diff --git a/src/specify_cli/commands/event.py b/src/specify_cli/commands/event.py
index d1576c2c70..47bcf86e70 100644
--- a/src/specify_cli/commands/event.py
+++ b/src/specify_cli/commands/event.py
@@ -27,14 +27,25 @@ def event_run(
     # Read payload from stdin if available (capped at 1 MiB to prevent DoS).
     MAX_STDIN_BYTES = 1 * 1024 * 1024
     if not sys.stdin.isatty():
-        raw = sys.stdin.read(MAX_STDIN_BYTES)
-        if not sys.stdin.eof:
-            raise typer.Exit(
-                code=1,
-                message="stdin payload exceeds 1 MiB limit; "
+        # Read from the underlying binary buffer so the cap counts encoded
+        # bytes, not decoded characters — `sys.stdin.read()` on a text stream
+        # counts Unicode characters, which lets multibyte payloads (e.g. a
+        # few hundred thousand emoji) exceed 1 MiB on the wire while still
+        # passing the length check. Reading one byte past the cap tells us
+        # whether more data was waiting beyond it.
+        raw = sys.stdin.buffer.read(MAX_STDIN_BYTES + 1)
+        if len(raw) > MAX_STDIN_BYTES:
+            typer.echo(
+                "stdin payload exceeds 1 MiB limit; "
                 "truncate or pipe a smaller payload",
+                err=True,
             )
-        payload = raw
+            raise typer.Exit(code=1)
+        try:
+            payload = raw.decode("utf-8")
+        except UnicodeDecodeError:
+            typer.echo("stdin payload must be valid UTF-8", err=True)
+            raise typer.Exit(code=1) from None
     else:
         payload = "{}"
 
diff --git a/tests/test_event_command.py b/tests/test_event_command.py
new file mode 100644
index 0000000000..0a431fb5c6
--- /dev/null
+++ b/tests/test_event_command.py
@@ -0,0 +1,145 @@
+"""`specify event run` must read piped stdin without crashing.
+
+`event_run` (src/specify_cli/commands/event.py) capped its stdin read at 1
+MiB to prevent a DoS (#3857), but the truncation check read a `.eof`
+attribute that does not exist on any Python file-like object (including
+`sys.stdin`) — every piped-stdin invocation raised `AttributeError` instead
+of running, regardless of payload size. Piped stdin is the command's
+documented primary use case (it is how a native hook feeds it a JSON
+payload), so this broke the feature entirely rather than only rejecting
+oversized payloads. Even the intended oversized-payload branch was broken a
+second way: `typer.Exit(code=1, message=...)` — `typer.Exit` accepts no
+`message` keyword argument, so that path raised `TypeError` instead of a
+clean CLI error.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import patch
+
+import pytest
+import typer
+from typer.testing import CliRunner
+
+from specify_cli import app
+from specify_cli.commands.event import event_run
+
+
+def test_event_run_reads_piped_stdin_payload():
+    """A normal, under-the-cap piped payload must reach the handler intact."""
+    with patch(
+        "specify_cli.events.resolve_and_run_event_command", return_value=0
+    ) as mock_run:
+        result = CliRunner().invoke(
+            app,
+            ["event", "run", "some-command", "session_start"],
+            input='{"key": "value"}',
+        )
+
+    assert result.exit_code == 0, result.output
+    assert mock_run.called
+    payload_arg = mock_run.call_args[0][2]
+    assert payload_arg == '{"key": "value"}'
+
+
+def test_event_run_empty_pipe_reads_empty_payload():
+    """An empty (but non-TTY) piped stream must not crash; it forwards `""`.
+
+    CliRunner always provides a non-TTY stdin, even when no `input=` is
+    given, so this exercises the piped-input branch with zero bytes — not
+    the TTY fallback. See `test_event_run_tty_uses_empty_object` below for
+    the actual TTY case.
+    """
+    with patch(
+        "specify_cli.events.resolve_and_run_event_command", return_value=0
+    ) as mock_run:
+        result = CliRunner().invoke(
+            app,
+            ["event", "run", "some-command", "session_start"],
+        )
+
+    assert result.exit_code == 0, result.output
+    assert mock_run.called
+    payload_arg = mock_run.call_args[0][2]
+    assert payload_arg == ""
+
+
+def test_event_run_tty_uses_empty_object(monkeypatch):
+    """A real TTY (no piped input at all) must fall back to `"{}"`."""
+
+    class FakeTtyStdin:
+        def isatty(self):
+            return True
+
+    monkeypatch.setattr("specify_cli.commands.event.sys.stdin", FakeTtyStdin())
+
+    with patch(
+        "specify_cli.events.resolve_and_run_event_command", return_value=0
+    ) as mock_run:
+        with pytest.raises(typer.Exit):
+            event_run(command_name="some-command", event_name="session_start", timeout=120)
+
+    assert mock_run.called
+    payload_arg = mock_run.call_args[0][2]
+    assert payload_arg == "{}"
+
+
+def test_event_run_oversized_stdin_reports_clean_error():
+    """A payload exceeding the 1 MiB cap must exit 1 with the limit message,
+    not crash with AttributeError (missing `.eof`) or TypeError (`typer.Exit`
+    does not accept `message=`)."""
+    oversized = "x" * (1 * 1024 * 1024 + 10)
+    with patch(
+        "specify_cli.events.resolve_and_run_event_command", return_value=0
+    ) as mock_run:
+        result = CliRunner().invoke(
+            app,
+            ["event", "run", "some-command", "session_start"],
+            input=oversized,
+        )
+
+    assert result.exit_code == 1, result.output
+    assert "1 MiB limit" in result.output
+    assert not mock_run.called
+
+
+def test_event_run_invalid_utf8_reports_clean_error():
+    """A piped payload that isn't valid UTF-8 must exit 1 with the encoding
+    error message, not propagate a raw `UnicodeDecodeError`, and the handler
+    must never be invoked with undecodable data."""
+    with patch(
+        "specify_cli.events.resolve_and_run_event_command", return_value=0
+    ) as mock_run:
+        result = CliRunner().invoke(
+            app,
+            ["event", "run", "some-command", "session_start"],
+            input=b"\xff\xfe",
+        )
+
+    assert result.exit_code == 1, result.output
+    assert "must be valid UTF-8" in result.output
+    assert not mock_run.called
+
+
+def test_event_run_multibyte_payload_enforces_byte_limit():
+    """The 1 MiB cap must be enforced in encoded bytes, not decoded characters.
+
+    300,000 emoji is ~1.14 MiB of UTF-8 (4 bytes each) but only 300,000
+    *characters* — comfortably under the 1,048,576 character cap a text-mode
+    `sys.stdin.read(MAX_STDIN_BYTES)` would have applied. Reading from the
+    binary buffer instead must still reject it.
+    """
+    oversized = "\U0001F600" * 300_000  # 😀, 4 bytes each in UTF-8
+    assert len(oversized) < 1 * 1024 * 1024  # under the old, wrong character cap
+    with patch(
+        "specify_cli.events.resolve_and_run_event_command", return_value=0
+    ) as mock_run:
+        result = CliRunner().invoke(
+            app,
+            ["event", "run", "some-command", "session_start"],
+            input=oversized,
+        )
+
+    assert result.exit_code == 1, result.output
+    assert "1 MiB limit" in result.output
+    assert not mock_run.called