diff --git a/nixd/include/nixd/AST/ParseAST.h b/nixd/include/nixd/AST/ParseAST.h
index 2590cb60d..6bf9d1be3 100644
--- a/nixd/include/nixd/AST/ParseAST.h
+++ b/nixd/include/nixd/AST/ParseAST.h
@@ -212,14 +212,6 @@ class ParseAST {
 
   [[nodiscard]] Links documentLink(const std::string &File) const;
 
-  // Completion
-
-  [[nodiscard]] lspserver::CompletionItem
-  toCompletionItem(const nix::Symbol &V) const;
-
-  [[nodiscard]] std::vector<lspserver::CompletionItem>
-  completion(const lspserver::Position &Pos) const;
-
   [[nodiscard]] LocationContext getContext(lspserver::Position Pos) const;
 };
 } // namespace nixd
diff --git a/nixd/include/nixd/Sema/CompletionBuilder.h b/nixd/include/nixd/Sema/CompletionBuilder.h
new file mode 100644
index 000000000..55ec1c28a
--- /dev/null
+++ b/nixd/include/nixd/Sema/CompletionBuilder.h
@@ -0,0 +1,49 @@
+#pragma once
+
+#include "nixd/AST/EvalAST.h"
+#include "nixd/AST/ParseAST.h"
+
+#include "lspserver/Protocol.h"
+
+#include <vector>
+
+namespace nixd {
+
+using CompletionResult = lspserver::CompletionList;
+
+class CompletionBuilder {
+  CompletionResult Result;
+  size_t Limit = 1000;
+
+public:
+  /// { a = 1; b = 2; }.|
+  ///                   ^ "a", "b"
+  void addAttrFields(const EvalAST &AST, const lspserver::Position &Pos,
+                     nix::EvalState &State);
+
+  /// Symbols from let-bindings, rec-attrs.
+  void addSymbols(const ParseAST &AST, const nix::Expr *Node);
+
+  ///
+  /// (
+  /// { foo ? 1 # <-- Lambda formal
+  /// , bar ? 2
+  /// }: foo + bar) { | }
+  ///                 ^ "foo", "bar"
+  void addLambdaFormals(const EvalAST &AST, nix::EvalState &State,
+                        const nix::Expr *Node);
+
+  /// with pkgs; [ ]
+  void addEnv(nix::EvalState &State, nix::Env &NixEnv);
+
+  void addEnv(const EvalAST &AST, nix::EvalState &State, const nix::Expr *Node);
+
+  /// builtins.*
+  void addStaticEnv(const nix::SymbolTable &STable, const nix::StaticEnv &SEnv);
+
+  CompletionResult &getResult() { return Result; }
+
+  void setLimit(size_t Limit) { this->Limit = Limit; }
+};
+
+} // namespace nixd
diff --git a/nixd/include/nixd/Support/CompletionHelper.h b/nixd/include/nixd/Support/CompletionHelper.h
deleted file mode 100644
index 848f63c24..000000000
--- a/nixd/include/nixd/Support/CompletionHelper.h
+++ /dev/null
@@ -1,18 +0,0 @@
-#pragma once
-
-#include "lspserver/Protocol.h"
-
-#include <nix/eval.hh>
-
-#include <vector>
-
-namespace nixd {
-
-struct CompletionHelper {
-  using Items = std::vector<lspserver::CompletionItem>;
-  static void fromEnv(nix::EvalState &State, nix::Env &NixEnv, Items &Items);
-  static void fromStaticEnv(const nix::SymbolTable &STable,
-                            const nix::StaticEnv &SEnv, Items &Items);
-};
-
-} // namespace nixd
diff --git a/nixd/lib/AST/ParseAST.cpp b/nixd/lib/AST/ParseAST.cpp
index 616edcc74..548dbc2e5 100644
--- a/nixd/lib/AST/ParseAST.cpp
+++ b/nixd/lib/AST/ParseAST.cpp
@@ -279,29 +279,6 @@ ParseAST::Links ParseAST::documentLink(const std::string &File) const {
   return V.Result;
 }
 
-lspserver::CompletionItem
-ParseAST::toCompletionItem(const nix::Symbol &V) const {
-  lspserver::CompletionItem R;
-  R.kind = lspserver::CompletionItemKind::Interface;
-  R.label = symbols()[V];
-  return R;
-}
-
-std::vector<lspserver::CompletionItem>
-ParseAST::completion(const lspserver::Position &Pos) const {
-  std::vector<lspserver::CompletionItem> Items;
-  if (const auto *Node = lookupContainMin(Pos)) {
-    std::vector<nix::Symbol> Symbols;
-    collectSymbols(Node, Symbols);
-
-    // Insert symbols to our completion list.
