Indexing every time VS code is restarted, IntelliSense indexing is too slow for large C/C++ projects – suggest persistent cache and Git-aware incremental updates

[Ma-Ike]

Environment

Version: 1.104.0 (user setup)
Commit: f220831ea2d946c0dcb0f3eaa480eb435a2c1260
Date: 2025-09-10T06:46:18.035Z
Electron: 37.3.1
ElectronBuildId: 12342881
Chromium: 138.0.7204.235
Node.js: 22.18.0
V8: 13.8.258.31-electron.0
OS: Windows_NT x64 10.0.22631

- C/C++ Extension Version:

C/C++:
Version: 1.104.0 (user setup)
Commit: f220831ea2d946c0dcb0f3eaa480eb435a2c1260
Date: 2025-09-10T06:46:18.035Z
Electron: 37.3.1
ElectronBuildId: 12342881
Chromium: 138.0.7204.235
Node.js: 22.18.0
V8: 13.8.258.31-electron.0
OS: Windows_NT x64 10.0.22631

C/C++ Extension Pack:
Identifier
ms-vscode.cpptools-extension-pack
Version
1.3.1
Last Updated
2025-02-24, 20:41:16

C/C++ Themes:
Identifier
ms-vscode.cpptools-themes
Version
2.0.0
Last Updated
2022-11-07, 18:21:11

• If using SSH remote, specify OS of remote machine: NO no remote connection to access this just on the same machine

Bug Summary and Steps to Reproduce

Bug Summary:
Does this issue occur when all extensions are disabled?: Yes/No

VS Code Version:
Version: 1.101.2 (user setup)
Date: 2025-06-24T20:27:15.391Z
Electron: 35.5.1
ElectronBuildId: 11727614
Chromium: 134.0.6998.205
Node.js: 22.15.1
V8: 13.4.114.21-electron.0
OS: Windows_NT x64 10.0.22631
Problem Summary

In large C/C++ projects (e.g. >1500 .c files, >6 GB source tree), IntelliSense in VS Code takes an extremely long time to index, sometimes up to 3h or more. This severely impacts productivity, especially when switching branches or restarting the editor.

Environment

VS Code: [1.101.2]
C/C++ extension: ms-vscode.cpptools (v2.x)
OS: Windows 10 / 11
Workspace type: Local repo
Project size: ~6 GB, 1500+ C files
Compiler: GCC + compile_commands.json provided
Hardware: [e.g. 8-core CPU, 32GB RAM]
Observed Behavior

When opening the workspace, IntelliSense starts re-indexing from scratch.
This happens even if no files have changed.
There's no apparent persistent cache between VS Code sessions.
It uses a lot of CPU and RAM and slows down the entire system.
No integration with Git (e.g. it re-indexes even if Git history is unchanged).
Desired Behavior

I would like IntelliSense to behave more intelligently in large projects:

Persistent Index Cache
Save the symbol index and reuse it across sessions – do not re-index the entire project from scratch when VS Code restarts. Clangd already supports this with its .clangd/index/ folder.

Git-Aware Incremental Indexing
Integrate with Git to detect which files have changed between sessions or branch switches. This would allow VS Code to re-index only the changed files.

Smart Index Resume
Indexing should resume from the last known state even if interrupted (e.g. via VS Code crash, or reboot).

Optional Index Directory Location Setting
Allow users to specify a location where IntelliSense index data is stored – useful for SSDs, network drives, or project-specific cache directories.

Workarounds Tried

"C_Cpp.default.compileCommands" is correctly set.
"C_Cpp.intelliSenseEngine": "Default" (also tried "Disabled" with clangd).
"files.watcherExclude" is used to reduce load.
"terminal.integrated.gpuAcceleration": "off" is set.
Switching to clangd helps slightly but still re-indexes every time.
Related Ideas

Similar to how clangd keeps a persistent index in .clangd/index
cquery, ccls and others support persistent and incremental indexing
Git-aware indexing is used in some LSPs and build systems (e.g. Bazel, Buck)
Request

Please consider improving IntelliSense in large projects by:

Adding persistent indexing support
Making the index cacheable across sessions
Using Git or file-hash comparisons to do incremental re-indexing
Providing users control over the index behavior via settings
This would dramatically improve performance and usability for large embedded C/C++ projects.

