From 6cee37718834fa7d0d51b1a7bb14f70ad0ec7ce8 Mon Sep 17 00:00:00 2001
From: Adam Turner <turner@hudson-trading.com>
Date: Thu, 30 Apr 2026 21:13:13 +0100
Subject: [PATCH 1/2] Respect JSON output mode for syntax errors

---
 mypy/build.py                  | 9 +++++++--
 mypy/errors.py                 | 6 +++++-
 test-data/unit/outputjson.test | 7 +++++++
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/mypy/build.py b/mypy/build.py
index 5c1c6c62bffde..38242e4d2cafd 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -493,7 +493,12 @@ def build_inner(
 
     source_set = BuildSourceSet(sources)
     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,
+    )
     # Record import errors so that they can be replayed by the workers.
     if workers:
         errors.global_watcher = True
@@ -516,7 +521,7 @@ def build_inner(
         plugin=plugin,
         plugins_snapshot=snapshot,
         errors=errors,
-        error_formatter=None if options.output is None else OUTPUT_CHOICES.get(options.output),
+        error_formatter=error_formatter,
         flush_errors=flush_errors,
         fscache=fscache,
         stdout=stdout,
diff --git a/mypy/errors.py b/mypy/errors.py
index 70458e187f363..19f01205d5565 100644
--- a/mypy/errors.py
+++ b/mypy/errors.py
@@ -500,6 +500,7 @@ def __init__(
         *,
         read_source: Callable[[str], list[str] | None] | None = None,
         hide_error_codes: bool | None = None,
+        error_formatter: ErrorFormatter | None = None,
     ) -> None:
         self.options = options
         self.hide_error_codes = (
@@ -507,6 +508,7 @@ def __init__(
         )
         # We use fscache to read source code when showing snippets.
         self.read_source = read_source
+        self.error_formatter = error_formatter
         self.initialize()
 
     def initialize(self) -> None:
@@ -1129,7 +1131,9 @@ def new_messages(self) -> list[str]:
         for path in self.error_info_map.keys():
             if path not in self.flushed_files:
                 error_tuples = self.file_messages(path)
-                msgs.extend(self.format_messages(path, error_tuples))
+                msgs.extend(
+                    self.format_messages(path, error_tuples, formatter=self.error_formatter)
+                )
         return msgs
 
     def targets(self) -> set[str]:
diff --git a/test-data/unit/outputjson.test b/test-data/unit/outputjson.test
index 39741b3d1a8bf..18048a33399ff 100644
--- a/test-data/unit/outputjson.test
+++ b/test-data/unit/outputjson.test
@@ -42,3 +42,10 @@ bar('42')
 {"file": "main", "line": 12, "column": 12, "end_line": 12, "end_column": 15, "message": "Revealed type is \"Overload(def (), def (x: int))\"", "hint": null, "code": "misc", "severity": "note"}
 {"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 testOutputJsonSyntaxError]
+# flags: --output=json
+def foo<
+[out]
+{"file": "main", "line": 2, "column": 8, "end_line": 2, "end_column": 9, "message": "Expected '('", "hint": null, "code": "syntax", "severity": "error"}
+!!! Mypy crashed !!!

From 071f7df6a540be03205f7d8382094502d14843b0 Mon Sep 17 00:00:00 2001
From: Adam Turner <turner@hudson-trading.com>
Date: Thu, 30 Apr 2026 21:13:13 +0100
Subject: [PATCH 2/2] Fix test for 3.10

---
 test-data/unit/outputjson.test | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/test-data/unit/outputjson.test b/test-data/unit/outputjson.test
index 18048a33399ff..89bcb99d3224c 100644
--- a/test-data/unit/outputjson.test
+++ b/test-data/unit/outputjson.test
@@ -45,7 +45,7 @@ bar('42')
 
 [case testOutputJsonSyntaxError]
 # flags: --output=json
-def foo<
+klass foo
 [out]
-{"file": "main", "line": 2, "column": 8, "end_line": 2, "end_column": 9, "message": "Expected '('", "hint": null, "code": "syntax", "severity": "error"}
+{"file": "main", "line": 2, "column": 7, "end_line": 2, "end_column": 8, "message": "Invalid syntax", "hint": null, "code": "syntax", "severity": "error"}
 !!! Mypy crashed !!!