fix: fold inner exceptions into IDE test failure output

[thomhurst]

Summary

Fixes #1327. Upstream improvement for the platform itself: microsoft/testfx#11202.

Rider and Visual Studio showed only the outermost exception for a failed test, while dotnet run and dotnet test printed the full chain. JetBrains closed RSRP-499947 as a TUnit-side problem, and that is correct:

• Microsoft.Testing.Platform's server-mode (JSON-RPC) serializer sends a failed node as error.message (Explanation ?? Exception.Message) and error.stacktrace (Exception.StackTrace). It never walks InnerException.
• The console reporter and the dotnet test IPC consumer flatten the chain themselves, so the CLI was never affected.
• TUnit handed MTP the raw exception with an explanation of [Category] outer.Message, so IDE clients only ever saw the outer exception. xUnit (XunitException), MSTest (MSTestTestNodeException) and NUnit (via the VSTest bridge) all fold the chain into Message and StackTrace before handing the exception to MTP.

The TRX report had the same gap in every client mode: TrxExceptionProperty was built from the outermost exception's Message and StackTrace only.

Fix

• New internal FlattenedException (derives from TUnitFailedException). Message is the outer message plus one ---> Type: Message line per inner exception. StackTrace is the outer trace plus each thrown inner exception's trace under a --- Inner exception stack trace (Type) --- separator. Every member of an AggregateException is included.
• Folding rules that keep the output readable: an inner message the enclosing exception already embeds is not repeated (hook wrappers splice the cause into their own message, and Assert.Multiple lists every failure in the outer message before attaching them as an AggregateException, which previously came out three times); an inner AggregateException gets no line of its own; an inner exception that was never thrown gets no stack-trace separator; when the outer trace is a filtered TUnit wrapper trace, the inner traces are filtered the same way instead of reintroducing engine frames below the hint.
• InnerException is null so consumers that walk the chain never print it twice, the original exception stays reachable through WrappedException, and Data is copied so MTP's assert.expected/assert.actual fallback still works. The HTML, GitHub and JUnit reporters already call TUnitFailedException.Unwrap, so their output is unchanged.
• TUnitMessageBus.GetFailureStateProperty applies the wrapper for non-console clients only. A multi-member AggregateException (for example several failing [After] hooks) is reported whole on both branches so every sibling is listed; a single-member one is still reduced to its real cause. Console and dotnet test output for nested exceptions is unchanged (verified manually against NestedExceptionTests in both modes); a detailed console run (--detailed-stacktrace) of a multi-member aggregate now lists every sibling instead of only the first, because MTP's terminal reporter flattens a top-level aggregate itself.
• The timeout explanation, which is what IDE clients receive as error.message, now folds the diagnostic exception's own inner messages in for non-console clients instead of only the immediate one.
• TUnitTestFramework.ReportUnhandledException (discovery-level failures such as a throwing [Before(TestDiscovery)] hook) applies the same fold for non-console clients.
• TestExtensions.ToTestNode builds TrxExceptionProperty through the same FlattenedException.Wrap, so TRX ErrorInfo now carries the whole chain for console and IDE runs alike. In console runs the state property holds the TestFailedException wrapper, whose InnerException is only the first aggregate member, so the fold reads the chain from TUnitFailedException.WrappedException while keeping the wrapper's filtered message and stack trace in front.
• The console/IDE client-id check that TUnitMessageBus and VerbosityService each had a copy of now lives in ClientInfoExtensions.IsConsoleClient.
• TUnitFailedException gains an internal constructor for the pre-computed message and stack trace. No public API change.

Known trade-off, shared with xUnit and MSTest: in IDE sessions the state property's exception is the wrapper type, so an MTP consumer that reads exception.GetType() without TUnitFailedException.Unwrap (for example the OpenTelemetry result handler) reports FlattenedException for nested failures.

Server-mode payload for NestedExceptionTests before:

error.message:    [Test Failure] Thrown from Method1
error.stacktrace: <outer frames only>

After:

error.message:    [Test Failure] Thrown from Method1
                   ---> System.ArgumentException: Thrown from Method2
                   ---> System.InvalidOperationException: Thrown from Method3
error.stacktrace: <outer frames>
                  --- Inner exception stack trace (System.ArgumentException) ---
                  <Method2 frames>
                  --- Inner exception stack trace (System.InvalidOperationException) ---
                  <Method3 frames>

