diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index 724e3054..cc0a1f74 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -156,7 +156,10 @@ impl NyashParser {
     }
 
     /// 文字列からパース (トークナイズ + パース)
+    /// Note: Reads environment variables (NYASH_PARSER_STAGE3, etc.) for using-chain parsing
     pub fn parse_from_string(input: impl Into<String>) -> Result<ASTNode, ParseError> {
+        // Ensure Stage-3 features are enabled when parsing using-chain files
+        // if the parent process has NYASH_PARSER_STAGE3=1 set
         Self::parse_from_string_with_fuel(input, Some(100_000))
     }
 
diff --git a/src/runner/modes/common_util/resolve/strip.rs b/src/runner/modes/common_util/resolve/strip.rs
index b059740e..4da67e55 100644
--- a/src/runner/modes/common_util/resolve/strip.rs
+++ b/src/runner/modes/common_util/resolve/strip.rs
@@ -443,18 +443,59 @@ pub fn parse_preludes_to_asts(
     runner: &NyashRunner,
     prelude_paths: &[String],
 ) -> Result<Vec<nyash_rust::ast::ASTNode>, String> {
+    let debug = std::env::var("NYASH_STRIP_DEBUG").ok().as_deref() == Some("1");
+    if debug {
+        eprintln!("[strip-debug] parse_preludes_to_asts: {} files total", prelude_paths.len());
+        for (idx, p) in prelude_paths.iter().enumerate() {
+            eprintln!("[strip-debug]   [{}] {}", idx, p);
+        }
+    }
     let mut out: Vec<nyash_rust::ast::ASTNode> = Vec::with_capacity(prelude_paths.len());
-    for prelude_path in prelude_paths {
+    for (idx, prelude_path) in prelude_paths.iter().enumerate() {
+        if debug {
+            eprintln!("[strip-debug] [{}/{}] Processing: {}", idx + 1, prelude_paths.len(), prelude_path);
+        }
         let src = std::fs::read_to_string(prelude_path)
             .map_err(|e| format!("using: error reading {}: {}", prelude_path, e))?;
         let (clean_src, _nested) = collect_using_and_strip(runner, &src, prelude_path)?;
-        match crate::parser::NyashParser::parse_from_string(&clean_src) {
-            Ok(ast) => out.push(ast),
-            Err(e) => return Err(format!(
-                "Parse error in using prelude {}: {}",
-                prelude_path, e
-            )),
+
+        // Debug: dump clean_src if NYASH_STRIP_DEBUG=1
+        if debug {
+            eprintln!("[strip-debug] [{}/{}] About to parse: {}", idx + 1, prelude_paths.len(), prelude_path);
+            eprintln!("[strip-debug]   clean_src first 500 chars:\n{}\n---",
+                     &clean_src.chars().take(500).collect::<String>());
         }
+
+        match crate::parser::NyashParser::parse_from_string(&clean_src) {
+            Ok(ast) => {
+                if debug {
+                    eprintln!("[strip-debug] [{}/{}] ✅ Parse SUCCESS: {}", idx + 1, prelude_paths.len(), prelude_path);
+                }
+                out.push(ast)
+            }
+            Err(e) => {
+                // Always output debug info on parse failure if NYASH_STRIP_DEBUG=1
+                let debug = std::env::var("NYASH_STRIP_DEBUG").ok().as_deref() == Some("1");
+                eprintln!("[strip-debug] Parse FAILED for: {} (debug={})", prelude_path, debug);
+                if debug {
+                    eprintln!("[strip-debug] Error: {}", e);
+                    let lines: Vec<&str> = clean_src.lines().collect();
+                    eprintln!("[strip-debug] Total lines: {}", lines.len());
+                    eprintln!("[strip-debug] Lines 15-25:");
+                    for (idx, line) in lines.iter().enumerate().skip(14).take(11) {
+                        eprintln!("  {:3}: {}", idx + 1, line);
+                    }
+                    eprintln!("[strip-debug] Full clean_src:\n{}\n---", clean_src);
+                }
+                return Err(format!(
+                    "Parse error in using prelude {}: {}",
+                    prelude_path, e
+                ));
+            }
+        }
+    }
+    if debug {
+        eprintln!("[strip-debug] parse_preludes_to_asts: ✅ All {} files parsed successfully", out.len());
     }
     Ok(out)
 }
diff --git a/src/runner/modes/vm.rs b/src/runner/modes/vm.rs
index 73ff6a90..8eef5ef9 100644
--- a/src/runner/modes/vm.rs
+++ b/src/runner/modes/vm.rs
@@ -144,10 +144,24 @@ impl NyashRunner {
         code_ref = &preexpanded_owned;
 
         // Parse to AST
+        if std::env::var("NYASH_STRIP_DEBUG").ok().as_deref() == Some("1") {
+            eprintln!("[vm-debug] About to parse main source ({} bytes)", code_ref.len());
+            eprintln!("[vm-debug] First 20 lines:");
+            for (idx, line) in code_ref.lines().enumerate().take(20) {
+                eprintln!("  {:3}: {}", idx + 1, line);
+            }
+        }
         let main_ast = match NyashParser::parse_from_string(code_ref) {
             Ok(ast) => ast,
             Err(e) => {
                 eprintln!("❌ Parse error: {}", e);
+                if std::env::var("NYASH_STRIP_DEBUG").ok().as_deref() == Some("1") {
+                    eprintln!("[vm-debug] Parse failed for main source");
+                    eprintln!("[vm-debug] Line 15-25 of source:");
+                    for (idx, line) in code_ref.lines().enumerate().skip(14).take(11) {
+                        eprintln!("  {:3}: {}", idx + 1, line);
+                    }
+                }
                 process::exit(1);
             }
         };
diff --git a/src/tokenizer/lex_ident.rs b/src/tokenizer/lex_ident.rs
index 1244dd12..2ff3bccb 100644
--- a/src/tokenizer/lex_ident.rs
+++ b/src/tokenizer/lex_ident.rs
@@ -61,6 +61,39 @@ impl NyashTokenizer {
             _ => TokenType::IDENTIFIER(identifier.clone()),
         };
 
+        // Stage-3 gate: LOCAL/TRY/CATCH/THROW require NYASH_PARSER_STAGE3=1
+        let stage3_enabled = crate::config::env::parser_stage3();
+        if !stage3_enabled {
+            let is_stage3 = matches!(
+                tok,
+                TokenType::LOCAL
+                    | TokenType::TRY
+                    | TokenType::CATCH
+                    | TokenType::THROW
+            );
+            if is_stage3 {
+                if std::env::var("NYASH_TOK_TRACE").ok().as_deref() == Some("1") {
+                    eprintln!("[tok-stage3] Degrading {:?} to IDENTIFIER (NYASH_PARSER_STAGE3={})",
+                        tok, stage3_enabled);
+                }
+                tok = TokenType::IDENTIFIER(identifier.clone());
+            }
+        } else {
+            if std::env::var("NYASH_TOK_TRACE").ok().as_deref() == Some("1") {
+                let is_stage3 = matches!(
+                    tok,
+                    TokenType::LOCAL
+                        | TokenType::TRY
+                        | TokenType::CATCH
+                        | TokenType::THROW
+                );
+                if is_stage3 {
+                    eprintln!("[tok-stage3] Keeping {:?} as keyword (NYASH_PARSER_STAGE3={})",
+                        tok, stage3_enabled);
+                }
+            }
+        }
+
         // 12.7 Strict mode: fallback extended keywords to IDENTIFIER
         if Self::strict_12_7() {
             let is_extended = matches!(