diff --git a/claim_test.go b/claim_test.go
new file mode 100644
index 0000000..6bad3a4
--- /dev/null
+++ b/claim_test.go
@@ -0,0 +1,91 @@
+package main
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func postMsg(t *testing.T, ts *httptest.Server, id, body string) (int, map[string]any) {
+	t.Helper()
+	return post(t, ts.URL+"/api/nests/"+id+"/messages", body)
+}
+
+func postRequest(t *testing.T, ts *httptest.Server, id, mode, text string) string {
+	t.Helper()
+	code, out := postMsg(t, ts, id, `{"author":"owner","kind":"request","mode":"`+mode+`","text":"`+text+`"}`)
+	if code != http.StatusCreated {
+		t.Fatalf("request: %d %v", code, out)
+	}
+	return out["msg_id"].(string)
+}
+
+func TestSoloSecondClaimRefused(t *testing.T) {
+	ts, s := newTestNestServer(t)
+	id := newNest(t, ts, "Lab", `"Hugin","Munin"`)
+	r := postRequest(t, ts, id, "solo", "count the eggs")
+
+	code1, c1 := postMsg(t, ts, id, `{"author":"Hugin","kind":"claim","refers_to":"`+r+`","text":"on it"}`)
+	if code1 != http.StatusCreated || c1["to"] != "owner" {
+		t.Fatalf("first claim should win and address requester: %d %v", code1, c1)
+	}
+	code2, out := postMsg(t, ts, id, `{"author":"Munin","kind":"claim","refers_to":"`+r+`","text":"mine too"}`)
+	if code2 != http.StatusConflict {
+		t.Fatalf("second solo claim must be refused 409, got %d %v", code2, out)
+	}
+	if msg, _ := out["message"].(string); !strings.Contains(msg, "Hugin") {
+		t.Fatalf("refusal must name the owner Hugin, got %q", msg)
+	}
+	if len(s.msgs.All(id)) != 2 { // request + Hugin's claim, not Munin's
+		t.Fatalf("refused claim must not be stored: %+v", s.msgs.All(id))
+	}
+}
+
+func TestConspiracyMultipleClaims(t *testing.T) {
+	ts, s := newTestNestServer(t)
+	id := newNest(t, ts, "Lab", `"Hugin","Munin"`)
+	r := postRequest(t, ts, id, "conspiracy", "is this plan risky")
+
+	for _, c := range []string{"Hugin", "Munin"} {
+		code, out := postMsg(t, ts, id, `{"author":"`+c+`","kind":"claim","refers_to":"`+r+`","text":"lens `+c+`"}`)
+		if code != http.StatusCreated || out["refers_to"] != r {
+			t.Fatalf("conspiracy claim by %s refused: %d %v", c, code, out)
+		}
+	}
+	claims := 0
+	for _, m := range s.msgs.All(id) {
+		if m.Kind == "claim" && m.RefersTo == r {
+			claims++
+		}
+	}
+	if claims != 2 {
+		t.Fatalf("want 2 conspiracy claims stored, got %d", claims)
+	}
+}
+
+func TestSplitSubtaskFirstClaimWins(t *testing.T) {
+	ts, _ := newTestNestServer(t)
+	id := newNest(t, ts, "Lab", `"Hugin","Munin"`)
+	r := postRequest(t, ts, id, "split", "build and test")
+	subOf := func(text string) string {
+		code, out := postMsg(t, ts, id, `{"author":"owner","kind":"request","mode":"solo","text":"`+text+`","refers_to":"`+r+`"}`)
+		if code != http.StatusCreated {
+			t.Fatalf("subtask: %d %v", code, out)
+		}
+		return out["msg_id"].(string)
+	}
+	a := subOf("build")
+	b := subOf("test")
+	// same subtask: second claim refused
+	if code, _ := postMsg(t, ts, id, `{"author":"Hugin","kind":"claim","refers_to":"`+a+`","text":"b1"}`); code != http.StatusCreated {
+		t.Fatalf("first subtask claim: %d", code)
+	}
+	if code, _ := postMsg(t, ts, id, `{"author":"Munin","kind":"claim","refers_to":"`+a+`","text":"b2"}`); code != http.StatusConflict {
+		t.Fatalf("second claim on same subtask must 409, got %d", code)
+	}
+	// different subtask: allowed
+	if code, _ := postMsg(t, ts, id, `{"author":"Munin","kind":"claim","refers_to":"`+b+`","text":"t1"}`); code != http.StatusCreated {
+		t.Fatalf("claim on other subtask: %d", code)
+	}
+}