diff --git a/agents/runtime.go b/agents/runtime.go
index 4739e7f..01b0dbe 100644
--- a/agents/runtime.go
+++ b/agents/runtime.go
@@ -486,7 +486,7 @@ func (a *Agent) handleTaskEvent(ctx context.Context, msg bus.AgentMessage) {
 		Role: coretypes.RoleUser, Content: msgCtx.Content,
 	})
 
-	reply, err := a.runLLM(ctx, msgCtx)
+	reply, err := a.runLLM(ctx, msgCtx, roomID)
 
 	// Build the result to send back via bus
 	result := orchestration.TaskResult{
@@ -571,13 +571,13 @@ func (a *Agent) handleEvent(ctx context.Context, msgCtx decision.MessageContext,
 			// RBAC check for commands
 			if !a.acl.CanDo(msgCtx.SenderID, "command:"+cmdName) {
 				a.logger.Info("command_denied", "command", cmdName, "sender", msgCtx.SenderID)
-				_ = a.matrix.SendReplyMarkdown(ctx, roomID, msgCtx.EventID,
+				_ = a.sendReply(ctx, roomID, msgCtx.EventID, msgCtx.ThreadID,
 					"No tienes permisos para ejecutar este comando.")
 				return
 			}
 			a.logger.Info("command_executed", "command", cmdName)
 			reply := handler(ctx, msgCtx)
-			_ = a.matrix.SendReplyMarkdown(ctx, roomID, msgCtx.EventID, reply)
+			_ = a.sendReply(ctx, roomID, msgCtx.EventID, msgCtx.ThreadID, reply)
 			return
 		}
 
@@ -591,7 +591,7 @@ func (a *Agent) handleEvent(ctx context.Context, msgCtx decision.MessageContext,
 		} else {
 			// Unknown command — never falls through to rules or LLM
 			a.logger.Info("command_unknown", "command", msgCtx.Command)
-			_ = a.matrix.SendReplyMarkdown(ctx, roomID, msgCtx.EventID,
+			_ = a.sendReply(ctx, roomID, msgCtx.EventID, msgCtx.ThreadID,
 				fmt.Sprintf("Comando desconocido: `!%s`. Usa `!help` para ver comandos disponibles.", msgCtx.Command))
 			return
 		}
@@ -601,7 +601,7 @@ func (a *Agent) handleEvent(ctx context.Context, msgCtx decision.MessageContext,
 	// RBAC check for LLM access ("ask" action)
 	if !a.acl.CanDo(msgCtx.SenderID, "ask") {
 		a.logger.Info("ask_denied", "sender", msgCtx.SenderID)
-		_ = a.matrix.SendReplyMarkdown(ctx, roomID, msgCtx.EventID,
+		_ = a.sendReply(ctx, roomID, msgCtx.EventID, msgCtx.ThreadID,
 			"No tienes permisos para interactuar con este agente.")
 		return
 	}
@@ -636,17 +636,30 @@ func (a *Agent) handleEvent(ctx context.Context, msgCtx decision.MessageContext,
 
 // executeActions expands LLM actions and runs the effects runner.
 func (a *Agent) executeActions(ctx context.Context, roomID string, msgCtx decision.MessageContext, actions []decision.Action) {
+	// Auto-thread: if configured and message is not already in a thread,
+	// start a new thread rooted at the user's message.
+	if a.cfg.Matrix.Threads.AutoThread && msgCtx.ThreadID == "" && msgCtx.EventID != "" {
+		msgCtx.ThreadID = msgCtx.EventID
+	}
+
 	// Sanitize user input before sending to LLM
 	sanitized, rejected := a.sanitizeInput(msgCtx.Content, roomID, msgCtx.SenderID)
 	if rejected {
 		a.runner.Execute(ctx, roomID, []decision.Action{{
 			Kind:  decision.ActionKindReply,
-			Reply: &decision.ReplyAction{Content: "Tu mensaje fue rechazado por el filtro de seguridad.", InReplyTo: msgCtx.EventID},
+			Reply: &decision.ReplyAction{Content: "Tu mensaje fue rechazado por el filtro de seguridad.", InReplyTo: msgCtx.EventID, ThreadID: msgCtx.ThreadID},
 		}})
 		return
 	}
 	msgCtx.Content = sanitized
 
