diff --git a/lib/system.nim b/lib/system.nim
index ffdc59661d..28a0415a12 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -1703,19 +1703,19 @@ when not (notJSnotNims and defined(nimSeqsV2)):
   # Needed so modules imported by system (e.g. syncio) can reference these without guards.
   when notJSnotNims:
     # mm:refc: string = ptr NimStringDesc with data: UncheckedArray[char]
-    proc beginStore*(s: var string; ensuredLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, tags: [].} =
+    proc beginStore*(s: var string; ensuredLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, raises: [], tags: [].} =
       let ns = cast[NimString](s)
       if ns == nil: nil
       else: cast[ptr UncheckedArray[char]](addr ns.data[start])
-    proc endStore*(s: var string) {.inline, noSideEffect, tags: [].} = discard
+    proc endStore*(s: var string) {.inline, noSideEffect, raises: [], tags: [].} = discard
     template readRawData*(s: string; start = 0): ptr UncheckedArray[char] =
       let ns = cast[NimString](s)
       if ns == nil: nil
       else: cast[ptr UncheckedArray[char]](addr ns.data[start])
   else:
     # JS/nimscript: callers are guarded by whenNotVmJsNims/when not defined(js)
-    proc beginStore*(s: var string; ensuredLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, tags: [].} = nil
-    proc endStore*(s: var string) {.inline, noSideEffect, tags: [].} = discard
+    proc beginStore*(s: var string; ensuredLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, raises: [], tags: [].} = nil
+    proc endStore*(s: var string) {.inline, noSideEffect, raises: [], tags: [].} = discard
     template readRawData*(s: string; start = 0): ptr UncheckedArray[char] = nil
 
 when not defined(js):
diff --git a/lib/system/strs_v2.nim b/lib/system/strs_v2.nim
index b99b433257..6942b69a6d 100644
--- a/lib/system/strs_v2.nim
+++ b/lib/system/strs_v2.nim
@@ -176,18 +176,18 @@ proc nimAsgnStrV2(a: var NimStringV2, b: NimStringV2) {.compilerRtl.} =
     a.len = b.len
     copyMem(unsafeAddr a.p.data[0], unsafeAddr b.p.data[0], b.len+1)
 
-proc nimPrepareStrMutationImpl(s: var NimStringV2) =
+proc nimPrepareStrMutationImpl(s: var NimStringV2) {.raises: [], tags: [].} =
   let oldP = s.p
   # can't mutate a literal, so we need a fresh copy here:
   s.p = allocPayload(s.len)
   s.p.cap = s.len
   copyMem(unsafeAddr s.p.data[0], unsafeAddr oldP.data[0], s.len+1)
 
-proc nimPrepareStrMutationV2(s: var NimStringV2) {.compilerRtl, inl.} =
+proc nimPrepareStrMutationV2(s: var NimStringV2) {.compilerRtl, inl, raises: [], tags: [].} =
   if s.p != nil and (s.p.cap and strlitFlag) == strlitFlag:
     nimPrepareStrMutationImpl(s)
 
-proc prepareMutation*(s: var string) {.inline, tags: [].} =
+proc prepareMutation*(s: var string) {.inline, raises: [], tags: [].} =
   # string literals are "copy on write", so you need to call
   # `prepareMutation` before modifying the strings via `addr`.
   {.cast(noSideEffect).}:
@@ -216,21 +216,25 @@ func capacity*(self: string): int {.inline.} =
   let str = cast[ptr NimStringV2](unsafeAddr self)
   result = if str.p != nil: str.p.cap and not strlitFlag else: 0
 
-proc beginStore*(s: var string; ensuredLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, tags: [].} =
+proc beginStore*(s: var string; ensuredLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, raises: [], tags: [].} =
   ## Returns a writable pointer for bulk write of `ensuredLen` bytes starting at `start`.
   ## Call `endStore(s)` afterwards for portability.
   {.cast(noSideEffect).}: prepareMutation(s)
-  if s.len == 0: nil
-  else: cast[ptr UncheckedArray[char]](addr s[start])
+  let str = cast[ptr NimStringV2](unsafeAddr s)
+  if str.p == nil: nil
+  else: cast[ptr UncheckedArray[char]](addr str.p.data[start])
 
-proc endStore*(s: var string) {.inline, noSideEffect, tags: [].} =
+proc endStore*(s: var string) {.inline, noSideEffect, raises: [], tags: [].} =
   ## No-op for non-SSO strings; call after bulk writes via `beginStore`.
   discard
 
+proc rawDataImpl(str: ptr NimStringV2; start: int): ptr UncheckedArray[char] {.inline, noSideEffect, raises: [], tags: [].} =
+  if str.p == nil: nil
+  else: cast[ptr UncheckedArray[char]](addr str.p.data[start])
+
 template readRawData*(s: string; start = 0): ptr UncheckedArray[char] =
   ## Returns a pointer to `s[start]` for read-only raw access.
   ## Template ensures no copy of `s`; ptr is valid while `s` is alive.
-  let str = cast[ptr NimStringV2](unsafeAddr s)
-  if str.p == nil: nil else: cast[ptr UncheckedArray[char]](addr str.p.data[start])
+  rawDataImpl(cast[ptr NimStringV2](unsafeAddr s), start)
 
 {.pop.}
diff --git a/lib/system/strs_v3.nim b/lib/system/strs_v3.nim
index b4e426908c..ff22e24ecc 100644
--- a/lib/system/strs_v3.nim
+++ b/lib/system/strs_v3.nim
@@ -675,7 +675,7 @@ proc completeStore(s: var SmallString) {.compilerproc, inline.} =
 proc completeStore*(s: var string) {.inline.} =
   completeStore(cast[ptr SmallString](addr s)[])
 
-proc beginStore*(s: var string; ensuredLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, tags: [].} =
+proc beginStore*(s: var string; ensuredLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, raises: [], tags: [].} =
   ## Prepares `s` for a bulk write of `ensuredLen` bytes starting at `start`.
   ## The caller must ensure `s.len >= start + ensuredLen` (e.g. via `newString` or `setLen`).
   ## Call `endStore(s)` afterwards to sync the inline cache.
@@ -688,7 +688,7 @@ proc beginStore*(s: var string; ensuredLen: int; start = 0): ptr UncheckedArray[
     else:
       result = cast[ptr UncheckedArray[char]](cast[uint](inlinePtr(ss[])) + uint(start))
 
-proc endStore*(s: var string) {.inline, noSideEffect, tags: [].} =
+proc endStore*(s: var string) {.inline, noSideEffect, raises: [], tags: [].} =
   ## Syncs the inline cache after bulk writes via `beginStore`. No-op for short/medium strings.
   {.cast(noSideEffect).}: completeStore(cast[ptr SmallString](addr s)[])