Suppress banner and default stdout/stderr to failed in LLM environments

[Evangelink]

Summary

Extends LLMEnvironmentDetector so that AI agent CLIs (Claude Code, Gemini CLI, GitHub Copilot CLI, Aider, Cursor, etc.) also get quieter output defaults out of the box. This reduces token consumption on every test run an agent triggers, without changing anything for humans or CI.

When an LLM environment is detected (existing detection — see LLMEnvironmentDetector.DetectionRules), the following defaults flip:

Option	Default (human)	Default (LLM)
Banner (--no-banner)	shown	suppressed (both platform + framework banner)
--show-stdout	All	Failed
--show-stderr	All	Failed

ANSI mode is already disabled in LLM environments today; this PR extends the same opt-in mechanism to the two largest contributors of run-time noise.

Any explicit user-provided value still wins (e.g. --show-stdout all overrides the new default).

Why opt-in (instead of #7664's universal change)

Issue #7664 proposes changing --show-stdout / --show-stderr defaults to Failed for everyone, which is a breaking change for human and CI workflows. This PR takes the LLM-opt-in subset:

• No behavior change for humans or CI.
• Agents win immediately, without waiting for a major version.
• Easy escape hatch: unset the LLM env var (or pass an explicit value).
• Compatible with [MTP][Breaking] Change the default of --show-stdout and --show-stderr to failed. #7664 if/when it ships — the LLM defaults would simply become a no-op.

Implementation notes

• LLMEnvironmentDetector refactored from static to instance with an injected IEnvironment. Detection rules remain static. This makes the new behaviors testable via env-var injection.
• TerminalOutputDevice hoists isLLMEnvironment once next to the existing inCI lookup and threads it into GetShowOutputMode.
• DisplayBannerIfEnabledAsync short-circuits when LLM is detected (mirrors existing --no-banner / DOTNET_NOLOGO behavior; suppresses both platform and framework banners).
• Test infrastructure (WellKnownEnvironmentVariables.ToSkipEnvironmentVariables) broadly filters the 23 LLM env vars when spawning child processes, so existing tests are not affected by the parent agent host.
• Help-text wording for --show-stdout / --show-stderr was intentionally not changed (still says Default is 'All'.) to keep this PR tight and avoid 13-XLF churn. Defer to a wording-only follow-up.

Tests

New acceptance tests (all green locally on net8.0 / net10.0 / net462):

• NoBannerTests.UsingLLMEnvironmentVar_TheBannerDoesNotAppear — sets CLAUDECODE=1, asserts banner is suppressed.
• ShowOutputOptionTests.ShowStdout_DefaultInLLMEnvironment_ShowsStandardOutputOnlyForFailedTests
• ShowOutputOptionTests.ShowStderr_DefaultInLLMEnvironment_ShowsErrorOutputOnlyForFailedTests
• ShowOutputOptionTests.ShowStdout_All_InLLMEnvironment_StillShowsAllStandardOutput (explicit override still wins; stderr is symmetric and covered by impl)

Existing NoBannerTests, AnsiOptionTests, HelpInfoTests, HelpInfoAllExtensionsTests, and the full ShowOutputOptionTests suite still pass.

Related: #7664

[Copilot]

Pull request overview

This PR extends Microsoft.Testing.Platform’s “LLM environment” detection so that known AI-agent CLI shells automatically get quieter default console output (banner suppressed; --show-stdout/--show-stderr default to Failed), while preserving existing behavior for humans/CI and ensuring explicit CLI arguments still take precedence.

Changes:

• Refactors LLMEnvironmentDetector to be instance-based with an injected IEnvironment, enabling deterministic testing.
• Updates console defaults in LLM environments: suppress banner and default stdout/stderr output to Failed when the option is not explicitly provided.
• Hardens acceptance test isolation by filtering LLM-related environment variables from spawned processes, and adds new acceptance tests validating the LLM-default behaviors.