Tests

• tests/TUnit.Engine.Tests/FlattenedExceptionTests.cs: 18 unit tests covering message and stack-trace folding, frame ordering, AggregateException siblings (root and inner), the Assert.Multiple shape, embedded inner messages, inner exceptions without a stack trace, inner-trace filtering behind the console wrapper, Data copying, the already-flattened no-op, Unwrap and ToString.
• tests/TUnit.RpcTests/Tests.cs: three server-mode regressions on net8.0 and net10.0. RunTests_WithNestedException_ReportsInnerExceptionsToIdeClients asserts error.message and error.stacktrace contain every inner exception (on main the payload contains only the outer exception). RunTests_WithAggregateException_ReportsEverySiblingToIdeClients asserts both members of a thrown AggregateException and their inner chains are reported. RunTests_WithTimeoutDiagnosticChain_ReportsEveryDiagnosticMessageToIdeClients asserts a timed-out test's nested diagnostic messages reach error.message.
• tests/TUnit.TestProject/Bugs/_1327/IdeExceptionReportingTests.cs: the aggregate and nested-timeout fixtures those RPC tests drive.
• tests/TUnit.Engine.Tests/NestedExceptionTrxTests.cs: runs NestedExceptionTests and the aggregate fixture through the console host with --report-trx (the aggregate both with and without --detailed-stacktrace) and asserts the TRX ErrorInfo message and stack trace contain every inner exception, every aggregate member, and the separators in outer-to-inner order.
• Existing StackTraceFilterTests, GitHubReporterTests, Issue6688Tests, DefaultTimeoutClassificationTests and DataSourceExceptionPropagationTests still pass.

https://claude.ai/code/session_01LvWBihkYCSQiPDXcTVatEk

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📝 Walkthrough

Walkthrough

The change adds FlattenedException to combine nested and aggregate exception details. TUnitMessageBus uses it for IDE clients while preserving console behavior. TRX output and regression tests verify complete messages and stack traces.

Changes

Exception reporting

Layer / File(s)	Summary
Flatten exception chains src/TUnit.Engine/Exceptions/FlattenedException.cs, src/TUnit.Engine/Exceptions/TUnitFailedException.cs, tests/TUnit.Engine.Tests/FlattenedExceptionTests.cs	FlattenedException combines messages and stack traces from nested and aggregate exceptions. Tests cover wrapping, ordering, aggregate members, missing stack traces, and idempotency.
Route exceptions to IDE clients src/TUnit.Engine/TUnitMessageBus.cs, tests/TUnit.TestProject/Bugs/_1327/IdeExceptionReportingTests.cs, tests/TUnit.RpcTests/Tests.cs	TUnitMessageBus preserves complete aggregate and timeout diagnostics for non-console clients. Regression tests cover aggregate failures and nested timeout diagnostics.
Include complete details in TRX output src/TUnit.Engine/Extensions/TestExtensions.cs, tests/TUnit.Engine.Tests/NestedExceptionTrxTests.cs	TRX exception properties now contain combined messages and stack traces. Tests cover nested exceptions, aggregate members, and detailed stack traces.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant TestExecution
  participant TUnitMessageBus
  participant FlattenedException
  participant IDEClient
  TestExecution->>TUnitMessageBus: report test failure
  TUnitMessageBus->>FlattenedException: wrap exception chain
  FlattenedException-->>TUnitMessageBus: return combined details
  TUnitMessageBus->>IDEClient: serialize exception details

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Merge Risk: ⚪ Minimal · up to 045a3

Exception reporting now retains nested and aggregate failure details for IDE and TRX consumers without an identified regression in console handling.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The production changes and regression tests directly implement issue #1327. The BOM removal in tests/TUnit.RpcTests/Tests.cs has no connection to exception reporting and is an unrelated cleanup chan…	Revert the BOM-only change in tests/TUnit.RpcTests/Tests.cs, or document a separate issue that requires this file normalization.
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.45% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 44 functions across 8 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue #1327 requires inner exceptions in test-failure output. FlattenedException combines nested messages and stack traces, including all AggregateException members. IDE payloads and TRX output us…
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: including inner exceptions in IDE test failure output.

Full details: Out of Scope Changes check

Explanation

The production changes and regression tests directly implement issue #1327. The BOM removal in tests/TUnit.RpcTests/Tests.cs has no connection to exception reporting and is an unrelated cleanup change.

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Inner causes join the flow
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[greptile-apps]

Greptile Summary

This PR folds nested exception details into the message and stack trace supplied to IDE and TRX consumers that do not traverse InnerException.

