diff --git a/compiler/ast2nif.nim b/compiler/ast2nif.nim
index b2525510d5..e95d9b9fbc 100644
--- a/compiler/ast2nif.nim
+++ b/compiler/ast2nif.nim
@@ -198,6 +198,10 @@ type
     depSuffixes: HashSet[string]  # module suffixes already emitted as `(import ...)` deps
     emittedBackendTypes: HashSet[int32]  # backend-local type items already def'd this module
     emittedBackendSyms: HashSet[int32]   # backend-local sym items already def'd this module
+    lowering: bool  # serializing the `lower` stage's `.t.nif` (per-entry self-contained)
+    emittedFieldSyms: HashSet[ItemId]    # lowering: derived env-field syms already def'd this entry
+    inTypeReclist: int   # >0 while writing a type's OWN reclist: fields must be SELF-CONTAINED
+                         # defs (the type can be seek-loaded in isolation), not entry-deduped uses
 
 
 proc isLocalSym(sym: PSym): bool {.inline.} =
@@ -377,7 +381,13 @@ proc writeTypeDef(w: var Writer; dest: var TokenBuf; typ: PType) =
     writeType(w, dest, typ.typeInstImpl)
     #if typ.kind in {tyProc, tyIterator} and typ.nImpl != nil and typ.nImpl.kind != nkFormalParams:
 
+    # The reclist holds this type's OWN fields. A type can be force-loaded by
+    # name in isolation (cg seeks the `.t.nif`/`.s.nif` index entry), so its
+    # fields must be DEFS here, not entry-deduped SymUses whose def lives
+    # elsewhere in the `(lowered)` entry and is never read by the seek.
+    inc w.inTypeReclist
     writeNode(w, dest, typ.nImpl)
+    dec w.inTypeReclist
     writeSym(w, dest, typ.ownerFieldImpl)
     writeSym(w, dest, typ.symImpl)
 
@@ -497,6 +507,18 @@ proc writeSymDef(w: var Writer; dest: var TokenBuf; sym: PSym) =
   writeLoc w, dest, sym.locImpl
   writeNode(w, dest, sym.constraintImpl)
   writeSym(w, dest, sym.instantiatedFromImpl)
+  # The TRANSFORMED body (ic_ideas.md 2-way body): a routine run at compile time
+  # (macro / VM transform / `static`) already has its lowered body — closure
+  # `:env` and all — computed during sem; serialize it so the backend reuses it
+  # instead of re-deriving (the divergence behind the t17.275 env class). An
+  # empty `.` here means "same as the semchecked body OR to be found in the
+  # `.t.nif`" (the `lower` stage fills that gap). Non-routines / not-yet-
+  # transformed routines write the empty marker. (`transformedBodyImpl` only
+  # exists in the routine branch of the `TSym` variant.)
+  if sym.kindImpl in routineKinds:
+    writeNode(w, dest, sym.transformedBodyImpl)
+  else:
+    dest.addDotToken
   dest.addParRi
 
 
@@ -517,9 +539,24 @@ proc shouldWriteSymDef(w: var Writer; sym: PSym): bool {.inline.} =
       return true  # Normal case for global symbols
   return false
 
+proc isLoweredPerEntryField(w: Writer; sym: PSym): bool {.inline.} =
+  ## In the `lower` stage every entry (each `(lowered)` body AND each `@bk` type
+  ## def, which carries its fields inline) is loaded independently, so it must be
+  ## SELF-CONTAINED. A derived closure-env FIELD is module-homed (its id derives
+  ## from the captured local, NOT `@bk` — see itemids.derivedFieldId) so
+  ## `shouldWriteSymDef` would seal it after the first entry and later entries
+  ## would reference a def that their indexed copy does not contain. Re-emit it
+  ## as a full def per entry, deduped within the entry via `emittedFieldSyms`.
+  w.lowering and sym.kindImpl == skField and not sym.itemId.isBackendMinted
+
 proc writeSym(w: var Writer; dest: var TokenBuf; sym: PSym) =
   if sym == nil:
     dest.addDotToken()
+  elif isLoweredPerEntryField(w, sym):
+    if not w.emittedFieldSyms.containsOrIncl(sym.itemId):
+      writeSymDef(w, dest, sym)
+    else:
+      dest.addSymUse pool.syms.getOrIncl(w.toNifSymName(sym)), NoLineInfo
   elif sym.itemId.isBackendMinted:
     # Process-local backend sym (closure env field / hidden `:env` param): emit a
     # MODULE-LOCAL `@bk` def the first time, reference it after. Per-Writer dedup.
@@ -556,11 +593,18 @@ proc writeSymNode(w: var Writer; dest: var TokenBuf; n: PNode; sym: PSym) =
   # dedup shared with `writeSym`), regardless of module: their itemId.module is
   # the systemModule of `vmTransfIdgen`, so `shouldWriteSymDef` (which gates on
   # currentModule) would otherwise only ever emit a SymUse → dangling def.
+  let perEntryField = isLoweredPerEntryField(w, sym)
+  # A field reached while writing its own type's reclist MUST be a self-contained
+  # def: the type can be seek-loaded by name in isolation, so a deduped SymUse
+  # (whose def lives elsewhere in the entry) would resolve to nil.
+  let reclistField = w.lowering and w.inTypeReclist > 0 and sym.kindImpl == skField
   let wantDef =
-    if sym.itemId.isBackendMinted: not w.emittedBackendSyms.containsOrIncl(sym.itemId.item)
+    if reclistField: true
+    elif sym.itemId.isBackendMinted: not w.emittedBackendSyms.containsOrIncl(sym.itemId.item)
+    elif perEntryField: not w.emittedFieldSyms.containsOrIncl(sym.itemId)
     else: shouldWriteSymDef(w, sym)
   if wantDef:
-    if not sym.itemId.isBackendMinted: sym.state = Sealed
+    if not sym.itemId.isBackendMinted and not perEntryField and not reclistField: sym.state = Sealed
     if nodeTyp != n.sym.typImpl:
       dest.buildTree hiddenTypeTag, trLineInfo(w, n.info):
         writeType(w, dest, nodeTyp)
@@ -674,6 +718,7 @@ var implTag = registerTag("implementation")
 var reexpModTag = registerTag("reexpmod")
 var offerTag = registerTag("offer")
 var typeOfferTag = registerTag("toffer")
+var loweredTag = registerTag("lowered")
 
 proc registerNifAstTags*() =
   ## (Re)registers ast2nif's NIF tags explicitly. The top-level `registerTag`
@@ -706,6 +751,7 @@ proc registerNifAstTags*() =
   reexpModTag = registerTag("reexpmod")
   offerTag = registerTag("offer")
   typeOfferTag = registerTag("toffer")
+  loweredTag = registerTag("lowered")
 
 proc writeNode(w: var Writer; dest: var TokenBuf; n: PNode; forAst = false) =
   if n == nil:
@@ -1441,7 +1487,7 @@ proc writeNifModule*(config: ConfigRef; thisModule: int32; n: PNode;
   content.addParRi()
 
   let m = modname(w.currentModule, w.infos.config)
-  let nifFilename = AbsoluteFile(m).changeFileExt(".nif")
+  let nifFilename = AbsoluteFile(m).changeFileExt(".s.nif")
   let d = completeGeneratedFilePath(config, nifFilename).string
 
   var dest = createTokenBuf(600)
@@ -1472,6 +1518,71 @@ proc writeNifModule*(config: ConfigRef; thisModule: int32; n: PNode;
     writeImplCookie(config, thisModule, dest, ifaceHex)
     writeEdgesFile(config, thisModule, implDeps)
 
