diff --git a/pipe/function.go b/pipe/function.go
index a80947f..2970a12 100644
--- a/pipe/function.go
+++ b/pipe/function.go
@@ -33,20 +33,32 @@ type StageFunc func(ctx context.Context, env Env, stdin io.Reader, stdout io.Wri
 // FunctionOption configures a Function stage.
 type FunctionOption func(*goStage)
 
+// WithStdinRequirement returns a FunctionOption declaring the stage's stdin
+// requirement.
+func WithStdinRequirement(requirement StreamRequirement) FunctionOption {
+	return func(s *goStage) {
+		s.requirements.Stdin = requirement
+	}
+}
+
+// WithStdoutRequirement returns a FunctionOption declaring the stage's stdout
+// requirement.
+func WithStdoutRequirement(requirement StreamRequirement) FunctionOption {
+	return func(s *goStage) {
+		s.requirements.Stdout = requirement
+	}
+}
+
 // ForbidStdin returns a FunctionOption declaring that the stage must not be
 // connected to stdin.
 func ForbidStdin() FunctionOption {
-	return func(s *goStage) {
-		s.requirements.Stdin = StreamForbidden
-	}
+	return WithStdinRequirement(StreamForbidden)
 }
 
 // ForbidStdout returns a FunctionOption declaring that the stage must not be
 // connected to stdout.
 func ForbidStdout() FunctionOption {
-	return func(s *goStage) {
-		s.requirements.Stdout = StreamForbidden
-	}
+	return WithStdoutRequirement(StreamForbidden)
 }
 
 // Function returns a pipeline `Stage` that will run a `StageFunc` in
diff --git a/pipe/pipeline.go b/pipe/pipeline.go
index 878cdc6..3078c10 100644
--- a/pipe/pipeline.go
+++ b/pipe/pipeline.go
@@ -99,6 +99,16 @@ func WithDir(dir string) Option {
 // WithStdin assigns stdin to the first command in the pipeline. The
 // caller retains ownership of stdin; the pipeline will not close it,
 // even if `Start()` returns an error.
+//
+// If the first stage is a `Command` and stdin is not an `*os.File`,
+// `exec.Cmd` has to copy stdin through an internal goroutine, and
+// `Cmd.Wait()` waits for that copy to finish. This is fine for bounded
+// readers such as `strings.Reader` and `bytes.Reader`, and for
+// `*os.File` values, which are passed to the command directly. But a
+// borrowed, non-file reader that can block forever can also block the
+// pipeline forever if the command exits without consuming all of its
+// stdin. See `TestPipelineIOPipeStdinThatIsNeverClosed` for the known
+// limitation.
 func WithStdin(stdin io.Reader) Option {
 	return func(p *Pipeline) {
 		p.stdin = Input(stdin)
@@ -238,6 +248,12 @@ func (p *Pipeline) Start(ctx context.Context) error {
 
 	atomic.StoreUint32(&p.started, 1)
 	ctx, p.cancel = context.WithCancel(ctx)
+	startedOK := false
+	defer func() {
+		if !startedOK {
+			p.cancel()
+		}
+	}()
 
 	if len(p.stages) == 0 {
 		if p.stdout == nil {
@@ -290,11 +306,15 @@ func (p *Pipeline) Start(ctx context.Context) error {
 		requirements := s.Requirements()
 		if err := requirements.Stdin.Validate(); err != nil {
 			closePipes()
-			return fmt.Errorf("stdin: %w", err)
+			return fmt.Errorf(
+				"stage %q has invalid stdin requirement: %w", s.Name(), err,
+			)
 		}
 		if err := requirements.Stdout.Validate(); err != nil {
 			closePipes()
-			return fmt.Errorf("stdout: %w", err)
+			return fmt.Errorf(
+				"stage %q has invalid stdout requirement: %w", s.Name(), err,
+			)
 		}
 
 		stageJoiners[i].nextStage = s
@@ -362,10 +382,19 @@ func (p *Pipeline) Start(ctx context.Context) error {
 		}
 	}
 
