diff --git a/app/backend/main.py b/app/backend/main.py
index c48c535b..29a28957 100644
--- a/app/backend/main.py
+++ b/app/backend/main.py
@@ -16,7 +16,7 @@ from starlette.routing import Route
 from haiku.rag.client import HaikuRAG
 from haiku.rag.config import load_yaml_config
 from haiku.rag.config.models import AppConfig
-from haiku.rag.skills.rag import AGENT_PREAMBLE, create_skill
+from haiku.rag.skills.rag import create_skill, get_agent_preamble
 from haiku.rag.utils import get_model
 from haiku.skills import (
     SkillDeps,
@@ -74,7 +74,9 @@ toolset = SkillToolset(skills=[skill])
 
 agent = Agent(
     get_model(Config.qa.model, Config),
-    instructions=build_system_prompt(toolset.skill_catalog, preamble=AGENT_PREAMBLE),
+    instructions=build_system_prompt(
+        toolset.skill_catalog, preamble=get_agent_preamble(Config)
+    ),
     toolsets=[toolset],
     deps_type=SkillDeps,
 )
diff --git a/haiku_rag_slim/haiku/rag/chat/app.py b/haiku_rag_slim/haiku/rag/chat/app.py
index 1905510a..da58774f 100644
--- a/haiku_rag_slim/haiku/rag/chat/app.py
+++ b/haiku_rag_slim/haiku/rag/chat/app.py
@@ -30,7 +30,7 @@ from textual.worker import Worker
 from haiku.rag.chat.widgets.chat_history import ChatHistory, CitationWidget
 from haiku.rag.client import HaikuRAG
 from haiku.rag.config import get_config
-from haiku.rag.skills.rag import AGENT_PREAMBLE, RAGState
+from haiku.rag.skills.rag import RAGState, get_agent_preamble
 from haiku.skills.agent import (
     SkillToolset,
     run_agui_stream,
@@ -157,7 +157,8 @@ class ChatApp(App):
         self._agent = Agent(
             self._model,
             instructions=build_system_prompt(
-                self._toolset.skill_catalog, preamble=AGENT_PREAMBLE
+                self._toolset.skill_catalog,
+                preamble=get_agent_preamble(self.config),
             ),
             toolsets=[self._toolset],
         )
diff --git a/haiku_rag_slim/haiku/rag/skills/rag.py b/haiku_rag_slim/haiku/rag/skills/rag.py
index db3e82c6..e9e52772 100644
--- a/haiku_rag_slim/haiku/rag/skills/rag.py
+++ b/haiku_rag_slim/haiku/rag/skills/rag.py
@@ -24,6 +24,13 @@ CRITICAL RULES:
 _RAG_TOOLS = ["search", "list_documents", "get_document", "ask", "research"]
 
 
+def get_agent_preamble(config: AppConfig) -> str:
+    """Build the main agent preamble, prepending domain_preamble if configured."""
+    if config.prompts.domain_preamble:
+        return f"{config.prompts.domain_preamble}\n\n{AGENT_PREAMBLE}"
+    return AGENT_PREAMBLE
+
+
 class RAGState(BaseModel):
     citations: list[Citation] = Field(default_factory=list)
     qa_history: list[QAHistoryEntry] = Field(default_factory=list)
@@ -88,11 +95,15 @@ def create_skill(
     tools = create_skill_tools(db_path, config, RAGState, _RAG_TOOLS)
     extras = create_skill_extras(db_path, config)
 
+    skill_instructions = instructions()
+    if config.prompts.domain_preamble and skill_instructions:
+        skill_instructions = f"{config.prompts.domain_preamble}\n\n{skill_instructions}"
+
     return Skill(
         metadata=skill_metadata(),
         source=SkillSource.ENTRYPOINT,
         path=_skill_path,
-        instructions=instructions(),
+        instructions=skill_instructions,
         tools=list(tools.values()),
         extras=extras,
         state_type=STATE_TYPE,
diff --git a/haiku_rag_slim/haiku/rag/skills/rlm.py b/haiku_rag_slim/haiku/rag/skills/rlm.py
index 7efa403e..503d9d4a 100644
--- a/haiku_rag_slim/haiku/rag/skills/rlm.py
+++ b/haiku_rag_slim/haiku/rag/skills/rlm.py
@@ -69,11 +69,15 @@ def create_skill(
     tools = create_skill_tools(db_path, config, RLMState, ["analyze"])
     extras = create_skill_extras(db_path, config)
 