• Adds FlattenedException to preserve complete nested and aggregate failure details.
• Applies folding to IDE failures, timeout diagnostics, discovery-level failures, and TRX output.
• Preserves assertion metadata and avoids duplicate output from known embedding wrappers.
• Centralizes console-client detection.
• Adds unit, RPC, and TRX regression coverage for nested exceptions and aggregate siblings.

Confidence Score: 5/5

The PR appears safe to merge; the earlier aggregate, timeout, substring-suppression, and duplicate-message findings are resolved in the current code.

No new actionable failures or repository-rule violations remain, and every previous review thread was resolved with the corresponding behavior now covered by focused regression tests.

Important Files Changed

Filename	Overview
src/TUnit.Engine/Exceptions/FlattenedException.cs	Implements exception-chain folding, aggregate traversal, duplicate suppression, stack-trace filtering, and assertion metadata preservation.
src/TUnit.Engine/TUnitMessageBus.cs	Selects console versus IDE exception reporting and preserves aggregate siblings and timeout diagnostics.
src/TUnit.Engine/Extensions/TestExtensions.cs	Folds nested exception details into TRX ErrorInfo fields.
src/TUnit.Engine/Framework/TUnitTestFramework.cs	Applies equivalent folding to non-console discovery-level failures.
tests/TUnit.Engine.Tests/FlattenedExceptionTests.cs	Covers nested chains, aggregates, wrapper embedding, stack traces, metadata, unwrapping, and formatting.
tests/TUnit.RpcTests/Tests.cs	Verifies complete nested, aggregate, and timeout diagnostics in server-mode payloads.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Test or discovery exception] --> B{Console client?}
  B -->|Yes| C[Keep structured exception chain]
  B -->|No| D[Flatten inner messages and stack traces]
  C --> E[MTP console reporting]
  D --> F[IDE JSON-RPC failure payload]
  C --> G[TRX conversion]
  D --> G
  G --> H[Fold chain into TRX ErrorInfo]

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Inline comments:
In `@src/TUnit.Engine/TUnitMessageBus.cs`:
- Line 156: Update the non-console assignment in the message handling flow to
pass the original exception variable e to FlattenedException.Wrap, while
retaining unwrapped for categorization and console output. Add an RPC regression
case covering an AggregateException with multiple inner members and verify the
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📒 Files selected for processing (5)

• src/TUnit.Engine/Exceptions/FlattenedException.cs
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[github-actions]

Code Review

Solid design overall: FlattenedException cleanly folds a chain into Message/StackTrace for MTP's server-mode serializer while leaving InnerException null to avoid double-printing for consumers that do walk the chain, and console output is correctly left untouched. Good regression test coverage for the happy path in FlattenedExceptionTests.cs.

Two issues remain outstanding — both already flagged by CodeRabbit's automated review but not yet addressed in the latest commit (af24fd4):

1. Aggregate sibling exceptions are still dropped for IDE clients (bug)

src/TUnit.Engine/TUnitMessageBus.cs:141-154:

var unwrapped = e is AggregateException { InnerExceptions.Count: > 0 } agg
    ? agg.InnerExceptions[0]
    : e;
...
var reported = IsConsole ? unwrapped : FlattenedException.Wrap(unwrapped);

unwrapped collapses an AggregateException down to InnerExceptions[0] before FlattenedException.Wrap ever sees it. FlattenedException.GetInnerExceptions has special-case handling for AggregateException.InnerExceptions, but that branch is now unreachable from this call site: Wrap only ever receives a single, already-unwrapped exception, so it falls into the plain InnerException chain-walk instead.

Concretely: two [After(Test)] hooks that both throw get bundled by TestCoordinator into an AggregateException with 2+ InnerExceptions. Through this path, only the first hook's exception (and its own InnerException chain) reaches Rider/VS - the second hook's failure is silently dropped, even though the PR's stated goal (and FlattenedExceptionTests, which exercises Wrap directly on an AggregateException) implies all siblings should show up.

Suggested fix: pass e (or the original aggregate) into FlattenedException.Wrap when it's an AggregateException, rather than pre-selecting InnerExceptions[0], so the existing sibling-handling logic in FlattenedException actually runs. The unwrapped single-exception value can still be kept for categorization (FailureCategorizer.Categorize) if that logic depends on a single exception.

2. Unrelated BOM change

tests/TUnit.RpcTests/Tests.cs gets a UTF-8 byte-order mark prepended as a side effect of editing line 1 (the using statement). This is unrelated to the exception-flattening change and just adds diff noise for the file's next editor. Worth stripping the BOM before merge (re-save as UTF-8 without BOM, or use an editor/format-on-save that preserves the original encoding).

