diff --git a/server/cancel_subagent_test.go b/server/cancel_subagent_test.go
new file mode 100644
index 00000000..1a85f0c8
--- /dev/null
+++ b/server/cancel_subagent_test.go
@@ -0,0 +1,110 @@
+package server
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"shelley.exe.dev/db"
+	"shelley.exe.dev/db/generated"
+	"shelley.exe.dev/llm"
+)
+
+// TestCancelParentCancelsSubagents verifies that cancelling a parent conversation
+// also cancels any active subagent conversations.
+func TestCancelParentCancelsSubagents(t *testing.T) {
+	server, database, _ := newTestServer(t)
+
+	// Create parent conversation
+	conversation, err := database.CreateConversation(context.Background(), nil, true, nil, nil, db.ConversationOptions{})
+	if err != nil {
+		t.Fatalf("failed to create conversation: %v", err)
+	}
+	parentID := conversation.ConversationID
+
+	// Start the parent conversation. The predictable model will:
+	// 1. See "subagent: worker bash: sleep 30" and invoke the subagent tool
+	// 2. The subagent will receive "bash: sleep 30" and start a long sleep
+	chatReq := ChatRequest{
+		Message: "subagent: worker bash: sleep 30",
+		Model:   "predictable",
+	}
+	chatBody, _ := json.Marshal(chatReq)
+
+	req := httptest.NewRequest("POST", "/api/conversation/"+parentID+"/chat", strings.NewReader(string(chatBody)))
+	req.Header.Set("Content-Type", "application/json")
+	w := httptest.NewRecorder()
+	server.handleChatConversation(w, req, parentID)
+	if w.Code != http.StatusAccepted {
+		t.Fatalf("expected 202, got %d: %s", w.Code, w.Body.String())
+	}
+
+	// Wait for the subagent to exist and start working
+	var subagentID string
+	waitFor(t, 10*time.Second, func() bool {
+		subagents, err := database.GetSubagents(context.Background(), parentID)
+		if err != nil || len(subagents) == 0 {
+			return false
+		}
+		subagentID = subagents[0].ConversationID
+		return server.IsAgentWorking(subagentID)
+	})
+
+	t.Logf("subagent %s is working", subagentID)
+
+	// Verify parent is also working (blocked on the subagent tool call)
+	if !server.IsAgentWorking(parentID) {
+		t.Fatal("expected parent to be working")
+	}
+
+	// Cancel the parent
+	cancelReq := httptest.NewRequest("POST", "/api/conversation/"+parentID+"/cancel", nil)
+	cancelW := httptest.NewRecorder()
+	server.handleCancelConversation(cancelW, cancelReq, parentID)
+	if cancelW.Code != http.StatusOK {
+		t.Fatalf("cancel expected 200, got %d: %s", cancelW.Code, cancelW.Body.String())
+	}
+
+	// Wait for parent to stop working
+	waitFor(t, 5*time.Second, func() bool {
+		return !server.IsAgentWorking(parentID)
+	})
+
+	// The subagent must also stop working
+	waitFor(t, 5*time.Second, func() bool {
+		return !server.IsAgentWorking(subagentID)
+	})
+
+	// Verify subagent has a cancellation end-of-turn message
+	var subMsgs []generated.Message
+	err = database.Queries(context.Background(), func(q *generated.Queries) error {
+		var qerr error
+		subMsgs, qerr = q.ListMessages(context.Background(), subagentID)
+		return qerr
+	})
+	if err != nil {
+		t.Fatalf("failed to list subagent messages: %v", err)
+	}
+
+	foundEndTurn := false
+	for _, msg := range subMsgs {
+		if msg.Type != string(db.MessageTypeAgent) || msg.LlmData == nil {
+			continue
+		}
+		var llmMsg llm.Message
+		if err := json.Unmarshal([]byte(*msg.LlmData), &llmMsg); err != nil {
+			continue
+		}
+		if llmMsg.EndOfTurn {
+			foundEndTurn = true
+			break
+		}
+	}
+	if !foundEndTurn {
+		t.Error("expected subagent to have an end-of-turn message after parent cancellation")
+	}
+}
diff --git a/server/handlers.go b/server/handlers.go
index 1ae85a39..4a10a4ef 100644
--- a/server/handlers.go
+++ b/server/handlers.go
@@ -908,6 +908,9 @@ func (s *Server) handleCancelConversation(w http.ResponseWriter, r *http.Request
 		return
 	}
 
+	// Cancel active subagent conversations first
+	s.cancelSubagents(ctx, conversationID)
+
 	// Cancel the conversation
 	if err := manager.CancelConversation(ctx); err != nil {
 		s.logger.Error("Failed to cancel conversation", "conversationID", conversationID, "error", err)
diff --git a/server/server.go b/server/server.go
index c2c65592..be017495 100644
--- a/server/server.go
+++ b/server/server.go
@@ -1126,6 +1126,28 @@ func (s *Server) getWorkingConversations() map[string]bool {
 	return working
 }
 
+// cancelSubagents cancels all active subagent conversations for the given parent.
+func (s *Server) cancelSubagents(ctx context.Context, parentID string) {
+	subagents, err := s.db.GetSubagents(ctx, parentID)
+	if err != nil {
+		s.logger.Error("Failed to get subagents for cancellation", "parentID", parentID, "error", err)
+		return
+	}
+
+	for _, sub := range subagents {
+		s.mu.Lock()
+		mgr, ok := s.activeConversations[sub.ConversationID]
+		s.mu.Unlock()
+		if !ok || !mgr.IsAgentWorking() {
+			continue
+		}
+		s.logger.Info("Cancelling subagent", "subagentID", sub.ConversationID, "parentID", parentID)
+		if err := mgr.CancelConversation(ctx); err != nil {
+			s.logger.Error("Failed to cancel subagent", "subagentID", sub.ConversationID, "error", err)
+		}
+	}
+}
+
 // IsAgentWorking returns whether the agent is currently working on the given conversation.
 // Returns false if the conversation doesn't have an active manager.
 func (s *Server) IsAgentWorking(conversationID string) bool {