diff --git a/src/ast/compiler/macros.rs b/src/ast/compiler/macros.rs
index 97b5e02..6d15d1b 100644
--- a/src/ast/compiler/macros.rs
+++ b/src/ast/compiler/macros.rs
@@ -662,7 +662,7 @@ mod tests {
     use crate::ast::compiler::binder::{CompilerScope, LocalInfo};
     use crate::ast::compiler::Binder;
     use crate::ast::diagnostics::Diagnostics;
-    use crate::ast::nodes::{Address, GlobalIdx, VirtualId};
+    use crate::ast::nodes::{Address, GlobalIdx};
     use crate::ast::parser::Parser;
     use crate::ast::types::{NodeIdentity, Purity, SourceLocation, StaticType, Value};
 
diff --git a/src/ast/lexer.rs b/src/ast/lexer.rs
index 170a489..64b77d1 100644
--- a/src/ast/lexer.rs
+++ b/src/ast/lexer.rs
@@ -309,7 +309,7 @@ mod tests {
     #[test]
     fn test_lex_invisible_chars() {
         // U+200B Zero Width Space should be ignored inside identifiers
-        let source = "a​b";
+        let source = "a\u{200B}b";
         let mut lexer = Lexer::new(source);
         let token = lexer.next_token().unwrap();
         if let TokenKind::Identifier(id) = token.kind {
diff --git a/src/ast/mcp_server.rs b/src/ast/mcp_server.rs
index 1ec9cbc..a64613f 100644
--- a/src/ast/mcp_server.rs
+++ b/src/ast/mcp_server.rs
@@ -1,3 +1,5 @@
+use std::cell::RefCell;
+
 use rmcp::{
     ServerHandler,
     handler::server::{router::tool::ToolRouter, wrapper::Parameters},
@@ -24,6 +26,15 @@ pub struct SymbolInput {
     pub name: String,
 }
 
+thread_local! {
+    /// Persistent environment for REPL-style evaluation across multiple calls.
+    /// Lives in a thread-local because `Environment` uses `Rc<RefCell<...>>`
+    /// internally and is not `Send`/`Sync`, while `rmcp` requires the server
+    /// struct to be `Send + Sync`. Safe because we run on a single-threaded
+    /// Tokio runtime.
+    static REPL_ENV: RefCell<Option<Environment>> = const { RefCell::new(None) };
+}
+
 /// MCP server exposing the Myc compiler as a set of tools for LLMs.
 #[derive(Clone)]
 pub struct MycMcpServer {
@@ -164,6 +175,34 @@ impl MycMcpServer {
             None => format!("No documentation found for symbol '{}'.", name),
         }
     }
+
+    /// Evaluates a Myc expression in a persistent REPL session.
+    /// Bindings from previous calls are preserved.
+    #[tool(description = "Evaluate a Myc expression in a persistent REPL session. \
+        Unlike eval_myc, bindings (variables, functions, macros) from previous calls \
+        are preserved across invocations. Use repl_reset to start a fresh session. \
+        Example workflow: call repl_eval with '(def x 42)', then '(+ x 1)' returns 43.")]
+    fn repl_eval(&self, Parameters(CodeInput { code }): Parameters<CodeInput>) -> String {
+        REPL_ENV.with(|cell| {
+            let mut env_opt = cell.borrow_mut();
+            let env = env_opt.get_or_insert_with(Self::make_env);
+            match env.run_script(&code) {
+                Ok(value) => format!("{}", value),
+                Err(e) => format!("Error: {}", e),
+            }
+        })
+    }
+
+    /// Resets the REPL session, discarding all accumulated bindings.
+    #[tool(description = "Reset the persistent REPL session, discarding all variables, \
+        functions, and macros defined in previous repl_eval calls. \
+        The next repl_eval call will start with a fresh environment.")]
+    fn repl_reset(&self) -> String {
+        REPL_ENV.with(|cell| {
+            *cell.borrow_mut() = None;
+        });
+        "REPL session reset.".to_string()
+    }
 }
 
 #[tool_handler]
@@ -180,7 +219,9 @@ impl ServerHandler for MycMcpServer {
             `list_builtins` to see all available built-in functions, \
             `check_syntax` to validate code, \
             `eval_myc` to run expressions, and \
-            `dump_ast` to inspect the compiled AST.",
+            `dump_ast` to inspect the compiled AST. \
+            For interactive sessions, use `repl_eval` to evaluate expressions with \
+            persistent bindings across calls, and `repl_reset` to start fresh.",
         )
     }
 }