diff --git a/lib/pure/streams.nim b/lib/pure/streams.nim
index a1fffa5d95..9cf7c01fed 100644
--- a/lib/pure/streams.nim
+++ b/lib/pure/streams.nim
@@ -259,7 +259,7 @@ proc readDataStr*(s: Stream, buffer: var string, slice: Slice[int]): int =
     result = s.readDataStrImpl(s, buffer, slice)
   else:
     # fallback
-    result = s.readData(beginStore(buffer, slice.b + 1 - slice.a, slice.a), slice.b + 1 - slice.a)
+    result = s.readData(beginStore(buffer, slice.b + 1, slice.a), slice.b + 1 - slice.a)
     endStore(buffer)
 
 template jsOrVmBlock(caseJsOrVm, caseElse: untyped): untyped =
@@ -1226,7 +1226,7 @@ else: # after 1.3 or JS not defined
       jsOrVmBlock:
         buffer[slice.a..<slice.a+result] = s.data[s.pos..<s.pos+result]
       do:
-        copyMem(beginStore(buffer, result, slice.a), readRawData(s.data, s.pos), result)
+        copyMem(beginStore(buffer, slice.a + result, slice.a), readRawData(s.data, s.pos), result)
         endStore(buffer)
       inc(s.pos, result)
     else:
@@ -1267,16 +1267,16 @@ else: # after 1.3 or JS not defined
     var s = StringStream(s)
     if bufLen <= 0:
       return
-    if s.pos + bufLen > s.data.len:
-      setLen(s.data, s.pos + bufLen)
     when defined(js):
+      if s.pos + bufLen > s.data.len:
+        setLen(s.data, s.pos + bufLen)
       try:
         s.data[s.pos..<s.pos+bufLen] = cast[ptr string](buffer)[][0..<bufLen]
       except:
         raise newException(Defect, "could not write to string stream, " &
           "did you use a non-string buffer pointer?", getCurrentException())
     elif not defined(nimscript):
-      copyMem(beginStore(s.data, bufLen, s.pos), buffer, bufLen)
+      copyMem(beginStore(s.data, s.pos + bufLen, s.pos), buffer, bufLen)
       endStore(s.data)
     inc(s.pos, bufLen)
 
@@ -1346,7 +1346,7 @@ proc fsReadData(s: Stream, buffer: pointer, bufLen: int): int =
 
 proc fsReadDataStr(s: Stream, buffer: var string, slice: Slice[int]): int =
   let len = slice.b + 1 - slice.a
-  result = readBuffer(FileStream(s).f, beginStore(buffer, len, slice.a), len)
+  result = readBuffer(FileStream(s).f, beginStore(buffer, slice.b + 1, slice.a), len)
   endStore(buffer)
 
 proc fsPeekData(s: Stream, buffer: pointer, bufLen: int): int =
diff --git a/lib/std/formatfloat.nim b/lib/std/formatfloat.nim
index 44f745c264..8778ba5766 100644
--- a/lib/std/formatfloat.nim
+++ b/lib/std/formatfloat.nim
@@ -18,12 +18,12 @@ proc addCstringN(result: var string, buf: cstring; buflen: int) =
   # no nimvm support needed, so it doesn't need to be fast here either
   let oldLen = result.len
   let newLen = oldLen + buflen
-  result.setLen newLen
   {.cast(noSideEffect).}:
-    when declared(completeStore):
-      c_memcpy(beginStore(result, buflen, oldLen), buf, buflen.csize_t)
+    when declared(beginStore):
+      c_memcpy(beginStore(result, newLen, oldLen), buf, buflen.csize_t)
       endStore(result)
     else:
+      result.setLen newLen
       discard c_memcpy(result[oldLen].addr, buf, buflen.csize_t)
 
 import std/private/[dragonbox, schubfach]
diff --git a/lib/std/strbasics.nim b/lib/std/strbasics.nim
index 50e645b266..beaf9d89a3 100644
--- a/lib/std/strbasics.nim
+++ b/lib/std/strbasics.nim
@@ -84,7 +84,7 @@ func setSlice*(s: var string, slice: Slice[int]) =
       when not declared(moveMem):
         impl()
       else:
-        let p = beginStore(s, last - first + 1)
+        let p = beginStore(s, s.len)
         moveMem(p, addr p[first], last - first + 1)
         endStore(s)
   s.setLen(last - first + 1)
diff --git a/lib/std/syncio.nim b/lib/std/syncio.nim
index 70a0c711cb..3d812ae127 100644
--- a/lib/std/syncio.nim
+++ b/lib/std/syncio.nim
@@ -485,7 +485,7 @@ proc readLine*(f: File, line: var string): bool {.tags: [ReadIOEffect],
     while true:
       # fixes #9634; this pattern may need to be abstracted as a template if reused;
       # likely other io procs need this for correctness.
-      fgetsSuccess = c_fgets(cast[cstring](beginStore(line, sp, pos)), sp.cint, f) != nil
+      fgetsSuccess = c_fgets(cast[cstring](beginStore(line, pos + sp, pos)), sp.cint, f) != nil
       endStore(line)
       if fgetsSuccess: break
       when not defined(nimscript):
diff --git a/lib/system.nim b/lib/system.nim
index 49e600aae7..c76d096426 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -1703,7 +1703,8 @@ 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, raises: [], tags: [].} =
+    proc beginStore*(s: var string; newLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, raises: [], tags: [].} =
+      {.cast(noSideEffect).}: s.setLen(newLen)
       let ns = cast[NimString](s)
       if ns == nil: nil
       else: cast[ptr UncheckedArray[char]](addr ns.data[start])
@@ -1714,7 +1715,7 @@ when not (notJSnotNims and defined(nimSeqsV2)):
       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, raises: [], tags: [].} = nil
+    proc beginStore*(s: var string; newLen: 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
 