+proc collectLoweredLocals(w: var Writer; n: PNode) =
+  ## Record every transform-created local (a `Complete`, non-`@bk` sym owned by
+  ## the module being serialized) in `w.locals` so it is written SUFFIX-LESS as
+  ## an inline def. Pre-existing entities (params, result, callees) were sealed
+  ## before serialization, so they are skipped here and emit as index SymUses.
+  if n == nil: return
+  if n.kind == nkSym and n.sym != nil:
+    let s = n.sym
+    if not s.itemId.isBackendMinted and s.itemId.module == w.currentModule and
+        s.state == Complete:
+      w.locals.incl s.itemId
+  for i in 0 ..< n.safeLen:
+    collectLoweredLocals(w, n[i])
+
+proc serializeLoweredBodies*(config: ConfigRef; ownerModule: int32;
+                             entries: openArray[tuple[name: string; body: PNode]];
+                             hooks: openArray[LogEntry];
+                             outfile: string) =
+  ## Write the `lower` backend stage's `.t.nif`. Three sections, all covered by
+  ## the file's embedded index (which the `cg` loader registers as the module's
+  ## SECOND index/stream — see loadLoweredBodies):
+  ## - `(repdestroy/repcopy/... key sym)`: the type-bound ops the lower stage
+  ##   lifted while transforming (closure-env `=destroy` etc.). `cg`'s
+  ##   `registerLoadedHooks` re-attaches them so `injectDestructorCalls` (kept in
+  ##   cg) resolves them via `getAttachedOp`'s key fallback.
+  ## - the hook ROUTINES as full `(sd)`: NEW in the lower stage (no `.s.nif`
+  ##   signature), so serialize sig + transformed body whole; cg loads them via
+  ##   the `.t.nif` index when the HookEntry's SymUse resolves, then demand-emits.
+  ## - owned routines' transformed bodies as `(lowered "<nifname>" <body>)`: only
+  ##   the body (signature comes from the `.s.nif`); applyLoweredBodies sets it on
+  ##   the existing sym.
+  ## A `@bk` closure-env type/sym referenced anywhere is emitted inline AND
+  ## indexed, so it resolves through the embedded index regardless of section.
+  var w = Writer(infos: LineInfoWriter(config: config), currentModule: ownerModule)
+  w.inProc = 1   # we are serializing routine *bodies*
+  w.lowering = true
+  var dest = createTokenBuf(256)
+  createStmtList(dest, NoLineInfo)
+  for op in hooks:
+    writeOp(w, dest, op)
+  var emittedHooks = initHashSet[int32]()
+  for op in hooks:
+    if op.sym != nil and op.sym.kindImpl in routineKinds and
+        not emittedHooks.containsOrIncl(op.sym.itemId.item):
+      w.emittedBackendTypes.clear()
+      w.emittedBackendSyms.clear()
+      w.emittedFieldSyms.clear()
+      w.locals.clear()
+      writeSymDef(w, dest, op.sym)
+  for e in entries:
+    # Each `(lowered ...)` entry is loaded independently, so it must be
+    # SELF-CONTAINED: reset the per-Writer backend-local (@bk) dedup so every
+    # entry re-emits the def of any closure-env type/sym it references.
+    w.emittedBackendTypes.clear()
+    w.emittedBackendSyms.clear()
+    w.emittedFieldSyms.clear()
+    w.locals.clear()
+    collectLoweredLocals(w, e.body)
+    dest.addParLe loweredTag, NoLineInfo
+    dest.addStrLit e.name
+    writeNode(w, dest, e.body)
+    dest.addParRi
+  dest.addParRi()
+  writeFile(dest, outfile)
+
 # --------------------------- Loader (lazy!) -----------------------------------------------
 
 proc nodeKind(n: Cursor): TNodeKind {.inline.} =
@@ -1533,6 +1644,15 @@ type
     suffix: string
     contentStart: int  # stream offset of the module body, so a full-AST load can
                        # rewind after lazy symbol loads moved the cursor
+    # The module's `.t.nif` (the `lower` stage's transformed bodies): a SECOND
+    # embedded-index + stream consulted when the main `.s.nif` index misses. The
+    # transformed bodies' backend-minted (`@bk`) entities (closure-env types/syms,
+    # temporaries) live ONLY here; they are named with this module's suffix, so
+    # `createTypeStub`/`loadSymStub` look them up here on a `.s.nif` miss and load
+    # them from `tStream` (see applyLoweredBodies). "Just use NIF's embedded index."
+    hasTIndex: bool
+    tStream: nifstreams.Stream
+    tIndex: Table[string, NifIndexEntry]
 
   DecodeContext* = object
     infos: LineInfoWriter
@@ -1568,8 +1688,8 @@ proc loadedState(c: DecodeContext): ItemState {.inline.} =
   if c.infos.config.cmd == cmdNifC: Complete else: Sealed
 
 proc cursorFromIndexEntry(c: var DecodeContext; module: FileIndex; entry: NifIndexEntry;
-                          buf: var TokenBuf): Cursor =
-  let s = addr c.mods[module].stream
+                          buf: var TokenBuf; fromT = false): Cursor =
+  let s = if fromT: addr c.mods[module].tStream else: addr c.mods[module].stream
   s.r.jumpTo entry.offset
   # A seek-load is self-contained: its tokens must decode their relative line
   # info against `entry.info` ALONE. The stream's `parents` stack can be left at
@@ -1631,7 +1751,7 @@ proc moduleId(c: var DecodeContext; suffix: string; flags: set[LoadFlag] = {}):
   # but haven't had their NIF index loaded yet
   let hasEntry = c.mods.hasKey(result)
   if not hasEntry or AlwaysLoadInterface in flags:
-    let modFile = (getNimcacheDir(c.infos.config) / RelativeFile(suffix & ".nif")).string
+    let modFile = (getNimcacheDir(c.infos.config) / RelativeFile(suffix & ".s.nif")).string
     if not fileExists(modFile):
       raiseAssert "NIF file not found for module suffix '" & suffix & "': " & modFile &
         ". This can happen when loading a module from NIF that references another module " &
@@ -1723,10 +1843,17 @@ proc createTypeStub(c: var DecodeContext; t: SymId): PType =
     let realSuffix = if isBk: suffix[0 ..< suffix.len - BackendLocalMarker.len] else: suffix
     let modIdx = moduleId(c, realSuffix).int32
     let id = if isBk: backendItemId(modIdx, itemVal) else: itemId(modIdx, itemVal)
-    let ii = addr c.mods[id.module.FileIndex].index
-    let offs = ii[].getOrDefault(name)
+    let modFi = id.module.FileIndex
+    let ii = addr c.mods[modFi].index
+    var offs = ii[].getOrDefault(name)
     if offs.offset == 0:
-      raiseAssert "symbol has no offset: " & name
+      # A backend-minted (`@bk`) type produced by the `lower` stage lives in this
+      # module's `.t.nif`, not its `.s.nif` index. Resolve it through the `.t.nif`
+      # embedded index (loadType picks `tStream` for a tIndex-only name).
+      if c.mods[modFi].hasTIndex:
+        offs = c.mods[modFi].tIndex.getOrDefault(name)
+      if offs.offset == 0:
+        raiseAssert "symbol has no offset: " & name
     result = PType(itemId: id, uniqueId: id, kind: TTypeKind(k), state: Partial)
     c.types[name] = (result, offs)
 
@@ -1821,7 +1948,16 @@ proc loadSymStub(c: var DecodeContext; t: SymId; thisModule: string;
     inc val[]
     let id = if isBk: backendItemId(module.int32, val[]) else: itemId(module.int32, val[])
 
-    let offs = c.mods[module].index.getOrDefault(symAsStr)
+    var offs = c.mods[module].index.getOrDefault(symAsStr)
+    if offs.offset == 0 and c.mods[module].hasTIndex:
+      # A sym produced by the `lower` stage lives in this module's `.t.nif`, not
+      # its `.s.nif` index. This covers BOTH backend-minted (`@bk`) entities
+      # (closure `:env` param, `:tmp`/`res` temporaries) AND module-homed derived
+      # closure-env FIELDS (e.g. a captured `i`): the latter have a normal
+      # module suffix but are added to the env object only in the backend, so
+      # they are indexed solely in `.t.nif`. Resolve through that embedded index
+      # (loadSym picks `tStream` for a tIndex-only name).
+      offs = c.mods[module].tIndex.getOrDefault(symAsStr)
     if offs.offset == 0:
       # Only module/package self-syms are never written as `(sd)` entries, so a
       # missing index offset means this is such a sym — typically the OWNER of an
@@ -1902,7 +2038,12 @@ proc loadTypeFromCursor(c: var DecodeContext; n: var Cursor; t: PType; localSyms
     raiseAssert "(td) expected"
 
   var scanCursor = n  # copy cursor at start of type
-  let typesModule = parseSymName(pool.syms[n.firstSon.symId]).module
+  var typesModule = parseSymName(pool.syms[n.firstSon.symId]).module
+  if typesModule.endsWith(BackendLocalMarker):
+    # A backend-minted (`@bk`) type's name carries the marker in its module part;
+    # strip it so the nested-local pre-scan resolves the real module, not a
+    # nonexistent `<suffix>@bk.nif`.
+    typesModule = typesModule[0 ..< typesModule.len - BackendLocalMarker.len]
   extractLocalSymsFromTree(c, scanCursor, typesModule, localSyms)
 
