diff --git a/api/core/dify_builder/placeholder_agent.py b/api/core/dify_builder/placeholder_agent.py
index 54d59ce1adc..18607b322ca 100644
--- a/api/core/dify_builder/placeholder_agent.py
+++ b/api/core/dify_builder/placeholder_agent.py
@@ -175,6 +175,15 @@ class PlaceholderAgent:
             )
         ]
 
+    def learn_from_build(
+        self,
+        goal_text: str,
+        requirements: dict[str, Any],
+        plan_items: list[str],
+        built_node_ids: list[str],
+    ) -> str:
+        return "Reusable skill: a Start->Knowledge->LLM->End summary workflow"
+
     # -- Edit cognition (Slice 3; fixed, deterministic canned output) --
 
     def analyze_impact(self, goal_text: str, graph: Graph) -> dict[str, Any]:
diff --git a/api/core/dify_builder/ports.py b/api/core/dify_builder/ports.py
index 81ce663b8c9..1d4513cbd9f 100644
--- a/api/core/dify_builder/ports.py
+++ b/api/core/dify_builder/ports.py
@@ -91,6 +91,14 @@ class DifyBuilderAgent(Protocol):
 
     def propose_build_repair(self, built_node_ids: list[str]) -> list[MutationIntent]: ...
 
+    def learn_from_build(
+        self,
+        goal_text: str,
+        requirements: dict[str, Any],
+        plan_items: list[str],
+        built_node_ids: list[str],
+    ) -> str: ...
+
     # -- Edit cognition (Slice 3; canned in PlaceholderAgent) --
 
     def analyze_impact(self, goal_text: str, graph: Graph) -> dict[str, Any]: ...
diff --git a/api/tests/unit_tests/core/dify_builder/fakes.py b/api/tests/unit_tests/core/dify_builder/fakes.py
index e417f96008c..94bac27fc1d 100644
--- a/api/tests/unit_tests/core/dify_builder/fakes.py
+++ b/api/tests/unit_tests/core/dify_builder/fakes.py
@@ -367,6 +367,10 @@ class StubAgent:
     def propose_build_repair(self, _built_node_ids):
         return []
 
+    def learn_from_build(self, _goal_text, _requirements, _plan_items, _built_node_ids):
+        self.learn_calls = getattr(self, "learn_calls", 0) + 1
+        return "stub-skill"
+
     def analyze_impact(self, _goal_text, _graph):
         return {"edit_rules": {}, "target_node_ids": []}
 
diff --git a/api/tests/unit_tests/core/dify_builder/test_placeholder_agent.py b/api/tests/unit_tests/core/dify_builder/test_placeholder_agent.py
index be4952de014..91c3e47bffc 100644
--- a/api/tests/unit_tests/core/dify_builder/test_placeholder_agent.py
+++ b/api/tests/unit_tests/core/dify_builder/test_placeholder_agent.py
@@ -180,3 +180,10 @@ def test_propose_build_repair_targets_the_llm_node():
     assert len(intents) == 1
     assert intents[0].op == "set_node_config"
     assert intents[0].args["node_id"] == BUILD_LLM_ID
+
+
+def test_placeholder_learn_from_build_returns_descriptor():
+    from core.dify_builder.placeholder_agent import PlaceholderAgent
+
+    out = PlaceholderAgent().learn_from_build("summarize a PDF", {}, ["step"], ["llm"])
+    assert isinstance(out, str) and out != ""
diff --git a/api/tests/unit_tests/core/dify_builder/test_ports.py b/api/tests/unit_tests/core/dify_builder/test_ports.py
index a5e3216e52b..68492f9ad85 100644
--- a/api/tests/unit_tests/core/dify_builder/test_ports.py
+++ b/api/tests/unit_tests/core/dify_builder/test_ports.py
@@ -131,6 +131,9 @@ class _StubAgent:
     def propose_build_repair(self, _built_node_ids):
         return []
 
+    def learn_from_build(self, _goal_text, _requirements, _plan_items, _built_node_ids):
+        return "stub"
+
     def analyze_impact(self, _goal_text, _graph):
         return {"edit_rules": {}, "target_node_ids": []}
 
@@ -267,3 +270,23 @@ def test_pcstate_is_the_state_module_type_used_by_compare_and_advance():
     # compare_and_advance's `next` parameter is typed PcState; sanity-check
     # the import path ports.py takes matches state.py's canonical enum.
     assert PcState.FIX_DIAGNOSE == "fix.diagnose"
+
+
+def test_agent_missing_learn_from_build_is_not_a_dify_builder_agent():
+    from core.dify_builder.ports import DifyBuilderAgent
+
+    class _Missing:  # a former-complete agent lacking only learn_from_build
+        def diagnose(self, *a): ...
+        def diagnose_checklist(self, *a): ...
+        def propose_repair(self, *a): ...
+        def generate_mock_inputs(self, *a): ...
+        def analyze_goal(self, *a): ...
+        def propose_plan_v1(self, *a): ...
+        def discover_resources(self, *a): ...
+        def bind_resources(self, *a): ...
+        def build_nodes(self, *a): ...
+        def propose_build_repair(self, *a): ...
+        def analyze_impact(self, *a): ...
+        def propose_edit_plan(self, *a): ...
+        def build_edit_intents(self, *a): ...
+    assert not isinstance(_Missing(), DifyBuilderAgent)