Tidy up the AttachedFiles plumbing

Pure cleanup, no behaviour change:

* buildTaskSystemMessage now uses a single strings.Builder throughout

  instead of mixing string concatenation with a one-off builder for the

  attached_files block.

* CreateUserMessageWithAttachment introduces a noAttachment closure so

  the seven best-effort fallback paths share one return statement instead

  of repeating session.UserMessage(userContent), "" everywhere.

* branch.go drops a redundant cloneStringSlice on the result of

  AttachedFilesSnapshot, which already returns an independent copy.

* The over-eager three-line comments at the AddAttachedFile call sites

  and on top of newSubSession are replaced with a single line each that

  states the contract.

Assisted-By: docker-agent

Author: dgageot

pkg/app/app.go

@@ -140,9 +140,7 @@ func (a *App) SendFirstMessage() tea.Cmd {
 		func() tea.Msg {
 			// Use the shared PrepareUserMessage function for consistent attachment handling
 			userMsg, attachedPath := cli.PrepareUserMessage(context.Background(), a.runtime, *a.firstMessage, a.firstMessageAttach)
-			// Record the attached file on the session so sub-agents created via
-			// task transfer can reference it through their inherited
-			// AttachedFiles list.
+			// Inherit the attachment in any sub-session created by this turn.
 			a.session.AddAttachedFile(attachedPath)
 
 			// If the message has multi-content (attachments), we need to handle it specially
@@ -304,9 +302,9 @@ func (a *App) Run(ctx context.Context, cancel context.CancelFunc, message string
 			for _, att := range attachments {
 				switch {
 				case att.FilePath != "":
-					// Record the attached path on the session so that sub-agents
-					// created via task transfer inherit it. The editor already
-					// resolves @-mentions to absolute paths before this point.
+					// The editor resolves @-mentions to absolute paths before this
+					// point; remember them so sub-agents created via task transfer
+					// inherit the same file references.
 					a.session.AddAttachedFile(att.FilePath)
 					// File-reference attachment: read and classify from disk.
 					a.processFileAttachment(ctx, att, &textBuilder, &binaryParts)

pkg/cli/runner.go

@@ -403,21 +403,28 @@ func ParseAttachCommand(userInput string) (messageText, attachPath string) {
 // large to inline, etc.). Callers should record successful attachments via
 // session.Session.AddAttachedFile so sub-agents inherit the file context.
 func CreateUserMessageWithAttachment(userContent, attachmentPath string) (*session.Message, string) {
-	if attachmentPath == "" {
+	// noAttachment returns the message without any attachment. It's used both
+	// when the caller didn't supply a path and as the fallback for every
+	// best-effort failure below (unreadable file, unsupported type, etc.) so
+	// the user always gets at least a plain text message.
+	noAttachment := func() (*session.Message, string) {
 		return session.UserMessage(userContent), ""
 	}
 
-	// Validate file exists
+	if attachmentPath == "" {
+		return noAttachment()
+	}
+
 	absPath, err := filepath.Abs(attachmentPath)
 	if err != nil {
 		slog.Warn("Failed to get absolute path for attachment", "path", attachmentPath, "error", err)
-		return session.UserMessage(userContent), ""
+		return noAttachment()
 	}
 
 	fi, err := os.Stat(absPath)
 	if err != nil {
 		slog.Warn("Attachment file not accessible", "path", absPath, "error", err)
-		return session.UserMessage(userContent), ""
+		return noAttachment()
 	}
 
 	// Ensure we have some text content when attaching a file
@@ -435,12 +442,12 @@ func CreateUserMessageWithAttachment(userContent, attachmentPath string) (*sessi
 		// Text files are inlined directly as text content.
 		if fi.Size() > chat.MaxInlineFileSize {
 			slog.Warn("Attachment text file too large to inline", "path", absPath, "size", fi.Size())
-			return session.UserMessage(userContent), ""
+			return noAttachment()
 		}
 		content, err := chat.ReadFileForInline(absPath)
 		if err != nil {
 			slog.Warn("Failed to read attachment file", "path", absPath, "error", err)
-			return session.UserMessage(userContent), ""
+			return noAttachment()
 		}
 		multiContent = append(multiContent, chat.MessagePart{
 			Type: chat.MessagePartTypeText,
@@ -452,7 +459,7 @@ func CreateUserMessageWithAttachment(userContent, attachmentPath string) (*sessi
 		mimeType := chat.DetectMimeType(absPath)
 		if !chat.IsSupportedMimeType(mimeType) {
 			slog.Warn("Unsupported attachment file type", "path", absPath, "mime_type", mimeType)
-			return session.UserMessage(userContent), ""
+			return noAttachment()
 		}
 		if chat.IsImageMimeType(mimeType) {
 			// Read, resize if needed, and inline as base64 data URL.
@@ -461,12 +468,12 @@ func CreateUserMessageWithAttachment(userContent, attachmentPath string) (*sessi
 			imgData, readErr := os.ReadFile(absPath)
 			if readErr != nil {
 				slog.Warn("Failed to read image attachment", "path", absPath, "error", readErr)
-				return session.UserMessage(userContent), ""
+				return noAttachment()
 			}
 			resized, resizeErr := chat.ResizeImage(imgData, mimeType)
 			if resizeErr != nil {
 				slog.Warn("Image resize failed for attachment", "path", absPath, "error", resizeErr)
-				return session.UserMessage(userContent), ""
+				return noAttachment()
 			}
 			dataURL := fmt.Sprintf("data:%s;base64,%s", resized.MimeType, base64.StdEncoding.EncodeToString(resized.Data))
 			multiContent = append(multiContent, chat.MessagePart{

