diff --git a/changelog.md b/changelog.md
index ff16a22..03ffb3a 100644
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,13 @@
 # Changelog
 
+## 2026-03-05 - 4.2.0 - feat(mod_compiler)
+add diagnostic interception of fs operations to detect and report unexpected file system changes in previously compiled output directories during compilation
+
+- Wraps fs.unlinkSync, fs.rmSync, fs.rmdirSync, fs.renameSync and fs.writeFileSync to record operations targeting other successful output directories during a compile.
+- Enabled only when there are previously compiled output dirs and when not running in quiet or JSON mode; original fs methods are restored after compilation.
+- Logs up to 30 intercepted operations and prints a summary count if any ops were observed; intercepted calls still perform the original fs action (diagnostic-only).
+- No functional change to compilation behavior beyond additional diagnostic reporting.
+
 ## 2026-03-05 - 4.1.26 - fix(compiler)
 fsync output directories after unpack to avoid XFS delayed logging causing corrupt or invisible directory entries during subsequent TypeScript emits
 
diff --git a/ts/00_commitinfo_data.ts b/ts/00_commitinfo_data.ts
index 25fae25..621ee5f 100644
--- a/ts/00_commitinfo_data.ts
+++ b/ts/00_commitinfo_data.ts
@@ -3,6 +3,6 @@
  */
 export const commitinfo = {
   name: '@git.zone/tsbuild',
-  version: '4.1.26',
+  version: '4.2.0',
   description: 'A tool for compiling TypeScript files using the latest nightly features, offering flexible APIs and a CLI for streamlined development.'
 }
diff --git a/ts/mod_compiler/classes.tscompiler.ts b/ts/mod_compiler/classes.tscompiler.ts
index dafb279..ff35196 100644
--- a/ts/mod_compiler/classes.tscompiler.ts
+++ b/ts/mod_compiler/classes.tscompiler.ts
@@ -380,9 +380,69 @@ export class TsCompiler {
         fileCount: absoluteFiles.length,
       };
 
+      // Diagnostic: intercept fs operations during compilation to detect
+      // any unexpected deletions in previously compiled output directories
+      const watchedDirs = successfulOutputDirs.filter(d => d !== destDir);
+      const origUnlink = fs.unlinkSync;
+      const origRm = fs.rmSync;
+      const origRmdir = fs.rmdirSync;
+      const origRename = fs.renameSync;
+      const origWriteFile = fs.writeFileSync;
+      let interceptedOps: string[] = [];
+      if (watchedDirs.length > 0 && !isQuiet && !isJson) {
+        (fs as any).unlinkSync = (p: string, ...args: any[]) => {
+          if (watchedDirs.some(d => String(p).startsWith(d + '/'))) {
+            interceptedOps.push(`unlink: ${p}`);
+          }
+          return origUnlink.call(fs, p, ...args);
+        };
+        (fs as any).rmSync = (p: string, ...args: any[]) => {
+          if (watchedDirs.some(d => String(p).startsWith(d + '/'))) {
+            interceptedOps.push(`rm: ${p}`);
+          }
+          return origRm.call(fs, p, ...args);
+        };
+        (fs as any).rmdirSync = (p: string, ...args: any[]) => {
+          if (watchedDirs.some(d => String(p).startsWith(d + '/'))) {
+            interceptedOps.push(`rmdir: ${p}`);
+          }
+          return origRmdir.call(fs, p, ...args);
+        };
+        (fs as any).renameSync = (src: string, dest: string, ...args: any[]) => {
+          if (watchedDirs.some(d => String(src).startsWith(d + '/') || String(dest).startsWith(d + '/'))) {
+            interceptedOps.push(`rename: ${src} → ${dest}`);
+          }
+          return origRename.call(fs, src, dest, ...args);
+        };
+        (fs as any).writeFileSync = (p: string, ...args: any[]) => {
+          if (watchedDirs.some(d => String(p).startsWith(d + '/'))) {
+            interceptedOps.push(`write: ${p}`);
+          }
+          return origWriteFile.call(fs, p, ...args);
+        };
+      }
+
       const result = await this.compileFiles(absoluteFiles, options, taskInfo);
       emittedFiles.push(...result.emittedFiles);
       errorSummaries.push(result.errorSummary);
+
+      // Restore original fs methods and report any intercepted operations
+      if (watchedDirs.length > 0 && !isQuiet && !isJson) {
+        (fs as any).unlinkSync = origUnlink;
+        (fs as any).rmSync = origRm;
+        (fs as any).rmdirSync = origRmdir;
+        (fs as any).renameSync = origRename;
+        (fs as any).writeFileSync = origWriteFile;
+        if (interceptedOps.length > 0) {
+          console.log(`   ⚠️ [diag] ${interceptedOps.length} ops on previous output dirs during compilation:`);
+          for (const op of interceptedOps.slice(0, 30)) {
+            console.log(`      ${op.replace(this.cwd + '/', '')}`);
+          }
+          if (interceptedOps.length > 30) {
+            console.log(`      ... and ${interceptedOps.length - 30} more`);
+          }
+        }
+      }
       diagSnap('post-compile');
 
       // Diagnostic: log emitted files that went to unexpected directories