Gate experimental polyglot languages behind per-language feature flags #14428

sebastienros · 2026-02-10T15:42:50Z

Summary

Add per-language experimental feature flags for Python, Go, Java, and Rust polyglot languages. When polyglotSupportEnabled is true, only C# and TypeScript are available by default. Experimental languages require an additional flag to be enabled.

New Feature Flags

Flag	Language	Default
`experimentalPolyglot:python`	Python	`false`
`experimentalPolyglot:go`	Go	`false`
`experimentalPolyglot:java`	Java	`false`
`experimentalPolyglot:rust`	Rust	`false`

Behavior Matrix

`polyglotSupportEnabled`	Per-language flag	Languages Available
`false` (default)	N/A	C# only
`true`	none	C#, TypeScript
`true`	`experimentalPolyglot:python=true`	C#, TypeScript, Python
`true`	`experimentalPolyglot:go=true`	C#, TypeScript, Go

Per-language flags are additive on top of polyglotSupportEnabled.

Changes

KnownFeatures.cs — Added 4 new feature flag constants and metadata entries
ILanguageDiscovery.cs — Added IsExperimental property to LanguageInfo record
DefaultLanguageDiscovery.cs — Injected IFeatures, filters experimental languages based on per-language flags in all discovery methods
NewCommand.cs / InitCommand.cs — Updated --language option description
Dockerfile.go/java/rust/python — CI polyglot validation enables per-language experimental flags
Extension schemas — Added experimentalPolyglot object with per-language properties
DefaultLanguageDiscoveryTests.cs — Added tests for experimental language filtering

Usage

# Enable polyglot + Python
aspire config set features:polyglotSupportEnabled true --global
aspire config set features:experimentalPolyglot:python true --global

Add per-language experimental feature flags for Python, Go, Java, and Rust. When polyglotSupportEnabled is true, only C# and TypeScript are available by default. Experimental languages require an additional flag: - experimentalPolyglot:python - experimentalPolyglot:go - experimentalPolyglot:java - experimentalPolyglot:rust Changes: - Add IsExperimental property to LanguageInfo record - Add 4 new feature flags to KnownFeatures - Filter experimental languages in DefaultLanguageDiscovery based on flags - Update CI Dockerfiles to enable per-language flags - Update extension JSON schemas with new flag definitions - Update --language option description in NewCommand/InitCommand - Add tests for experimental language filtering

github-actions · 2026-02-10T15:43:01Z

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 14428

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Copilot

Pull request overview

This PR introduces per-language feature flags to gate experimental polyglot language support in Aspire.Cli, so that polyglot can be enabled broadly while still requiring explicit opt-in for specific experimental languages.

Changes:

Added new feature flags (experimentalPolyglot:{python,go,java,rust}) and surfaced them in CLI feature metadata and extension JSON schemas.
Extended language discovery metadata with an IsExperimental marker and filtered experimental languages based on feature flags.
Updated CLI help text, CI polyglot validation Dockerfiles, and added/updated tests to cover experimental language filtering.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
tests/Aspire.Cli.Tests/Projects/DefaultLanguageDiscoveryTests.cs	Updates construction of language discovery and adds tests for experimental language gating.
src/Aspire.Cli/Projects/ILanguageDiscovery.cs	Adds `IsExperimental` to `LanguageInfo` to support gating logic.
src/Aspire.Cli/Projects/DefaultLanguageDiscovery.cs	Implements experimental-language filtering via per-language feature flags.
src/Aspire.Cli/KnownFeatures.cs	Defines new experimental polyglot feature flags and metadata.
src/Aspire.Cli/Commands/NewCommand.cs	Updates `--language` help text to reflect default language options.
src/Aspire.Cli/Commands/InitCommand.cs	Updates `--language` help text to reflect default language options.
extension/schemas/aspire-settings.schema.json	Adds `features.experimentalPolyglot` schema entries for settings validation/intellisense.
extension/schemas/aspire-global-settings.schema.json	Adds `features.experimentalPolyglot` schema entries for global settings validation/intellisense.
.github/workflows/polyglot-validation/Dockerfile.*	Enables the relevant experimental polyglot flag in each language validation container.