+	// Resolve memory key: use thread root as context key when inside a thread,
+	// so parallel threads in the same room have independent conversation windows.
+	memKey := roomID
+	if msgCtx.ThreadID != "" {
+		memKey = msgCtx.ThreadID
+	}
+
 	expanded := make([]decision.Action, 0, len(actions))
 	for _, act := range actions {
 		if act.Kind == decision.ActionKindLLM {
@@ -654,35 +667,35 @@ func (a *Agent) executeActions(ctx context.Context, roomID string, msgCtx decisi
 				a.logger.Warn("LLM action requested but no LLM configured")
 				expanded = append(expanded, decision.Action{
 					Kind:  decision.ActionKindReply,
-					Reply: &decision.ReplyAction{Content: "Este bot no tiene LLM configurado.", InReplyTo: msgCtx.EventID},
+					Reply: &decision.ReplyAction{Content: "Este bot no tiene LLM configurado.", InReplyTo: msgCtx.EventID, ThreadID: msgCtx.ThreadID},
 				})
 				continue
 			}
 			// Memory: load window + append user message before LLM call
-			a.ensureWindowLoaded(ctx, roomID)
-			a.appendToWindow(roomID, coretypes.Message{
+			a.ensureWindowLoaded(ctx, memKey)
+			a.appendToWindow(memKey, coretypes.Message{
 				Role: coretypes.RoleUser, Content: msgCtx.Content,
 			})
-			a.persistMessage(ctx, roomID, coretypes.RoleUser, msgCtx.Content)
+			a.persistMessage(ctx, memKey, coretypes.RoleUser, msgCtx.Content)
 
-			reply, err := a.runLLM(ctx, msgCtx)
+			reply, err := a.runLLM(ctx, msgCtx, memKey)
 			if err != nil {
 				a.logger.Error("llm error", "err", err)
 				expanded = append(expanded, decision.Action{
 					Kind:  decision.ActionKindReply,
-					Reply: &decision.ReplyAction{Content: "Sorry, I encountered an error.", InReplyTo: msgCtx.EventID},
+					Reply: &decision.ReplyAction{Content: "Sorry, I encountered an error.", InReplyTo: msgCtx.EventID, ThreadID: msgCtx.ThreadID},
 				})
 			} else {
 				expanded = append(expanded, decision.Action{
 					Kind:  decision.ActionKindReply,
-					Reply: &decision.ReplyAction{Content: reply, InReplyTo: msgCtx.EventID},
+					Reply: &decision.ReplyAction{Content: reply, InReplyTo: msgCtx.EventID, ThreadID: msgCtx.ThreadID},
 				})
 
 				// Memory: append assistant reply after LLM call
-				a.appendToWindow(roomID, coretypes.Message{
+				a.appendToWindow(memKey, coretypes.Message{
 					Role: coretypes.RoleAssistant, Content: reply,
 				})
-				a.persistMessage(ctx, roomID, coretypes.RoleAssistant, reply)
+				a.persistMessage(ctx, memKey, coretypes.RoleAssistant, reply)
 			}
 		} else {
 			expanded = append(expanded, act)
@@ -692,7 +705,7 @@ func (a *Agent) executeActions(ctx context.Context, roomID string, msgCtx decisi
 	a.runner.Execute(ctx, roomID, expanded)
 }
 
-func (a *Agent) runLLM(ctx context.Context, msgCtx decision.MessageContext) (string, error) {
+func (a *Agent) runLLM(ctx context.Context, msgCtx decision.MessageContext, memKey string) (string, error) {
 	a.logger.Debug("calling LLM",
 		"model", a.cfg.LLM.Primary.Model,
 		"provider", a.cfg.LLM.Primary.Provider,
@@ -702,7 +715,7 @@ func (a *Agent) runLLM(ctx context.Context, msgCtx decision.MessageContext) (str
 	systemPrompt := a.cfg.Agent.Description
 