Show a summary per file

File	Description
test/Utilities/Microsoft.Testing.TestInfrastructure/WellKnownEnvironmentVariables.cs	Adds an LLM env-var allowlist and filters these vars from child processes so acceptance tests aren’t impacted by an ambient agent shell.
test/IntegrationTests/MSTest.Acceptance.IntegrationTests/ShowOutputOptionTests.cs	Adds acceptance coverage ensuring stdout/stderr defaults flip to Failed in LLM environments and explicit overrides still win.
test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/NoBannerTests.cs	Adds acceptance coverage ensuring the banner is suppressed when an LLM env var is set.
src/Platform/Microsoft.Testing.Platform/OutputDevice/TerminalOutputDevice.cs	Defaults --show-stdout/--show-stderr to Failed when LLM is detected and the option is absent.
src/Platform/Microsoft.Testing.Platform/Hosts/TestHostBuilder.Utilities.cs	Skips displaying the banner when LLM environment is detected.
src/Platform/Microsoft.Testing.Platform/Helpers/LLMEnvironmentDetector.cs	Refactors detector to use injected IEnvironment and supports reuse for user-facing defaults.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 1

[Evangelink]

Review Summary

#	Dimension	Result
1	Algorithmic Correctness	✅ LGTM — detection logic, banner suppression, and GetShowOutputMode fallback are all correct. Explicit CLI values correctly take precedence.
2	Threading & Concurrency	✅ LGTM — DetectionRules is static readonly; per-call instance creation avoids shared mutable state.
3	Security & IPC	✅ LGTM — env-var reads only; no injection surface.
4	Public API Compatibility	✅ LGTM — LLMEnvironmentDetector is internal; WellKnownEnvironmentVariables is in a test-utilities project with no PublicAPI analyzer.
5	Performance	⚠️ MODERATE — IsLLMEnvironment() → GetLLMEnvironment() collects all matches into a string?[] and joins them, only to check is not null. The old code cached the result in a static property; the new instance design re-evaluates all 19+ rules on every call (at least twice per test startup). See inline comment on LLMEnvironmentDetector.cs:70.
6	Cross-TFM Compatibility	✅ LGTM — only standard IEnvironment and LINQ used.
7	Resource Management	✅ LGTM — no disposables introduced.
8	Defensive Coding	✅ LGTM — null guards on constructor parameters.
9	Localization & Resources	✅ LGTM — no new user-facing strings added to resources.
10	Test Isolation	✅ LGTM — ToSkipEnvironmentVariables spreads LLMEnvironmentVariables so acceptance tests are isolated from ambient agent shells; new tests set the env var explicitly.
11	Assertion Quality	✅ LGTM — AssertOutputContains / AssertOutputDoesNotContain are correct for the behavior under test.
12	Flakiness Patterns	✅ LGTM — no timing-dependent code.
13	Test Completeness	✅ LGTM — banner, stdout default, stderr default, and explicit-override tests are all covered. Stdout and stderr coverage is symmetric.
14	Data-Driven Coverage	✅ LGTM — AllForDynamicData covers all TFMs.
15	Code Structure	✅ LGTM — clear guard-clause structure.
16	Naming & Conventions	✅ LGTM.
17	Documentation Accuracy	i️ NIT — --show-stdout/--show-stderr help text still says "Default is 'All'." which is wrong in LLM environments. Intentionally deferred but should be tracked. See inline comment on TerminalOutputDevice.cs:241.
18	Analyzer & Code Fix Quality	N/A — no src/Analyzers/ changes.
19	IPC Wire Compatibility	N/A — no serialized types changed.
20	Build Infrastructure	✅ LGTM — no new dependencies.
21	Scope & PR Discipline	✅ LGTM — OR_APP_NAME is stripped entirely from child-process env even though LLM detection only triggers for specific values (Aider, plandex, OpenHands); this is acceptable since the variable is tool-specific, and enumerating every possible value is not feasible.

19 dimensions clean · 1 moderate · 1 nit · 0 blocking

The PR is well-structured and the core logic is correct. The one actionable item before merge is the performance concern: collapsing IsLLMEnvironment() to a short-circuiting Any(...) call (or adding a per-instance cached field) avoids unnecessary allocations and re-evaluation. The help-text gap should be tracked in a follow-up issue with a TODO(#XXXX) comment per the repo's TODO policy.

Generated by Expert Code Review (on open) for issue #8771 · sonnet46 2.8M

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2