-    std::transform(
-        Symbols.begin(), Symbols.end(), std::back_inserter(Items),
-        [this](const nix::Symbol &V) { return toCompletionItem(V); });
-  };
-  return Items;
-}
-
 namespace {
 struct AttrDefVisitor : RecursiveASTVisitor<AttrDefVisitor> {
   const ParseAST &AST;
diff --git a/nixd/lib/Sema/CompletionBuilder.cpp b/nixd/lib/Sema/CompletionBuilder.cpp
new file mode 100644
index 000000000..67da84115
--- /dev/null
+++ b/nixd/lib/Sema/CompletionBuilder.cpp
@@ -0,0 +1,134 @@
+#include "nixd/Sema/CompletionBuilder.h"
+#include "nixd/AST/ParseAST.h"
+#include "nixd/Nix/Eval.h"
+
+#include "lspserver/Protocol.h"
+
+#include <stdexcept>
+
+namespace nixd {
+
+using lspserver::CompletionItem;
+using lspserver::CompletionItemKind;
+
+void CompletionBuilder::addAttrFields(const EvalAST &AST,
+                                      const lspserver::Position &Pos,
+                                      nix::EvalState &State) {
+  try {
+    if (const auto *Node = AST.lookupEnd(Pos)) {
+      auto Value = AST.getValueEval(Node, State);
+      if (Value.type() == nix::ValueType::nAttrs) {
+        // Traverse attribute bindings
+        for (auto Binding : *Value.attrs) {
+          CompletionItem R;
+          R.label = State.symbols[Binding.name];
+          R.kind = CompletionItemKind::Field;
+          Result.items.emplace_back(std::move(R));
+          if (Result.items.size() > Limit) {
+            Result.isIncomplete = true;
+            break;
+          }
+        }
+      }
+    }
+  } catch (std::out_of_range &) {
+  }
+}
+
+void CompletionBuilder::addSymbols(const ParseAST &AST, const nix::Expr *Node) {
+  std::vector<nix::Symbol> Symbols;
+  AST.collectSymbols(Node, Symbols);
+  // Insert symbols to our completion list.