 	// Build messages: conversation history from window (includes current user msg)
-	messages := a.getWindowMessages(msgCtx.RoomID)
+	messages := a.getWindowMessages(memKey)
 	if len(messages) == 0 {
 		// Fallback if memory is disabled: just the current message
 		messages = []coretypes.Message{
@@ -873,6 +886,15 @@ func (a *Agent) persistMessage(ctx context.Context, roomID string, role coretype
 	}
 }
 
+// sendReply sends a markdown reply that respects thread context.
+// If threadID is non-empty, the reply is sent as part of that thread.
+func (a *Agent) sendReply(ctx context.Context, roomID, eventID, threadID, markdown string) error {
+	if threadID != "" {
+		return a.matrix.SendThreadMarkdown(ctx, roomID, threadID, eventID, markdown)
+	}
+	return a.matrix.SendReplyMarkdown(ctx, roomID, eventID, markdown)
+}
+
 // parseSeverity converts a config string to sanitize.Severity.
 func parseSeverity(s string) sanitize.Severity {
 	switch s {
diff --git a/dev/issues/README.md b/dev/issues/README.md
index 63f0e9c..070f5fe 100644
--- a/dev/issues/README.md
+++ b/dev/issues/README.md
@@ -16,7 +16,7 @@ afectados y notas de implementacion.
 | 9  | Command system               | [0009-command_system.md](completed/0009-command_system.md)           | completado |
 | 10 | Access control               | [0010-access-control.md](completed/0010-access-control.md)           | completado |
 | 11 | Markdown rendering           | [0011-markdown-rendering.md](completed/0011-markdown-rendering.md)   | completado |
-| 12 | Threads                      | [0012-threads.md](0012-threads.md)                                   | pendiente  |
+| 12 | Threads                      | [0012-threads.md](completed/0012-threads.md)                         | completado |
 | 13 | Hot reload                   | [0013-hot-reload.md](0013-hot-reload.md)                             | pendiente  |
 | 14 | Template agent standardize   | [0014-template-agent-standardize.md](0014-template-agent-standardize.md) | pendiente |
 | 15 | Multi-platform Telegram      | [0015-multi-platform-telegram.md](0015-multi-platform-telegram.md)   | pendiente  |
diff --git a/dev/issues/0012-threads.md b/dev/issues/completed/0012-threads.md
similarity index 100%
rename from dev/issues/0012-threads.md
rename to dev/issues/completed/0012-threads.md
diff --git a/internal/config/schema.go b/internal/config/schema.go
index cd05d9f..b461d1c 100644
--- a/internal/config/schema.go
+++ b/internal/config/schema.go
@@ -198,6 +198,13 @@ type MatrixCfg struct {
 	Encryption      EncryptionCfg  `yaml:"encryption"`
 	Rooms           RoomsCfg       `yaml:"rooms"`
 	Filters         FiltersCfg     `yaml:"filters"`
+	Threads         ThreadsCfg     `yaml:"threads"`
+}
+
+// ThreadsCfg controls Matrix thread support (m.thread).
+type ThreadsCfg struct {
+	Enabled    bool `yaml:"enabled"`     // respond in threads when message is in a thread (default true)
+	AutoThread bool `yaml:"auto_thread"` // auto-create a thread for each new conversation (default false)
 }
 
 type EncryptionCfg struct {
diff --git a/shell/effects/runner.go b/shell/effects/runner.go
index 9b5f154..5788df6 100644
--- a/shell/effects/runner.go
+++ b/shell/effects/runner.go
@@ -24,6 +24,7 @@ type MatrixSender interface {
 	SendText(ctx context.Context, roomID, text string) error
 	SendMarkdown(ctx context.Context, roomID, markdown string) error
 	SendReplyMarkdown(ctx context.Context, roomID, inReplyTo, markdown string) error
+	SendThreadMarkdown(ctx context.Context, roomID, threadRootID, inReplyTo, markdown string) error
 	SendTyping(ctx context.Context, roomID string, typing bool) error
 }
 