No other concerns - the TUnitFailedException constructor addition and the console/non-console branching look correct.

[github-actions]

Review

Nice, focused fix. FlattenedException is a clean way to fold the chain into Message/StackTrace for MTP's server-mode serializer while keeping InnerException null (so chain-walking consumers don't double-print) and the original exception reachable via WrappedException/Unwrap. Gating on IsConsole reuses the existing client-detection pattern from VerbosityService, and the TRX fix in TestExtensions.ToTestNode correctly applies folding unconditionally since TRX has no chain walker at all. Test coverage for the happy path (FlattenedExceptionTests) is thorough.

Two issues were already raised by earlier automated reviews on this PR and are still present in the current head (af24fd4/d3e2b18):

1. AggregateException siblings are still dropped before FlattenedException ever sees them

src/TUnit.Engine/TUnitMessageBus.cs:144-156:

var unwrapped = e is AggregateException { InnerExceptions.Count: > 0 } agg
    ? agg.InnerExceptions[0]
    : e;
...
var reported = IsConsole ? unwrapped : FlattenedException.Wrap(unwrapped);

unwrapped collapses a top-level AggregateException down to InnerExceptions[0] (pre-existing behavior) before Wrap is called, so FlattenedException.GetInnerExceptions's aggregate-sibling handling is unreachable from this call site — Wrap only ever sees a single already-unwrapped exception and falls into the plain InnerException walk. FlattenedExceptionTests.AggregateException_IncludesEverySiblingAndTheirInnerChains exercises Wrap directly with a raw AggregateException, which is why it passes despite this gap — it doesn't reflect what production actually calls.

This is now wider than just IDE output: since TestExtensions.ToTestNode builds TrxExceptionProperty from this same reported/unwrapped value (stateProperty's exception), a TRX report for a test that fails via AggregateException (e.g. two [After(Test)] hooks each throwing, or Task.WhenAll failures) will also only ever record the first sibling — undermining the "TRX now carries the whole chain" half of this PR's own stated goal, not just the Rider/VS path.

Suggested fix: pass e (the original, possibly-aggregate exception) into FlattenedException.Wrap, and keep unwrapped only where a single exception is needed (FailureCategorizer.Categorize). This lets FlattenedException's existing AggregateException.InnerExceptions branch actually run for real failures.

2. Stray UTF-8 BOM in tests/TUnit.RpcTests/Tests.cs

The using TUnit.RpcTests.Models; line at the top of the file picked up a UTF-8 BOM (EF BB BF) as a side effect of the edit, unrelated to the exception-flattening change. Worth stripping before merge to avoid unnecessary diff noise for the next editor of that file.

3. (Minor, new) Timeout explanation still only surfaces one level of nesting

src/TUnit.Engine/TUnitMessageBus.cs:157-171: for FailureCategory.Assertion/Error, the explanation passed to FailedTestNodeStateProperty/ErrorTestNodeStateProperty is built from reported.Message (the fully folded chain). For FailureCategory.Timeout, though, the explanation is built separately from diagnosticException.Message, where diagnosticException = unwrapped.InnerException — only one hop, not the flattened chain, and not derived from reported at all. Since MTP's server-mode serializer prefers Explanation over Exception.Message for error.message when an explanation is present, an IDE user hitting a timeout with a multi-level nested cause will still only see the immediate cause's message, unlike assertion/error failures which now get the full chain. Consider building the timeout explanation from reported.Message (or otherwise folding the chain) for consistency with the rest of this fix.

Items 1 and 2 have already been flagged twice by other automated reviewers on this PR without a response in the latest commits — worth confirming whether that's an intentional follow-up or just missed.

[thomhurst]

Addressed the review findings in 49cd25b:

• Aggregate siblings dropped (greptile, Codex, CodeRabbit, review bot): GetFailureStateProperty now folds a multi-member AggregateException whole for non-console clients, so every sibling and its inner chain reaches the IDE. A single-member aggregate is still reduced to its real cause so error.message does not start with "One or more errors occurred". New fixture IdeExceptionReportingTests.AggregateFailures plus RPC regression RunTests_WithAggregateException_ReportsEverySiblingToIdeClients (net8.0 and net10.0).
• Timeout explanation one level deep (greptile, Codex): for non-console clients the diagnostic part of the timeout explanation is now FlattenedException.CombineMessages(diagnosticException), so nested diagnostic messages reach error.message. Console output is unchanged because the terminal reporter walks the chain itself. New fixture IdeExceptionReportingTests.Timeout_With_Nested_Diagnostic plus RPC regression RunTests_WithTimeoutDiagnosticChain_ReportsEveryDiagnosticMessageToIdeClients.
• Stray BOM in tests/TUnit.RpcTests/Tests.cs (review bot): removed.

