From 857001b1c2c9e8214e94404006a7bf06d96bddd5 Mon Sep 17 00:00:00 2001
From: Adam Turner <turner@hudson-trading.com>
Date: Thu, 7 May 2026 15:03:16 +0100
Subject: [PATCH] Fix JSON output mode for syntax errors in parallel mode

---
 mypy/build_worker/worker.py    |  7 ++++++-
 mypy/test/testoutput.py        |  8 +++++++-
 test-data/unit/outputjson.test | 16 ++++++++++++++++
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/mypy/build_worker/worker.py b/mypy/build_worker/worker.py
index 4a0bac99b440b..e9f7a026037e4 100644
--- a/mypy/build_worker/worker.py
+++ b/mypy/build_worker/worker.py
@@ -113,7 +113,12 @@ def main(argv: list[str]) -> None:
     fscache = FileSystemCache()
     fscache.set_package_root(options.package_root)
     cached_read = fscache.read
-    errors = Errors(options, read_source=lambda path: read_py_file(path, cached_read))
+    error_formatter = None if options.output is None else OUTPUT_CHOICES.get(options.output)
+    errors = Errors(
+        options,
+        read_source=lambda path: read_py_file(path, cached_read),
+        error_formatter=error_formatter,
+    )
 
     ctx = ServerContext(options, disable_error_code, enable_error_code, errors, fscache)
     try:
diff --git a/mypy/test/testoutput.py b/mypy/test/testoutput.py
index 41f6881658c8c..e18302cbf92a0 100644
--- a/mypy/test/testoutput.py
+++ b/mypy/test/testoutput.py
@@ -8,6 +8,7 @@
 
 import os
 import os.path
+import re
 
 from mypy import api
 from mypy.defaults import PYTHON3_VERSION
@@ -24,7 +25,12 @@ def run_case(self, testcase: DataDrivenTestCase) -> None:
 
 def test_output_json(testcase: DataDrivenTestCase) -> None:
     """Runs Mypy in a subprocess, and ensures that `--output=json` works as intended."""
-    mypy_cmdline = ["--output=json"]
+    program_text = "\n".join(testcase.input)
+    flags_match = re.search("# flags: (.*)$", program_text, flags=re.MULTILINE)
+    if flags_match is not None:
+        mypy_cmdline = flags_match.group(1).split()
+    else:
+        mypy_cmdline = ["--output=json"]
     mypy_cmdline.append(f"--python-version={'.'.join(map(str, PYTHON3_VERSION))}")
 
     # Write the program to a file.
diff --git a/test-data/unit/outputjson.test b/test-data/unit/outputjson.test
index 89bcb99d3224c..ec716d97b8f1e 100644
--- a/test-data/unit/outputjson.test
+++ b/test-data/unit/outputjson.test
@@ -43,9 +43,25 @@ bar('42')
 {"file": "main", "line": 14, "column": 0, "end_line": 14, "end_column": 9, "message": "No overload variant of \"foo\" matches argument type \"str\"", "hint": "Possible overload variants:\n    def foo() -> None\n    def foo(x: int) -> None", "code": "call-overload", "severity": "error"}
 {"file": "main", "line": 17, "column": 0, "end_line": 17, "end_column": 9, "message": "Too many arguments for \"bar\"", "hint": null, "code": "call-arg", "severity": "error"}
 
+[case testOutputJsonParallel]
+# flags: --output=json --num-workers=2
+def foo() -> None:
+    pass
+
+foo(1)
+[out]
+{"file": "main", "line": 5, "column": 0, "end_line": 5, "end_column": 6, "message": "Too many arguments for \"foo\"", "hint": null, "code": "call-arg", "severity": "error"}
+
 [case testOutputJsonSyntaxError]
 # flags: --output=json
 klass foo
 [out]
 {"file": "main", "line": 2, "column": 7, "end_line": 2, "end_column": 8, "message": "Invalid syntax", "hint": null, "code": "syntax", "severity": "error"}
 !!! Mypy crashed !!!
+
+[case testOutputJsonSyntaxErrorParallel]
+# flags: --output=json --num-workers=2
+break
+[out]
+{"file": "main", "line": 2, "column": 0, "end_line": 2, "end_column": 5, "message": "\"break\" outside loop", "hint": null, "code": null, "severity": "error"}
+!!! Mypy crashed !!!