+	startedOK = true
 	return nil
 }
 
 func (p *Pipeline) Output(ctx context.Context) ([]byte, error) {
+	if p.hasStarted() {
+		panic("attempt to get output from a pipeline that has already started")
+	}
+
+	if err := p.stdout.Close(); err != nil {
+		return nil, fmt.Errorf("closing previous stdout: %w", err)
+	}
+
 	var buf bytes.Buffer
 	p.stdout = Output(&buf)
 	err := p.Run(ctx)
diff --git a/pipe/pipeline_test.go b/pipe/pipeline_test.go
index e956964..fe73eea 100644
--- a/pipe/pipeline_test.go
+++ b/pipe/pipeline_test.go
@@ -54,6 +54,23 @@ func TestPipelineEmptyOutput(t *testing.T) {
 	}
 }
 
+func TestPipelineOutputClosesConfiguredStdoutCloser(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	stdout := &closeTrackingWriter{}
+	p := pipe.New(
+		pipe.WithStdin(strings.NewReader("hello world\n")),
+		pipe.WithStdoutCloser(stdout),
+	)
+
+	out, err := p.Output(ctx)
+	if assert.NoError(t, err) {
+		assert.Equal(t, "hello world\n", string(out))
+		assert.Equal(t, "", stdout.buf.String())
+		assert.True(t, stdout.closed, "WithStdoutCloser destination should be closed")
+	}
+}
+
 func TestPipelineEmptyWithStdoutCloser(t *testing.T) {
 	t.Parallel()
 	ctx := context.Background()
@@ -951,6 +968,24 @@ func TestFunctionOptionsForbidStreams(t *testing.T) {
 	})
 }
 
+func TestFunctionOptionsSetStreamRequirements(t *testing.T) {
+	t.Parallel()
+
+	stage := pipe.Function(
+		"file-preferring",
+		func(_ context.Context, _ pipe.Env, _ io.Reader, _ io.Writer) error {
+			return nil
+		},
+		pipe.WithStdinRequirement(pipe.StreamPreferFile),
+		pipe.WithStdoutRequirement(pipe.StreamPreferFile),
+	)
+
+	assert.Equal(t, pipe.StageRequirements{
+		Stdin:  pipe.StreamPreferFile,
+		Stdout: pipe.StreamPreferFile,
+	}, stage.Requirements())
+}
+
 func TestStreamForbiddenStdin(t *testing.T) {
 	t.Parallel()
 	ctx := context.Background()
@@ -1024,7 +1059,10 @@ func TestInvalidStreamRequirements(t *testing.T) {
 				Stdin: pipe.StreamRequirement(123),
 			},
 		})
-		require.ErrorContains(t, p.Run(ctx), `stdin: invalid stream requirement 123`)
+		require.ErrorContains(
+			t, p.Run(ctx),
+			`stage "source" has invalid stdin requirement: invalid stream requirement 123`,
+		)
 		assert.True(t, stdout.closed, "WithStdoutCloser destination should be closed")
 	})
 
@@ -1038,7 +1076,10 @@ func TestInvalidStreamRequirements(t *testing.T) {
 				Stdout: pipe.StreamRequirement(123),
 			},
 		})
-		require.ErrorContains(t, p.Run(ctx), `stdout: invalid stream requirement 123`)
+		require.ErrorContains(
+			t, p.Run(ctx),
+			`stage "sink" has invalid stdout requirement: invalid stream requirement 123`,
+		)
 		assert.True(t, stdout.closed, "WithStdoutCloser destination should be closed")
 	})
 }
@@ -1071,7 +1112,10 @@ func TestInvalidStreamRequirement(t *testing.T) {
 			Stdin: pipe.StreamRequirement(99),
 		},
 	})
-	require.ErrorContains(t, p.Run(ctx), `stdin: invalid stream requirement 99`)
+	require.ErrorContains(
+		t, p.Run(ctx),
+		`stage "invalid" has invalid stdin requirement: invalid stream requirement 99`,
+	)
 }
 
 func TestFunctionNoInput(t *testing.T) {