diff --git a/README.md b/README.md
index d855dca..d361d67 100644
--- a/README.md
+++ b/README.md
@@ -681,6 +681,29 @@ Downloads the binary body of a message. The authenticated identity must be a par
 | `404`  | Message not found or data file not available |
 | `403`  | Authenticated user is not a participant |
 
+### GET `/fmsg/:id/thread`
+
+Returns the message's thread as plain text: the direct ancestor lineage
+(each message's `pid` followed to the root) plus the message itself, root
+first. Intended for "copy thread" UI actions and for feeding a whole thread
+to an agent as context.
+
+The authenticated identity must be a participant of the *requested*
+message. Each message is preceded by a one-line separator with its sender
+and send time. Ancestors the caller is not a participant of appear as
+`[message not visible to you]` (a recipient added mid-thread cannot read
+what came before); non-text bodies appear as a `[non-text message: <type>,
+<size> bytes]` placeholder. The walk is capped at 100 hops.
+
+**Response:** `200 OK`, `text/plain; charset=utf-8`.
+
+**Errors:**
+
+| Status | Condition |
+| ------ | --------- |
+| `404`  | Message not found |
+| `403`  | Authenticated user is not a participant of the requested message |
+
 ### POST `/fmsg/:id/attach`
 
 Uploads a file attachment for a draft message. Only the owner may upload, and the message must not have been sent.
diff --git a/cmd/fmsg-webapi/main.go b/cmd/fmsg-webapi/main.go
index be59384..4fa2df3 100644
--- a/cmd/fmsg-webapi/main.go
+++ b/cmd/fmsg-webapi/main.go
@@ -193,6 +193,7 @@ func main() {
 		fmsg.POST("/:id/read", msgHandler.MarkRead)
 		fmsg.POST("/:id/add-to", msgHandler.AddRecipients)
 		fmsg.GET("/:id/data", msgHandler.DownloadData)
+		fmsg.GET("/:id/thread", msgHandler.ThreadText)
 