   inc n  # move past (td
@@ -1941,8 +2082,22 @@ proc loadType*(c: var DecodeContext; t: PType) =
   if t.state != Partial: return
   t.state = c.loadedState
   var buf = createTokenBuf(30)
-  let typeName = typeToNifSym(t, c.infos.config)
-  var n = cursorFromIndexEntry(c, t.itemId.module.FileIndex, c.types[typeName][1], buf)
+  # A backend-minted (`@bk`) closure-env type produced by the `lower` stage lives
+  # ONLY in the `.t.nif` and is keyed by its `@bk` name (see nifTypeName), not the
+  # canonical `typeToNifSym` (which asserts non-`@bk`). Reconstruct that name so a
+  # Partial `@bk` stub that escaped the inline pre-scan can still be force-loaded.
+  let typeName =
+    if t.uniqueId.isBackendMinted:
+      "`t" & $ord(t.kind) & "." & $t.uniqueId.item & "." &
+        modname(t.itemId.module, c.infos.config) & BackendLocalMarker
+    else:
+      typeToNifSym(t, c.infos.config)
+  let modFi = t.itemId.module.FileIndex
+  # A name resolved through the `.t.nif` (tIndex) — a `lower`-stage closure-env
+  # type — must seek in `tStream`, not the `.s.nif` stream.
+  let fromT = c.mods[modFi].hasTIndex and not c.mods[modFi].index.hasKey(typeName) and
+              c.mods[modFi].tIndex.hasKey(typeName)
+  var n = cursorFromIndexEntry(c, modFi, c.types[typeName][1], buf, fromT = fromT)
   var localSyms = initTable[string, PSym]()
   loadTypeFromCursor(c, n, t, localSyms)
 
@@ -2027,6 +2182,15 @@ proc loadSymFromCursor(c: var DecodeContext; s: PSym; n: var Cursor; thisModule:
   loadLoc c, n, s.locImpl
   s.constraintImpl = loadNode(c, n, thisModule, localSyms)
   s.instantiatedFromImpl = loadSymStub(c, n, thisModule, localSyms)
+  # The TRANSFORMED body slot (see writeSymDef). Reconstruct it ONLY in the
+  # backend (`cmdNifC`), where `transformBody` short-circuits on it; during
+  # frontend sem (`cmdM`) skip the tokens — a dependent never needs a foreign
+  # routine's lowered body, and reconstructing one must not perturb effect/
+  # exception inference (the "never change frontend node-typing for IC" rule).
+  if c.infos.config.cmd == cmdNifC and s.kindImpl in routineKinds:
+    s.transformedBodyImpl = loadNode(c, n, thisModule, localSyms)
+  else:
+    skip n
   skipParRi n
 
 proc loadSym*(c: var DecodeContext; s: PSym) =
@@ -2035,7 +2199,10 @@ proc loadSym*(c: var DecodeContext; s: PSym) =
   var buf = createTokenBuf(30)
   let symsModule = s.itemId.module.FileIndex
   let nifname = globalName(s, c.infos.config)
-  var n = cursorFromIndexEntry(c, symsModule, c.syms[nifname][1], buf)
+  # A `@bk` sym resolved through the `.t.nif` (tIndex) seeks in `tStream`.
+  let fromT = c.mods[symsModule].hasTIndex and not c.mods[symsModule].index.hasKey(nifname) and
+              c.mods[symsModule].tIndex.hasKey(nifname)
+  var n = cursorFromIndexEntry(c, symsModule, c.syms[nifname][1], buf, fromT = fromT)
 
   expect n, ParLe
   if n.tagId != sdefTag:
@@ -2053,6 +2220,149 @@ proc loadSym*(c: var DecodeContext; s: PSym) =
   inc n
   loadSymFromCursor(c, s, n, c.mods[symsModule].suffix, localSyms)
 
+proc sealLoadedBackendEntities*(c: var DecodeContext) =
+  ## Before the `lower` stage serializes its transformed bodies, mark every
+  ## index-loaded sym/type `Sealed`. The backend loads them `Complete` (mutable
+  ## for the transform); without this, `writeNode`'s `shouldWriteSymDef` would
+  ## emit a duplicate `(sd)`/`(td)` def for a param/result/existing-local/owner
+  ## type — and the `cg` body-loader would then bind the body's `result` to a
+  ## FRESH sym instead of the one in `prc.ast[resultPos]` (the #6/#7 "result
+  ## cannot be captured" class). Sealed ⟹ SymUse ⟹ resolved via `cg`'s module
+  ## index. The transform-CREATED entities are absent from `c.syms`/`c.types`
+  ## (or are backend-minted), so they stay `Complete`/`@bk` and still get the
+  ## inline defs the loader's local-sym pre-scan needs.
+  for _, v in c.syms:
+    if v[0] != nil and v[0].state == Complete: v[0].state = Sealed
+  for _, v in c.types:
+    if v[0] != nil and v[0].state == Complete: v[0].state = Sealed
+
+proc preloadLoweredDefs(c: var DecodeContext; n: var Cursor; thisModule: string;
+                        localSyms: var Table[string, PSym]) =
+  ## One pre-scan over a `.t.nif` body that fully loads EVERY inline def it
+  ## carries before the body is decoded: suffix-less locals into `localSyms`,
+  ## backend-minted `@bk` syms into `c.syms`, `@bk` types into `c.types`. These
+  ## transform-created entities have NO module index entry, so a later reference
+  ## — in the body, or inside a loaded type that `typeKey`/codegen later walks
+  ## (where the index-based force-load would assert) — must find them already
+  ## loaded. The writer emits defs before uses, so a forward walk that loads each
+  ## in place suffices.
+  if n.kind != ParLe:
+    inc n
+    return
+  var depth = 0
+  while true:
+    if n.kind == ParLe:
+      if n.tagId == sdefTag:
+        let nm = n.firstSon
+        if nm.kind == SymbolDef:
+          let symName = pool.syms[nm.symId]
+          let sn = parseSymName(symName)
+          if sn.module.len == 0:
+            if symName notin localSyms:
+              let module = moduleId(c, thisModule)
+              let val = addr c.mods[module].symCounter
+              inc val[]
+              let sym = PSym(itemId: itemId(module.int32, val[]), kindImpl: skStub,
+                             name: c.cache.getIdent(sn.name), disamb: sn.count.int32,
+                             state: Complete)
+              localSyms[symName] = sym
+              inc n
+              loadSymFromCursor(c, sym, n, thisModule, localSyms)
+              sym.state = c.loadedState
+              continue
+          elif sn.module.endsWith(BackendLocalMarker):
+            let sym = c.loadSymStub(nm.symId, thisModule, localSyms)
+            if sym.state == Partial:
+              sym.state = c.loadedState
+              inc n
+              loadSymFromCursor(c, sym, n, thisModule, localSyms)
+              continue
+      elif n.tagId == tdefTag:
+        let nm = n.firstSon
+        if nm.kind == SymbolDef and pool.syms[nm.symId].endsWith(BackendLocalMarker):
+          discard loadTypeStub(c, n, localSyms)
+          continue
+      inc depth
+    elif n.kind == ParRi:
+      dec depth
+      if depth == 0:
+        inc n
+        break
+    inc n
+
+proc resolveHookSym*(c: var DecodeContext; symId: nifstreams.SymId): PSym
+
+proc repTagToOp(tagId: TagId): (bool, TTypeAttachedOp) =
+  ## Map a `(rep…)` hook tag to its attached-op kind (and whether it IS one).
+  if tagId == repDestroyTag: (true, attachedDestructor)
+  elif tagId == repCopyTag: (true, attachedAsgn)
+  elif tagId == repWasMovedTag: (true, attachedWasMoved)
+  elif tagId == repDupTag: (true, attachedDup)
+  elif tagId == repSinkTag: (true, attachedSink)
+  elif tagId == repTraceTag: (true, attachedTrace)
+  elif tagId == repDeepCopyTag: (true, attachedDeepCopy)
+  else: (false, attachedDestructor)
+
+proc loadLoweredBodies*(c: var DecodeContext; module: FileIndex; suffix: string;
+                        infile: string; loadBodies = true):
+    tuple[bodies: seq[tuple[name: string; body: PNode]]; hooks: seq[LogEntry]] =
+  ## Reconstruct the `lower` stage's `.t.nif`. Registers its embedded index as the
+  ## module's SECOND index/stream (`tIndex`/`tStream`) so every backend-minted
+  ## (`@bk`) closure-env entity resolves through NIF's own index — `createTypeStub`/
+  ## `loadSymStub` fall through to it on a `.s.nif` miss, `loadType`/`loadSym` seek
+  ## `tStream`. Returns the owned routines' transformed bodies (the `(lowered …)`
+  ## entries) and the lifted type-bound ops (`(rep… key sym)`); the hook ROUTINES'
+  ## `(sd)` defs are loaded lazily through the index when their op's sym resolves.
+  result = (@[], @[])
+  if not fileExists(infile): return
+  var tstream = nifstreams.open(infile)
+  let tindex = readEmbeddedIndex(tstream)  # leaves the cursor at the content start
+  let contentStart = offset(tstream.r)
+  c.mods[module].tStream = tstream
+  c.mods[module].tIndex = tindex
+  c.mods[module].hasTIndex = true
+  # Parse the WHOLE content up front: the per-body loads below lazily seek
+  # `tStream` for `@bk` entities, which would otherwise clobber a live walk cursor.
+  var buf = createTokenBuf(256)
+  tstream.r.jumpTo contentStart
+  nifcursors.parse(tstream, buf, NoLineInfo)
+  var n = beginRead(buf)
+  if n.kind != ParLe: return
+  inc n          # into (stmts -> flags dot
+  inc n          # -> type dot
+  inc n          # -> first entry or ParRi
+  while n.kind == ParLe:
+    if n.tagId == loweredTag:
+      if not loadBodies:
+        # A dependency: register its `.t.nif` (tIndex + hooks) so the consumer
+        # can destroy the dep's closures, but don't reconstruct its bodies (only
+        # the dep's OWN cg emits them).
+        skip n
+        continue
+      inc n        # -> StringLit name
+      let name = pool.strings[n.litId]
+      inc n        # -> body tree
+      var localSyms = initTable[string, PSym]()
+      var scanCursor = n
+      extractLocalSymsFromTree(c, scanCursor, suffix, localSyms)
+      let body = loadNode(c, n, suffix, localSyms)
+      result.bodies.add (name, body)
+      skipParRi n  # close (lowered ...)
+    else:
+      let (isHook, op) = repTagToOp(n.tagId)
+      if isHook:
+        inc n      # -> StringLit key
+        let key = pool.strings[n.litId]
+        inc n      # -> Symbol sym
+        let sym = resolveHookSym(c, n.symId)
+        inc n
+        if sym != nil:
+          result.hooks.add LogEntry(kind: HookEntry, op: op, module: module.int,
+                                    key: key, sym: sym)
+        skipParRi n
+      else:
+        skip n     # a hook routine's `(sd)` def — loaded lazily via the index
+
 
