fix(config): honor system_prompt.prefix from config.toml (#482)

A [system_prompt] prefix set in config.toml was silently ignored: (1)
load_config()'s section allowlist dropped the [system_prompt],
[memory_files], [compression], and [skills] blocks entirely; (2)
SystemPromptConfig had no prefix field; (3) the builder never prepended
one.

Add the four blocks to the allowlist, add prefix: str = "" to
SystemPromptConfig, and prepend a "prefix" PromptSection at the front of
SystemPromptBuilder's frozen sections so it leads build() output and is
exposed via sections() (#457). Empty prefix emits no section — existing
configs are byte-for-byte unchanged.

Rebuilt on top of #457 (which refactored the builder to PromptSection
objects); the original #452 by @SoulSniper-V2 patched the pre-#457
method. Credit to @SoulSniper-V2. Adds the regression tests the original
PR lacked: config parse + prefix-prepended + empty-prefix-unchanged.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/openjarvis/core/config.py

@@ -1434,6 +1434,7 @@ class MemoryFilesConfig:
 class SystemPromptConfig:
     """Limits and strategy for system-prompt assembly."""
 
+    prefix: str = ""
     soul_max_chars: int = 4000
     memory_max_chars: int = 2500
     user_max_chars: int = 1500
@@ -1810,6 +1811,10 @@ def load_config(path: Optional[Path] = None) -> JarvisConfig:
             "agent_manager",
             "digest",
             "proactive",
+            "memory_files",
+            "system_prompt",
+            "compression",
+            "skills",
         )
         for section_name in top_sections:
             if section_name in data:

src/openjarvis/prompt/builder.py

@@ -125,14 +125,26 @@ class SystemPromptBuilder:
 
     def _build_frozen_sections(self) -> list[PromptSection]:
         sections: list[PromptSection] = []
-        sections.append(
-            PromptSection(
-                name="agent_template",
-                content=self._agent_template,
-                source="agent_template",
-                cache_segment="frozen_prefix",
+        # Config-driven persona prefix from [system_prompt] prefix (#401),
+        # prepended ahead of the agent template so it leads the frozen prefix.
+        if self._sp_config.prefix:
+            sections.append(
+                PromptSection(
+                    name="prefix",
+                    content=self._sp_config.prefix,
+                    source="system_prompt.prefix",
+                    cache_segment="frozen_prefix",
+                )
+            )
+        if self._agent_template:
+            sections.append(
+                PromptSection(
+                    name="agent_template",
+                    content=self._agent_template,
+                    source="agent_template",
+                    cache_segment="frozen_prefix",
+                )
             )
-        )
         sections.extend(self._persona_prompt_sections())
         # XML skill catalog (preferred over legacy markdown list)
         if self._skill_catalog_xml:

tests/core/test_config.py

@@ -106,6 +106,25 @@ class TestTomlLoading:
         assert cfg.engine.lemonade.host == "http://custom-lemonade:19000"
         assert cfg.engine.lemonade_host == "http://custom-lemonade:19000"
 
+    def test_system_prompt_block_parsed(self, tmp_path: Path) -> None:
+        """Regression for #401: the [system_prompt] block (and its prefix)
+        must reach the runtime config, not be dropped by load_config()."""
+        toml_file = tmp_path / "config.toml"
+        toml_file.write_text(
+            '[system_prompt]\nprefix = "You are Jarvis."\nsoul_max_chars = 999\n'
+        )
+        cfg = load_config(toml_file)
+        assert cfg.system_prompt.prefix == "You are Jarvis."
+        assert cfg.system_prompt.soul_max_chars == 999
+
+    def test_config_without_system_prompt_block_defaults(self, tmp_path: Path) -> None:
+        """Backward compatibility: a config lacking [system_prompt] keeps
+        the empty-prefix default (no behavior change for existing configs)."""
+        toml_file = tmp_path / "config.toml"
+        toml_file.write_text('[engine]\ndefault = "ollama"\n')
+        cfg = load_config(toml_file)
+        assert cfg.system_prompt.prefix == ""
+
 
 class TestGenerateToml:
     def test_contains_engine_section(self) -> None:

tests/prompt/test_builder.py

@@ -37,6 +37,53 @@ def test_build_frozen_prefix(memory_dir: Path):
     assert "Alice" in prompt
 
 
+def test_config_prefix_prepended(memory_dir: Path):
+    """Regression for #401: a configured system_prompt.prefix leads the
+    assembled prompt, ahead of the agent template, and is exposed as a
+    'prefix' section."""
+    from openjarvis.prompt.builder import SystemPromptBuilder
+
+    builder = SystemPromptBuilder(
+        agent_template="You are Jarvis.",
+        memory_files_config=MemoryFilesConfig(
+            soul_path=str(memory_dir / "SOUL.md"),
+            memory_path=str(memory_dir / "MEMORY.md"),
+            user_path=str(memory_dir / "USER.md"),
+        ),
+        system_prompt_config=SystemPromptConfig(prefix="ALWAYS ANSWER AS JARVIS."),
+    )
+    prompt = builder.build()
+    assert prompt.startswith("ALWAYS ANSWER AS JARVIS.")
+    assert "You are Jarvis." in prompt
+    # Prefix is visible in the inspection API too (#457), as a frozen section.
+    section_names = [s.name for s in builder.sections()]
+    assert section_names[0] == "prefix"
+    assert builder.sections()[0].cache_segment == "frozen_prefix"
+
+
+def test_empty_prefix_leaves_prompt_unchanged(memory_dir: Path):
+    """Backward compatibility: the default empty prefix adds no section and
+    leaves build() output identical to having no prefix configured."""
+    from openjarvis.prompt.builder import SystemPromptBuilder
+
+    def _make(prefix: str) -> SystemPromptBuilder:
+        return SystemPromptBuilder(
+            agent_template="You are Jarvis.",
+            memory_files_config=MemoryFilesConfig(
+                soul_path=str(memory_dir / "SOUL.md"),
+                memory_path=str(memory_dir / "MEMORY.md"),
+                user_path=str(memory_dir / "USER.md"),
+            ),
+            system_prompt_config=SystemPromptConfig(prefix=prefix),
+        )
+
+    assert _make("").build() == _make("").build()
+    # No 'prefix' section is emitted when prefix is empty.
+    assert "prefix" not in [s.name for s in _make("").sections()]
+    # And the first section is the agent template, as before.
+    assert _make("").sections()[0].name == "agent_template"
+
+
 def test_frozen_prefix_stability(memory_dir: Path):
     from openjarvis.prompt.builder import SystemPromptBuilder