diff --git a/internal/http/agents.go b/internal/http/agents.go
index 675332e7..5741ad05 100644
--- a/internal/http/agents.go
+++ b/internal/http/agents.go
@@ -368,6 +368,18 @@ func (h *AgentsHandler) handleUpdate(w http.ResponseWriter, r *http.Request) {
 	allowed := filterAllowedKeys(updates, agentAllowedFields)
 	allowed["restrict_to_workspace"] = true
 
+	// If agent_key is being changed, enforce the slug format. The router
+	// cache uses `tenantID:agentKey` as its canonical key and splits on the
+	// last colon for exact-segment invalidation — a colon inside agent_key
+	// would silently break invalidation. Slug regex already rejects colons
+	// and any other shell/path-unfriendly characters.
+	if newKey, ok := allowed["agent_key"].(string); ok && newKey != "" {
+		if !isValidSlug(newKey) {
+			writeError(w, http.StatusBadRequest, protocol.ErrInvalidRequest, i18n.T(locale, i18n.MsgInvalidSlug, "agent_key"))
+			return
+		}
+	}
+
 	// Validate v3 flag values in other_config (must be boolean).
 	if oc, ok := allowed["other_config"]; ok && oc != nil {
 		switch v := oc.(type) {
diff --git a/internal/http/validate_slug_test.go b/internal/http/validate_slug_test.go
new file mode 100644
index 00000000..1cc13352
--- /dev/null
+++ b/internal/http/validate_slug_test.go
@@ -0,0 +1,41 @@
+package http
+
+import "testing"
+
+// TestIsValidSlug covers the slug predicate used by agent_key, skill slug,
+// provider name, and MCP server name validation. The slug format is the
+// router cache's canonical anchor — the cache splits on the last colon for
+// exact-segment invalidation, so the predicate MUST reject any character
+// that would collide with that split (notably `:`).
+func TestIsValidSlug(t *testing.T) {
+	cases := []struct {
+		name  string
+		input string
+		want  bool
+	}{
+		{"simple lowercase", "agent", true},
+		{"with hyphen", "goctech-leader", true},
+		{"with digits", "agent-42", true},
+		{"single char", "a", true},
+		{"starts with digit", "1-agent", true},
+
+		{"empty", "", false},
+		{"uppercase rejected", "Agent", false},
+		{"starts with hyphen", "-agent", false},
+		{"ends with hyphen", "agent-", false},
+		{"colon rejected", "weird:key", false},
+		{"slash rejected", "weird/key", false},
+		{"whitespace rejected", "weird key", false},
+		{"dot rejected", "weird.key", false},
+		{"tab rejected", "weird\tkey", false},
+		{"underscore rejected", "weird_key", false},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := isValidSlug(tc.input); got != tc.want {
+				t.Errorf("isValidSlug(%q) = %v, want %v", tc.input, got, tc.want)
+			}
+		})
+	}
+}