diff --git a/spacy/cli/_util.py b/spacy/cli/_util.py
index c46abffe5..0f4e9f599 100644
--- a/spacy/cli/_util.py
+++ b/spacy/cli/_util.py
@@ -583,6 +583,10 @@ def setup_gpu(use_gpu: int, silent=None) -> None:
 
 
 def walk_directory(path: Path, suffix: Optional[str] = None) -> List[Path]:
+    """Given a directory and a suffix, recursively find all files matching the suffix.
+    Directories or files with names beginning with a . are ignored, but hidden flags on
+    filesystems are not checked.
+    When provided with a suffix `None`, there is no suffix-based filtering."""
     if not path.is_dir():
         return [path]
     paths = [path]
diff --git a/spacy/cli/convert.py b/spacy/cli/convert.py
index 7f365ae2c..68d454b3e 100644
--- a/spacy/cli/convert.py
+++ b/spacy/cli/convert.py
@@ -28,6 +28,8 @@ CONVERTERS: Mapping[str, Callable[..., Iterable[Doc]]] = {
     "json": json_to_docs,
 }
 
+AUTO = "auto"
+
 
 # File types that can be written to stdout
 FILE_TYPES_STDOUT = ("json",)
@@ -49,7 +51,7 @@ def convert_cli(
     model: Optional[str] = Opt(None, "--model", "--base", "-b", help="Trained spaCy pipeline for sentence segmentation to use as base (for --seg-sents)"),
     morphology: bool = Opt(False, "--morphology", "-m", help="Enable appending morphology to tags"),
     merge_subtokens: bool = Opt(False, "--merge-subtokens", "-T", help="Merge CoNLL-U subtokens"),
-    converter: str = Opt("auto", "--converter", "-c", help=f"Converter: {tuple(CONVERTERS.keys())}"),
+    converter: str = Opt(AUTO, "--converter", "-c", help=f"Converter: {tuple(CONVERTERS.keys())}"),
     ner_map: Optional[Path] = Opt(None, "--ner-map", "-nm", help="NER tag mapping (as JSON-encoded dict of entity types)", exists=True),
     lang: Optional[str] = Opt(None, "--lang", "-l", help="Language (if tokenizer required)"),
     concatenate: bool = Opt(None, "--concatenate", "-C", help="Concatenate output to a single file"),
@@ -70,8 +72,8 @@ def convert_cli(
     output_dir: Union[str, Path] = "-" if output_dir == Path("-") else output_dir
     silent = output_dir == "-"
     msg = Printer(no_print=silent)
-    verify_cli_args(msg, input_path, output_dir, file_type.value, converter, ner_map)
     converter = _get_converter(msg, converter, input_path)
+    verify_cli_args(msg, input_path, output_dir, file_type.value, converter, ner_map)
     convert(
         input_path,
         output_dir,
@@ -100,7 +102,7 @@ def convert(
     model: Optional[str] = None,
     morphology: bool = False,
     merge_subtokens: bool = False,
-    converter: str = "auto",
+    converter: str,
     ner_map: Optional[Path] = None,
     lang: Optional[str] = None,
     concatenate: bool = False,
@@ -212,18 +214,22 @@ def verify_cli_args(
         input_locs = walk_directory(input_path, converter)
         if len(input_locs) == 0:
             msg.fail("No input files in directory", input_path, exits=1)
-        file_types = list(set([loc.suffix[1:] for loc in input_locs]))
-        if converter == "auto" and len(file_types) >= 2:
-            file_types_str = ",".join(file_types)
-            msg.fail("All input files must be same type", file_types_str, exits=1)
-    if converter != "auto" and converter not in CONVERTERS:
+    if converter not in CONVERTERS:
         msg.fail(f"Can't find converter for {converter}", exits=1)
 
