diff --git a/langfun/core/concurrent.py b/langfun/core/concurrent.py
index 6a96fa4..56e10c1 100644
--- a/langfun/core/concurrent.py
+++ b/langfun/core/concurrent.py
@@ -921,5 +921,11 @@ def _progress_control(
     raise ValueError(f'Unsupported progress bar type: {progress_bar}')
 
 
+def get_executor(
+    resource_id: str,
+    max_workers: int | None = None) -> concurrent.futures.ThreadPoolExecutor:
+  """Gets a thread pool executor associated with a resource id."""
+  return _executor_pool.get(resource_id, max_workers)
+
 # The global executor pool based on resource IDs.
 _executor_pool = ExecutorPool()
diff --git a/langfun/core/eval/base.py b/langfun/core/eval/base.py
index ac54db6..c159098 100644
--- a/langfun/core/eval/base.py
+++ b/langfun/core/eval/base.py
@@ -1087,7 +1087,7 @@ def _dryrun(
       )
       error = e
 
-    copy.audit(example, output_message, error, dryrun=True)
+    copy.audit(1, example, output_message, error, dryrun=True)
     result = copy.finalize()
 
     if verbose:
@@ -1124,19 +1124,20 @@ def _run(
     with lf.use_settings(debug=debug, cache=self.cache):
       self._reset()
 
-      def _process(example: Any):
+      def _process(idx_and_example: Any):
         # NOTE(daiyip): set the `input` symbol of the globals to None, so LLM
         # generated code with calls to `input` will raise an error, thus not
         # blocking the evaluation.
+        _, example = idx_and_example
         with lf_coding.context(input=None):
           output_message = self.process(example, **(self.additional_args or {}))
           self.process_output(example, output_message)
           return output_message
 
       try:
-        for example, message, error in lf.concurrent_map(
+        for (idx, example), message, error in lf.concurrent_map(
             _process,
-            examples,
+            enumerate(examples),
             max_workers=self.max_workers,
             show_progress=progress_bar or False,
             status_fn=self._status,
@@ -1148,7 +1149,7 @@ def _process(example: Any):
                 if isinstance(error, lf_structured.MappingError)
                 else None
             )
-          self.audit(example, message, error)
+          self.audit(idx + 1, example, message, error)
       finally:
         # Save cache upon completion or interruption.
         if self.dir and self.cache:
@@ -1437,6 +1438,7 @@ def _format_rate(self, rate: float) -> str:
 
   def audit(
       self,
+      example_idx: int,
       example: Any,
       message: lf.Message | None,
       error: Exception | None = None,
@@ -1445,6 +1447,7 @@ def audit(
     """Audits the example against the output. Subclasses should override.
 
     Args:
+      example_idx: 1-based index of the example in its dataset.
       example: The input object.
       message: The entire message returned by the LM, which could be used to
         trace the LM input, response and parsed structure. If error is raised
@@ -1465,7 +1468,7 @@ def audit(
     else:
       assert message is not None
       output = message.text if self.schema is None else message.result
-      self.audit_processed(example, output, message, dryrun=dryrun)
+      self.audit_processed(example_idx, example, output, message, dryrun=dryrun)
 
     # Audit usage.
     if message is not None:
@@ -1482,7 +1485,8 @@ def audit_usage(self, message: lf.Message, dryrun: bool = False) -> None:
         self._num_usages += 1
 
   def audit_processed(
-      self, example: Any, output: Any, message: lf.Message, dryrun: bool = False
+      self, example_idx: int, example: Any, output: Any, message: lf.Message,
+      dryrun: bool = False
   ) -> None:
     """Audits a successfully processed example. Subclass should override."""
 
diff --git a/langfun/core/eval/matching.py b/langfun/core/eval/matching.py
index ee1dd31..5fad74b 100644
--- a/langfun/core/eval/matching.py
+++ b/langfun/core/eval/matching.py
@@ -41,8 +41,8 @@ def answer(self, output: Any, example: Any) -> Any:
     """Returns the answer from the structure output."""
 
   @property
-  def matches(self) -> list[tuple[Any, Any, lf.Message]]:
-    """Returns the matches examples, outputs and the output messages."""
+  def matches(self) -> list[tuple[int, Any, Any, lf.Message]]:
+    """Returns the matches IDs, examples, outputs and the output messages."""
     return self._matches
 
   @property
@@ -57,7 +57,7 @@ def match_rate(self) -> float:
     return self.num_matches / self.num_completed
 
   @property
-  def mismatches(self) -> list[tuple[Any, Any, lf.Message]]:
+  def mismatches(self) -> list[tuple[int, Any, Any, lf.Message]]:
     """Returns the mismatches examples, outputs and output messages."""
     return self._mismatches
 
@@ -87,7 +87,8 @@ def _reset(self) -> None:
     self._mismatches = []
 
   def audit_processed(
-      self, example: Any, output: Any, message: lf.Message, dryrun: bool = False
+      self, example_idx: int, example: Any, output: Any, message: lf.Message,
+      dryrun: bool = False
   ) -> None:
     groundtruth = self.groundtruth(example)
     answer = self.answer(output, example)
@@ -107,9 +108,9 @@ def audit_processed(
       )
 
     if self.match(answer, groundtruth):
-      self._matches.append((example, output, message))
+      self._matches.append((example_idx, example, output, message))
     else:
-      self._mismatches.append((example, output, message))
+      self._mismatches.append((example_idx, example, output, message))
 
   def match(self, answer: Any, groundtruth: Any) -> bool:
     """Matches answer against the groundtruth. Subclasses can override."""
@@ -247,7 +248,7 @@ def _maybe_html(v, root_indent: int):
       # Fall back to the default format.
       return None
 
-    for i, (example, output, message) in enumerate(self.matches):
+    for i, (_, example, output, message) in enumerate(self.matches):
       bgcolor = 'white' if i % 2 == 0 else '#DDDDDD'
       s.write(f'<tr style="background-color: {bgcolor}"><td>{i + 1}</td>')
       input_str = lf.repr_utils.escape_quoted(
@@ -282,7 +283,7 @@ def _render_mismatches(self, s: io.StringIO) -> None:
         '</tr>'
     )
 
-    for i, (example, output, message) in enumerate(self.mismatches):
+    for i, (_, example, output, message) in enumerate(self.mismatches):
       bgcolor = 'white' if i % 2 == 0 else '#DDDDDD'
       s.write(f'<tr style="background-color: {bgcolor}"><td>{i + 1}</td>')
       input_str = pg.format(example, verbose=False, max_bytes_len=32)
diff --git a/langfun/core/eval/scoring.py b/langfun/core/eval/scoring.py
index 44793ff..ec5097a 100644
--- a/langfun/core/eval/scoring.py
+++ b/langfun/core/eval/scoring.py
@@ -62,8 +62,10 @@ def _reset(self) -> None:
     self._scored = []
 
   def audit_processed(
-      self, example: Any, output: Any, message: lf.Message, dryrun: bool = False
+      self, example_idx: int, example: Any, output: Any, message: lf.Message,
+      dryrun: bool = False
   ) -> None:
+    del example_idx
     score = self.score(example, output)
 
     if dryrun: