diff --git a/compiler/semdata.nim b/compiler/semdata.nim
index 15d8b14fe7..32c98cdb31 100644
--- a/compiler/semdata.nim
+++ b/compiler/semdata.nim
@@ -637,6 +637,11 @@ proc renderNotLValue*(n: PNode): string =
   elif n.kind in {nkHiddenStdConv, nkHiddenSubConv} and n.len == 2:
     result = typeToString(n.typ.skipTypes(abstractVar)) & "(" & result & ")"
 
+proc isSsoStringIndex*(conf: ConfigRef; n: PNode): bool =
+  result = conf.usesSso() and n.kind == nkBracketExpr and n.len >= 1 and
+      n[0].typ != nil and
+      n[0].typ.skipTypes(abstractVar + abstractInst - {tyTypeDesc}).kind == tyString
+
 proc isAssignable(c: PContext, n: PNode): TAssignableResult =
   result = parampatterns.isAssignable(c.p.owner, n)
 
diff --git a/compiler/sempass2.nim b/compiler/sempass2.nim
index 35901ed960..9c84b721ad 100644
--- a/compiler/sempass2.nim
+++ b/compiler/sempass2.nim
@@ -809,6 +809,10 @@ proc trackOperandForIndirectCall(tracked: PEffects, n: PNode, formals: PType; ar
         markSideEffect(tracked, a, n.info)
   let paramType = if formals != nil and argIndex < formals.signatureLen: formals[argIndex] else: nil
   if paramType != nil and paramType.kind in {tyVar}:
+    let arg = n.skipAddr()
+    if isSsoStringIndex(tracked.config, arg):
+      localError(tracked.config, arg.info,
+        "expression '$1' is immutable, not 'var'" % renderNotLValue(arg))
     invalidateFacts(tracked.guards, n)
     if n.kind == nkSym and isLocalSym(tracked, n.sym):
       makeVolatile(tracked, n.sym)
diff --git a/lib/system.nim b/lib/system.nim
index c76d096426..63989b1502 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -2694,7 +2694,9 @@ when hasAlloc or defined(nimscript):
     setLen(x, xl+item.len)
     var j = xl-1
     while j >= i:
-      when defined(gcArc) or defined(gcOrc) or defined(gcYrc) or defined(gcAtomicArc):
+      when defined(nimsso):
+        x[j+item.len] = x[j]
+      elif defined(gcArc) or defined(gcOrc) or defined(gcYrc) or defined(gcAtomicArc):
         x[j+item.len] = move x[j]
       else:
         shallowCopy(x[j+item.len], x[j])
diff --git a/lib/system/indices.nim b/lib/system/indices.nim
index 6230b36788..8f20af5ec5 100644
--- a/lib/system/indices.nim
+++ b/lib/system/indices.nim
@@ -59,16 +59,35 @@ template `[]=`*(s: string; i: int; val: char) = arrPut(s, i, val)
 template `^^`(s, i: untyped): untyped =
   (when i is BackwardsIndex: s.len - int(i) else: int(i))
 
-template spliceImpl(s, a, L, b: typed): untyped =
+template spliceStringImpl(s, a, L, b: typed): untyped =
   # make room for additional elements or cut:
   var shift = b.len - max(0,L)  # ignore negative slice size
   var newLen = s.len + shift
   if shift > 0:
     # enlarge:
     setLen(s, newLen)
-    for i in countdown(newLen-1, a+b.len): movingCopy(s[i], s[i-shift])
+    for i in countdown(newLen-1, a+b.len):
+      s[i] = s[i-shift]
   else:
-    for i in countup(a+b.len, newLen-1): movingCopy(s[i], s[i-shift])
+    for i in countup(a+b.len, newLen-1):
+      s[i] = s[i-shift]
+    # cut down:
+    setLen(s, newLen)
+  # fill the hole:
+  for i in 0 ..< b.len: s[a+i] = b[i]
+
+template spliceSeqImpl(s, a, L, b: typed): untyped =
+  # make room for additional elements or cut:
+  var shift = b.len - max(0,L)  # ignore negative slice size
+  var newLen = s.len + shift
+  if shift > 0:
+    # enlarge:
+    setLen(s, newLen)
+    for i in countdown(newLen-1, a+b.len):
+      movingCopy(s[i], s[i-shift])
+  else:
+    for i in countup(a+b.len, newLen-1):
+      movingCopy(s[i], s[i-shift])
     # cut down:
     setLen(s, newLen)
   # fill the hole:
@@ -102,7 +121,7 @@ proc `[]=`*[T, U: Ordinal](s: var string, x: HSlice[T, U], b: string) {.systemRa
   if L == b.len:
     for i in 0..<L: s[i+a] = b[i]
   else:
-    spliceImpl(s, a, L, b)
+    spliceStringImpl(s, a, L, b)
 
 proc `[]`*[Idx, T; U, V: Ordinal](a: array[Idx, T], x: HSlice[U, V]): seq[T] {.systemRaisesDefect.} =
   ## Slice operation for arrays.
@@ -162,4 +181,4 @@ proc `[]=`*[T; U, V: Ordinal](s: var seq[T], x: HSlice[U, V], b: openArray[T]) {
   if L == b.len:
     for i in 0 ..< L: s[i+a] = b[i]
   else:
-    spliceImpl(s, a, L, b)
+    spliceSeqImpl(s, a, L, b)
diff --git a/lib/system/strs_v3.nim b/lib/system/strs_v3.nim
index 173737edea..58462ba522 100644
--- a/lib/system/strs_v3.nim
+++ b/lib/system/strs_v3.nim
@@ -224,13 +224,14 @@ proc cmpStringPtrs(a, b: ptr SmallString): int {.inline.} =
         minLen - AlwaysAvail)
     if result == 0: result = aslen - bslen
     return
-  # At least one is long. Hot prefix: inlinePtr[0..AlwaysAvail-1] mirrors heap data.
-  let pfxLen = min(min(aslen, bslen), AlwaysAvail)
-  result = cmpInlineBytes(inlinePtrOf(a), inlinePtrOf(b), pfxLen)
-  if result != 0: return
+  # At least one is long. Hot prefix mirrors heap data, but only up to fullLen:
+  # shrinking can leave stale bytes in the inline cache past the logical length.
   let la = if aslen > PayloadSize: a.more.fullLen else: aslen
   let lb = if bslen > PayloadSize: b.more.fullLen else: bslen
   let minLen = min(la, lb)
+  let pfxLen = min(minLen, AlwaysAvail)
+  result = cmpInlineBytes(inlinePtrOf(a), inlinePtrOf(b), pfxLen)
+  if result != 0: return
   if minLen <= AlwaysAvail:
     result = la - lb
     return
diff --git a/tests/errmsgs/tsso_string_index_var.nim b/tests/errmsgs/tsso_string_index_var.nim
new file mode 100644
index 0000000000..401491e0a5
--- /dev/null
+++ b/tests/errmsgs/tsso_string_index_var.nim
@@ -0,0 +1,13 @@
+discard """
+  cmd: "nim check --strings:sso --mm:orc --hints:off $file"
+  action: "reject"
+  nimout: '''
+tsso_string_index_var.nim(13, 12) Error: expression 's[0]' is immutable, not 'var'
+'''
+"""
+
+proc passByVar(c: var char) =
+  c = 'x'
+
+var s = "abc"
+passByVar(s[0])