@@ -63,15 +64,15 @@ func (r *Runner) executeOne(ctx context.Context, roomID string, a decision.Actio
 		if a.Reply == nil {
 			return Result{Action: a, Err: fmt.Errorf("nil reply action")}
 		}
-		target := roomID
-		if a.Reply.ThreadID != "" {
-			target = a.Reply.ThreadID
-		}
 		var err error
-		if a.Reply.InReplyTo != "" {
-			err = r.matrix.SendReplyMarkdown(ctx, target, a.Reply.InReplyTo, a.Reply.Content)
-		} else {
-			err = r.matrix.SendMarkdown(ctx, target, a.Reply.Content)
+		switch {
+		case a.Reply.ThreadID != "":
+			// Thread reply: send as part of the thread with fallback in_reply_to
+			err = r.matrix.SendThreadMarkdown(ctx, roomID, a.Reply.ThreadID, a.Reply.InReplyTo, a.Reply.Content)
+		case a.Reply.InReplyTo != "":
+			err = r.matrix.SendReplyMarkdown(ctx, roomID, a.Reply.InReplyTo, a.Reply.Content)
+		default:
+			err = r.matrix.SendMarkdown(ctx, roomID, a.Reply.Content)
 		}
 		return Result{Action: a, Output: a.Reply.Content, Err: err}
 