+  std::transform(Symbols.begin(), Symbols.end(),
+                 std::back_inserter(Result.items), [&](const nix::Symbol &V) {
+                   lspserver::CompletionItem R;
+                   R.kind = CompletionItemKind::Interface;
+                   R.label = AST.symbols()[V];
+                   return R;
+                 });
+}
+
+void CompletionBuilder::addLambdaFormals(const EvalAST &AST,
+                                         nix::EvalState &State,
+                                         const nix::Expr *Node) {
+
+  // Nix only accepts attr set formals
+  if (!dynamic_cast<const nix::ExprAttrs *>(Node))
+    return;
+
+  try {
+    const auto *Parent = AST.parent(Node);
+    if (const auto *SomeExprCall =
+            dynamic_cast<const nix::ExprCall *>(Parent)) {
+      auto Value = AST.getValueEval(SomeExprCall->fun, State);
+      if (!Value.isLambda())
+        return;
+      auto *Fun = Value.lambda.fun;
+      if (!Fun->hasFormals())
+        return;
+      for (auto Formal : Fun->formals->formals) {
+        CompletionItem R;
+        R.label = State.symbols[Formal.name];
+        R.kind = CompletionItemKind::Constructor;
+        Result.items.emplace_back(std::move(R));
+      }
+    }
+  } catch (std::out_of_range &) {
+  }
+}
+
+void CompletionBuilder::addEnv(nix::EvalState &State, nix::Env &NixEnv) {
+  if (NixEnv.type == nix::Env::HasWithExpr) {
+    nix::Value *V = State.allocValue();
+    State.evalAttrs(*NixEnv.up, (nix::Expr *)NixEnv.values[0], *V, nix::noPos,
+                    "<borked>");
+    NixEnv.values[0] = V;
+    NixEnv.type = nix::Env::HasWithAttrs;
+  }
+  if (NixEnv.type == nix::Env::HasWithAttrs) {
+    for (const auto &SomeAttr : *NixEnv.values[0]->attrs) {
+      std::string Name = State.symbols[SomeAttr.name];
+      lspserver::CompletionItem R;
+      R.label = Name;
+      R.kind = lspserver::CompletionItemKind::Variable;
+      Result.items.emplace_back(std::move(R));
+
+      if (Result.items.size() > Limit) {
+        Result.isIncomplete = true;
+        break;
+      }
+    }
+  }
+  if (NixEnv.up)
+    addEnv(State, *NixEnv.up);
+}
+
+void CompletionBuilder::addEnv(const EvalAST &AST, nix::EvalState &State,
+                               const nix::Expr *Node) {
+  try {
+    // Eval this node, for reaching deeper envs (e.g. with expressions).
+    AST.getValueEval(Node, State);
+    if (auto *ExprEnv = AST.searchUpEnv(Node))
+      addEnv(State, *ExprEnv);
+  } catch (std::out_of_range &) {
+  }
+}
+
+void CompletionBuilder::addStaticEnv(const nix::SymbolTable &STable,
+                                     const nix::StaticEnv &SEnv) {
+  for (auto [Symbol, Displ] : SEnv.vars) {
+    std::string Name = STable[Symbol];
+    if (Name.starts_with("__"))
+      continue;
+
+    CompletionItem R;
+    R.label = Name;
+    R.kind = CompletionItemKind::Constant;
+    Result.items.emplace_back(std::move(R));
+  }
+
+  if (SEnv.up)
+    addStaticEnv(STable, SEnv);
+}
+
+} // namespace nixd
diff --git a/nixd/lib/Sema/meson.build b/nixd/lib/Sema/meson.build
new file mode 100644
index 000000000..2313d3d56
--- /dev/null
+++ b/nixd/lib/Sema/meson.build
@@ -0,0 +1,17 @@
+libnixdSemaDeps = [ nixd_lsp_server
+                  , nix_all
+                  , nixdAST
+                  , llvm
+                  ]
+
+libnixdSema = library('nixdSema'
+, 'CompletionBuilder.cpp'
+, include_directories: nixd_inc
+, dependencies: libnixdSemaDeps
+, install: true
+)
+
+nixdSema = declare_dependency( include_directories: nixd_inc
+                             , link_with: libnixdSema
+                             , dependencies: libnixdSemaDeps
+                             )
diff --git a/nixd/lib/Server/EvalWorker.cpp b/nixd/lib/Server/EvalWorker.cpp
index 9b829d4aa..9a72a9fdf 100644
--- a/nixd/lib/Server/EvalWorker.cpp
+++ b/nixd/lib/Server/EvalWorker.cpp
@@ -1,9 +1,9 @@
 #include "nixd/Server/EvalWorker.h"
 #include "nixd/Nix/Init.h"
 #include "nixd/Nix/Value.h"
