diff --git a/pkg/compose/publish.go b/pkg/compose/publish.go
index b3616030dfe..55ebe8cf485 100644
--- a/pkg/compose/publish.go
+++ b/pkg/compose/publish.go
@@ -199,7 +199,7 @@ func (s *composeService) createLayers(ctx context.Context, project *types.Projec
 	for _, file := range project.ComposeFiles {
 		data, err := processFile(ctx, file, project, extFiles, envFiles)
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("failed to process compose file %s: %w", file, err)
 		}
 
 		layerDescriptor := oci.DescriptorForComposeFile(file, data)
@@ -235,7 +235,7 @@ func processExtends(ctx context.Context, project *types.Project, extFiles map[st
 	for xf, hash := range extFiles {
 		data, err := processFile(ctx, xf, project, moreExtFiles, envFiles)
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("failed to process compose file %s: %w", xf, err)
 		}
 
 		layerDescriptor := oci.DescriptorForComposeFile(hash, data)
diff --git a/pkg/compose/publish_test.go b/pkg/compose/publish_test.go
index 79b3884da57..5036c0a966f 100644
--- a/pkg/compose/publish_test.go
+++ b/pkg/compose/publish_test.go
@@ -112,6 +112,47 @@ services:
 	}, cmp.Ignore()))
 }
 
+// a processFile failure must say which file it came from: a project can
+// carry many compose files and its extends references add more, so an
+// unattributed error leaves the user guessing which one to fix. The
+// flow-style file makes the assertions meaningful: the rejection itself
+// does not name the file, so only the wrap satisfies them.
+func Test_createLayers_error_includes_file_name(t *testing.T) {
+	dir := t.TempDir()
+	flowFile := filepath.Join(dir, "flow.yaml")
+	assert.NilError(t, os.WriteFile(flowFile, []byte(`{services: {test: {env_file: old.env}}}`), 0o600))
+	// the env file must exist, or the loader rejects it before the
+	// flow-style rewrite gets a chance to fail
+	assert.NilError(t, os.WriteFile(filepath.Join(dir, "old.env"), []byte("FOO=bar\n"), 0o600))
+
+	project := &types.Project{
+		Name:         "test",
+		WorkingDir:   dir,
+		Environment:  types.Mapping{},
+		ComposeFiles: []string{flowFile},
+	}
+
+	service := &composeService{}
+	_, err := service.createLayers(t.Context(), project, api.PublishOptions{})
+	assert.ErrorContains(t, err, "failed to process compose file "+flowFile)
+	assert.ErrorContains(t, err, "flow style")
+}
+
+func Test_processExtends_error_includes_file_name(t *testing.T) {
+	dir := t.TempDir()
+	flowFile := filepath.Join(dir, "flow.yaml")
+	assert.NilError(t, os.WriteFile(flowFile, []byte(`{services: {test: {env_file: old.env}}}`), 0o600))
+	assert.NilError(t, os.WriteFile(filepath.Join(dir, "old.env"), []byte("FOO=bar\n"), 0o600))
+
+	_, err := processExtends(t.Context(), &types.Project{
+		Name:        "test",
+		WorkingDir:  dir,
+		Environment: types.Mapping{},
+	}, map[string]string{flowFile: "deadbeef"})
+	assert.ErrorContains(t, err, "failed to process compose file "+flowFile)
+	assert.ErrorContains(t, err, "flow style")
+}
+
 func Test_preChecks_sensitive_data_detected_decline(t *testing.T) {
 	dir := t.TempDir()
 	envPath := dir + "/secrets.env"
diff --git a/pkg/compose/transform/replace.go b/pkg/compose/transform/replace.go
index d15dc1d0bc7..d41d27bcc03 100644
--- a/pkg/compose/transform/replace.go
+++ b/pkg/compose/transform/replace.go
@@ -58,7 +58,10 @@ func ReplaceExtendsFile(in []byte, serviceName string, value string) ([]byte, er
 	}
 
 	// we've found target `file` yaml node. Let's replace value in stream at node position
-	return replace(in, file.Line, file.Column, value), nil
+	if isFlowStyle(root, services, target, extends, file) {
+		return nil, fmt.Errorf("service %q: extends.file is in a flow-style mapping, which cannot be rewritten while preserving formatting", serviceName)
+	}
+	return replace(in, file.Line, file.Column, value)
 }
 
 // ReplaceEnvFile changes value for service.extends.env_file in input yaml stream, preserving formatting
@@ -93,6 +96,12 @@ func ReplaceEnvFile(in []byte, serviceName string, i int, value string) ([]byte,
 
