diff --git a/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/compiler/parser/soft/IdentifierSpec.scala b/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/compiler/parser/soft/IdentifierSpec.scala
new file mode 100644
index 000000000..3eebf0fa3
--- /dev/null
+++ b/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/compiler/parser/soft/IdentifierSpec.scala
@@ -0,0 +1,34 @@
+package org.alephium.ralph.lsp.access.compiler.parser.soft
+
+import org.alephium.ralph.lsp.access.compiler.parser.soft.ast.Token
+import org.alephium.ralph.lsp.access.compiler.parser.soft.TestParser._
+import org.alephium.ralph.lsp.access.util.TestFastParse._
+import org.scalatest.matchers.should.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+
+class IdentifierSpec extends AnyWordSpec with Matchers {
+
+  "disallow reserved tokens to be used as identifier" when {
+    val reserved =
+      Token.reserved.diff(Seq(Token.Hash)) // Remove hash because `let hash = #` is valid
+
+    "tail has space" in {
+      reserved foreach {
+        reserved =>
+          assertIsFastParseError {
+            parseIdentifier(s"${reserved.lexeme} ")
+          }
+      }
+    }
+
+    "tail is end-of-file" in {
+      reserved foreach {
+        reserved =>
+          assertIsFastParseError {
+            parseIdentifier(reserved.lexeme)
+          }
+      }
+    }
+  }
+
+}
diff --git a/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/compiler/parser/soft/TestParser.scala b/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/compiler/parser/soft/TestParser.scala
index 495d1ada6..dd00a30fa 100644
--- a/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/compiler/parser/soft/TestParser.scala
+++ b/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/compiler/parser/soft/TestParser.scala
@@ -60,6 +60,9 @@ object TestParser {
   def parseReservedTokenOrError(remove: Token.Reserved*)(code: String): Either[Parsed.Failure, Token.Reserved] =
     runAnyParserOrError(TokenParser.Reserved(remove: _*)(_))(code)
 
+  def parseIdentifier(code: String): SoftAST.IdentifierAST =
+    runSoftParser(IdentifierParser.parseOrFail(_))(code)
+
   def findAnnotation(identifier: String)(code: String): Option[SoftAST.Annotation] =
     findAnnotation(
       identifier = identifier,
@@ -118,7 +121,8 @@ object TestParser {
     result match {
       case Left(error) =>
         // Print a formatted error so it's easier to debug.
-        fail(CompilerError.FastParseError(error).toFormatter().format(Some(Console.RED)))
+        val throwable = CompilerError.FastParseError(error)
+        fail(throwable.toFormatter().format(Some(Console.RED)), throwable)
 
       case Right(ast) =>
         ast
diff --git a/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/util/TestFastParse.scala b/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/util/TestFastParse.scala
new file mode 100644
index 000000000..36873e62f
--- /dev/null
+++ b/compiler-access/src/test/scala/org/alephium/ralph/lsp/access/util/TestFastParse.scala
@@ -0,0 +1,18 @@
+package org.alephium.ralph.lsp.access.util
+
+import org.alephium.ralph.error.CompilerError
+import org.scalatest.TryValues._
+import org.scalatest.matchers.should.Matchers._
+import org.scalatest.Assertion
+
+import scala.util.Try
+
+object TestFastParse {
+
+  def assertIsFastParseError[A](f: => A): Assertion =
+    Try(f)
+      .failure
+      .exception
+      .getCause shouldBe a[CompilerError.FastParseError]
+
+}