 template withNode(c: var DecodeContext; n: var Cursor; result: PNode; kind: TNodeKind; body: untyped) =
   let info = c.infos.oldLineInfo(n.info)
@@ -2126,9 +2436,47 @@ proc loadNode(c: var DecodeContext; n: var Cursor; thisModule: string;
           loadSymFromCursor(c, sym, n, thisModule, localSyms)
           sym.state = c.loadedState  # mark as fully loaded
           result = newSymNode(sym, info)
-        else:
+        elif sn.module.endsWith(BackendLocalMarker):
+          # A backend-minted (`@bk`) def lives ONLY inline in this `.t.nif` body
+          # (not in any module index): create/find its cached stub and FILL it
+          # from the sdef instead of skipping (which would leave the skModule/
+          # Partial stub `loadSymStub` made unresolved).
           sym = c.loadSymStub(name.symId, thisModule, localSyms)
-          skip n  # skip the entire sdef for indexed symbols
+          if sym.state == Partial:
+            sym.state = c.loadedState
+            inc n  # skip `sd` tag
+            loadSymFromCursor(c, sym, n, thisModule, localSyms)
+          else:
+            skip n
+          result = newSymNode(sym, info)
+          result.flags.incl nfLazyType
+        else:
+          # A module-homed inline sdef. Normally its def lives in that module's
+          # index and is loaded lazily, so we skip the inline copy. BUT a
+          # transform-created closure-env FIELD (`x0.0.clo`) is module-homed yet
+          # lives ONLY inline in this `.t.nif` reclist — it has no index entry.
+          # Skipping it leaves a nil-typed `skModule` fallback stub (from
+          # loadSymStub's "no offset" path) and codegen of the env struct then
+          # dereferences a nil field type. Detect the unindexed case and FILL
+          # the sym from the inline def instead.
+          let m = moduleId(c, sn.module)
+          let indexed = c.mods[m].index.hasKey(symName) or
+                        (c.mods[m].hasTIndex and c.mods[m].tIndex.hasKey(symName))
+          if indexed:
+            sym = c.loadSymStub(name.symId, thisModule, localSyms)
+            skip n  # skip the entire sdef for indexed symbols
+          else:
+            sym = c.syms.getOrDefault(symName)[0]
+            if sym == nil:
+              let val = addr c.mods[m].symCounter
+              inc val[]
+              sym = PSym(itemId: itemId(m.int32, val[]), kindImpl: skStub,
+                         name: c.cache.getIdent(sn.name), disamb: sn.count.int32,
+                         state: Partial)
+            c.syms[symName] = (sym, NifIndexEntry())
+            sym.state = c.loadedState
+            inc n  # skip `sd` tag
+            loadSymFromCursor(c, sym, n, thisModule, localSyms)
           result = newSymNode(sym, info)
           result.flags.incl nfLazyType
       of typeDefTagName:
@@ -2289,7 +2637,7 @@ proc moduleSymbolStubs*(c: var DecodeContext; module: FileIndex): seq[PSym] =
 
 proc toNifFilename*(conf: ConfigRef; f: FileIndex): string =
   let suffix = moduleSuffix(conf, f)
-  result = toGeneratedFile(conf, AbsoluteFile(suffix), ".nif").string
+  result = toGeneratedFile(conf, AbsoluteFile(suffix), ".s.nif").string
 
 proc resolveSym(c: var DecodeContext; symAsStr: string; alsoConsiderPrivate: bool): PSym =
   result = c.syms.getOrDefault(symAsStr)[0]
@@ -2311,6 +2659,11 @@ proc resolveSym(c: var DecodeContext; symAsStr: string; alsoConsiderPrivate: boo
     # Try the format without module suffix
     let localKey = sn.name & "." & $sn.count & "."
     offs = c.mods[module].index.getOrDefault(localKey)
+  if offs.offset == 0 and c.mods[module].hasTIndex:
+    # A `lower`-stage entity (an `@bk` hook routine, OR a module-homed derived
+    # closure-env field added only in the backend) lives in the module's
+    # `.t.nif`, not its `.s.nif`: resolve it through the second (tIndex) index.
+    offs = c.mods[module].tIndex.getOrDefault(symAsStr)
   if offs.offset == 0:
     return nil
   if not alsoConsiderPrivate and offs.vis == Hidden:
diff --git a/compiler/cnif.nim b/compiler/cnif.nim
index 0caebd8682..ad51f47f8c 100644
--- a/compiler/cnif.nim
+++ b/compiler/cnif.nim
@@ -358,6 +358,52 @@ proc readCnifHeads*(f: string): CnifHeads =
   endRead(c)
   result.valid = sawMeta and version == CnifVersion
 
+proc writeLoweredArtifact*(outfile: string; entries: openArray[string]) =
+  ## The `.t.nif` "lowered" artifact: one `(lowered "<nifname>" <body>)` per
+  ## routine the module OWNS, written by the per-module `lower` backend stage
+  ## for the `cg` stage to read instead of re-deriving the transformed body
+  ## (re-derivation in each parallel `cg` process is what makes a closure
+  ## `:env`'s identity diverge across modules — see transf.transformBody).
+  ##
+  ## SKELETON: every body is the empty-marker `.` ("transformed body == sem
+  ## body"), so `cg` falls back to its own `transformBody` and output stays
+  ## byte-identical. The real transformed body fills this slot in a later step;
+  ## the `.` then means "unchanged by lowering" (the dedup Araq sketched).
+  var b = nifbuilder.open(outfile)
+  b.withTree "stmts":
+    for name in entries:
+      b.withTree "lowered":
+        b.addStrLit name
+        b.addEmpty()
+  b.close()
+
+proc readLoweredArtifact*(f: string): seq[string] =
+  ## The routine NIF names recorded in a `.t.nif`. (Bodies are not returned
+  ## yet: the skeleton records only empty-markers; reading proves the artifact
+  ## round-trips and that the `cg` rule depends on it.)
+  result = @[]
+  if not fileExists(f): return
+  var pool = newPool()
+  var tags = newTagPool()
+  let stmtsTag = tags.registerTag("stmts")
+  let loweredTag = tags.registerTag("lowered")
+  var buf = parseFromFile(f, 1000, pool, tags)
+  var c = beginRead(buf)
+  if c.kind != TagLit or c.cursorTagId != stmtsTag:
+    endRead(c)
+    return
+  c.loopInto:
+    if c.kind == TagLit and c.cursorTagId == loweredTag:
+      c.loopInto:
+        if c.kind == StrLit:
+          result.add strVal(c)
+          inc c
+        else:
+          skip c
+    else:
+      skip c
+  endRead(c)
+
 type
   CnifLiveness* = object
     defs*: int      ## proc definitions emitted across all modules
diff --git a/compiler/deps.nim b/compiler/deps.nim
index 08a272aa6b..3dec7fc507 100644
--- a/compiler/deps.nim
+++ b/compiler/deps.nim
@@ -51,7 +51,7 @@ proc parsedFile(c: DepContext; f: FilePair): string =
   getNimcacheDir(c.config).string / f.modname & ".p.nif"
 