pkg/runtime/agent_delegation.go

@@ -54,23 +54,21 @@ func validateAgentInList(currentAgent, targetAgent, action, listDesc string, age
 // it can use them directly without scanning the workspace or guessing from a
 // bare filename.
 func buildTaskSystemMessage(task, expectedOutput string, attachedFiles []string) string {
-	msg := "You are a member of a team of agents. Your goal is to complete the following task:"
-	msg += fmt.Sprintf("\n\n<task>\n%s\n</task>", task)
+	var b strings.Builder
+	b.WriteString("You are a member of a team of agents. Your goal is to complete the following task:")
+	fmt.Fprintf(&b, "\n\n<task>\n%s\n</task>", task)
 	if expectedOutput != "" {
-		msg += fmt.Sprintf("\n\n<expected_output>\n%s\n</expected_output>", expectedOutput)
+		fmt.Fprintf(&b, "\n\n<expected_output>\n%s\n</expected_output>", expectedOutput)
 	}
 	if len(attachedFiles) > 0 {
-		var b strings.Builder
 		b.WriteString("\n\nThe user attached these files in the original conversation. They are available for you to read at these absolute paths; prefer them over any bare filenames mentioned in <task>:\n<attached_files>")
 		for _, p := range attachedFiles {
-			b.WriteString("\n- ")
-			b.WriteString(p)
+			fmt.Fprintf(&b, "\n- %s", p)
 		}
 		b.WriteString("\n</attached_files>")
-		msg += b.String()
 	}
-	msg += "\n\nIf the task references files, treat any absolute paths in <task> as authoritative and use them as-is. If a referenced file is given by name only (e.g. \"foo.go\"), do not guess: search the workspace or ask the calling agent for the absolute path before reading or modifying the file."
-	return msg
+	b.WriteString("\n\nIf the task references files, treat any absolute paths in <task> as authoritative and use them as-is. If a referenced file is given by name only (e.g. \"foo.go\"), do not guess: search the workspace or ask the calling agent for the absolute path before reading or modifying the file.")
+	return b.String()
 }
 
 // SubSessionConfig describes how to build and run a child session.
@@ -110,11 +108,11 @@ type SubSessionConfig struct {
 // session. It consolidates the session options that were previously duplicated
 // across handleTaskTransfer and RunAgent.
 func newSubSession(parent *session.Session, cfg SubSessionConfig, childAgent *agent.Agent) *session.Session {
-	// Inherit the parent's attached files so that delegated agents can read
-	// the same files the user attached to the original conversation, without
-	// having to scan the workspace or guess from a bare filename. This works
-	// recursively: sub-sub-sessions also inherit because we copy the parent
-	// session's AttachedFiles, and the child session is itself populated.
+	// Sub-agents start in a fresh session, so they don't see the user's
+	// original messages or attached files. Snapshot the parent's attached
+	// files once and propagate them both to the system prompt (so the agent
+	// is told about them) and to the child session (so further nested
+	// transfers keep inheriting them).
 	attachedFiles := parent.AttachedFilesSnapshot()
 
 	sysMsg := cfg.SystemMessage

pkg/session/branch.go

@@ -90,7 +90,7 @@ func copySessionMetadata(dst, src *Session, title string) {
 	dst.Permissions = clonePermissionsConfig(src.Permissions)
 	dst.AgentModelOverrides = cloneStringMap(src.AgentModelOverrides)
 	dst.CustomModelsUsed = cloneStringSlice(src.CustomModelsUsed)
-	dst.AttachedFiles = cloneStringSlice(src.AttachedFilesSnapshot())
+	dst.AttachedFiles = src.AttachedFilesSnapshot()
 }
 
 // generateBranchTitle creates a title for a branched session based on the parent title.