From 74ed64aadbb4d5a316b233f3469219202b25efeb Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Wed, 5 Aug 2026 09:56:44 +0000
Subject: [PATCH 1/2] Initial plan


From cab72e184a8994aed19353ec3099728dff29f02d Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Wed, 5 Aug 2026 09:58:48 +0000
Subject: [PATCH 2/2] Fix write mode file truncation

Co-authored-by: ccamel <9574336+ccamel@users.noreply.github.com>
---
 engine/builtin_test.go | 19 ++++++++-----------
 engine/stream.go       |  4 ++--
 2 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/engine/builtin_test.go b/engine/builtin_test.go
index 7a947ed..21052fb 100644
--- a/engine/builtin_test.go
+++ b/engine/builtin_test.go
@@ -3703,8 +3703,10 @@ func TestOpen(t *testing.T) {
 		defer func() {
 			assert.NoError(t, os.Remove(n))
 		}()
+		assert.NoError(t, os.WriteFile(n, []byte("longer existing payload"), 0o644))
 
 		v := NewVariable()
+		var s *Stream
 
 		ok, err := Open(&vm, NewAtom(n), atomWrite, v, List(&compound{
 			functor: atomAlias,
@@ -3712,7 +3714,7 @@ func TestOpen(t *testing.T) {
 		}), func(env *Env) *Promise {
 			ref, ok := env.lookup(v)
 			assert.True(t, ok)
-			s, ok := ref.(*Stream)
+			s, ok = ref.(*Stream)
 			assert.True(t, ok)
 
 			l, ok := vm.streams.lookup(atomOutput)
@@ -3722,20 +3724,15 @@ func TestOpen(t *testing.T) {
 			_, err := fmt.Fprintf(s.sink, "test\n")
 			assert.NoError(t, err)
 
-			f, err := os.Open(n)
-			assert.NoError(t, err)
-			defer func() {
-				assert.NoError(t, f.Close())
-			}()
-
-			b, err := io.ReadAll(f)
-			assert.NoError(t, err)
-			assert.Equal(t, "test\n", string(b))
-
 			return Bool(true)
 		}, nil).Force(context.Background())
 		assert.NoError(t, err)
 		assert.True(t, ok)
+		assert.NoError(t, s.Close())
+
+		b, err := os.ReadFile(n)
+		assert.NoError(t, err)
+		assert.Equal(t, "test\n", string(b))
 	})
 
 	t.Run("read_write", func(t *testing.T) {
diff --git a/engine/stream.go b/engine/stream.go
index c6fe303..646a36e 100644
--- a/engine/stream.go
+++ b/engine/stream.go
@@ -468,9 +468,9 @@ const (
 	// ioModeRead means you can read from the stream.
 	ioModeRead = ioMode(os.O_RDONLY)
 	// ioModeWrite means you can write to the stream.
-	ioModeWrite = ioMode(os.O_CREATE | os.O_WRONLY)
+	ioModeWrite = ioMode(os.O_CREATE | os.O_WRONLY | os.O_TRUNC)
 	// ioModeAppend means you can append to the stream.
-	ioModeAppend = ioMode(os.O_APPEND) | ioModeWrite
+	ioModeAppend = ioMode(os.O_APPEND | os.O_CREATE | os.O_WRONLY)
 	// ioModeReadWrite means you can both read from and write to an existing stream.
 	ioModeReadWrite = ioMode(os.O_RDWR)
 )