+    skill_instructions = instructions()
+    if config.prompts.domain_preamble and skill_instructions:
+        skill_instructions = f"{config.prompts.domain_preamble}\n\n{skill_instructions}"
+
     return Skill(
         metadata=skill_metadata(),
         source=SkillSource.ENTRYPOINT,
         path=_skill_path,
-        instructions=instructions(),
+        instructions=skill_instructions,
         tools=list(tools.values()),
         extras=extras,
         state_type=STATE_TYPE,
diff --git a/tests/skills/test_rag.py b/tests/skills/test_rag.py
index a82aff31..9bcda719 100644
--- a/tests/skills/test_rag.py
+++ b/tests/skills/test_rag.py
@@ -2,6 +2,7 @@ from unittest.mock import AsyncMock
 
 from haiku.rag.agents.research.models import Citation, ResearchReport
 from haiku.rag.client import HaikuRAG
+from haiku.rag.config.models import AppConfig
 from haiku.rag.skills.rag import (
     STATE_NAMESPACE,
     STATE_TYPE,
@@ -52,6 +53,55 @@ class TestRAGModuleAPI:
         assert skill.instructions == instructions()
 
 
+class TestGetAgentPreamble:
+    def test_without_domain_preamble(self):
+        from haiku.rag.skills.rag import AGENT_PREAMBLE, get_agent_preamble
+
+        config = AppConfig()
+        assert get_agent_preamble(config) == AGENT_PREAMBLE
+
+    def test_with_domain_preamble(self):
+        from haiku.rag.config.models import PromptsConfig
+        from haiku.rag.skills.rag import AGENT_PREAMBLE, get_agent_preamble
+
+        config = AppConfig(
+            prompts=PromptsConfig(
+                domain_preamble="This knowledge base contains C-146 aircraft documents."
+            )
+        )
+        result = get_agent_preamble(config)
+        assert result.startswith(
+            "This knowledge base contains C-146 aircraft documents."
+        )
+        assert AGENT_PREAMBLE in result
+
+
+class TestDomainPreambleInSkillInstructions:
+    def test_create_skill_without_domain_preamble(self, test_app_config, temp_db_path):
+        from haiku.rag.skills.rag import create_skill, instructions
+
+        skill = create_skill(config=test_app_config, db_path=temp_db_path)
+        assert skill.instructions == instructions()
+
+    def test_create_skill_with_domain_preamble(self, temp_db_path):
+        from haiku.rag.config.models import PromptsConfig
+        from haiku.rag.skills.rag import create_skill, instructions
+
+        config = AppConfig(
+            prompts=PromptsConfig(
+                domain_preamble="This knowledge base contains C-146 aircraft documents."
+            )
+        )
+        skill = create_skill(config=config, db_path=temp_db_path)
+        assert skill.instructions is not None
+        assert skill.instructions.startswith(
+            "This knowledge base contains C-146 aircraft documents."
+        )
+        base_instructions = instructions()
+        assert base_instructions is not None
+        assert base_instructions in skill.instructions
+
+
 class TestRAGSkillCreation:
     def test_create_skill_returns_valid_skill(self, test_app_config, temp_db_path):
         from haiku.rag.skills.rag import create_skill
diff --git a/tests/skills/test_rlm.py b/tests/skills/test_rlm.py
index c106222a..d0b26474 100644
--- a/tests/skills/test_rlm.py
+++ b/tests/skills/test_rlm.py
@@ -2,6 +2,7 @@ from unittest.mock import AsyncMock
 
 from haiku.rag.agents.rlm.models import RLMResult
 from haiku.rag.client import HaikuRAG
+from haiku.rag.config.models import AppConfig
 from haiku.rag.skills.rlm import (
     STATE_NAMESPACE,
     STATE_TYPE,
@@ -91,6 +92,32 @@ class TestRLMSkillCreation:
         assert skill.metadata.name == "rag-rlm"
 
 
+class TestDomainPreambleInRLMSkillInstructions:
+    def test_create_skill_without_domain_preamble(self, test_app_config, temp_db_path):
+        from haiku.rag.skills.rlm import create_skill, instructions
+
+        skill = create_skill(config=test_app_config, db_path=temp_db_path)
+        assert skill.instructions == instructions()
+
+    def test_create_skill_with_domain_preamble(self, temp_db_path):
+        from haiku.rag.config.models import PromptsConfig
+        from haiku.rag.skills.rlm import create_skill, instructions
+
+        config = AppConfig(
+            prompts=PromptsConfig(
+                domain_preamble="This knowledge base contains C-146 aircraft documents."
+            )
+        )
+        skill = create_skill(config=config, db_path=temp_db_path)
+        assert skill.instructions is not None
+        assert skill.instructions.startswith(
+            "This knowledge base contains C-146 aircraft documents."
+        )
+        base_instructions = instructions()
+        assert base_instructions is not None
+        assert base_instructions in skill.instructions
+
+
 class TestAnalyzeTool:
     async def test_analyze_returns_result(self, rag_db, monkeypatch):
         from haiku.rag.skills.rlm import create_skill