diff --git a/CHANGELOG.md b/CHANGELOG.md
index f9ae3a0f8..d68675bb1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Fix MCP OAuth credentials cache not invalidating when the server URL changes.
+- Add `isSubagent` condition variable for chat system instructions
 
 ## 0.112.0
 
diff --git a/src/eca/features/prompt.clj b/src/eca/features/prompt.clj
index 9b2ea1fd7..5a48b62fd 100644
--- a/src/eca/features/prompt.clj
+++ b/src/eca/features/prompt.clj
@@ -92,17 +92,21 @@
      (str "\n<additionalContext from=\"chatStart\">\n" startup-ctx "\n</additionalContext>\n\n"))
    "</contexts>"))
 
-(defn ^:private ->base-selmer-ctx [all-tools db]
-  (merge
-   {:workspaceRoots (shared/workspaces-as-str db)}
-   (reduce
-    (fn [m tool]
-      (assoc m (keyword (str "toolEnabled_" (:full-name tool))) true))
-    {}
-    all-tools)))
+(defn ^:private ->base-selmer-ctx
+  ([all-tools db]
+   (->base-selmer-ctx all-tools nil db))
+  ([all-tools chat-id db]
+   (merge
+    {:workspaceRoots (shared/workspaces-as-str db)
+     :isSubagent (boolean (get-in db [:chats chat-id :subagent]))}
+    (reduce
+     (fn [m tool]
+       (assoc m (keyword (str "toolEnabled_" (:full-name tool))) true))
+     {}
+     all-tools))))
 
 (defn build-chat-instructions [refined-contexts rules skills repo-map* agent-name config chat-id all-tools db]
-  (let [selmer-ctx (->base-selmer-ctx all-tools db)]
+  (let [selmer-ctx (->base-selmer-ctx all-tools chat-id db)]
     (multi-str
      (selmer/render (eca-chat-prompt agent-name config) selmer-ctx)
      (when (seq rules)
diff --git a/test/eca/features/prompt_test.clj b/test/eca/features/prompt_test.clj
index 68ec86ca8..aa0de343e 100644
--- a/test/eca/features/prompt_test.clj
+++ b/test/eca/features/prompt_test.clj
@@ -58,3 +58,17 @@
       (is (string/includes? result "<resource uri=\"custom://my-resource\">some-cool-content</resource>"))
       (is (string/includes? result "</contexts>"))
       (is (string? result)))))
+
+(deftest build-instructions-subagent-condition-test
+  (let [config {:prompts {:chat "{% if isSubagent %}SUBAGENT{% endif %}{% if not isSubagent %}MAIN{% endif %}"}}]
+    (testing "renders subagent-only content for subagent chats"
+      (let [db (assoc-in (h/db) [:chats "sub-chat" :subagent] {:name "explorer"})
+            result (prompt/build-chat-instructions [] [] [] (delay "TREE") "code" config "sub-chat" [] db)]
+        (is (string/includes? result "SUBAGENT"))
+        (is (not (string/includes? result "MAIN")))))
+
+    (testing "renders main-agent-only content for non-subagent chats"
+      (let [db (assoc-in (h/db) [:chats "main-chat"] {:id "main-chat"})
+            result (prompt/build-chat-instructions [] [] [] (delay "TREE") "code" config "main-chat" [] db)]
+        (is (string/includes? result "MAIN"))
+        (is (not (string/includes? result "SUBAGENT")))))))