Console-mode aggregate reporting is unchanged by design: SimplifyStacktrace wraps the aggregate in TestFailedException before this method runs, and MTP's terminal reporter prints the chain from there.

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[github-actions]

Code review

Re-reviewed at head 49cd25b against the three issues raised by earlier automated reviews (Codex, CodeRabbit, Greptile, and a prior review-bot pass) — all three are now fixed:

1. Aggregate siblings dropped — TUnitMessageBus.GetFailureStateProperty now passes the original AggregateException into FlattenedException.Wrap when it has more than one member (src/TUnit.Engine/TUnitMessageBus.cs:158), instead of pre-reducing to InnerExceptions[0] before wrapping. FlattenedException.GetInnerExceptions's AggregateException branch is reachable again, so every sibling (and each sibling's own inner chain) reaches IDE clients and TRX output. Verified the depth-first ordering in FlattenedException.AppendInnerMessages/AppendInnerStackTraces matches what RunTests_WithAggregateException_ReportsEverySiblingToIdeClients asserts.
2. Timeout explanation only one level deep — the diagnostic message is now built via FlattenedException.CombineMessages(diagnosticException) for non-console clients (TUnitMessageBus.cs:171), so nested diagnostic causes are folded in rather than just diagnosticException.Message.
3. Stray UTF-8 BOM in tests/TUnit.RpcTests/Tests.cs — confirmed removed (file starts with using, no BOM bytes).

Design holds up on a closer read:

• FlattenedException.Wrap's "no inner exception → return the same instance" fast path, combined with CombineMessages/CombineStackTraces short-circuiting on InnerException is null, is what makes reusing the same folding helpers safe on both the raw chain (console path in TestExtensions.ToTestNode) and an already-flattened exception (IDE path) — the second call is genuinely a no-op rather than double-folding.
• TUnitFailedException's new internal ctor correctly leaves StackTrace unfiltered (no FilterStackTrace call), which matches existing behavior for non-console/IDE consumers — filtering has only ever applied to the console path via TestFailedException, so this isn't a regression.
• Console-mode aggregate reporting is intentionally left as first-sibling-only per the author's note, and FailureCategorizer.Categorize still classifies off the single unwrapped exception, which is reasonable since a category label is inherently singular.

No new issues found. Good test coverage across the three layers (FlattenedExceptionTests, NestedExceptionTrxTests, and the three new TUnit.RpcTests server-mode regressions), and the new TUnit.TestProject fixtures reuse the existing NestedExceptionTests/EngineTest(ExpectedResult.Failure) conventions correctly.

[thomhurst]

Upstream improvement raised so the platform flattens the chain itself in server mode: microsoft/testfx#11202. This PR stays necessary for every Microsoft.Testing.Platform version shipped so far.

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Actionable comments posted: 1

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Inline comments:
In `@tests/TUnit.RpcTests/Tests.cs`:
- Line 231: Update the aggregate test selector in the test discovery query to
match the generated method format by replacing the ".AggregateFailures."
fragment check with ".AggregateFailures(". Preserve the existing
IdeExceptionReportingTests filter and all other selector behavior.

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• src/TUnit.Engine/Exceptions/FlattenedException.cs
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• tests/TUnit.Engine.Tests/FlattenedExceptionTests.cs
• tests/TUnit.RpcTests/Tests.cs
• tests/TUnit.TestProject/Bugs/_1327/IdeExceptionReportingTests.cs

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[thomhurst]

Addressed in 23bbd67:

• Console-mode TRX dropped aggregate siblings (Codex P2): correct. SimplifyStacktrace hands the TRX path a TestFailedException whose InnerException is only the first aggregate member. FlattenedException now folds from TUnitFailedException.WrappedException (the original aggregate) while keeping the wrapper's filtered message and stack trace in front; a FlattenedException is excluded so an already folded exception stays a no-op. New unit tests ConsoleWrapperAroundAggregate_FoldsEverySiblingFromWrappedOriginal and AlreadyFlattened_CombineIsNoOp, plus NestedExceptionTrxTests.Trx_ErrorInfo_Includes_Every_Aggregate_Member through the console host.
• RPC selector .AggregateFailures. (CodeRabbit): not a bug, see the inline reply. Parameterless methods carry no parentheses in the uid and the regression passes on both frameworks.