Support inherit key in agent config to inherit settings from another agent

Author: ericdallo

AGENTS.md

@@ -31,4 +31,4 @@ ECA Agent Guide (AGENTS.md)
 - Avoid adding too many comments, only add essential or when you think is really important to mention something. 
 - ECA's protocol specification of client <-> server lives in docs/protocol.md
 - If changing ECA config structure, remember to update its docs/config.json
-- When adding support to a new feature or fixing a existing github issue, add a entry to Unreleased in CHANGELOG.md if not already there, be concise like the rest.
+- When adding support to a new feature or fixing a existing github issue, add a entry to Unreleased in CHANGELOG.md if not already there as last entry, be concise like the rest.

CHANGELOG.md

@@ -6,6 +6,7 @@
   - Plugins can provide skills, MCP servers, agents, commands, hooks, rules, and arbitrary config overrides.
   - Add commands `/plugins` and `/plugin-install`.
 - Fix race condition where stopping a prompt and immediately sending a new one could cause two concurrent prompts with no way to stop the older one.
+- Support `inherit` key in agent config to inherit settings from another agent.
 
 ## 0.111.0
 

docs/config.json

@@ -567,6 +567,12 @@
       "description": "A named agent configuration.",
       "markdownDescription": "A named agent configuration.",
       "properties": {
+        "inherit": {
+          "type": "string",
+          "description": "Name of another agent to inherit configuration from. The current agent's settings are deep-merged on top of the inherited agent's settings.",
+          "markdownDescription": "Name of another agent to inherit configuration from. The current agent's settings are deep-merged on top of the inherited agent's settings.",
+          "examples": ["code", "plan", "explorer"]
+        },
         "mode": {
           "type": "string",
           "description": "Agent mode. 'primary' for standalone agents, 'subagent' for agent-spawnable agents.",

src/eca/config.clj

@@ -316,6 +316,31 @@
                         (last args)))
          maps))
 
+(defn ^:private resolve-agent-inheritance
+  "Resolves :inherit keys in agent configs. When an agent has :inherit \"other\",
+   its config is deep-merged on top of the parent agent's config (child wins).
+   The :inherit key is stripped from the resolved config."
+  [agents]
+  (reduce-kv
+   (fn [result agent-name agent-config]
+     (if-let [parent-name (:inherit agent-config)]
+       (let [parent-config (get agents parent-name)]
+         (cond
+           (= parent-name agent-name)
+           (do (logger/warn logger-tag (format "Agent '%s' inherits from itself, ignoring inherit" agent-name))
+               (assoc result agent-name (dissoc agent-config :inherit)))
+
+           (nil? parent-config)
+           (do (logger/warn logger-tag (format "Agent '%s' inherits from unknown agent '%s', ignoring inherit" agent-name parent-name))
+               (assoc result agent-name (dissoc agent-config :inherit)))
+
+           :else
+           (assoc result agent-name (deep-merge (dissoc parent-config :inherit)
+                                                (dissoc agent-config :inherit)))))
+       (assoc result agent-name agent-config)))
+   {}
+   agents))
+
 (defn ^:private eca-version* []
   (string/trim (slurp (io/resource "ECA_VERSION"))))
 
@@ -493,7 +518,8 @@
                 (if (or (seq md-agent-configs) (seq plugin-agents))
                   (update config :agent (fn [existing]
                                           (merge md-agent-configs plugin-agents existing)))
-                  config))))))
+                  config)))
+        (update :agent resolve-agent-inheritance))))
 
 (def all (memoize/ttl all* :ttl/threshold ttl-cache-config-ms))
 

test/eca/config_test.clj

@@ -166,6 +166,65 @@
       (with-redefs [logger/warn (fn [_ _] nil)]
         (is (= "code" (config/validate-agent-name "anything" config)))))))
 
+(deftest resolve-agent-inheritance-test
+  (testing "basic inheritance copies parent fields to child"
+    (let [agents {"plan" {:mode "primary"
+                          :disabledTools ["edit_file" "write_file"]
+                          :toolCall {:approval {:byDefault "ask"}}}
+                  "my-plan" {:inherit "plan"
+                             :description "custom plan"}}]
+      (is (match?
+           {"plan" {:mode "primary"
+                    :disabledTools ["edit_file" "write_file"]}
+            "my-plan" {:mode "primary"
+                       :disabledTools ["edit_file" "write_file"]
+                       :toolCall {:approval {:byDefault "ask"}}
+                       :description "custom plan"}}
+           (#'config/resolve-agent-inheritance agents)))))
+
+  (testing "child values override parent values"
+    (let [agents {"code" {:mode "primary"
+                          :disabledTools ["preview_file_change"]
+                          :defaultModel "anthropic/claude-sonnet-4-6"}
+                  "my-code" {:inherit "code"
+                             :defaultModel "openai/gpt-5"}}]
+      (is (match?
+           {"my-code" {:mode "primary"
+                       :disabledTools ["preview_file_change"]
+                       :defaultModel "openai/gpt-5"}}
+           (#'config/resolve-agent-inheritance agents)))))
+
+  (testing "missing parent is skipped with warning"
+    (let [agents {"child" {:inherit "nonexistent"
+                           :mode "primary"}}]
+      (is (match?
+           {"child" {:mode "primary"}}
+           (#'config/resolve-agent-inheritance agents)))
+      (is (not (contains? (get (#'config/resolve-agent-inheritance agents) "child") :inherit)))))
+
+  (testing "self-inheritance is skipped"
+    (let [agents {"self" {:inherit "self"
+                          :mode "primary"}}]
+      (is (match?
+           {"self" {:mode "primary"}}
+           (#'config/resolve-agent-inheritance agents)))
+      (is (not (contains? (get (#'config/resolve-agent-inheritance agents) "self") :inherit)))))
+
+  (testing "agent without inherit is unchanged"
+    (let [agents {"code" {:mode "primary"
+                          :disabledTools ["preview_file_change"]}}]
+      (is (match?
+           {"code" {:mode "primary"
+                    :disabledTools ["preview_file_change"]}}
+           (#'config/resolve-agent-inheritance agents)))))
+
+  (testing "inherit key is stripped from resolved config"
+    (let [agents {"plan" {:mode "primary"}
+                  "child" {:inherit "plan"
+                           :description "my child"}}
+          resolved (#'config/resolve-agent-inheritance agents)]
+      (is (not (contains? (get resolved "child") :inherit))))))
+
 (deftest diff-keeping-vectors-test
   (testing "like clojure.data/diff"
     (is (= {:b 3}