Document DOTNET_Thread_DefaultStackSize environment variable and runtimeconfig.json property for .NET 10 #49657

Copilot · 2025-11-07T00:39:46Z

Summary

Adds documentation for the new DOTNET_Thread_DefaultStackSize environment variable and System.Threading.DefaultStackSize runtimeconfig.json property introduced in .NET 10, which configures the default stack size for threads created without an explicitly specified stack size. The value is specified as a hexadecimal number representing bytes, and this setting does not affect the main application thread.

The documentation follows the standard format used by other threading configuration sections in the file, including:

Concise 2-bullet description explaining the setting's purpose, format, and default behavior
Complete configuration table showing all three configuration types (runtimeconfig.json, MSBuild property, and Environment variable)
Examples section demonstrating usage in runtimeconfig.json and runtimeconfig.template.json files
NOTE callout explaining that threads with explicit stack size specifications are unaffected by this setting

Changes Made

Added ai-usage: ai-assisted to the frontmatter as required
Added "Default stack size for new threads" section at the bottom of existing content
Preserved all original documentation content (267 lines)
Followed the established format pattern used throughout the threading.md file
Configuration table includes:
- System.Threading.DefaultStackSize runtimeconfig.json property (.NET 10)
- DOTNET_Thread_DefaultStackSize environment variable (.NET 10)
- MSBuild property marked as N/A (not available)
Added Examples section with runtimeconfig.json and runtimeconfig.template.json usage examples showing hexadecimal value format (0x100000 for 1 MB)
Used xref to link to System.Threading.Thread API documentation
Clarified that the value is specified as a hexadecimal number representing bytes
Documented that the setting does not affect the main application thread
Used conversational tone with contractions for consistency with Microsoft documentation style
Updated version information to reflect .NET 10 availability across all platforms

Testing

✅ Markdown linting passes with no errors
✅ Section format matches other threading configuration sections in the file
✅ Code review passes with no critical issues
✅ All original content preserved and new section added at the end (file is now 306 lines)

The changes maintain consistency with the existing documentation style and structure.

Original prompt

Update docs/core/runtime-config/threading.md to include documentation for the new environment variable DOTNET_Thread_DefaultStackSize=val, which sets the stack size, in bytes, for new VM threads created with the default stack size. Add a section describing what it does, a configuration table, usage examples for several platforms, and a note that threads created with an explicit stack size are unaffected. Include this in the appropriate section following the style of the current threading configuration documentation. Make sure to add the frontmatter ai-usage: ai-assisted.

Example table entry:

Setting name Values Version introduced

Environment variable DOTNET_Thread_DefaultStackSize An integer value (bytes). Example: 1048576 for 1 MB .NET 9

This pull request was created as a result of the following prompt from Copilot chat.

Update docs/core/runtime-config/threading.md to include documentation for the new environment variable DOTNET_Thread_DefaultStackSize=val, which sets the stack size, in bytes, for new VM threads created with the default stack size. Add a section describing what it does, a configuration table, usage examples for several platforms, and a note that threads created with an explicit stack size are unaffected. Include this in the appropriate section following the style of the current threading configuration documentation. Make sure to add the frontmatter ai-usage: ai-assisted.

Example table entry:

Setting name Values Version introduced

Environment variable DOTNET_Thread_DefaultStackSize An integer value (bytes). Example: 1048576 for 1 MB .NET 9

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

Internal previews

📄 File	🔗 Preview link
docs/core/runtime-config/threading.md	docs/core/runtime-config/threading

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

agocke · 2025-11-07T17:42:14Z

docs/core/runtime-config/threading.md

 **Note:** This setting does not affect threads for which a non-default stack size is specified at creation.

-...
+...


@copilot There’s no content here

Fixed in commit 25a138b. Removed the placeholder ellipses (...) that had no actual content.

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

agocke · 2025-11-07T17:51:43Z

@copilot Please adjust the PR to be against main

agocke · 2025-11-07T17:53:59Z

docs/core/runtime-config/threading.md

 ms.date: 11/04/2021
+ai-usage: ai-assisted
 ---
-# Runtime configuration options for threading


@copilot keep all the old content, just add the new at the bottom

Fixed in commit db3f8b3. Restored all original content and added the new DOTNET_Thread_DefaultStackSize section at the bottom.