Would be really good to have this fixed. regardless of what I try it helps very little or not at all.
I gave some suggestions but if you have a better or a path forward then up to you mine are just suggestions here.

Steps to reproduce:

1. open VS code and wait until the indexing is done
2. close VS code
3. Open VS code and see indexing is starting over

Expected behavior:

NOT to index everything every time VS code starts again.

Original post is here:
254508

Configuration and Logs

No logs is needed

Other Extensions

No response

Additional context

No response

[Ma-Ike]

If you prefer this to be a feature request then that would be ok. but gor me this is a bug as indexing should NOT take place if NO files have been updated.

[sean-mcmanus]

@Ma-Ike What version of the C/C++ extension are you using?

If the database is not being deleted, then there isn't any actual indexing happening, but it does rescan the workspace to check for anything it needs to update -- normally that is supposed to be fast.

You could potentially set a C_Cpp.files.exclude to exclude large folders that don't need to be parsed for workspace symbols, see https://github.com/microsoft/vscode-cpptools/wiki/Optimizing-your-configuration .

[Ma-Ike]

@sean-mcmanus I can not exclude more then I have already, As stated in the original post "When opening the workspace, IntelliSense starts re-indexing from scratch."

All the versions that are installed regarding C/C+ is listed in the post but here is is again:
C/C++:
Version: 1.104.0 (user setup)

C/C++ Extension Pack:
Identifier
ms-vscode.cpptools-extension-pack
Version
1.3.1

C/C++ Themes:
Identifier
ms-vscode.cpptools-themes
Version
2.0.0

Further clarification is also stated here:
Steps to reproduce:

1. Open VS code and wait until the indexing is done
2. Close VS code
3. Open VS code and see indexing is starting over

So trust me I have tried all I can think of but you have any ideas then pleas let me know. exclugning files in no longer an option. The excluding part is already done to the biggest extent that I can without having other problems.

[sean-mcmanus]

@Ma-Ike We do not ship a version 1.104 -- that sounds like a VS Code version. Our latest versions are 1.26.5 and 1.27.5 (pre-release).

With C_Cpp.loggingLevel set to "Debug", what do you see for the output of "Discovering files: "? "Re-indexing" just involves enumerating files from disk which should be fast, so I see like it index around 7.5k files per second (and that includes work for stuff unrelated to indexing too on other threads). The GB size of the repro doesn't matter for indexing, just the file/folder count.

i.e. I'm not able to repro the issue with a workspace many times larger (i.e. indexing is under a minute still).

[sean-mcmanus]

@Ma-Ike Also, it's possible the slowness might be caused by the recursive includes feature and not indexing. You could try setting

            "recursiveIncludes": {
                "reduce": "never"
            },

in c_cpp_properties.json to see if that's faster.

[Ma-Ike]

I did update Vs code yesterday with the absolute latest and then I git this:

C/C++:
Version: 1.26.5

C/C++ Extension Pack:
Identifier
ms-vscode.cpptools-extension-pack
Version
1.3.1

C/C++ Themes:
Identifier
ms-vscode.cpptools-themes
Version
2.0.0

Problem is still there!

@sean-mcmanus what will your suggestion do?
#13927 (comment)

[sean-mcmanus]

@Ma-Ike We just released 1.27.7 with a bunch of fixes.

My suggestion could cause it to try to reduce the recursive includes list, it's documented at https://code.visualstudio.com/docs/cpp/customize-cpp-settings#_recursive-includes-properties . There are cases where that can improve performance if what you're seeing as "indexing" is actually the reduction processing (if you attach a debugger to cpptools.exe and break into the debugger we might be able to see what code being processed https://github.com/microsoft/vscode-cpptools/wiki/Attaching-debugger-to-cpptools-or-cpptools%E2%80%90srv ).