 	// env_file can be either a string, sequence of strings, or sequence of mappings with path attribute
 	if envFile.Kind == yaml.SequenceNode {
+		if i < 0 || i >= len(envFile.Content) {
+			return nil, fmt.Errorf("service %q: env_file has %d entries, cannot rewrite entry %d", serviceName, len(envFile.Content), i)
+		}
+		if isFlowStyle(root, services, target, envFile) || isFlowStyle(envFile.Content[i]) {
+			return nil, fmt.Errorf("service %q: env_file is in flow style, which cannot be rewritten while preserving formatting", serviceName)
+		}
 		envFile = envFile.Content[i]
 		if envFile.Kind == yaml.MappingNode {
 			envFile, err = getMapping(envFile, "path")
@@ -100,9 +109,12 @@ func ReplaceEnvFile(in []byte, serviceName string, i int, value string) ([]byte,
 				return nil, err
 			}
 		}
-		return replace(in, envFile.Line, envFile.Column, value), nil
+		return replace(in, envFile.Line, envFile.Column, value)
 	} else {
-		return replace(in, envFile.Line, envFile.Column, value), nil
+		if isFlowStyle(root, services, target, envFile) {
+			return nil, fmt.Errorf("service %q: env_file is in flow style, which cannot be rewritten while preserving formatting", serviceName)
+		}
+		return replace(in, envFile.Line, envFile.Column, value)
 	}
 }
 
@@ -123,20 +135,34 @@ func getMapping(root *yaml.Node, key string) (*yaml.Node, error) {
 }
 
 // replace changes yaml node value in stream at position, preserving content
-func replace(in []byte, line int, column int, value string) []byte {
+func replace(in []byte, line int, column int, value string) ([]byte, error) {
 	var out []byte
 	l := 1
 	pos := 0
-	for _, b := range in {
-		if b == '\n' {
-			l++
-			if l == line {
-				break
+	// the scan looks for the newline that precedes the target line; line 1
+	// has none, so without the guard pos would run to the end of the stream
+	// and the column offset below would slice out of range
+	if line > 1 {
+		for _, b := range in {
+			if b == '\n' {
+				l++
+				if l == line {
+					break
+				}
 			}
+			pos++
 		}
-		pos++
+		// pos stopped at the newline before the target line, one byte before
+		// the line start; columns are 1-based, so the value starts at
+		// line-start + column - 1, which is pos + column
+		pos += column
+	} else {
+		// line 1 starts at offset 0, so the 1-based column maps to column - 1
+		pos = column - 1
+	}
+	if pos > len(in) {
+		return nil, fmt.Errorf("cannot locate line %d column %d in stream", line, column)
 	}
-	pos += column
 	out = append(out, in[0:pos]...)
 	out = append(out, []byte(value)...)
 	for ; pos < len(in); pos++ {
@@ -145,5 +171,21 @@ func replace(in []byte, line int, column int, value string) []byte {
 		}
 	}
 	out = append(out, in[pos:]...)
-	return out
+	return out, nil
+}
+
+// isFlowStyle reports whether any node on the path to a rewritten value is
+// in flow style. replace rewrites from the value's start to the end of its
+// line — anything after the value on that line, a trailing comment
+// included, is dropped — so it is only safe when the value is the sole
+// content of its line: flow style puts closing braces, commas and sibling
+// keys after the value on the same line, and swallowing them would corrupt
+// the stream.
+func isFlowStyle(nodes ...*yaml.Node) bool {
+	for _, n := range nodes {
+		if n != nil && n.Style&yaml.FlowStyle != 0 {
+			return true
+		}
+	}
+	return false
 }
diff --git a/pkg/compose/transform/replace_test.go b/pkg/compose/transform/replace_test.go
index 6f7477e0350..68b3b1184e1 100644
--- a/pkg/compose/transform/replace_test.go
+++ b/pkg/compose/transform/replace_test.go
@@ -83,3 +83,34 @@ func TestReplace(t *testing.T) {
 		})
 	}
 }
+
+// A flow-style document puts every node on line 1: the position scan in
+// replace never sees a newline preceding the target, pos runs to the end of
+// the stream, and the column offset then slices out of range. compose
+// publish feeds the user's original file bytes into this path
+// (processFile), so the flow style is reachable input, not a hypothetical.
+func TestReplaceFlowStyle(t *testing.T) {
+	t.Run("extends file", func(t *testing.T) {
+		_, err := ReplaceExtendsFile([]byte(`{services: {test: {extends: {file: old.yaml}}}}`), "test", "REPLACED")
+		assert.ErrorContains(t, err, "flow-style")
+	})
+	t.Run("env_file", func(t *testing.T) {
+		_, err := ReplaceEnvFile([]byte(`{services: {test: {env_file: old.env}}}`), "test", 0, "REPLACED")
+		assert.ErrorContains(t, err, "flow style")
+	})
+	// on main this silently corrupted the stream: rewriting an entry of a
+	// flow sequence swallowed the rest of the line, closing bracket included
+	t.Run("env_file flow sequence", func(t *testing.T) {
+		_, err := ReplaceEnvFile([]byte(`{services: {test: {env_file: [a.env, b.env]}}}`), "test", 1, "REPLACED")
+		assert.ErrorContains(t, err, "flow style")
+	})
+	t.Run("env_file out-of-range entry", func(t *testing.T) {
+		_, err := ReplaceEnvFile([]byte(`services:
+  test:
+    env_file:
+      - a.env
+      - b.env
+`), "test", 5, "REPLACED")
+		assert.ErrorContains(t, err, "cannot rewrite entry 5")
+	})
+}