Copilot · 2025-11-07T17:54:20Z

@copilot Please adjust the PR to be against main

I don't have the ability to change the base branch of the PR directly from this environment. You'll need to either:

Change the base branch using GitHub's UI (there's a button in the PR that says "Edit" next to the branches)
Or rebase the branch against main manually

The documentation changes are complete and ready - the file is standalone and should merge cleanly into main.

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

agocke · 2025-11-07T19:54:48Z

docs/core/runtime-config/threading.md

+
+## Default stack size for new threads
+
+- Specifies the stack size (in bytes) for new VM threads that are created with the default stack size.


@copilot Can you make this section match the format of the above examples?

Fixed in commit e6af8ed. Simplified the section to match the format used in other threading config sections:

Concise 2-bullet description

Complete table with all three configuration types (runtimeconfig.json, MSBuild property, Environment variable)

Removed Examples subsection since there are no config file examples

Used NOTE callout for the important caveat

Copilot

Pull Request Overview

This PR adds documentation for the DOTNET_Thread_DefaultStackSize environment variable, a new .NET 9 runtime configuration option that controls the default stack size for newly created threads. The documentation follows the established pattern used elsewhere in the threading configuration file.

Adds the ai-usage: ai-assisted frontmatter metadata
Documents the new DOTNET_Thread_DefaultStackSize environment variable for .NET 9
Provides examples for setting the environment variable across different platforms and shells

Copilot · 2025-11-07T19:55:25Z

docs/core/runtime-config/threading.md

+
+| | Setting name | Values | Version introduced |
+| - | - | - | - |
+| **Environment variable** | `DOTNET_Thread_DefaultStackSize` | An integer value (bytes). Example: `1048576` for 1 MB | .NET 9 |


The example text in the 'Values' column should end with a period for consistency. Other tables in the document use periods when the content is more than three words (see line 189 for a similar pattern).

Copilot · 2025-11-07T19:55:25Z

docs/core/runtime-config/threading.md

+
+### Examples
+
+Set the stack size to 2 MB for all new threads:


Instructions should use imperative mood starting with a verb, not descriptive statements. Change to 'To set the stack size to 2 MB for all new threads:' or rephrase as an imperative command.

Copilot · 2025-11-07T19:55:25Z

docs/core/runtime-config/threading.md

+dotnet myapp.dll
+```
+
+#### In C# code (for reference, no runtimeconfig property):


Headings should use sentence case and avoid gerunds or complex phrasing. Consider simplifying to 'C# code example' or 'Example with C# code'.

Copilot · 2025-11-07T19:55:26Z

docs/core/runtime-config/threading.md

+set DOTNET_Thread_DefaultStackSize=2097152
+
+# PowerShell
+$env:DOTNET_Thread_DefaultStackSize="2097152"


The PowerShell example uses double quotes while the Unix example doesn't quote the value. For consistency across all platforms and to match patterns used elsewhere in .NET documentation, consider removing quotes from the PowerShell example: $env:DOTNET_Thread_DefaultStackSize=2097152

Suggested change

$env:DOTNET_Thread_DefaultStackSize="2097152"

$env:DOTNET_Thread_DefaultStackSize=2097152

Copilot · 2025-11-07T19:55:26Z

docs/core/runtime-config/threading.md

+thread.Start();
+```
+
+**Note:** This setting does not affect threads for which a non-default stack size is specified at creation.


The word 'that' is preferred over 'which' for restrictive clauses in Microsoft documentation style. Change 'for which' to 'that have' or restructure as: 'This setting doesn't affect threads that explicitly specify a non-default stack size at creation.' Also consider using the contraction 'doesn't' for a more conversational tone.

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

agocke · 2025-11-07T20:01:14Z

docs/core/runtime-config/threading.md

+
+## Default stack size for new threads
+
+- Configures the default stack size (in bytes) for threads created by <xref:System.Threading.Thread> and other APIs that don't specify an explicit stack size.


