diff --git a/plain2code.py b/plain2code.py
index a7c93045..ed5796b2 100644
--- a/plain2code.py
+++ b/plain2code.py
@@ -482,7 +482,7 @@ def main():  # noqa: C901
     codeplainAPI = codeplain_api.CodeplainAPI(args.api_key, console)
     codeplainAPI.api_url = args.api
 
-    run_state = RunState(spec_filename=args.filename, replay_with=args.replay_with)
+    run_state = RunState(spec_filename=args.filename, replay_with=args.replay_with, job_tag=args.job_tag)
 
     # Without a terminal nobody can watch or drive the TUI, so render as --headless does:
     # progress on stdout, and a non-zero exit status when the render stops.
diff --git a/plain2code_arguments.py b/plain2code_arguments.py
index 0aacbeb5..e7d9c559 100644
--- a/plain2code_arguments.py
+++ b/plain2code_arguments.py
@@ -19,6 +19,7 @@
 DEFAULT_BUILD_FOLDER = "plain_modules"
 DEFAULT_BUILD_DEST = "dist"
 DEFAULT_CONFORMANCE_TESTS_DEST = "dist_conformance_tests"
+MAX_JOB_TAG_LENGTH = 300
 
 UNIT_TESTS_SCRIPT_NAME = "unittests_script"
 CONFORMANCE_TESTS_SCRIPT_NAME = "conformance_tests_script"
@@ -76,6 +77,15 @@ def non_empty_string(s):
     return s
 
 
+def job_tag_string(s):
+    s = str(s)
+    if not s:
+        raise argparse.ArgumentTypeError("The job tag cannot be empty.")
+    if len(s) > MAX_JOB_TAG_LENGTH:
+        raise argparse.ArgumentTypeError(f"The job tag cannot be longer than {MAX_JOB_TAG_LENGTH} characters.")
+    return s
+
+
 def frid_string(s):
     """Validate that the FRID is an integer."""
     if not s:
@@ -363,6 +373,15 @@ def create_parser():
         help="",
     )
 
+    _add_arg(
+        parser,
+        "--job-tag",
+        type=job_tag_string,
+        default=None,
+        help=f"Tag sent to the server with every request and attached to the render's LangSmith traces "
+        f"(max {MAX_JOB_TAG_LENGTH} characters).",
+    )
+
     _add_arg(
         parser,
         "--template-dir",
@@ -450,6 +469,15 @@ def create_parser():
     return parser
 
 
+def _validate_job_tag(args, parser):
+    # Values from config.yaml bypass argparse's type conversion, so validate them here.
+    if args.job_tag is not None:
+        try:
+            args.job_tag = job_tag_string(args.job_tag)
+        except argparse.ArgumentTypeError as e:
+            parser.error(f"--job-tag: {e}")
+
+
 def parse_arguments(command_line: Optional[Sequence[str]] = None):
     parser = create_parser()
 
@@ -496,6 +524,8 @@ def parse_arguments(command_line: Optional[Sequence[str]] = None):
     if not args.log_to_file and args.argument_sources.get("log_file_name") != "default":
         parser.error("--log-file-name cannot be used when --log-to-file is False.")
 
+    _validate_job_tag(args, parser)
+
     if args.full_plain and args.dry_run:
         parser.error("--full-plain and --dry-run are mutually exclusive")
 
diff --git a/plain2code_state.py b/plain2code_state.py
index 19cc21ac..a295cb97 100644
--- a/plain2code_state.py
+++ b/plain2code_state.py
@@ -8,7 +8,7 @@
 class RunState:
     """Contains information about the identifiable state of the rendering process."""
 