 proc semmedFile(c: DepContext; f: FilePair): string =
-  getNimcacheDir(c.config).string / f.modname & ".nif"
+  getNimcacheDir(c.config).string / f.modname & ".s.nif"
 
 proc ifaceFile(c: DepContext; f: FilePair): string =
   ## Interface-cookie sidecar written by `nim m` (ast2nif.writeIfaceCookie,
@@ -941,9 +941,11 @@ proc generateBackendBuildFile(c: DepContext; forwardedArgs: seq[string]): string
   # Per-node output paths.
   var cnifFiles = newSeq[string](c.nodes.len)
   var cFiles = newSeq[string](c.nodes.len)
+  var tFiles = newSeq[string](c.nodes.len)
   for i, node in c.nodes:
     cFiles[i] = backendCFile(c, node)
     cnifFiles[i] = cFiles[i] & ".nif"
+    tFiles[i] = cFiles[i] & ".t.nif"
 
   var b = nifbuilder.open(result)
   defer: b.close()
@@ -979,9 +981,28 @@ proc generateBackendBuildFile(c: DepContext; forwardedArgs: seq[string]): string
     b.addStrLit s
     b.endTree()
 
-  # cg: one rule per module. Inputs are the project (slot 0) and every semmed
-  # NIF (so the whole program loads and the rule is ordered after the frontend);
-  # the main module additionally depends on every other `.c.nif` (init metas).
+  # lower: one rule per module. Transforms (eventually) the routines the module
+  # OWNS once, in the owner's id space, into `<module>.t.nif`, so the `cg` stage
+  # reads them instead of re-deriving (which makes a closure `:env`'s identity
+  # diverge across the parallel `cg` processes). Runs per module in parallel on
+  # the shallow backend dep-graph. Inputs mirror `cg` (project + every semmed
+  # NIF) so the rule is ordered after the frontend.
+  for i, node in c.nodes:
+    b.addTree "do"
+    b.addIdent "nim_nifc"
+    b.withTree "args":
+      b.addStrLit "--icBackendStage:lower"
+      b.addStrLit "--icBackendModule:" & node.files[0].modname
+    inputStr mainNif
+    for n2 in c.nodes:
+      inputStr c.semmedFile(n2.files[0])
+    outputStr tFiles[i]
+    b.endTree()
+
+  # cg: one rule per module. Inputs are the project (slot 0), every semmed
+  # NIF (so the whole program loads and the rule is ordered after the frontend)
+  # and this module's `.t.nif` (its lowered bodies); the main module additionally
+  # depends on every other `.c.nif` (init metas).
   for i, node in c.nodes:
     b.addTree "do"
     b.addIdent "nim_nifc"
@@ -991,6 +1012,7 @@ proc generateBackendBuildFile(c: DepContext; forwardedArgs: seq[string]): string
     inputStr mainNif
     for n2 in c.nodes:
       inputStr c.semmedFile(n2.files[0])
+    inputStr tFiles[i]
     if node.id == 0:
       for j in 0 ..< c.nodes.len:
         if c.nodes[j].id != 0:
diff --git a/compiler/injectdestructors.nim b/compiler/injectdestructors.nim
index 2b5ae6421e..7f5b00ee8b 100644
--- a/compiler/injectdestructors.nim
+++ b/compiler/injectdestructors.nim
@@ -245,6 +245,18 @@ proc genOp(c: var Con; t: PType; kind: TTypeAttachedOp; dest, ri: PNode): PNode
     let canon = c.graph.canonTypes.getOrDefault(h)
     if canon != nil:
       op = getAttachedOp(c.graph, canon, kind)
+  if (op == nil or op.ast.isGenericRoutine) and icLoweredBodies(c.graph.config):
+    # IC: injectDestructorCalls is demand-driven and runs HERE (cg), not in the
+    # `lower` stage, so a structural, env-agnostic op the lower stage never had
+    # reason to serialize — most often a closure PROC type's `=destroy`/`=sink`
+    # (which act on the `(ClP_0, ClE_0)` tuple, NOT the concrete env) — must be
+    # lifted on demand, exactly as the lazy path's cg does. This is safe now:
+    # closure-env identity resolves via `attachedOps[itemId]`/env-erased typeKey,
+    # env objects load complete, and atomicRefOp's type-erased path covers any
+    # still-incomplete env (so the lift never walks a nil field).
+    excl t.flagsImpl, tfCheckedForDestructor
+    createTypeBoundOps(c.graph, nil, t, dest.info, c.idgen)
+    op = getAttachedOp(c.graph, t, kind)
   if op == nil:
     #echo dest.typ.id
     globalError(c.graph.config, dest.info, "internal error: '" & AttachedOpToStr[kind] &
diff --git a/compiler/lambdalifting.nim b/compiler/lambdalifting.nim
index 9e00179fbb..3418bad729 100644
--- a/compiler/lambdalifting.nim
+++ b/compiler/lambdalifting.nim
@@ -176,7 +176,7 @@ proc closureParams(routine: PSym): PNode =
     result = routine.typ.n
     routine.ast[paramsPos] = result
 
-proc addHiddenParam(routine: PSym, param: PSym) =
+proc addHiddenParam*(routine: PSym, param: PSym) =
   assert param.kind == skParam
   var params = closureParams(routine)
   # -1 is correct here as param.position is 0 based but we have at position 0
diff --git a/compiler/liftdestructors.nim b/compiler/liftdestructors.nim
index 3684ec437a..60408a5861 100644
--- a/compiler/liftdestructors.nim
+++ b/compiler/liftdestructors.nim
@@ -761,6 +761,28 @@ proc cyclicType*(g: ModuleGraph, t: PType): bool =
   of tyProc: result = t.callConv == ccClosure
   else: result = false
 
+proc recHasNilFieldType(n: PNode): bool =
+  case n.kind
+  of nkSym: result = n.sym == nil or n.sym.typ == nil
+  of nkRecList, nkRecCase:
+    for ch in n:
+      if recHasNilFieldType(ch): return true
+    result = false
+  else: result = false
+
+proc isTypeErasedEnvRef(config: ConfigRef; elemType: PType): bool =
+  ## IC: a foreign closure-env object can load (cross-module) with nil-typed
+  ## derived fields when its hook key diverges across the NIF boundary. Per the
+  ## closure type-erasure principle, destroying such a `ref` must go through RTTI
+  ## (`nimDestroyAndDispose` / `nimDecRefIsLastCyclicDyn`), NEVER a statically
+  ## lifted concrete env destructor — the producer module emits that destructor
+  ## and registers it in the env object's type info. Detect the incomplete load
+  ## so `atomicRefOp` takes the dynamic-dispatch path and never walks the nil
+  ## field (which would SIGSEGV).
+  if not config.icLoweredBodies: return false
+  let obj = elemType.skipTypes({tyGenericInst, tyAlias, tySink, tyOwned})
+  result = obj.kind == tyObject and obj.n != nil and recHasNilFieldType(obj.n)
+
 proc atomicRefOp(c: var TLiftCtx; t: PType; body, x, y: PNode) =
   #[ bug #15753 is really subtle. Usually the classical write barrier for reference
   counting looks like this::
@@ -793,13 +815,17 @@ proc atomicRefOp(c: var TLiftCtx; t: PType; body, x, y: PNode) =
   ]#
   var actions = newNodeI(nkStmtList, c.info)
   let elemType = t.elementType
+  # (b) type-erasure: a foreign closure env that loaded incomplete cross-module
+  # must be destroyed via RTTI, not by lifting its concrete (nil-fielded) object.
+  let erased = isTypeErasedEnvRef(c.g.config, elemType)
 
-  createTypeBoundOps(c.g, c.c, elemType, c.info, c.idgen)
+  if not erased:
+    createTypeBoundOps(c.g, c.c, elemType, c.info, c.idgen)
 
