slogt

slogt is a bridge between Go stdlib testing pkg and log/slog.

The problem: when tests execute, your slog output goes directly to stdout, unlike a call to t.Log, which is correlated with your test's execution.

func TestSlog_Ugly(t *testing.T) {
   log := slog.New(slog.NewTextHandler(os.Stdout, nil))
   t.Log("I am indented correctly")
   log.Info("But I am not")
}

Produces:

=== RUN   TestSlog_Ugly
    slogt_test.go:21: I am indented correctly
time=2026-05-23T10:00:00.000-06:00 level=INFO msg="But I am not"

Note the second line (produced via slog).

slogt bridges those packages.

func TestSlogt_Pretty(t *testing.T) {
	log := slogt.New(t)
	t.Log("I am indented correctly")
	log.Info("And so am I")
}

Produces:

=== RUN   TestSlogt_Pretty
    slogt_test.go:27: I am indented correctly
    time=2026-05-23T10:00:00.000-06:00 level=INFO msg="And so am I"

Usage

Run go get as per procedure:

go get -u github.com/neilotoole/slogt/v2

Note

slogt/v2 requires Go 1.25 or later (it routes output through testing.TB.Output).

Then, use slogt.New to get a *slog.Logger that you can use as you normally would.

func TestText(t *testing.T) {
   log := slogt.New(t)
   log.Info("hello world")
}

Produces:

=== RUN   TestText
    time=2026-05-23T10:00:00.000-06:00 level=INFO msg="hello world"

In practice, you would pass the *slog.Logger returned from slogt.New to the component under test. For example:

func TestApp(t *testing.T) {
  log := slogt.New(t)
  
  app := app.New(log, ...) // other dependencies

  result, err := app.DepositMoney(100)
  require.NoError(t, err)
  require.Equal(t, 100, result.Balance)
}

If the app.DepositMoney method logs anything, its output is routed to t.Output() and correlated with the running test.

Options

The default output is text, i.e. a slog.TextHandler. You can specify JSON using the slogt.JSON() option.

func TestJSON(t *testing.T) {
   log := slogt.New(t, slogt.JSON())
   log.Info("hello world")
}

Produces:

=== RUN   TestJSON
    {"time":"2026-05-23T10:00:00.000-06:00","level":"INFO","msg":"hello world"}

To switch the default handler:

func init() {
    slogt.SetDefault(slogt.JSON())
}

When Text() and JSON() aren't enough — a non-default level, AddSource: true, a custom ReplaceAttr, or a third-party handler — reach for slogt.Factory(). slogt calls your function with the test's writer (t.Output()), and you return any slog.Handler that writes to it. (Since v2 removed the exported Bridge type, Factory is the way to plug in a custom handler.)

func TestSomething(t *testing.T) {
    // This factory returns a slog.Handler using slog.LevelError.
    f := slogt.Factory(func(w io.Writer) slog.Handler {
       opts := &slog.HandlerOptions{
           Level: slog.LevelError,
       }
       return slog.NewTextHandler(w, opts)
    })

    log := slogt.New(t, f)
}

Showing the caller

By default, log lines carry no callsite (writing through t.Output() adds none). If you want the real caller, enable AddSource: true on the handler:

func TestCaller(t *testing.T) {
	f := slogt.Factory(func(w io.Writer) slog.Handler {
		opts := &slog.HandlerOptions{
			AddSource: true,
		}

		return slog.NewTextHandler(w, opts)
	})

	log := slogt.New(t, f)
	log.Info("Show me the real callsite")
}

Produces (note the correct source attribute — the actual call site, not slog's internals):

=== RUN   TestCaller
    time=2026-05-23T10:00:00.000-06:00 level=INFO source=.../slogt_test.go:118 msg="Show me the real callsite"

History

slogt originally had a notable deficiency: every log line was prefixed with a bogus callsite pointing into slog's internals (e.g. logger.go:230) rather than the real log.Info() call site. This was unavoidable — the only sink available was t.Log(string), a function call rather than an io.Writer, so slogt had to buffer slog's output and forward it through t.Log, which then attributed each line to slog's own internals (the frame that called Handle) rather than the real call site. t.Helper() could not mark enough frames to fix it.

That limitation directly motivated the upstream Go proposal golang/go#59928, which was accepted and shipped in Go 1.25 as testing.TB.Output(): an io.Writer that writes to the test log without prepending a source location. As of v2.0.0, slogt writes straight to t.Output(), so the bogus prefix is gone — and the real caller is available via AddSource: true (see Showing the caller).

Note

This deficiency is what motivated golang/go#59928. The original issue (by @earthboundkid):

In a test, you often want to mock out the logger. It would be nice to be able to call t.Slog() and get a log/slog logger that send output to t.Log() with the correct caller information.

See https://github.com/neilotoole/slogt for an example of a third party library providing this functionality, but note that it cannot provide correct caller information:

Alas, given the available functionality on testing.T (i.e. the Helper method), and how slog is implemented, there's no way to have the correct callsite printed.

It seems like this needs to be done on the Go side to fix the callsite.

Changelog

v2.0.0 (2026-05-23)

• Breaking: require Go 1.25 and route output through testing.TB.Output(), eliminating the bogus slog-internal callsite prefix (see History).
• Breaking: remove the exported Bridge type (an internal workaround that is no longer needed). The module path is now github.com/neilotoole/slogt/v2.
• Breaking: replace the mutable Default package var with the concurrency-safe SetDefault function.
• Drop the stale golang.org/x/exp dependency; test CI across Go 1.25.x and stable.

v1.1.0 (2023-08-12)

• Require Go 1.21; switch from golang.org/x/exp/slog to the standard library log/slog.

v1.0.1 (2023-05-31)

• Update README examples for the newer golang.org/x/exp/slog API.

v1.0.0 (2023-04-03)

• Initial release: bridge between testing.T and golang.org/x/exp/slog.