+#include "nixd/Sema/CompletionBuilder.h"
 #include "nixd/Server/EvalDraftStore.h"
 #include "nixd/Server/IPC.h"
-#include "nixd/Support/CompletionHelper.h"
 #include "nixd/Support/String.h"
 
 #include "lspserver/LSPServer.h"
@@ -163,93 +163,22 @@ void EvalWorker::onCompletion(const lspserver::CompletionParams &Params,
   auto Action = [Params, this](const nix::ref<EvalAST> &AST,
                                ReplyRAII<CompletionList> &&RR) {
     auto State = IER->Session->getState();
+    CompletionBuilder Builder;
     // Lookup an AST node, and get it's 'Env' after evaluation
-    CompletionHelper::Items Items;
     lspserver::log("current trigger character is {0}",
                    Params.context.triggerCharacter);
 
     if (Params.context.triggerCharacter == ".") {
-      const auto *Node = AST->lookupEnd(Params.position);
-      if (!Node)
-        return;
-      try {
-        auto Value = AST->getValueEval(Node, *IER->Session->getState());
-        if (Value.type() == nix::ValueType::nAttrs) {
-          // Traverse attribute bindings
-          for (auto Binding : *Value.attrs) {
-            Items.emplace_back(
-                CompletionItem{.label = State->symbols[Binding.name],
-                               .kind = CompletionItemKind::Field});
-            if (Items.size() > 5000)
-              break;
-          }
-        }
-      } catch (std::out_of_range &) {
-        RR.Response = error("no associated value on requested attrset.");
-        return;
-      }
+      Builder.addAttrFields(*AST, Params.position, *State);
     } else {
-
       const auto *Node = AST->lookupContainMin(Params.position);
-
-      // TODO: share the same code with static worker
-      std::vector<nix::Symbol> Symbols;
-      AST->collectSymbols(Node, Symbols);
-      // Insert symbols to our completion list.
-      std::transform(Symbols.begin(), Symbols.end(), std::back_inserter(Items),
-                     [&](const nix::Symbol &V) -> decltype(Items)::value_type {
-                       decltype(Items)::value_type R;
-                       R.kind = CompletionItemKind::Interface;
-                       R.label = State->symbols[V];
-                       return R;
-                     });
-
-      auto FromLambdaFormals = [&]() {
-        // Firstly, we check that if we are in "ExprAttrs", consider this
-        // is a
-        // special case for completion lambda formals
-        if (!dynamic_cast<const nix::ExprAttrs *>(Node))
-          return;
-        // Then, check that if the parent of evaluates to a "<LAMBDA>"
-        try {
-          const auto *Parent = AST->parent(Node);
-          if (const auto *SomeExprCall =
-                  dynamic_cast<const nix::ExprCall *>(Parent)) {
-            auto Value = AST->getValue(SomeExprCall->fun);
-            if (!Value.isLambda())
-              return;
-            // Then, filling the completion list using that lambda
-            // formals.
-            auto *Fun = Value.lambda.fun;
-            if (!Fun->hasFormals())
-              return;
-            for (auto Formal : Fun->formals->formals) {
-              CompletionItem Item;
-              Item.label = State->symbols[Formal.name];
-              Item.kind = CompletionItemKind::Constructor;
-              Items.emplace_back(std::move(Item));
-            }
-          }
-
-        } catch (...) {
-        }
-      };
-      FromLambdaFormals();
-      try {
-        if (!Node)
-          return;
-        auto *ExprEnv = AST->searchUpEnv(Node);
-        CompletionHelper::fromEnv(*State, *ExprEnv, Items);
-      } catch (std::out_of_range &) {
-      }
-      CompletionHelper::fromStaticEnv(State->symbols, *State->staticBaseEnv,
-                                      Items);
+      Builder.addSymbols(*AST, Node);
+      Builder.addLambdaFormals(*AST, *State, Node);
+      Builder.addEnv(*AST, *State, Node);
+      Builder.addStaticEnv(State->symbols, *State->staticBaseEnv);
     }
-    // Make the response.