   # YRC uses dedicated runtime procs for the entire write barrier:
   if c.g.config.selectedGC == gcYrc:
     let desc =
-      if isFinal(elemType):
+      if isFinal(elemType) and not erased:
         let ti = genBuiltin(c, mGetTypeInfoV2, "getTypeInfoV2", newNodeIT(nkType, x.info, elemType))
         ti.typ = getSysType(c.g, c.info, tyPointer)
         ti
@@ -814,31 +840,40 @@ proc atomicRefOp(c: var TLiftCtx; t: PType; body, x, y: PNode) =
       return
     else: discard # fall through for destructor, trace, wasMoved
 
-  let isCyclic = c.g.config.selectedGC in {gcOrc, gcYrc} and types.canFormAcycle(c.g, elemType)
+  # `erased` must short-circuit BEFORE canFormAcycle/isPureObject/isFinal, which
+  # walk the (nil-fielded) type graph. Assume cyclic + dynamic — the dyn runtime
+  # calls work for any ref via its type info.
+  let isCyclic = erased or
+    (c.g.config.selectedGC in {gcOrc, gcYrc} and types.canFormAcycle(c.g, elemType))
 
-  let isInheritableAcyclicRef = c.g.config.selectedGC in {gcOrc, gcYrc} and
+  let isInheritableAcyclicRef = (not erased) and c.g.config.selectedGC in {gcOrc, gcYrc} and
                       (not isPureObject(elemType)) and
                       tfAcyclic in skipTypes(elemType, abstractInst+{tyOwned}-{tyTypeDesc}).flags
   # dynamic Acyclic refs need to use dyn decRef
 
+  let useStatic = (not erased) and isFinal(elemType)
+
   let tmp =
     if isCyclic and c.kind in {attachedAsgn, attachedSink, attachedDup}:
       declareTempOf(c, body, x)
     else:
       x
 
-  if isFinal(elemType):
+  if useStatic:
     addDestructorCall(c, elemType, actions, genDeref(tmp, nkDerefExpr))
     var alignOf = genBuiltin(c, mAlignOf, "alignof", newNodeIT(nkType, c.info, elemType))
     alignOf.typ = getSysType(c.g, c.info, tyInt)
     actions.add callCodegenProc(c.g, "nimRawDispose", c.info, tmp, alignOf)
   else:
-    addDestructorCall(c, elemType, newNodeI(nkStmtList, c.info), genDeref(tmp, nkDerefExpr))
+    # `nimDestroyAndDispose` resolves the real destructor via the object's RTTI,
+    # so the env destructor the producer emitted runs — no static lift needed.
+    if not erased:
+      addDestructorCall(c, elemType, newNodeI(nkStmtList, c.info), genDeref(tmp, nkDerefExpr))
     actions.add callCodegenProc(c.g, "nimDestroyAndDispose", c.info, tmp)
 
   var cond: PNode
   if isCyclic:
-    if isFinal(elemType):
+    if useStatic:
       let typInfo = genBuiltin(c, mGetTypeInfoV2, "getTypeInfoV2", newNodeIT(nkType, x.info, elemType))
       typInfo.typ = getSysType(c.g, c.info, tyPointer)
       cond = callCodegenProc(c.g, "nimDecRefIsLastCyclicStatic", c.info, tmp, typInfo)
@@ -873,7 +908,7 @@ proc atomicRefOp(c: var TLiftCtx; t: PType; body, x, y: PNode) =
   of attachedDeepCopy: assert(false, "cannot happen")
   of attachedTrace:
     if isCyclic:
-      if isFinal(elemType):
+      if useStatic:
         let typInfo = genBuiltin(c, mGetTypeInfoV2, "getTypeInfoV2", newNodeIT(nkType, x.info, elemType))
         typInfo.typ = getSysType(c.g, c.info, tyPointer)
         body.add callCodegenProc(c.g, "nimTraceRef", c.info, genAddrOf(x, c.idgen), typInfo, y)
diff --git a/compiler/modulegraphs.nim b/compiler/modulegraphs.nim
index 98f7b58537..65c8a070f3 100644
--- a/compiler/modulegraphs.nim
+++ b/compiler/modulegraphs.nim
@@ -904,7 +904,7 @@ proc getBody*(g: ModuleGraph; s: PSym): PNode {.inline.} =
   assert result != nil
 
 when not defined(nimKochBootstrap):
-  proc registerLoadedHooks(g: ModuleGraph; logOps: seq[LogEntry]) =
+  proc registerLoadedHooks*(g: ModuleGraph; logOps: seq[LogEntry]) =
     let mainSuffix = getMainModuleSuffix(ast.program)
     for x in logOps:
       # A dependency's NIF may carry hooks whose syms belong to the module we
diff --git a/compiler/nifbackend.nim b/compiler/nifbackend.nim
index ed2567d91c..b83e25e152 100644
--- a/compiler/nifbackend.nim
+++ b/compiler/nifbackend.nim
@@ -26,6 +26,8 @@ import ast, options, lineinfos, modulegraphs, cgendata, cgen,
   pathutils, extccomp, msgs, modulepaths, idents, types, ast2nif, typekeys,
   cnif
 from cgmeth import generateIfMethodDispatchers
+from transf import transformBody
+from lambdalifting import getEnvParam, addHiddenParam, paramName
 import ic / replayer
 
 proc loadModuleDependencies(g: ModuleGraph; mainFileIdx: FileIndex;
@@ -145,6 +147,32 @@ proc signatureHasMetaType(t: PType; depth: int = 0): bool =
   for k in t.kids:
     if signatureHasMetaType(k, depth + 1): return true
 
+proc ownsRuntimeRoutine(s: PSym; modPos: int): bool =
+  ## A concrete, non-generic, runtime routine with a real body, OWNED by the
+  ## module at `modPos`. Shared by the `cg` stage's owned-routine seeding (so a
+  ## routine called only from other modules is still emitted by somebody) and
+  ## the `lower` stage's owned-routine enumeration, so both stages see exactly
+  ## the same set. The exclusions:
+  ## - nested/closure procs (owner is a proc, not a module): emitted via their
+  ##   enclosing routine's lambda-lifting, never standalone;
+  ## - generic instances (`sfFromGeneric`): emitted by demand, deduped by merge;
+  ## - `importc`/`compileTime`/`error`/forward sentinels and meta signatures:
+  ##   not real codegen targets.
+  ## A `{.closure.}` iterator IS a standalone runtime routine (unlike an inline
+  ## iterator, which is expanded at each call site) and must be emitted by its
+  ## owner — else a cross-module `for` over it links to nothing.
+  s.itemId.module == modPos and
+  (s.kind in {skProc, skFunc, skConverter, skMethod} or
+   (s.kind == skIterator and s.typ != nil and s.typ.callConv == ccClosure)) and
+  s.skipGenericOwner != nil and s.skipGenericOwner.kind == skModule and
+  s.magic == mNone and
+  sfFromGeneric notin s.flags and
+  {sfForward, sfImportc, sfCompileTime, sfError} * s.flags == {} and
+  s.typ != nil and not signatureHasMetaType(s.typ) and
+  s.ast != nil and s.ast.safeLen > bodyPos and
+  s.ast[genericParamsPos].kind == nkEmpty and
+  s.ast[bodyPos].kind != nkEmpty
+
 proc generateCodeForModule(g: ModuleGraph; precomp: PrecompiledModule) =
   ## Generate C code for a single module.
   let moduleId = precomp.module.position
@@ -170,34 +198,7 @@ proc generateCodeForModule(g: ModuleGraph; precomp: PrecompiledModule) =
   if g.config.cmd == cmdNifC and g.config.icBackendStage == "cg":
     let modPos = precomp.module.position
     for s in moduleSymbolStubs(ast.program, FileIndex modPos):
-      if s.itemId.module == modPos and
-         s.kind in {skProc, skFunc, skConverter, skMethod} and
-         # Only MODULE-level routines: a nested/closure proc (its owner is a
-         # proc) captures its enclosing scope and cannot be emitted standalone —
-         # the captured params have no loc → `expr: param not init`. Nested procs
-         # are emitted via their enclosing routine's lambda-lifting, so seeding
-         # the enclosing (module-level) routine already covers them.
-         s.skipGenericOwner != nil and s.skipGenericOwner.kind == skModule and
-         s.magic == mNone and
-         # Skip generic instances: they have no single owning-module top-level
-         # and are emitted by demand (emit-everywhere, deduped by the merge
-         # stage). An instance has an empty `genericParamsPos` just like a plain
-         # concrete proc, so only `sfFromGeneric` tells them apart; seeding one
-         # would force standalone codegen of an instance body whose `when T is X`
-         # branches were never folded for this path → `genMagicExpr: mIs`.
-         sfFromGeneric notin s.flags and
-         # Every other routine the module owns must be emitted here, exported or
-         # not: a non-exported helper is still reached from another module when a
-         # `template`/inline routine expands at a call site there (e.g. msgs'
-         # `internalErrorImpl` behind the `internalError` template), and that
-         # caller now only prototypes it. `{.error.}`/`compileTime` sentinels and
-         # bodyless forward decls are not real codegen targets.
-         {sfForward, sfImportc, sfCompileTime, sfError} * s.flags == {} and
-         s.typ != nil and not signatureHasMetaType(s.typ) and
-         s.ast != nil and s.ast.safeLen > bodyPos and
-         s.ast[genericParamsPos].kind == nkEmpty and
-         s.ast[bodyPos].kind != nkEmpty:
-        # a concrete, non-generic, runtime routine with a real body, owned here
+      if ownsRuntimeRoutine(s, modPos):
         requestProcDef(bmod, s)
 
 proc loadBackendModules(g: ModuleGraph; mainFileIdx: FileIndex):
@@ -323,6 +324,218 @@ proc findTargetModule(g: ModuleGraph; modules: seq[PrecompiledModule];
       cachedModuleSuffix(g.config, FileIndex precompSys.module.position) == suffix:
     return precompSys
 
