diff --git a/server/internal/daemon/execenv/codex_home.go b/server/internal/daemon/execenv/codex_home.go
index f8e40611ec..a2867fb8c6 100644
--- a/server/internal/daemon/execenv/codex_home.go
+++ b/server/internal/daemon/execenv/codex_home.go
@@ -1,6 +1,7 @@
 package execenv
 
 import (
+	"crypto/sha256"
 	"encoding/hex"
 	"fmt"
 	"io"
@@ -218,18 +219,22 @@ func codexSessionStoreDir(sharedHome, key string) string {
 // 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).
+// The map MUST be collision-free (distinct profiles are distinct daemons and
+// must never share a namespace) AND fixed-length (a profile can be as long as a
+// filesystem segment allows, ~255 bytes, so any length-expanding encoding would
+// overflow the 255-byte limit and fail to create the store dir). A lossy "drop
+// unsafe characters" scheme collides ("" vs "default", "staging.prod" vs
+// "stagingprod"); a full hex encoding doubles the length and overflows. So the
+// empty (default) profile gets a reserved bare literal, and every named profile
+// is the hex of its SHA-256 — a constant 64 hex chars, filesystem-safe and
+// collision-resistant — under a "p_" prefix the bare literal can never collide
+// with (MUL-4424).
 func codexSessionStoreNamespace(profile string) string {
 	if profile == "" {
 		return "default"
 	}
-	return "p_" + hex.EncodeToString([]byte(profile))
+	sum := sha256.Sum256([]byte(profile))
+	return "p_" + hex.EncodeToString(sum[:])
 }
 
 // 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 2bdf196349..2e482817a8 100644
--- a/server/internal/daemon/execenv/codex_home_sessions_test.go
+++ b/server/internal/daemon/execenv/codex_home_sessions_test.go
@@ -4,6 +4,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"testing"
 	"time"
 )
@@ -620,6 +621,25 @@ func TestCodexSessionStoreNamespaceInjective(t *testing.T) {
 	}
 }
 
+// TestCodexSessionStoreNamespace_FitsDirectorySegment guards Elon's round-8
+// blocker: a profile the CLI can persist as its own config-dir segment (up to
+// the ~255-byte filesystem limit) must yield a namespace that also fits one
+// segment and is creatable — a length-expanding encoding (full hex) overflowed
+// at 127 bytes. MUL-4424.
+func TestCodexSessionStoreNamespace_FitsDirectorySegment(t *testing.T) {
+	t.Parallel()
+	base := t.TempDir()
+	for _, profile := range []string{"", "staging", strings.Repeat("a", 127), strings.Repeat("z", 255)} {
+		ns := codexSessionStoreNamespace(profile)
+		if len(ns) > 255 {
+			t.Errorf("profile (len %d) -> namespace (len %d) exceeds the 255-byte single-segment limit", len(profile), len(ns))
+		}
+		if err := os.MkdirAll(filepath.Join(base, ns), 0o755); err != nil {
+			t.Errorf("namespace for profile (len %d) could not be created: %v", len(profile), err)
+		}
+	}
+}
+
 // 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