-    CompletionList List;
-    List.isIncomplete = false;
-    List.items = Items;
-    RR.Response = List;
+    if (!Builder.getResult().items.empty())
+      RR.Response = Builder.getResult();
   };
 
   withAST<CompletionList>(Params.textDocument.uri.file().str(),
diff --git a/nixd/lib/Server/meson.build b/nixd/lib/Server/meson.build
index 930e57205..fff8133ea 100644
--- a/nixd/lib/Server/meson.build
+++ b/nixd/lib/Server/meson.build
@@ -1,5 +1,6 @@
 libnixdServerDeps = [ nixd_lsp_server
                     , nix_all
+                    , nixdSema
                     , nixdAST
                     , nixdNix
                     , nixdSupport
diff --git a/nixd/lib/Support/CompletionHelper.cpp b/nixd/lib/Support/CompletionHelper.cpp
deleted file mode 100644
index 42925c9d7..000000000
--- a/nixd/lib/Support/CompletionHelper.cpp
+++ /dev/null
@@ -1,43 +0,0 @@
-#include "nixd/Support/CompletionHelper.h"
-#include "nixd/Nix/Eval.h"
-
-namespace nixd {
-
-void CompletionHelper::fromStaticEnv(const nix::SymbolTable &STable,
-                                     const nix::StaticEnv &SEnv, Items &Items) {
-  for (auto [Symbol, Displ] : SEnv.vars) {
-    std::string Name = STable[Symbol];
-    if (Name.starts_with("__"))
-      continue;
-
-    // Variables in static envs, let's mark it as "Constant".
-    Items.emplace_back(lspserver::CompletionItem{
-        .label = Name, .kind = lspserver::CompletionItemKind::Constant});
-  }
-  if (SEnv.up)
-    fromStaticEnv(STable, SEnv, Items);
-}
-
-void CompletionHelper::fromEnv(nix::EvalState &State, nix::Env &NixEnv,
-                               Items &Items) {
-  if (NixEnv.type == nix::Env::HasWithExpr) {
-    nix::Value *V = State.allocValue();
-    State.evalAttrs(*NixEnv.up, (nix::Expr *)NixEnv.values[0], *V, nix::noPos,
-                    "<borked>");
-    NixEnv.values[0] = V;
-    NixEnv.type = nix::Env::HasWithAttrs;
-  }
-  if (NixEnv.type == nix::Env::HasWithAttrs) {
-    for (const auto &SomeAttr : *NixEnv.values[0]->attrs) {
-      std::string Name = State.symbols[SomeAttr.name];
-      if (Items.size() > 5000)
-        break;
-      Items.emplace_back(lspserver::CompletionItem{
-          .label = Name, .kind = lspserver::CompletionItemKind::Variable});
-    }
-  }
-  if (NixEnv.up)
-    fromEnv(State, *NixEnv.up, Items);
-}
-
-} // namespace nixd
diff --git a/nixd/lib/Support/meson.build b/nixd/lib/Support/meson.build
index 4021dc90d..485d196e3 100644
--- a/nixd/lib/Support/meson.build
+++ b/nixd/lib/Support/meson.build
@@ -5,7 +5,6 @@ libnixdSupportDeps = [ nix_all
                      ]
 
 libnixdSupport = library('nixdSupport'
-, 'CompletionHelper.cpp'
 , 'Diagnostic.cpp'
 , 'JSONSerialization.cpp'
 , include_directories: nixd_inc
diff --git a/nixd/lib/meson.build b/nixd/lib/meson.build
index a3a8cc7b6..39df63d31 100644
--- a/nixd/lib/meson.build
+++ b/nixd/lib/meson.build
@@ -3,4 +3,5 @@ subdir('Nix')
 subdir('Parser')
 subdir('AST')
 subdir('Support')
+subdir('Sema')
 subdir('Server')