+proc findHiddenEnvParam(n: PNode; owner: PSym): PSym =
+  ## Locate the hidden env param (`:envP`) that belongs to `owner` in its loaded
+  ## transformed body, so it can be re-welded into `owner`'s signature. Matching
+  ## by `owner` is essential: a body can ALSO reference a callee's `:envP` (a
+  ## closure call passes the callee env), so the first `:envP` in DFS order is
+  ## not necessarily this proc's own.
+  if n == nil: return nil
+  if n.kind == nkSym:
+    if n.sym != nil and n.sym.kind == skParam and n.sym.name.s == paramName and
+        n.sym.owner == owner:
+      return n.sym
+  else:
+    for i in 0 ..< n.safeLen:
+      let r = findHiddenEnvParam(n[i], owner)
+      if r != nil: return r
+  return nil
+
+proc registerLoweredModule(g: ModuleGraph; m: PSym; applyBodies: bool) =
+  ## Load module `m`'s `<m>.t.nif` and register its `lower`-stage output: its
+  ## embedded index (so its closure-env `@bk` entities resolve) and its lifted
+  ## type-bound ops (so any cg that DESTROYS one of `m`'s closures finds the env
+  ## `=destroy` via getAttachedOp). For the cg TARGET (`applyBodies`), also set
+  ## each owned routine's `transformedBody` so cg's `transformBody` short-circuits
+  ## (transf.nim:1383) instead of re-deriving. `injectDestructorCalls` stays in cg.
+  let modPos = m.position
+  let bmod = BModuleList(g.backend).mods[modPos]
+  if bmod == nil: return
+  let artifact = getCFile(bmod).string & ".t.nif"
+  if not fileExists(artifact): return
+  let suffix = cachedModuleSuffix(g.config, FileIndex modPos)
+  let (bodies, hooks) = loadLoweredBodies(ast.program, FileIndex modPos, suffix,
+                                          artifact, loadBodies = applyBodies)
+  registerLoadedHooks(g, hooks)
+  if not applyBodies: return
+  var byName = initTable[string, PSym]()
+  for s in moduleSymbolStubs(ast.program, FileIndex modPos):
+    # Owned routines AND nested closure routines (the `:anonymous` procs the
+    # lower stage emits as their own entries) — both are index-resolvable syms
+    # of this module whose transformed body the lower stage authored.
+    if s.kind in routineKinds and s.itemId.module == modPos:
+      byName[globalName(s, g.config)] = s
+  for (name, body) in bodies:
+    let s = byName.getOrDefault(name)
+    # `.s.nif` wins: only fill from `.t.nif` if sem did not already transform it.
+    if s != nil and body != nil and s.transformedBody == nil:
+      s.transformedBody = body
+      # Lambda-lift in the lower stage gave this proc a hidden `:envP` env param
+      # (a captured-var closure env), but cg loaded the PRE-lift signature from
+      # `.s.nif`. The transformed body references that `@bk` `:envP`; re-weld it
+      # into the proc's params so genProc assigns it a loc (else "param not
+      # init"). `transformBody` short-circuits on the cached body, skipping the
+      # lift that normally adds it. This applies to BOTH a true `ccClosure` proc
+      # (env arrives via the closure ABI `ClE_0`, needs `tfCapturesEnv`) and a
+      # plain nested `nimcall` proc that merely captures (env is a regular last
+      # param). Match the env param by owner — a body can also reference a
+      # callee's `:envP`.
+      if s.typ != nil and getEnvParam(s) == nil:
+        let ep = findHiddenEnvParam(body, s)
+        if ep != nil:
+          # From-source, `ast[paramsPos]` and `typ.n` are the SAME node, but a
+          # NIF-loaded routine has two distinct param nodes. genProc reads
+          # `typ.n`, so unify them first — else addHiddenParam appends to
+          # `ast[paramsPos]` and the env param never reaches genProc's loc setup.
+          if s.typ.n != nil:
+            s.ast[paramsPos] = s.typ.n
+          addHiddenParam(s, ep)
+          # The lower stage's lambda-lift converts EVERY captured nested proc to a
+          # closure (collectNestedClosureBodies only emits `ccClosure` entries),
+          # and the serialized call sites use the closure ABI. cg loaded the
+          # pre-lift signature, which for a proc only ever CALLED (never used as a
+          # value) is still `nimcall`. Re-apply the lift's `ccClosure` +
+          # `tfCapturesEnv` so closureSetup maps the env param to `ClE_0` and the
+          # calls match.
+          s.typ.callConv = ccClosure
+          incl(s.typ, {tfCapturesEnv})
+
+proc applyLoweredBodies(g: ModuleGraph; modules: seq[PrecompiledModule];
+                        precompSys: PrecompiledModule; target: PrecompiledModule) =
+  ## Register every loaded module's `.t.nif` (env entities + lifted hooks),
+  ## applying transformed bodies only for the cg target.
+  if not icLoweredBodies(g.config): return  # Stage 0 (lazy): nothing to apply
+  if precompSys.module != nil:
+    registerLoweredModule(g, precompSys.module, applyBodies = false)
+  for m in modules:
+    if m.module != nil:
+      registerLoweredModule(g, m.module,
+        applyBodies = (m.module.position == target.module.position))
+
+proc collectNestedClosureBodies(g: ModuleGraph; idgen: IdGenerator; n: PNode;
+                                owner: PSym; seen: var IntSet;
+                                entries: var seq[tuple[name: string; body: PNode]]) =
+  ## A closure routine nested in `owner` (the `:anonymous` proc lambda-lifting
+  ## minted, plus any deeper nesting) gets its captured-var→env rewrite produced
+  ## as part of the OWNER's `transformBody`, but only the owner's body is emitted
+  ## as a `(lowered)` entry. The nested proc itself IS index-resolvable (it has a
+  ## `.s.nif` sdef from sem, with its PRE-lift body), so cg loads that and
+  ## re-derives — and the capture mapping is gone (it accesses `x` directly
+  ## instead of `ClE_0->x0`). Walk the transformed body and emit each nested
+  ## closure routine's transformed body as its OWN `(lowered)` entry so
+  ## applyLoweredBodies installs it and cg reuses it verbatim.
+  if n == nil: return
+  if n.kind == nkSym:
+    let s = n.sym
+    if s != nil and s.kind in routineKinds and s != owner and
+        not seen.containsOrIncl(s.id):
+      if s.ast != nil and getBody(g, s).kind != nkEmpty and
+          s.typ != nil and s.typ.callConv == ccClosure:
+        if s.transformedBody == nil:
+          s.transformedBody = transformBody(g, idgen, s, {})
+        entries.add (globalName(s, g.config), s.transformedBody)
+        collectNestedClosureBodies(g, idgen, s.transformedBody, s, seen, entries)
+  else:
+    for i in 0 ..< n.safeLen:
+      collectNestedClosureBodies(g, idgen, n[i], owner, seen, entries)
+
+proc generateLowerStage(g: ModuleGraph; mainFileIdx: FileIndex) =
+  ## Per-module backend lowering (`--icBackendStage:lower --icBackendModule:<suffix>`):
+  ## enumerate the routines this module OWNS and write them to `<module>.t.nif`.
+  ## Eventually this transforms each owned routine once, in the owner's id space,
+  ## so `cg` reads the result instead of re-deriving it (re-derivation per
+  ## parallel `cg` process is the root of the closure-`:env` identity drift).
+  ## Runs per module in parallel on the shallow backend dep-graph — NOT folded
+  ## into the dense, mostly-serial sem stage.
+  ##
+  ## gate `newSymNode`'s lazy-type marking to the backend (see astdef) — the
+  ## transform builds sym nodes off not-yet-typed stubs, exactly as the `cg`
+  ## stage does.
+  nifcBackendActive = true
+  let mainSuffix = cachedModuleSuffix(g.config, mainFileIdx)
+  let targetIsMain = g.config.icBackendModule.len == 0 or
+                     g.config.icBackendModule == mainSuffix
+  var modules: seq[PrecompiledModule]
+  var precompSys: PrecompiledModule
+  var target: PrecompiledModule
+  if targetIsMain:
+    var nifFiles: seq[string]
+    (modules, precompSys, nifFiles) = loadBackendModules(g, mainFileIdx)
+    if modules.len == 0:
+      rawMessage(g.config, errGenerated,
+        "Cannot load NIF file for main module: " & toFullPath(g.config, mainFileIdx))
+      return
+    target = findTargetModule(g, modules, precompSys, g.config.icBackendModule)
+  else:
+    (modules, precompSys, target) = loadDepClosure(g, g.config.icBackendModule)
+  if target.module == nil:
+    rawMessage(g.config, errGenerated,
+      "per-module lowering: module not found for suffix: " & g.config.icBackendModule)
+    return
+  let modPos = target.module.position
+  let tb = BModuleList(g.backend).mods[modPos]
+  if tb == nil:
+    rawMessage(g.config, errGenerated,
+      "per-module lowering: no backend module for suffix: " & g.config.icBackendModule)
+    return
+  let artifact = getCFile(tb).string & ".t.nif"
+  if icLoweredBodies(g.config):
+    # STAGE 1 (DEFAULT; `-d:icNoLowerBodies` opts out): transform every owned routine
+    # ONCE in this single process's id space and serialize the results, so `cg`
+    # reads them instead of re-deriving (the single-writer-per-owner that keeps
+    # closure-`:env` identity stable). `transformBody` with flags {} mirrors the
+    # cg call (cgen.nim:1409); we keep only its return value (it clears
+    # `transformedBody` for non-cached procs). `injectDestructorCalls` is NOT run
+    # — it stays in `cg` on the loaded body.
+    var entries: seq[tuple[name: string; body: PNode]] = @[]
+    # `transformBody`/lambda-lifting LIFTS the closure env's type-bound ops
+    # (`=destroy` etc.) into `g.opsLog`; snapshot its length so we can serialize
+    # exactly the ops THIS stage created (not those loaded from `.s.nif`).
+    let opsLogStart = g.opsLog.len
+    for s in moduleSymbolStubs(ast.program, FileIndex modPos):
+      if ownsRuntimeRoutine(s, modPos):
+        # `.s.nif` wins: a routine already transformed during sem (CT eval /
+        # macro / VM transform) carries its lowered body in the `.s.nif` slot —
+        # don't re-transform it here, just leave its `.t.nif` entry empty.
+        if s.transformedBody != nil: continue
+        let tbody = transformBody(g, tb.idgen, s, {})
+        entries.add (globalName(s, g.config), tbody)
+        var seenNested = initIntSet()
+        collectNestedClosureBodies(g, tb.idgen, tbody, s, seenNested, entries)
+    # Collect the hooks this stage lifted, and transform each hook ROUTINE's body
+    # too (it is itself lowered into NIFC). The hooks' `(sd)` + transformed body go
+    # into the `.t.nif`; `cg` re-attaches them so `injectDestructorCalls` resolves
+    # the loaded env's `=destroy`. Iterate to a fixpoint: a hook body can lift
+    # further hooks (a field's `=destroy`).
+    var hooks: seq[LogEntry] = @[]
+    var i = opsLogStart
+    while i < g.opsLog.len:
+      let e = g.opsLog[i]
+      if e.kind == HookEntry and e.sym != nil and e.sym.kind in routineKinds and
+          e.sym.transformedBody == nil:
+        hooks.add e
+        # Transform the hook routine's body and cache it on the sym so
+        # `writeSymDef` serializes it in the hook's `(sd)` transformed-body slot
+        # (`transformBody {}` returns the body but does not cache it).
+        e.sym.transformedBody = transformBody(g, tb.idgen, e.sym, {})
+      inc i
+    # Seal the index-loaded entities so their references in the bodies serialize
+    # as SymUses (resolved via the module index in cg), not duplicate defs.
+    sealLoadedBackendEntities(ast.program)
+    serializeLoweredBodies(g.config, modPos.int32, entries, hooks, artifact)
+    if isDefined(g.config, "icDceCheck"):
+      stderr.writeLine "[icLower] " & extractFilename(artifact) & " " &
+        $entries.len & " routines transformed, " & $hooks.len & " hooks"
+  else:
+    # DEFAULT (Stage 0, byte-neutral): record one empty-marker per owned routine.
+    # `cg` derives the transformed body itself, so output is unchanged; this only
+    # exercises the artifact + scheduling the transform-move builds on.
+    var names: seq[string] = @[]
+    for s in moduleSymbolStubs(ast.program, FileIndex modPos):
+      if ownsRuntimeRoutine(s, modPos):
+        names.add globalName(s, g.config)
+    writeLoweredArtifact(artifact, names)
+
 proc generateCgStage(g: ModuleGraph; mainFileIdx: FileIndex) =
   ## Per-module backend codegen (`--icBackendStage:cg --icBackendModule:<suffix>`):
   ## generate C for the single module named by `icBackendModule` and write only
@@ -365,6 +578,7 @@ proc generateCgStage(g: ModuleGraph; mainFileIdx: FileIndex) =
       "per-module codegen: module not found for suffix: " & g.config.icBackendModule)
     return
 