-    def __init__(self, spec_filename: str, replay_with: Optional[str] = None):
+    def __init__(self, spec_filename: str, replay_with: Optional[str] = None, job_tag: Optional[str] = None):
         self.replay: bool = replay_with is not None
         self.render_succeeded: bool = False
         self.render_cancelled: bool = False
@@ -19,6 +19,7 @@ def __init__(self, spec_filename: str, replay_with: Optional[str] = None):
         else:
             self.render_id: str = str(uuid.uuid4())
         self.spec_filename: str = spec_filename
+        self.job_tag: Optional[str] = job_tag
         self.call_count: int = 0
         self.unittest_batch_id: int = 0
         self.render_time_accumulated: int = 0
@@ -70,9 +71,12 @@ def set_last_render_start_timestamp(self):
         self.last_render_start_timestamp = time.monotonic()
 
     def to_dict(self):
-        return {
+        state = {
             "render_id": self.render_id,
             "call_count": self.call_count,
             "replay": self.replay,
             "spec_filename": self.spec_filename,
         }
+        if self.job_tag is not None:
+            state["job_tag"] = self.job_tag
+        return state
diff --git a/tests/test_arg_provenance.py b/tests/test_arg_provenance.py
index 0ef6807f..4764fec1 100644
--- a/tests/test_arg_provenance.py
+++ b/tests/test_arg_provenance.py
@@ -87,3 +87,23 @@ def test_mixed_sources(project):
     assert srcs["conformance_tests_dest"] == "cli"
     assert srcs["build_folder"] == "config"
     assert srcs["build_dest"] == "default"
+
+
+def test_job_tag_from_cli_and_config(project):
+    plain_file = os.path.join(project, "module.plain")
+    assert parse_arguments([plain_file]).job_tag is None
+    assert parse_arguments([plain_file, "--job-tag", "nightly"]).job_tag == "nightly"
+
+    (Path(project) / "config.yaml").write_text("job-tag: 123\n")
+    assert parse_arguments([plain_file]).job_tag == "123"
+    assert parse_arguments([plain_file, "--job-tag", "from_cli"]).job_tag == "from_cli"
+
+
+def test_job_tag_too_long_is_rejected(project):
+    plain_file = os.path.join(project, "module.plain")
+    with pytest.raises(SystemExit):
+        parse_arguments([plain_file, "--job-tag", "x" * 301])
+
+    (Path(project) / "config.yaml").write_text(f"job-tag: {'x' * 301}\n")
+    with pytest.raises(SystemExit):
+        parse_arguments([plain_file])
diff --git a/tests/test_plain2code_state.py b/tests/test_plain2code_state.py
index 71c1c714..e9612ee3 100644
--- a/tests/test_plain2code_state.py
+++ b/tests/test_plain2code_state.py
@@ -34,3 +34,8 @@ def test_add_to_render_time_is_cumulative_across_segments():
     run_state.add_to_render_time()
 
     assert run_state.render_time_accumulated == 7
+
+
+def test_to_dict_includes_job_tag_only_when_set():
+    assert "job_tag" not in RunState(spec_filename="x.plain").to_dict()
+    assert RunState(spec_filename="x.plain", job_tag="nightly").to_dict()["job_tag"] == "nightly"
diff --git a/tests/test_render_finished_report.py b/tests/test_render_finished_report.py
index 811e79df..b38210fb 100644
--- a/tests/test_render_finished_report.py
+++ b/tests/test_render_finished_report.py
@@ -97,6 +97,7 @@ def make_args(api_key="test-key", headless=False):
         render_from=None,
         headless=headless,
         replay_with=None,
+        job_tag=None,
         log_to_file=False,
         log_file_name="codeplain.log",
         conformance_tests_script=None,
diff --git a/tests/test_terminal_attachment.py b/tests/test_terminal_attachment.py
index 2b59960c..3da6e24c 100644
--- a/tests/test_terminal_attachment.py
+++ b/tests/test_terminal_attachment.py
@@ -84,6 +84,7 @@ def make_args():
         force_render=False,
         headless=False,
         replay_with=None,
+        job_tag=None,
         log_to_file=False,
         log_file_name="codeplain.log",
         render_conformance_tests=False,