Copilot · 2026-02-10T15:53:08Z

src/Aspire.Cli/Projects/DefaultLanguageDiscovery.cs

+        if (s_experimentalFeatureFlags.TryGetValue(language.LanguageId.Value, out var featureFlag))
+        {
+            return features.IsFeatureEnabled(featureFlag, false);
+        }
+
+        return true;


DefaultLanguageDiscovery now filters experimental languages only by the per-language flag, but the feature metadata/docs state these flags require polyglotSupportEnabled. As-is, enabling experimentalPolyglot:* will surface the language even when polyglotSupportEnabled is false. Consider updating IsLanguageEnabled to also require features.IsFeatureEnabled(KnownFeatures.PolyglotSupportEnabled, false) (at least for experimental languages) so runtime behavior matches the documented requirement.

Suggested change

if (s_experimentalFeatureFlags.TryGetValue(language.LanguageId.Value, out var featureFlag))

{

return features.IsFeatureEnabled(featureFlag, false);

}

return true;

// Experimental languages require polyglot support to be enabled globally.

if (!features.IsFeatureEnabled(KnownFeatures.PolyglotSupportEnabled, false))

{

return false;

}

if (s_experimentalFeatureFlags.TryGetValue(language.LanguageId.Value, out var featureFlag))

{

return features.IsFeatureEnabled(featureFlag, false);

}

// If there is no per-language feature flag defined, treat the experimental language as disabled.

return false;

Copilot · 2026-02-10T15:53:08Z

src/Aspire.Cli/Projects/DefaultLanguageDiscovery.cs

-/// - Configuration files (~/.aspire/languages.json)
-/// - NuGet search (Aspire.Hosting.Language.* packages)
-/// - Remote service endpoints
+/// Experimental languages (Go, Java, Rust) are filtered based on per-language feature flags.


The remarks mention experimental languages as “(Go, Java, Rust)”, but this file also marks Python as experimental. Update the remark to include Python (or reword to avoid listing specific languages) so the documentation matches behavior.

Suggested change

/// Experimental languages (Go, Java, Rust) are filtered based on per-language feature flags.

/// Experimental languages are filtered based on per-language feature flags.

Copilot · 2026-02-10T15:53:08Z

tests/Aspire.Cli.Tests/Projects/DefaultLanguageDiscoveryTests.cs

    public async Task GetAvailableLanguagesAsync_ReturnsPythonLanguage()
    {
-        var discovery = new DefaultLanguageDiscovery();
+        var features = new TestFeatures();


These tests enable the per-language experimental flag but do not enable KnownFeatures.PolyglotSupportEnabled. The PR description/feature metadata say experimental polyglot flags require polyglotSupportEnabled, so the tests should set both flags (or the implementation/docs should be adjusted for consistency).

Suggested change

var features = new TestFeatures();

var features = new TestFeatures();

features.SetFeature(KnownFeatures.PolyglotSupportEnabled, true);

Copilot · 2026-02-10T15:53:09Z

tests/Aspire.Cli.Tests/Projects/DefaultLanguageDiscoveryTests.cs

+        var features = new TestFeatures();
+        features.SetFeature(featureFlag, true);
+        var discovery = new DefaultLanguageDiscovery(features);
+


This parametrized test enables experimentalPolyglot:* flags but not polyglotSupportEnabled. If experimental language availability is intended to be additive on top of polyglotSupportEnabled (per PR description), set KnownFeatures.PolyglotSupportEnabled to true in the test setup as well.

Copilot · 2026-02-10T15:53:09Z

tests/Aspire.Cli.Tests/Projects/DefaultLanguageDiscoveryTests.cs

+        var features = new TestFeatures();
+        features.SetFeature(KnownFeatures.ExperimentalPolyglotGo, true);
+        var discovery = new DefaultLanguageDiscovery(features);


This test enables KnownFeatures.ExperimentalPolyglotGo but not KnownFeatures.PolyglotSupportEnabled. To match the documented requirement that experimental polyglot flags require polyglot support, the test should enable both (or update the implementation/docs).

