent", "issue"))
+		storeB := codexSessionStoreDir(home, codexSessionStoreKey(b, "agent", "issue"))
+		seedRolloutAt(t, filepath.Join(storeA, "2026", "06", "01", "rollout-2026-06-01T00-00-00-a.jsonl"), 16)
+		seedRolloutAt(t, filepath.Join(storeB, "2026", "06", "01", "rollout-2026-06-01T00-00-00-b.jsonl"), 16)
+		chtimesTree(t, storeA, time.Now().Add(-30*24*time.Hour))
+		chtimesTree(t, storeB, time.Now().Add(-30*24*time.Hour))
+
+		// Pruning profile a reclaims a's store only — b's must survive.
+		if removed, _ := PruneCodexSessionStores(a, 14*24*time.Hour, time.Now(), nil, testLogger()); removed != 1 {
+			t.Fatalf("prune %q removed=%d, want 1", a, removed)
+		}
+		assertAbsent(t, storeA)
+		assertPresent(t, storeB)
+
+		// Then pruning profile b reclaims b's store.
+		if removed, _ := PruneCodexSessionStores(b, 14*24*time.Hour, time.Now(), nil, testLogger()); removed != 1 {
+			t.Fatalf("prune %q removed=%d, want 1", b, removed)
+		}
+		assertAbsent(t, storeB)
+	}
+}
+
 // TestPruneCodexSessionStores_RemovesEmptyAgentDir confirms an agent directory
 // is cleaned up once its last issue store ages out, so the tree doesn't leave
 // empty <agent>/ shells behind.