diff --git a/shell/effects/runner_test.go b/shell/effects/runner_test.go
new file mode 100644
index 0000000..b5f0547
--- /dev/null
+++ b/shell/effects/runner_test.go
@@ -0,0 +1,172 @@
+package effects
+
+import (
+	"context"
+	"log/slog"
+	"testing"
+
+	"github.com/enmanuel/agents/pkg/decision"
+)
+
+// fakeMatrixSender records calls for assertions.
+type fakeMatrixSender struct {
+	calls []senderCall
+}
+
+type senderCall struct {
+	method       string
+	roomID       string
+	threadRootID string
+	inReplyTo    string
+	markdown     string
+}
+
+func (f *fakeMatrixSender) SendText(ctx context.Context, roomID, text string) error {
+	f.calls = append(f.calls, senderCall{method: "SendText", roomID: roomID, markdown: text})
+	return nil
+}
+
+func (f *fakeMatrixSender) SendMarkdown(ctx context.Context, roomID, markdown string) error {
+	f.calls = append(f.calls, senderCall{method: "SendMarkdown", roomID: roomID, markdown: markdown})
+	return nil
+}
+
+func (f *fakeMatrixSender) SendReplyMarkdown(ctx context.Context, roomID, inReplyTo, markdown string) error {
+	f.calls = append(f.calls, senderCall{method: "SendReplyMarkdown", roomID: roomID, inReplyTo: inReplyTo, markdown: markdown})
+	return nil
+}
+
+func (f *fakeMatrixSender) SendThreadMarkdown(ctx context.Context, roomID, threadRootID, inReplyTo, markdown string) error {
+	f.calls = append(f.calls, senderCall{method: "SendThreadMarkdown", roomID: roomID, threadRootID: threadRootID, inReplyTo: inReplyTo, markdown: markdown})
+	return nil
+}
+
+func (f *fakeMatrixSender) SendTyping(ctx context.Context, roomID string, typing bool) error {
+	return nil
+}
+
+func TestExecuteReply_PlainMarkdown(t *testing.T) {
+	sender := &fakeMatrixSender{}
+	runner := NewRunner(sender, nil, slog.Default())
+
+	actions := []decision.Action{{
+		Kind:  decision.ActionKindReply,
+		Reply: &decision.ReplyAction{Content: "hello"},
+	}}
+
+	results := runner.Execute(context.Background(), "!room:test", actions)
+	if len(results) != 1 || results[0].Err != nil {
+		t.Fatalf("unexpected results: %+v", results)
+	}
+	if len(sender.calls) != 1 {
+		t.Fatalf("expected 1 call, got %d", len(sender.calls))
+	}
+	c := sender.calls[0]
+	if c.method != "SendMarkdown" {
+		t.Errorf("expected SendMarkdown, got %s", c.method)
+	}
+	if c.roomID != "!room:test" {
+		t.Errorf("expected room !room:test, got %s", c.roomID)
+	}
+}
+
+func TestExecuteReply_WithInReplyTo(t *testing.T) {
+	sender := &fakeMatrixSender{}
+	runner := NewRunner(sender, nil, slog.Default())
+
+	actions := []decision.Action{{
+		Kind:  decision.ActionKindReply,
+		Reply: &decision.ReplyAction{Content: "hello", InReplyTo: "$evt1"},
+	}}
+
+	runner.Execute(context.Background(), "!room:test", actions)
+
+	if len(sender.calls) != 1 {
+		t.Fatalf("expected 1 call, got %d", len(sender.calls))
+	}
+	c := sender.calls[0]
+	if c.method != "SendReplyMarkdown" {
+		t.Errorf("expected SendReplyMarkdown, got %s", c.method)
+	}
+	if c.inReplyTo != "$evt1" {
+		t.Errorf("expected inReplyTo=$evt1, got %s", c.inReplyTo)
+	}
+}
+
+func TestExecuteReply_WithThread(t *testing.T) {
+	sender := &fakeMatrixSender{}
+	runner := NewRunner(sender, nil, slog.Default())
+
+	actions := []decision.Action{{
+		Kind: decision.ActionKindReply,
+		Reply: &decision.ReplyAction{
+			Content:   "thread reply",
+			ThreadID:  "$root",
+			InReplyTo: "$evt2",
+		},
+	}}
+
+	runner.Execute(context.Background(), "!room:test", actions)
+
+	if len(sender.calls) != 1 {
+		t.Fatalf("expected 1 call, got %d", len(sender.calls))
+	}
+	c := sender.calls[0]
+	if c.method != "SendThreadMarkdown" {
+		t.Errorf("expected SendThreadMarkdown, got %s", c.method)
+	}
+	if c.threadRootID != "$root" {
+		t.Errorf("expected threadRootID=$root, got %s", c.threadRootID)
+	}
+	if c.inReplyTo != "$evt2" {
+		t.Errorf("expected inReplyTo=$evt2, got %s", c.inReplyTo)
+	}
+	if c.roomID != "!room:test" {
+		t.Errorf("expected room !room:test, got %s", c.roomID)
+	}
+}
+
+func TestExecuteReply_ThreadWithoutInReplyTo(t *testing.T) {
+	sender := &fakeMatrixSender{}
+	runner := NewRunner(sender, nil, slog.Default())
+
+	actions := []decision.Action{{
+		Kind: decision.ActionKindReply,
+		Reply: &decision.ReplyAction{
+			Content:  "thread reply no fallback",
+			ThreadID: "$root",
+		},
+	}}
+
+	runner.Execute(context.Background(), "!room:test", actions)
+
+	if len(sender.calls) != 1 {
+		t.Fatalf("expected 1 call, got %d", len(sender.calls))
+	}
+	c := sender.calls[0]
+	if c.method != "SendThreadMarkdown" {
+		t.Errorf("expected SendThreadMarkdown, got %s", c.method)
+	}
+	// inReplyTo should be empty; SendThreadMarkdown will default to threadRootID
+	if c.inReplyTo != "" {
+		t.Errorf("expected empty inReplyTo, got %s", c.inReplyTo)
+	}
+}
+
+func TestExecuteReply_NilReply(t *testing.T) {
+	sender := &fakeMatrixSender{}
+	runner := NewRunner(sender, nil, slog.Default())
+
+	actions := []decision.Action{{
+		Kind:  decision.ActionKindReply,
+		Reply: nil,
+	}}
+
+	results := runner.Execute(context.Background(), "!room:test", actions)
+	if len(results) != 1 {
+		t.Fatalf("expected 1 result, got %d", len(results))
+	}
+	if results[0].Err == nil {
+		t.Error("expected error for nil reply")
+	}
+}
diff --git a/shell/matrix/client.go b/shell/matrix/client.go
index 68229f4..7155d49 100644
--- a/shell/matrix/client.go
+++ b/shell/matrix/client.go
@@ -320,6 +320,26 @@ func (c *Client) SendReplyMarkdown(ctx context.Context, roomID, inReplyTo, markd
 	return err
 }
 