Copilot · 2026-02-10T15:53:09Z

src/Aspire.Cli/Projects/DefaultLanguageDiscovery.cs

+            return features.IsFeatureEnabled(featureFlag, false);
+        }
+
+        return true;


IsLanguageEnabled returns true if an experimental language isn't present in s_experimentalFeatureFlags. That means a future contributor could mark a language as experimental but forget to add the mapping, and it would become enabled by default (opposite of the intended gating). Consider defaulting to false when IsExperimental is true and no feature-flag mapping exists.

Suggested change

return true;

return false;

Copilot · 2026-02-10T15:53:10Z

tests/Aspire.Cli.Tests/Projects/DefaultLanguageDiscoveryTests.cs

+    public void GetLanguageById_ReturnsExperimentalLanguageWhenFlagEnabled()
+    {
+        var features = new TestFeatures();
+        features.SetFeature(KnownFeatures.ExperimentalPolyglotRust, true);


This test enables KnownFeatures.ExperimentalPolyglotRust but not KnownFeatures.PolyglotSupportEnabled. If experimental polyglot languages are meant to be gated by both flags, the test should set polyglotSupportEnabled too (or the docs/implementation should be reconciled).

Suggested change

features.SetFeature(KnownFeatures.ExperimentalPolyglotRust, true);

features.SetFeature(KnownFeatures.ExperimentalPolyglotRust, true);

features.SetFeature(KnownFeatures.PolyglotSupportEnabled, true);

Copilot · 2026-02-10T15:53:10Z

src/Aspire.Cli/Commands/NewCommand.cs

            _languageOption = new Option<string?>("--language", "-l")
            {
-                Description = "The programming language for the AppHost (csharp, typescript, python)"
+                Description = "The programming language for the AppHost (csharp, typescript)"


The --language help text now lists only “csharp, typescript”, but the command can accept additional experimental languages when their feature flags are enabled. Consider tweaking the description to mention that more languages may be available when experimental polyglot flags are turned on, to avoid making --help misleading for users who enable them.

Suggested change

Description = "The programming language for the AppHost (csharp, typescript)"

Description = "The programming language for the AppHost (for example: csharp, typescript; additional experimental languages may be available when polyglot features are enabled)"

Copilot · 2026-02-10T15:53:10Z

src/Aspire.Cli/Commands/InitCommand.cs

            _languageOption = new Option<string?>("--language", "-l")
            {
-                Description = "The programming language for the AppHost (csharp, typescript, python)"
+                Description = "The programming language for the AppHost (csharp, typescript)"


The --language help text now lists only “csharp, typescript”, but init --language can also accept experimental languages when their feature flags are enabled. Consider mentioning the experimental flags in the option description so --help stays accurate for those scenarios.

Suggested change

Description = "The programming language for the AppHost (csharp, typescript)"

Description = "The programming language for the AppHost (csharp, typescript; additional experimental languages may be available when enabled via feature flags)."

github-actions · 2026-02-10T16:06:40Z

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Copilot AI review requested due to automatic review settings February 10, 2026 15:42

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sebastienros requested a review from davidfowl February 10, 2026 16:28

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Gate experimental polyglot languages behind per-language feature flags #14428

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	/// Experimental languages (Go, Java, Rust) are filtered based on per-language feature flags.
	/// Experimental languages are filtered based on per-language feature flags.

	var features = new TestFeatures();
	var features = new TestFeatures();
	features.SetFeature(KnownFeatures.PolyglotSupportEnabled, true);

	features.SetFeature(KnownFeatures.ExperimentalPolyglotRust, true);
	features.SetFeature(KnownFeatures.ExperimentalPolyglotRust, true);
	features.SetFeature(KnownFeatures.PolyglotSupportEnabled, true);

	Description = "The programming language for the AppHost (csharp, typescript)"
	Description = "The programming language for the AppHost (for example: csharp, typescript; additional experimental languages may be available when polyglot features are enabled)"

Gate experimental polyglot languages behind per-language feature flags #14428

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sebastienros commented Feb 10, 2026

Summary

New Feature Flags

Behavior Matrix

Changes

Usage

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