fix(builder): propagate compactc exit status

util-linux `script` exits 0 whatever the child did unless `-e` is
given, so every failed compile on Linux was reported as a success:
a green check, exit 0, and no artifact on disk. Surfaced when
compiling under `--feature-zkir-v3`, where zkir fails late.

The captured PTY output is also the only place the compiler's error
text lives, since `script` merges stderr into it, so the failure
path now falls back to stdout when stderr is empty.

Author: 0xisk

packages/builder/src/Compiler.ts

@@ -335,9 +335,13 @@ export class CompactCompiler {
           UIService.printOutput(summary, chalk.cyan);
         }
 
-        const cleanStderr = cleanForDisplay(execError.stderr);
-        if (cleanStderr) {
-          UIService.printOutput(cleanStderr, chalk.red);
+        // Under `script` the compiler's stderr is merged into the captured
+        // PTY output, so stdout is the only place the error text lives.
+        const details =
+          cleanForDisplay(execError.stderr) ||
+          cleanForDisplay(execError.stdout);
+        if (details) {
+          UIService.printOutput(details, chalk.red);
         }
       }
 

packages/builder/src/services/CompilerService.ts

@@ -41,7 +41,10 @@ function tokenizeFlags(flags: string): string[] {
  *
  * Handles both macOS and Linux `script` syntax:
  * - macOS: `script -q <file> <command> [args...]`
- * - Linux: `script -qc "<command> [args...]" <file>`
+ * - Linux: `script -qec "<command> [args...]" <file>`
+ *
+ * util-linux `script` exits 0 whatever the child did unless `-e` is given.
+ * BSD `script` on macOS already returns the child's status.
  *
  * Note: Circuit constraint output is only available when compiling WITHOUT
  * `--skip-zk`, as the k/rows values come from the ZK proving pass. When
@@ -61,11 +64,11 @@ async function spawnWithPty(
       // macOS: script -q <file> <command> [args...]
       scriptArgs = ['-q', tmpFile, 'compact', ...args];
     } else {
-      // Linux: script -qc "<command>" <file>
+      // Linux: script -qec "<command>" <file>
       const cmd = ['compact', ...args]
         .map((a) => (a.includes(' ') ? `'${a}'` : a))
         .join(' ');
-      scriptArgs = ['-qc', cmd, tmpFile];
+      scriptArgs = ['-qec', cmd, tmpFile];
     }
 
     const proc = spawn('script', scriptArgs, {

packages/builder/test/Compiler.test.ts

@@ -1153,6 +1153,45 @@ describe('CompactCompiler', () => {
       );
       expect(testMockExec).toHaveBeenCalledTimes(4);
     });
+
+    it('prints the captured output when the failure carries no stderr', async () => {
+      mockReaddir.mockResolvedValue([
+        {
+          name: 'MockElGamal.compact',
+          isFile: () => true,
+          isDirectory: () => false,
+        },
+      ] as any);
+      mockExistsSync.mockReturnValue(true);
+
+      const ptyError = new Error('compact exited with code 1') as Error & {
+        stdout: string;
+        stderr: string;
+      };
+      ptyError.stdout =
+        'Exception in thread "main" Unsupported test_eq: JubjubScalar == JubjubScalar\n';
+      ptyError.stderr = '';
+
+      const testMockExec = vi
+        .fn()
+        .mockResolvedValueOnce({ stdout: 'compact 0.1.0', stderr: '' })
+        .mockResolvedValueOnce({ stdout: 'compact 0.1.0', stderr: '' })
+        .mockResolvedValueOnce({ stdout: 'Compactc 0.26.0', stderr: '' })
+        .mockRejectedValueOnce(ptyError);
+
+      const logSpy = vi.spyOn(console, 'log').mockImplementation(() => {});
+      compiler = new CompactCompiler({}, testMockExec);
+
+      await expect(compiler.compile()).rejects.toThrow(CompilationError);
+
+      expect(logSpy).toHaveBeenCalledWith(
+        expect.stringContaining(
+          'Unsupported test_eq: JubjubScalar == JubjubScalar',
+        ),
+      );
+
+      logSpy.mockRestore();
+    });
   });
 
   describe('Real-world scenarios', () => {