+// SendThreadMarkdown sends a formatted message as part of a Matrix thread.
+// threadRootID is the event that started the thread (always the same for all messages in a thread).
+// inReplyTo is the specific event being replied to within the thread (used as fallback for non-thread clients).
+// If inReplyTo is empty, it defaults to threadRootID.
+func (c *Client) SendThreadMarkdown(ctx context.Context, roomID, threadRootID, inReplyTo, markdown string) error {
+	if inReplyTo == "" {
+		inReplyTo = threadRootID
+	}
+	html := mdToHTML(markdown)
+	content := event.MessageEventContent{
+		MsgType:       event.MsgText,
+		Body:          markdown,
+		Format:        event.FormatHTML,
+		FormattedBody: html,
+		RelatesTo:     (&event.RelatesTo{}).SetThread(id.EventID(threadRootID), id.EventID(inReplyTo)),
+	}
+	_, err := c.raw.SendMessageEvent(ctx, id.RoomID(roomID), event.EventMessage, content)
+	return err
+}
+
 // SendReaction sends a reaction to an event.
 func (c *Client) SendReaction(ctx context.Context, roomID, eventID, reaction string) error {
 	_, err := c.raw.SendReaction(ctx, id.RoomID(roomID), id.EventID(eventID), reaction)
diff --git a/shell/matrix/listener.go b/shell/matrix/listener.go
index 5b585cf..5486514 100644
--- a/shell/matrix/listener.go
+++ b/shell/matrix/listener.go
@@ -153,6 +153,17 @@ func (l *Listener) Run(ctx context.Context) error {
 		)
 		msgCtx.EventID = evt.ID.String()
 