+  applyLoweredBodies(g, modules, precompSys, target)
   generateCodeForModule(g, target)
   let bl = BModuleList(g.backend)
   # The main module also owns the whole-program method dispatchers + NimMain.
@@ -502,7 +716,10 @@ proc generateLinkStage(g: ModuleGraph; mainFileIdx: FileIndex) =
 proc generateCode*(g: ModuleGraph; mainFileIdx: FileIndex) =
   ## Main entry point for NIF-based C code generation.
   ## Traverses the module dependency graph and generates C code.
-  if g.config.icBackendStage == "cg":
+  if g.config.icBackendStage == "lower":
+    generateLowerStage(g, mainFileIdx)
+    return
+  elif g.config.icBackendStage == "cg":
     generateCgStage(g, mainFileIdx)
     return
   elif g.config.icBackendStage == "merge":
@@ -516,4 +733,4 @@ proc generateCode*(g: ModuleGraph; mainFileIdx: FileIndex) =
     return
   else:
     rawMessage(g.config, errGenerated,
-      "the per-module NIF backend requires --icBackendStage:cg|merge|emit|link")
+      "the per-module NIF backend requires --icBackendStage:lower|cg|merge|emit|link")
diff --git a/compiler/options.nim b/compiler/options.nim
index 48b5ffbc93..3d62b006b3 100644
--- a/compiler/options.nim
+++ b/compiler/options.nim
@@ -29,7 +29,7 @@ const
 
   nimEnableCovariance* = defined(nimEnableCovariance)
 
-  icFormatVersion* = "14"
+  icFormatVersion* = "16"
     ## Version of the IC cache format (the sem-NIF module layout written by
     ## ast2nif.nim plus the iface/impl/edges side files). Bump it whenever
     ## that layout changes: `commandIc` wipes a nimcache whose `ic.version`
@@ -786,6 +786,13 @@ template quitOrRaise*(conf: ConfigRef, msg = "") =
   else:
     quit(msg) # quits with QuitFailure
 
+proc icLoweredBodies*(conf: ConfigRef): bool {.inline.} =
+  ## Whether the `nim ic` backend uses the EAGER per-module `lower` stage
+  ## (transformBody serialized to `.t.nif`, cg reuses it) instead of the lazy
+  ## Stage-0 path (cg re-derives every transformed body). This is now the
+  ## DEFAULT; `-d:icNoLowerBodies` opts back into the lazy path for A/B testing.
+  not isDefined(conf, "icNoLowerBodies")
+
 proc importantComments*(conf: ConfigRef): bool {.inline.} = conf.cmd in cmdDocLike + {cmdIdeTools}
 proc usesWriteBarrier*(conf: ConfigRef): bool {.inline.} = conf.selectedGC >= gcRefc
 proc usesSso*(conf: ConfigRef): bool {.inline.} = conf.selectedStrings == stringSso