diff --git a/lib/system/strs_v2.nim b/lib/system/strs_v2.nim
index 58525c3d86..640e8eeb3f 100644
--- a/lib/system/strs_v2.nim
+++ b/lib/system/strs_v2.nim
@@ -236,13 +236,17 @@ 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, raises: [], tags: [].} =
-  ## Returns a writable pointer for bulk write of `ensuredLen` bytes starting at `start`.
+proc beginStore*(s: var string; newLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, raises: [], tags: [].} =
+  ## Sets s.len to `newLen` (new bytes are uninitialized), ensures unique
+  ## ownership, and returns a pointer to s[start] for bulk writing.
   ## Call `endStore(s)` afterwards for portability.
-  {.cast(noSideEffect).}: prepareMutation(s)
-  let str = cast[ptr NimStringV2](unsafeAddr s)
-  if str.p == nil: nil
-  else: cast[ptr UncheckedArray[char]](addr str.p.data[start])
+  ## To keep the current length, pass `s.len`.
+  {.cast(noSideEffect).}:
+    let p = cast[ptr NimStringV2](addr s)
+    setLengthStrV2Uninit(p[], newLen)
+    prepareMutation(s)
+    if p.p == nil: nil
+    else: cast[ptr UncheckedArray[char]](addr p.p.data[start])
 
 proc endStore*(s: var string) {.inline, noSideEffect, raises: [], tags: [].} =
   ## No-op for non-SSO strings; call after bulk writes via `beginStore`.
diff --git a/lib/system/strs_v3.nim b/lib/system/strs_v3.nim
index 13d258f8c0..173737edea 100644
--- a/lib/system/strs_v3.nim
+++ b/lib/system/strs_v3.nim
@@ -721,18 +721,37 @@ 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, 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`).
+proc beginStore*(s: var string; newLen: int; start = 0): ptr UncheckedArray[char] {.inline, noSideEffect, raises: [], tags: [].} =
+  ## Sets s.len to `newLen` (new bytes are uninitialized), ensures unique
+  ## ownership, and returns a pointer to s[start] for bulk writing.
   ## Call `endStore(s)` afterwards to sync the inline cache.
+  ## To keep the current length, pass `s.len`.
   {.cast(noSideEffect).}:
     let ss = cast[ptr SmallString](addr s)
     let slen = ssLen(ss[])
-    if slen > PayloadSize:
-      ensureUniqueLong(ss[], ss[].more.fullLen, ss[].more.fullLen)
+    let curLen = if slen > PayloadSize: ss[].more.fullLen else: slen
+    if newLen <= PayloadSize and slen <= PayloadSize:
+      # Stay inline/medium.
+      if newLen != curLen:
+        setSSLen(ss[], newLen)
+      result = cast[ptr UncheckedArray[char]](cast[uint](inlinePtr(ss[])) + uint(start))
+    elif slen <= PayloadSize:
+      # Inline/medium → long.
+      let newCap = resize(newLen)
+      let p = cast[ptr LongString](alloc(LongStringDataOffset + newCap + 1))
+      p.rc = 1
+      p.fullLen = newLen
+      p.capImpl = newCap
+      copyMem(addr p.data[0], inlinePtr(ss[]), curLen)
+      p.data[newLen] = '\0'
+      ss[].more = p
+      setSSLen(ss[], HeapSlen)
       result = cast[ptr UncheckedArray[char]](addr ss[].more.data[start])
     else:
-      result = cast[ptr UncheckedArray[char]](cast[uint](inlinePtr(ss[])) + uint(start))
+      # Already long: resize within heap (no transition back to inline).
+      ensureUniqueLong(ss[], curLen, newLen)
+      ss[].more.data[newLen] = '\0'
+      result = cast[ptr UncheckedArray[char]](addr ss[].more.data[start])
 
 proc endStore*(s: var string) {.inline, noSideEffect, raises: [], tags: [].} =
   ## Syncs the inline cache after bulk writes via `beginStore`. No-op for short/medium strings.