Feature #21360: Inconsistent Support for `Exception#cause` in `Fiber#raise` and `Thread#raise`

• Status: Closed
• Assignee: ioquatix (Samuel Williams)
• Target version: -

[ioquatix (Samuel Williams)]

The raise method supports setting the cause of an exception using the cause: keyword, but this behavior does not work as expected when calling Fiber#raise or Thread#raise, resulting in a TypeError. This breaks consistency with Kernel#raise and makes it difficult to attach causal chains to exceptions raised from other execution contexts.

The Problem

The following code behaves correctly when using Kernel#raise, correctly setting the cause:

cause = RuntimeError.new("cause")

begin
  raise RuntimeError, "boom", cause: cause
rescue => error
  pp error: error, cause: error.cause
end

Produces:

{error: #<RuntimeError: boom>, cause: #<RuntimeError: cause>}

However, using Fiber.current.raise or Thread.current.raise with the same arguments produces a TypeError:

begin
  Fiber.current.raise RuntimeError, "boom", cause: cause
rescue => error
  pp error: error, cause: error.cause
end

Results in:

{error: #<TypeError: backtrace must be an Array of String or an Array of Thread::Backtrace::Location>, cause: nil}

This occurs because the third argument is incorrectly interpreted as a backtrace, not as keyword arguments. A similar issue occurs with Thread#raise.

Proposed Solution

Update Fiber#raise and Thread#raise to accept and correctly interpret keyword arguments, including cause:, in the same manner as Kernel#raise. This would restore consistency across all raise implementations and allow causal exception chaining regardless of context. In other words, Fiber#raise and Thread#raise would be defined as the same interface as Kernel#raise.

[ioquatix (Samuel Williams)]

• Description updated (diff)

[Eregon (Benoit Daloze)]

• Related to Feature #21359: Introduce `Exception#cause=` for Post-Initialization Assignment added

[ioquatix (Samuel Williams)]

• Status changed from Open to Assigned
• Assignee set to ioquatix (Samuel Williams)

@matz (Yukihiro Matsumoto) accepted this change: https://github.com/ruby/dev-meeting-log/blob/master/2025/DevMeeting-2025-07-10.md#bug-21360-inconsistent-support-for-exceptioncause-in-fiberraise-and-threadraise-ioquatix

[ioquatix (Samuel Williams)]

• Tracker changed from Bug to Feature
• Backport deleted (3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN)

I'm changing this to a feature as it's net new code.

• Status changed from Assigned to Closed

Applied in changeset git|64f508ade8c8535b7d3ecdd217886aa52fddd43c.

---

Support cause: in Thread#raise and Fiber#raise. (#13967)

• Add support for cause: argument to Fiber#raise and Thread#raise.

The implementation behaviour is consistent with Kernel#raise and
Exception#initialize methods, allowing the cause: argument to be
passed to Fiber#raise and Thread#raise. This change ensures that
the cause: argument is handled correctly, providing a more consistent
and expected behavior when raising exceptions in fibers and threads.

[Feature #21360]

• Shared specs for Fiber/Thread/Kernel raise.

---

Co-authored-by: Samuel Williams [e-mail]