 		fmsg.POST("/:id/attach", attHandler.Upload)
 		fmsg.GET("/:id/attach/:filename", attHandler.Download)
diff --git a/internal/handlers/thread.go b/internal/handlers/thread.go
new file mode 100644
index 0000000..b174f4f
--- /dev/null
+++ b/internal/handlers/thread.go
@@ -0,0 +1,161 @@
+package handlers
+
+import (
+	"errors"
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5"
+)
+
+// threadMaxHops bounds the ancestor walk (defensive; a pid cycle cannot be
+// created through the API but the cap keeps the query finite regardless).
+const threadMaxHops = 100
+
+// threadEntry is one message on the direct pid lineage, root first.
+type threadEntry struct {
+	ID       int64
+	From     string
+	TimeSent *float64
+	Type     string
+	Size     int
+	Body     string // empty when unreadable by the caller or non-text
+	Readable bool   // caller is a participant of this message
+	Text     bool   // body is a text type that was inlined
+}
+
+// renderThreadText renders the lineage as plain text, root at the top. Each
+// message gets a one-line separator with sender and send time; non-text
+// bodies and messages the caller may not read become placeholders, so the
+// chain's shape stays visible without leaking content.
+func renderThreadText(entries []threadEntry) string {
+	var b strings.Builder
+	for i, e := range entries {
+		if i > 0 {
+			b.WriteString("\n\n")
+		}
+		ts := "draft"
+		if e.TimeSent != nil {
+			ts = time.Unix(int64(*e.TimeSent), 0).UTC().Format(time.RFC3339)
+		}
+		fmt.Fprintf(&b, "--- %s %s ---\n", e.From, ts)
+		switch {
+		case !e.Readable:
+			b.WriteString("[message not visible to you]")
+		case !e.Text:
+			fmt.Fprintf(&b, "[non-text message: %s, %d bytes]", e.Type, e.Size)
+		default:
+			b.WriteString(strings.TrimRight(e.Body, "\n"))
+		}
+	}
+	b.WriteString("\n")
+	return b.String()
+}
+
+// isTextType reports whether a MIME type's body can be inlined as text.
+func isTextType(mimeType string) bool {
+	t := strings.ToLower(strings.TrimSpace(mimeType))
+	return strings.HasPrefix(t, "text/") || t == "application/json"
+}
+
+// ThreadText handles GET /fmsg/:id/thread — returns the message's direct
+// ancestor lineage (pid walk to the root) plus the message itself as plain
+// text, root first. The caller must be a participant of the requested
+// message; ancestors the caller is not a participant of appear as
+// placeholders (a recipient added mid-thread cannot read what came before).
+func (h *MessageHandler) ThreadText(c *gin.Context) {
+	addrs, err := h.visibleAddrs(c)
+	if err != nil {
+		log.Printf("thread text: resolve visible addrs: %v", err)
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to retrieve thread"})
+		return
+	}
+	msgID, ok := parseID(c)
+	if !ok {
+		return
+	}
+	ctx := c.Request.Context()
+
+	rows, err := h.DB.Pool.Query(ctx, `
+		WITH RECURSIVE chain AS (
+			SELECT id, pid, from_addr, time_sent, type, size, filepath, 0 AS depth
+			FROM msg WHERE id = $1
+			UNION ALL
+			SELECT m.id, m.pid, m.from_addr, m.time_sent, m.type, m.size, m.filepath, c.depth + 1
+			FROM msg m JOIN chain c ON m.id = c.pid
+			WHERE c.depth < $3
+		)
+		SELECT c.id, c.from_addr, c.time_sent, c.type, c.size, c.filepath,
+		       (c.from_addr = ANY($2)
+		        OR EXISTS (SELECT 1 FROM msg_to t WHERE t.msg_id = c.id AND t.addr = ANY($2))
+		        OR EXISTS (SELECT 1 FROM msg_add_to a WHERE a.msg_id = c.id AND a.addr = ANY($2))) AS readable
+		FROM chain c ORDER BY c.depth DESC`,
+		msgID, addrs, threadMaxHops,
+	)
+	if err != nil {
+		log.Printf("thread text: walk %d: %v", msgID, err)
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to retrieve thread"})
+		return
+	}
+	defer rows.Close()
+
+	var entries []threadEntry
+	paths := map[int64]string{}
+	for rows.Next() {
+		var e threadEntry
+		var dataPath string
+		if err := rows.Scan(&e.ID, &e.From, &e.TimeSent, &e.Type, &e.Size, &dataPath, &e.Readable); err != nil {
+			log.Printf("thread text: scan: %v", err)
+			c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to retrieve thread"})
+			return
+		}
+		paths[e.ID] = dataPath
+		entries = append(entries, e)
+	}
+	if err := rows.Err(); err != nil && !errors.Is(err, pgx.ErrNoRows) {
+		log.Printf("thread text: rows %d: %v", msgID, err)
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to retrieve thread"})
+		return
+	}
+	if len(entries) == 0 {
+		c.JSON(http.StatusNotFound, gin.H{"error": "message not found"})
+		return
+	}
+
+	// Authorization gate is the requested message itself, same as GET
+	// /fmsg/:id — a non-participant learns nothing, not even chain shape.
+	if !entries[len(entries)-1].Readable {
+		c.JSON(http.StatusForbidden, gin.H{"error": "access denied"})
+		return
+	}
+
+	for i := range entries {
+		e := &entries[i]
+		if !e.Readable || !isTextType(e.Type) {
+			continue
+		}
+		dataPath := paths[e.ID]
+		if dataPath == "" {
+			continue
+		}
+		cleanPath, ok := safeDataPath(dataPath, h.DataDir)
+		if !ok {
+			log.Printf("thread text: path traversal attempt: %s", dataPath)
+			continue
+		}
+		raw, rerr := os.ReadFile(cleanPath)
+		if rerr != nil {
+			log.Printf("thread text: read %d: %v", e.ID, rerr)
+			continue
+		}
+		e.Body = string(raw)
+		e.Text = true
+	}
+
+	c.Data(http.StatusOK, "text/plain; charset=utf-8", []byte(renderThreadText(entries)))
+}
diff --git a/internal/handlers/thread_test.go b/internal/handlers/thread_test.go
new file mode 100644
index 0000000..4f93fcc
--- /dev/null
+++ b/internal/handlers/thread_test.go
@@ -0,0 +1,42 @@
+package handlers
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestRenderThreadText(t *testing.T) {
+	ts := float64(1754350000)
+	entries := []threadEntry{
+		{ID: 1, From: "@bob@example.com", TimeSent: &ts, Type: "text/markdown", Body: "root body\n", Readable: true, Text: true},
+		{ID: 2, From: "@alice@hairpin.local", TimeSent: &ts, Type: "image/png", Size: 123, Readable: true},
+		{ID: 3, From: "@bob@example.com", TimeSent: &ts, Type: "text/plain", Readable: false},
+		{ID: 4, From: "@alice@hairpin.local", TimeSent: &ts, Type: "text/plain", Body: "latest reply", Readable: true, Text: true},
+	}
+	got := renderThreadText(entries)
+
+	for _, want := range []string{
+		"--- @bob@example.com 2025-08-04T23:26:40Z ---\nroot body",
+		"[non-text message: image/png, 123 bytes]",
+		"[message not visible to you]",
+		"latest reply",
+	} {
+		if !strings.Contains(got, want) {
+			t.Fatalf("missing %q in:\n%s", want, got)
+		}
+	}
+	if strings.Index(got, "root body") > strings.Index(got, "latest reply") {
+		t.Fatal("root must come first (top to bottom)")
+	}
+}
+
+func TestIsTextType(t *testing.T) {
+	for typ, want := range map[string]bool{
+		"text/plain": true, "text/markdown": true, "TEXT/HTML": true,
+		"application/json": true, "image/png": false, "application/pdf": false, "": false,
+	} {
+		if isTextType(typ) != want {
+			t.Errorf("isTextType(%q) != %v", typ, want)
+		}
+	}
+}