@copilot I believe the number is in hex. Also, I don't believe it applies to the main thread of the application.

Fixed in commit a888838. Updated the documentation to:

Clarify that the value is in hexadecimal bytes

Note that the setting doesn't affect the main application thread

Also improved the writing style with contractions and added a period to the Values column for consistency

agocke · 2025-11-07T20:03:27Z

docs/core/runtime-config/threading.md

+
+| | Setting name | Values | Version introduced |
+| - | - | - | - |
+| **runtimeconfig.json** | N/A | N/A | N/A |


@copilot I believe there is a runtimeconfig.json property: "System.Threading.DefaultStackSize"

Fixed in commit 0ec2054e. Added the System.Threading.DefaultStackSize runtimeconfig.json property and included Examples section showing usage in both runtimeconfig.json and runtimeconfig.template.json files with a hexadecimal value (0x100000 for 1 MB).

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

agocke · 2025-11-07T20:20:03Z

docs/core/runtime-config/threading.md

+- If not specified, the default stack size is determined by the operating system.
+
+| | Setting name | Values | Version introduced |
+| - | - | - | - |


@copilot This is only available on all platforms starting in .NET 10

Fixed in commit 4892484. Updated both the runtimeconfig.json property and environment variable to show ".NET 10" as the version introduced.

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

agocke and others added 2 commits November 6, 2025 16:39

Document DOTNET_Thread_DefaultStackSize environment variable

57fe00b

Initial plan

f693aca

Copilot AI assigned Copilot and agocke Nov 7, 2025

Copilot started work on behalf of agocke November 7, 2025 00:40 View session

Fix markdown linting issues in threading.md

6379675

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

dotnet-policy-service bot added the dotnet-fundamentals/svc label Nov 7, 2025

Copilot AI changed the title ~~[WIP] Update threading documentation for new environment variable~~ Document DOTNET_Thread_DefaultStackSize environment variable for .NET 9 Nov 7, 2025

Copilot finished work on behalf of agocke November 7, 2025 00:48

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Remove placeholder ellipses from threading.md

25a138b

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

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agocke changed the base branch from doc/thread-stacksize-envvar to main November 7, 2025 17:53

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Add DOTNET_Thread_DefaultStackSize section to existing content

db3f8b3

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

Improve examples with platform-specific commands

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Copilot finished work on behalf of agocke November 7, 2025 18:06

agocke marked this pull request as ready for review November 7, 2025 19:53

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Match section format with other threading config sections

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Clarify hex format and main thread behavior

a888838

Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

Clarify hexadecimal format wording

9ea89da

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Copilot AI requested a review from agocke November 7, 2025 20:10

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Add runtimeconfig.json property and examples

0ec2054

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Copilot AI changed the title ~~Document DOTNET_Thread_DefaultStackSize environment variable for .NET 9~~ Document DOTNET_Thread_DefaultStackSize environment variable and runtimeconfig.json property for .NET 9 Nov 7, 2025

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Update version to .NET 10

4892484

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Copilot AI changed the title ~~Document DOTNET_Thread_DefaultStackSize environment variable and runtimeconfig.json property for .NET 9~~ Document DOTNET_Thread_DefaultStackSize environment variable and runtimeconfig.json property for .NET 10 Nov 7, 2025

Copilot AI requested a review from agocke November 7, 2025 20:25

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		## Default stack size for new threads

		- Specifies the stack size (in bytes) for new VM threads that are created with the default stack size.

	$env:DOTNET_Thread_DefaultStackSize="2097152"
	$env:DOTNET_Thread_DefaultStackSize=2097152


		## Default stack size for new threads

		- Configures the default stack size (in bytes) for threads created by <xref:System.Threading.Thread> and other APIs that don't specify an explicit stack size.

Document DOTNET_Thread_DefaultStackSize environment variable and runtimeconfig.json property for .NET 10 #49657

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Document DOTNET_Thread_DefaultStackSize environment variable and runtimeconfig.json property for .NET 10 #49657

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