+		// Extract thread root from m.relates_to (Matrix thread support).
+		if l.cfg.Threads.Enabled {
+			if relatesTo, ok := evt.Content.Raw["m.relates_to"].(map[string]any); ok {
+				if relType, _ := relatesTo["rel_type"].(string); relType == "m.thread" {
+					if threadRoot, _ := relatesTo["event_id"].(string); threadRoot != "" {
+						msgCtx.ThreadID = threadRoot
+					}
+				}
+			}
+		}
+
 		l.logger.Debug("message parsed",
 			"sender", msgCtx.SenderID,
 			"room", msgCtx.RoomID,
diff --git a/shell/matrix/thread_relates_test.go b/shell/matrix/thread_relates_test.go
new file mode 100644
index 0000000..2771a6b
--- /dev/null
+++ b/shell/matrix/thread_relates_test.go
@@ -0,0 +1,66 @@
+package matrix
+
+import (
+	"encoding/json"
+	"testing"
+
+	"maunium.net/go/mautrix/event"
+	"maunium.net/go/mautrix/id"
+)
+
+// TestThreadRelatesToStructure verifies the JSON structure produced by SetThread
+// matches the Matrix spec for m.thread events.
+func TestThreadRelatesToStructure(t *testing.T) {
+	rel := (&event.RelatesTo{}).SetThread(id.EventID("$root"), id.EventID("$last"))
+
+	data, err := json.Marshal(rel)
+	if err != nil {
+		t.Fatalf("marshal error: %v", err)
+	}
+
+	var m map[string]any
+	if err := json.Unmarshal(data, &m); err != nil {
+		t.Fatalf("unmarshal error: %v", err)
+	}
+
+	if m["rel_type"] != "m.thread" {
+		t.Errorf("expected rel_type=m.thread, got %v", m["rel_type"])
+	}
+	if m["event_id"] != "$root" {
+		t.Errorf("expected event_id=$root, got %v", m["event_id"])
+	}
+	if m["is_falling_back"] != true {
+		t.Errorf("expected is_falling_back=true, got %v", m["is_falling_back"])
+	}
+
+	inReplyTo, ok := m["m.in_reply_to"].(map[string]any)
+	if !ok {
+		t.Fatal("expected m.in_reply_to to be an object")
+	}
+	if inReplyTo["event_id"] != "$last" {
+		t.Errorf("expected m.in_reply_to.event_id=$last, got %v", inReplyTo["event_id"])
+	}
+}
+
+// TestThreadRelatesToStructure_SameRootAndFallback verifies that when root == fallback,
+// the structure is still correct.
+func TestThreadRelatesToStructure_SameRootAndFallback(t *testing.T) {
+	rel := (&event.RelatesTo{}).SetThread(id.EventID("$root"), id.EventID("$root"))
+
+	data, err := json.Marshal(rel)
+	if err != nil {
+		t.Fatalf("marshal error: %v", err)
+	}
+
+	var m map[string]any
+	if err := json.Unmarshal(data, &m); err != nil {
+		t.Fatalf("unmarshal error: %v", err)
+	}
+
+	if m["rel_type"] != "m.thread" {
+		t.Errorf("expected rel_type=m.thread, got %v", m["rel_type"])
+	}
+	if m["event_id"] != "$root" {
+		t.Errorf("expected event_id=$root, got %v", m["event_id"])
+	}
+}
diff --git a/shell/matrix/thread_test.go b/shell/matrix/thread_test.go
new file mode 100644
index 0000000..8d6d3dd
--- /dev/null
+++ b/shell/matrix/thread_test.go
@@ -0,0 +1,95 @@
+package matrix
+
+import (
+	"testing"
+)
+
+// extractThreadID is the pure logic extracted from the listener for testability.
+// It mirrors the extraction in listener.go's EventMessage handler.
+func extractThreadID(rawContent map[string]any) string {
+	relatesTo, ok := rawContent["m.relates_to"].(map[string]any)
+	if !ok {
+		return ""
+	}
+	relType, _ := relatesTo["rel_type"].(string)
+	if relType != "m.thread" {
+		return ""
+	}
+	threadRoot, _ := relatesTo["event_id"].(string)
+	return threadRoot
+}
+
+func TestExtractThreadID_ThreadMessage(t *testing.T) {
+	raw := map[string]any{
+		"body": "hello",
+		"m.relates_to": map[string]any{
+			"rel_type": "m.thread",
+			"event_id": "$root123",
+			"is_falling_back": true,
+			"m.in_reply_to": map[string]any{
+				"event_id": "$last456",
+			},
+		},
+	}
+
+	got := extractThreadID(raw)
+	if got != "$root123" {
+		t.Errorf("expected $root123, got %q", got)
+	}
+}
+
+func TestExtractThreadID_ReplyNotThread(t *testing.T) {
+	raw := map[string]any{
+		"body": "hello",
+		"m.relates_to": map[string]any{
+			"m.in_reply_to": map[string]any{
+				"event_id": "$evt789",
+			},
+		},
+	}
+
+	got := extractThreadID(raw)
+	if got != "" {
+		t.Errorf("expected empty string for non-thread reply, got %q", got)
+	}
+}
+
+func TestExtractThreadID_NoRelatesTo(t *testing.T) {
+	raw := map[string]any{
+		"body": "plain message",
+	}
+
+	got := extractThreadID(raw)
+	if got != "" {
+		t.Errorf("expected empty string for plain message, got %q", got)
+	}
+}
+
+func TestExtractThreadID_ReplaceRelation(t *testing.T) {
+	raw := map[string]any{
+		"body": "edited",
+		"m.relates_to": map[string]any{
+			"rel_type": "m.replace",
+			"event_id": "$original",
+		},
+	}
+
+	got := extractThreadID(raw)
+	if got != "" {
+		t.Errorf("expected empty string for m.replace, got %q", got)
+	}
+}
+
+func TestExtractThreadID_ThreadWithoutEventID(t *testing.T) {
+	raw := map[string]any{
+		"m.relates_to": map[string]any{
+			"rel_type": "m.thread",
+			// missing event_id
+		},
+	}
+
+	got := extractThreadID(raw)
+	if got != "" {
+		t.Errorf("expected empty string for thread without event_id, got %q", got)
+	}
+}