diff --git a/nixd/include/nixd/CommandLine/Configuration.h b/nixd/include/nixd/CommandLine/Configuration.h
new file mode 100644
index 000000000..817445b44
--- /dev/null
+++ b/nixd/include/nixd/CommandLine/Configuration.h
@@ -0,0 +1,13 @@
+/// \file
+/// \brief Allow default configuration being passed via CLI
+
+#include "nixd/Controller/Configuration.h"
+
+#include <exception>
+
+namespace nixd {
+
+/// \brief Parse the CLI flag and initialize the config nixd::DefaultConfig
+nixd::Configuration parseCLIConfig();
+
+} // namespace nixd
diff --git a/nixd/include/nixd/Support/Exception.h b/nixd/include/nixd/Support/Exception.h
new file mode 100644
index 000000000..7b014f78b
--- /dev/null
+++ b/nixd/include/nixd/Support/Exception.h
@@ -0,0 +1,18 @@
+#pragma once
+
+#include <llvm/Support/Error.h>
+
+#include <exception>
+
+namespace nixd {
+
+class LLVMErrorException : public std::exception {
+  llvm::Error E;
+
+public:
+  LLVMErrorException(llvm::Error E) : E(std::move(E)) {}
+
+  llvm::Error takeError() { return std::move(E); }
+};
+
+} // namespace nixd
diff --git a/nixd/include/nixd/Support/JSON.h b/nixd/include/nixd/Support/JSON.h
new file mode 100644
index 000000000..6f3a2404b
--- /dev/null
+++ b/nixd/include/nixd/Support/JSON.h
@@ -0,0 +1,38 @@
+#include "Exception.h"
+
+#include <llvm/ADT/StringRef.h>
+#include <llvm/Support/JSON.h>
+
+#include <exception>
+
+namespace nixd {
+
+class JSONParseException : public LLVMErrorException {
+public:
+  JSONParseException(llvm::Error E) : LLVMErrorException(std::move(E)) {}
+
+  [[nodiscard]] const char *what() const noexcept override {
+    return "JSON result cannot be parsed";
+  }
+};
+
+class JSONSchemaException : public LLVMErrorException {
+public:
+  JSONSchemaException(llvm::Error E) : LLVMErrorException(std::move(E)) {}
+
+  [[nodiscard]] const char *what() const noexcept override {
+    return "JSON schema mismatch";
+  }
+};
+
+llvm::json::Value parse(llvm::StringRef JSON);
+
+template <class T> T fromJSON(const llvm::json::Value &V) {
+  llvm::json::Path::Root R;
+  T Result;
+  if (!fromJSON(V, Result, R))
+    throw JSONSchemaException(R.getError());
+  return Result;
+}
+
+} // namespace nixd
diff --git a/nixd/lib/CommandLine/Configuration.cpp b/nixd/lib/CommandLine/Configuration.cpp
new file mode 100644
index 000000000..c3f38cfa8
--- /dev/null
+++ b/nixd/lib/CommandLine/Configuration.cpp
@@ -0,0 +1,32 @@
+/// \file
+/// \brief This file implements CLI initialized configuration.
+
+#include "nixd/CommandLine/Configuration.h"
+#include "nixd/CommandLine/Options.h"
+#include "nixd/Controller/Configuration.h"
+#include "nixd/Support/JSON.h"
+
+#include "lspserver/Logger.h"
+
+#include <llvm/Support/CommandLine.h>
+#include <llvm/Support/JSON.h>
+
+#include <string>
+
+using namespace nixd;
+using namespace llvm::cl;
+
+namespace {
+
+opt<std::string> DefaultConfigJSON{"config",
+                                   desc("JSON-encoded initial configuration"),
+                                   init(""), cat(NixdCategory)};
+
+} // namespace
+
+Configuration nixd::parseCLIConfig() {
+  if (DefaultConfigJSON.empty())
+    return {};
+
+  return nixd::fromJSON<Configuration>(nixd::parse(DefaultConfigJSON));
+}
diff --git a/nixd/lib/Controller/LifeTime.cpp b/nixd/lib/Controller/LifeTime.cpp
index 5912c79e9..fb693ca1a 100644
--- a/nixd/lib/Controller/LifeTime.cpp
+++ b/nixd/lib/Controller/LifeTime.cpp
@@ -5,9 +5,11 @@
 