 
 def _get_converter(msg, converter, input_path: Path):
     if input_path.is_dir():
-        input_path = walk_directory(input_path, converter)[0]
-    if converter == "auto":
+        if converter == AUTO:
+            input_locs = walk_directory(input_path, suffix=None)
+            file_types = list(set([loc.suffix[1:] for loc in input_locs]))
+            if len(file_types) >= 2:
+                file_types_str = ",".join(file_types)
+                msg.fail("All input files must be same type", file_types_str, exits=1)
+            input_path = input_locs[0]
+        else:
+            input_path = walk_directory(input_path, suffix=converter)[0]
+    if converter == AUTO:
         converter = input_path.suffix[1:]
     if converter == "ner" or converter == "iob":
         with input_path.open(encoding="utf8") as file_:
diff --git a/spacy/tests/test_cli.py b/spacy/tests/test_cli.py
index c6768a3fd..c88e20de2 100644
--- a/spacy/tests/test_cli.py
+++ b/spacy/tests/test_cli.py
@@ -4,6 +4,7 @@ from collections import Counter
 from typing import Tuple, List, Dict, Any
 import pkg_resources
 import time
+from pathlib import Path
 
 import spacy
 import numpy
@@ -15,7 +16,7 @@ from thinc.api import Config, ConfigValidationError
 
 from spacy import about
 from spacy.cli import info
-from spacy.cli._util import is_subpath_of, load_project_config
+from spacy.cli._util import is_subpath_of, load_project_config, walk_directory
 from spacy.cli._util import parse_config_overrides, string_to_list
 from spacy.cli._util import substitute_project_variables
 from spacy.cli._util import validate_project_commands
@@ -1185,3 +1186,26 @@ def test_upload_download_local_file():
         download_file(remote_file, local_file)
         with local_file.open(mode="r") as file_:
             assert file_.read() == content
+
+
+def test_walk_directory():
+    with make_tempdir() as d:
+        files = [
+            "data1.iob",
+            "data2.iob",
+            "data3.json",
+            "data4.conll",
+            "data5.conll",
+            "data6.conll",
+            "data7.txt",
+        ]
+
+        for f in files:
+            Path(d / f).touch()
+
+        assert (len(walk_directory(d))) == 7
+        assert (len(walk_directory(d, suffix=None))) == 7
+        assert (len(walk_directory(d, suffix="json"))) == 1
+        assert (len(walk_directory(d, suffix="iob"))) == 2
+        assert (len(walk_directory(d, suffix="conll"))) == 3
+        assert (len(walk_directory(d, suffix="pdf"))) == 0
diff --git a/spacy/tests/test_cli_app.py b/spacy/tests/test_cli_app.py
new file mode 100644
index 000000000..873a3ff66
--- /dev/null
+++ b/spacy/tests/test_cli_app.py
@@ -0,0 +1,33 @@
+import os
+from pathlib import Path
+from typer.testing import CliRunner
+
+from spacy.cli._util import app
+from .util import make_tempdir
+
+
+def test_convert_auto():
+    with make_tempdir() as d_in, make_tempdir() as d_out:
+        for f in ["data1.iob", "data2.iob", "data3.iob"]:
+            Path(d_in / f).touch()
+
+        # ensure that "automatic" suffix detection works
+        result = CliRunner().invoke(app, ["convert", str(d_in), str(d_out)])
+        assert "Generated output file" in result.stdout
+        out_files = os.listdir(d_out)
+        assert len(out_files) == 3
+        assert "data1.spacy" in out_files
+        assert "data2.spacy" in out_files
+        assert "data3.spacy" in out_files
+
+
+def test_convert_auto_conflict():
+    with make_tempdir() as d_in, make_tempdir() as d_out:
+        for f in ["data1.iob", "data2.iob", "data3.json"]:
+            Path(d_in / f).touch()
+
+        # ensure that "automatic" suffix detection warns when there are different file types
+        result = CliRunner().invoke(app, ["convert", str(d_in), str(d_out)])
+        assert "All input files must be same type" in result.stdout
+        out_files = os.listdir(d_out)
+        assert len(out_files) == 0