sphinx-build prints long message and traceback info for ExtensionError and ConfigError exceptions (regression)

Describe the bug

sphinx-build used to suppress python backtraces when an extension threw an ExtensionError. This allows the extension to report e.g. syntax errors in input files to the user in a user-friendly way. (QEMU has some extensions that do this: we might for instance report that the user made a syntax error in a file that the extension reads, including the filename and line number of that file.) Recent versions of sphinx-build seem to have regressed here, and now print a big long message implying that the user should report a bug in Sphinx.

Similarly, if a conf.py raises ConfigError a big message is printed. The reason ConfigError was introduced was to allow conf.py files to report problems in a friendly way without Sphinx putting in a confusing extra message implying that there's a bug (see issue #7108).

I think that when an extension throws ExtensionError, or a conf.py throws ConfigError:

• sphinx should not print any of this extra information about versions or loaded extensions
• sphinx should not save a traceback to a temp file
• even when -T is passed on the command line, it should not print the python traceback
• this should be the case whether using '-j auto' for parallelism or not

This allows config files and extensions to report issues to the user with the formatting and detail that they consider appropriate, and in a way that doesn't make the user believe that there's a Sphinx or extension bug involved.

Compare, for a simple extension that always throws ExtensionError from its run() method:

$ sphinx-build --version
sphinx-build 7.2.6
$ sphinx-build docs out
Running Sphinx v7.2.6
building [mo]: targets for 0 po files that are out of date
writing output... 
building [html]: targets for 1 source files that are out of date
updating environment: [new config] 1 added, 0 changed, 0 removed
reading sources... [100%] index
Extension error:
This is an expected error from TestExtn

with

$ sphinx-build --version
sphinx-build 9.1.0
$ sphinx-build docs out
Running Sphinx v9.1.0
loading translations [en]... done
building [mo]: targets for 0 po files that are out of date
writing output... 
building [html]: targets for 1 source files that are out of date
updating environment: [new config] 1 added, 0 changed, 0 removed
reading sources... [100%] index
Extension error!

Versions
========

* Platform:         linux; (Linux-6.17.0-1024-oem-x86_64-with-glibc2.39)
* Python version:   3.12.3 (CPython)
* Sphinx version:   9.1.0
* Docutils version: 0.22.4
* Jinja2 version:   3.1.6
* Pygments version: 2.20.0

Last Messages
=============

    writing output...

    building [html]: targets for 1 source files that are out of date
    updating environment:
    [new config]
    1 added, 0 changed, 0 removed

    reading sources... [100%]
    index

Loaded Extensions
=================

* sphinx.ext.mathjax (9.1.0)
* alabaster (1.0.0)
* sphinxcontrib.applehelp (2.0.0)
* sphinxcontrib.devhelp (2.0.0)
* sphinxcontrib.htmlhelp (2.1.0)
* sphinxcontrib.serializinghtml (2.0.0)
* sphinxcontrib.qthelp (2.0.0)
* test_extn (1.0)

Traceback
=========

      File "/home/pm215/sphinx-bug/docs/extn/test_extn.py", line 13, in run
        raise ExtensionError('This is an expected error from TestExtn')
    sphinx.errors.ExtensionError: This is an expected error from TestExtn


The full traceback has been saved in:
/tmp/sphinx-err-vr3zyjtx.log

To report this error to the developers, please open an issue at <https://github.com/sphinx-doc/sphinx/issues/>. Thanks!
Please also report this if it was a user error, so that a better error message can be provided next time.

And for ConfigError:

lanath:noble:sphinx-bug$ sphinx-build docs out
Running Sphinx v7.2.6

Configuration error:
problem in config

versus:

$ sphinx-build docs out
Running Sphinx v9.1.0

Configuration error!

Versions
========

* Platform:         linux; (Linux-6.17.0-1024-oem-x86_64-with-glibc2.39)
* Python version:   3.12.3 (CPython)
* Sphinx version:   9.1.0
* Docutils version: 0.22.4
* Jinja2 version:   3.1.6
* Pygments version: 2.20.0

Last Messages
=============

None.

Loaded Extensions
=================

None.

Traceback
=========

      File "/home/pm215/sphinx-bug/docs/conf.py", line 11, in <module>
        raise ConfigError("problem in config")
    sphinx.errors.ConfigError: problem in config


The full traceback has been saved in:
/tmp/sphinx-err-nx1lf1cp.log

To report this error to the developers, please open an issue at <https://github.com/sphinx-doc/sphinx/issues/>. Thanks!
Please also report this if it was a user error, so that a better error message can be provided next time.

How to Reproduce

Simple repro, with three files:

1: docs/conf.py:

# Minimal config file

import sys
from pathlib import Path
from sphinx.errors import ConfigError

sys.path.append(str(Path('extn').resolve()))
 
extensions = [ 'test_extn' ]

2: docs/index.rst:

Test document: invoke the test extension.

.. test_extn::

3: docs/extn/test_extn.py:

# Test extension that always throws an ExtensionError
import sphinx
from docutils.parsers.rst import directives, Directive
from sphinx.errors import ExtensionError

class TestExtnDirective(Directive):
    """This extension always throws an error"""
    required_argument = 0
    optional_arguments = 0
    has_content = False

    def run(self):
        raise ExtensionError('This is an expected error from TestExtn')

def setup(app):
    """ Register test_extn directive with Sphinx"""
    app.add_directive('test_extn', TestExtnDirective)

    return dict(
        version = '1.0',
        parallel_read_safe = True,
        parallel_write_safe = True
    )

Run with:

sphinx-build docs out

To test the ConfigError variant, add this line to the end of conf.py:

raise ConfigError("problem in config")

Environment Information

Platform:              linux; (Linux-6.17.0-1024-oem-x86_64-with-glibc2.39)
Python version:        3.12.3 (main, Jun 19 2026, 12:46:00) [GCC 13.3.0])
Python implementation: CPython
Sphinx version:        9.1.0
Docutils version:      0.22.4
Jinja2 version:        3.1.6
Pygments version:      2.20.0

Sphinx extensions

Additional context

No response

Start by running the sphinx-build docs out reproduction with docs/conf.py, docs/index.rst, and docs/extn/test_extn.py, then inspect the sphinx-build exception-handling path for ExtensionError and ConfigError. Done means both errors retain their user-facing messages without extra versions, extensions, traceback output, or temporary traceback files, including with -T, -j auto, and the ConfigError variant.