 #include "nixd-config.h"
 
+#include "nixd/CommandLine/Configuration.h"
 #include "nixd/CommandLine/Options.h"
 #include "nixd/Controller/Controller.h"
 #include "nixd/Eval/Launch.h"
+#include "nixd/Support/Exception.h"
 
 #include "lspserver/Protocol.h"
 
@@ -183,6 +185,13 @@ void Controller::
       evalExprWithProgress(*Client, getDefaultNixOSOptionsExpr(),
                            "nixos options");
   }
+  try {
+    Config = parseCLIConfig();
+  } catch (LLVMErrorException &Err) {
+    lspserver::elog("parse CLI config error: {0}, {1}", Err.what(),
+                    Err.takeError());
+    std::exit(-1);
+  }
   fetchConfig();
 }
 
diff --git a/nixd/lib/Support/JSON.cpp b/nixd/lib/Support/JSON.cpp
new file mode 100644
index 000000000..d92db13d6
--- /dev/null
+++ b/nixd/lib/Support/JSON.cpp
@@ -0,0 +1,10 @@
+#include "nixd/Support/JSON.h"
+
+#include <llvm/ADT/StringRef.h>
+
+llvm::json::Value nixd::parse(llvm::StringRef JSON) {
+  llvm::Expected<llvm::json::Value> E = llvm::json::parse(JSON);
+  if (!E)
+    throw JSONParseException(E.takeError());
+  return *E;
+}
diff --git a/nixd/lib/meson.build b/nixd/lib/meson.build
index 6dfc3588e..701850a24 100644
--- a/nixd/lib/meson.build
+++ b/nixd/lib/meson.build
@@ -4,6 +4,7 @@ libnixd_deps = [ nixd_lsp_server, nixf, llvm, nixt ]
 
 libnixd_lib = library(
     'nixd',
+    'CommandLine/Configuration.cpp',
     'CommandLine/Options.cpp',
     'Controller/AST.cpp',
     'Controller/CodeAction.cpp',
@@ -33,6 +34,7 @@ libnixd_lib = library(
     'Support/AutoCloseFD.cpp',
     'Support/AutoRemoveShm.cpp',
     'Support/ForkPiped.cpp',
+    'Support/JSON.cpp',
     'Support/StreamProc.cpp',
     dependencies: libnixd_deps,
     include_directories: libnixd_include,
diff --git a/nixd/tools/nixd/test/format/format.md b/nixd/tools/nixd/test/format/format.md
new file mode 100644
index 000000000..274aa83a4
--- /dev/null
+++ b/nixd/tools/nixd/test/format/format.md
@@ -0,0 +1,62 @@
+# RUN: nixd --lit-test -config='{ "formatting": { "command": ["%S/nixfmt"] } }' < %s | FileCheck %s
+
+<-- initialize(0)
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":0,
+   "method":"initialize",
+   "params":{
+      "processId":123,
+      "rootPath":"",
+      "capabilities":{
+      },
+      "trace":"off"
+   }
+}
+```
+
+```json
+{
+   "jsonrpc":"2.0",
+   "method":"textDocument/didOpen",
+   "params":{
+      "textDocument":{
+         "uri":"file:///format.nix",
+         "languageId":"nix",
+         "version":1,
+         "text":"{ stdenv,\npkgs}: \n let x=1; in { y = x; }"
+      }
+   }
+}
+```
+
+<-- textDocument/formatting
+
+```json
+{
+    "jsonrpc": "2.0",
+    "id": 2,
+    "method": "textDocument/formatting",
+    "params": {
+        "textDocument": {
+            "uri": "file:///format.nix"
+        },
+        "options": {
+            "tabSize": 2,
+            "insertSpaces": true,
+            "trimTrailingWhitespace": true,
+            "insertFinalNewline": true
+        }
+    }
+}
+```
+
+```
+CHECK: "newText": "Hello\n",
+```
+
+```json
+{"jsonrpc":"2.0","method":"exit"}
+```
diff --git a/nixd/tools/nixd/test/format/nixfmt b/nixd/tools/nixd/test/format/nixfmt
new file mode 100755
index 000000000..e5d398670
--- /dev/null
+++ b/nixd/tools/nixd/test/format/nixfmt
@@ -0,0 +1,2 @@
+#!/bin/sh
+echo "Hello"