diff --git a/src/httk/web/api.py b/src/httk/web/api.py
index 550a1b7..efa64f8 100644
--- a/src/httk/web/api.py
+++ b/src/httk/web/api.py
@@ -14,9 +14,10 @@ def create_asgi_app(
     srcdir: str | Path,
     *,
     baseurl: str | None = None,
+    compatibility_mode: bool = False,
     debug: bool = False,
 ) -> Starlette:
-    config = SiteConfig.from_srcdir(srcdir=srcdir, baseurl=baseurl)
+    config = SiteConfig.from_srcdir(srcdir=srcdir, baseurl=baseurl, compatibility_mode=compatibility_mode)
     engine = SiteEngine(config)
     return create_app(engine=engine, debug=debug)
 
@@ -27,9 +28,10 @@ def serve(
     host: str = "127.0.0.1",
     port: int = 8080,
     baseurl: str | None = None,
+    compatibility_mode: bool = False,
     debug: bool = False,
 ) -> None:
-    app = create_asgi_app(srcdir=srcdir, baseurl=baseurl, debug=debug)
+    app = create_asgi_app(srcdir=srcdir, baseurl=baseurl, compatibility_mode=compatibility_mode, debug=debug)
     run_dev_server(app=app, host=host, port=port)
 
 
@@ -37,7 +39,9 @@ def publish(
     srcdir: str | Path,
     outdir: str | Path,
     baseurl: str,
+    *,
+    compatibility_mode: bool = False,
 ) -> PublishReport:
-    config = SiteConfig.from_srcdir(srcdir=srcdir, baseurl=baseurl)
+    config = SiteConfig.from_srcdir(srcdir=srcdir, baseurl=baseurl, compatibility_mode=compatibility_mode)
     engine = SiteEngine(config)
     return publish_site(engine=engine, outdir=outdir)
diff --git a/src/httk/web/engine/site_engine.py b/src/httk/web/engine/site_engine.py
index bf4c1a6..2352de1 100644
--- a/src/httk/web/engine/site_engine.py
+++ b/src/httk/web/engine/site_engine.py
@@ -9,13 +9,22 @@
 from httk.web.model.page import PageResult, ResolvedRoute
 from httk.web.model.request import HttpRequestContext
 from httk.web.renderers import RENDERERS_BY_SUFFIX
-from httk.web.templating import JinjaTemplateEngine, TemplateRenderInput
+from httk.web.templating import (
+    HttkCompatTemplateEngine,
+    JinjaTemplateEngine,
+    TemplateEngine,
+    TemplateRenderInput,
+)
 
 
 class SiteEngine:
     def __init__(self, config: SiteConfig) -> None:
         self.config = config
-        self.template_engine = JinjaTemplateEngine(template_dir=config.template_dir)
+        self.template_engine: TemplateEngine
+        if config.compatibility_mode:
+            self.template_engine = HttkCompatTemplateEngine(template_dir=config.template_dir)
+        else:
+            self.template_engine = JinjaTemplateEngine(template_dir=config.template_dir)
         self.function_handler = PythonFunctionHandler(functions_dir=config.functions_dir)
 
     def resolve(self, route: str) -> ResolvedRoute:
diff --git a/src/httk/web/model/config.py b/src/httk/web/model/config.py
index dcb06ab..ee1fcec 100644
--- a/src/httk/web/model/config.py
+++ b/src/httk/web/model/config.py
@@ -11,10 +11,17 @@ class SiteConfig:
     template_subdir: str = "templates"
     functions_subdir: str = "functions"
     baseurl: str | None = None
+    compatibility_mode: bool = False
 
     @classmethod
-    def from_srcdir(cls, srcdir: str | Path, *, baseurl: str | None = None) -> Self:
-        return cls(srcdir=Path(srcdir).resolve(), baseurl=baseurl)
+    def from_srcdir(
+        cls,
+        srcdir: str | Path,
+        *,
+        baseurl: str | None = None,
+        compatibility_mode: bool = False,
+    ) -> Self:
+        return cls(srcdir=Path(srcdir).resolve(), baseurl=baseurl, compatibility_mode=compatibility_mode)
 
     @property
     def content_dir(self) -> Path:
diff --git a/src/httk/web/templating/__init__.py b/src/httk/web/templating/__init__.py
index c56023f..6ac66c7 100644
--- a/src/httk/web/templating/__init__.py
+++ b/src/httk/web/templating/__init__.py
@@ -1,4 +1,5 @@
-from .base import TemplateRenderInput
+from .base import TemplateEngine, TemplateRenderInput
+from .httk_compat import HttkCompatTemplateEngine
 from .jinja2_engine import JinjaTemplateEngine
 
-__all__ = ["TemplateRenderInput", "JinjaTemplateEngine"]
+__all__ = ["TemplateRenderInput", "TemplateEngine", "JinjaTemplateEngine", "HttkCompatTemplateEngine"]
diff --git a/src/httk/web/templating/base.py b/src/httk/web/templating/base.py
index 786a870..c8b3237 100644
--- a/src/httk/web/templating/base.py
+++ b/src/httk/web/templating/base.py
@@ -1,4 +1,5 @@
 from dataclasses import dataclass
+from typing import Protocol
 
 
 @dataclass(frozen=True)
@@ -7,3 +8,9 @@ class TemplateRenderInput:
     template_name: str | None
     base_template_name: str | None
     context: dict[str, object]
+
+
+class TemplateEngine(Protocol):
+    def render(self, render_input: TemplateRenderInput) -> str: ...
+
+    def render_fragment(self, *, template_name: str, context: dict[str, object]) -> str | None: ...
diff --git a/src/httk/web/templating/httk_compat.py b/src/httk/web/templating/httk_compat.py
new file mode 100644
index 0000000..f69ea1d
--- /dev/null
+++ b/src/httk/web/templating/httk_compat.py
@@ -0,0 +1,23 @@
+from pathlib import Path
+
+from .jinja2_engine import JinjaTemplateEngine
+
+LEGACY_TEMPLATE_SUFFIXES: tuple[str, ...] = (
+    ".httkweb.html",
+    ".html",
+    ".html.j2",
+    ".jinja",
+    ".j2",
+)
+
+
+class HttkCompatTemplateEngine(JinjaTemplateEngine):
+    """
+    Legacy-oriented template resolution for old httkweb projects.
+
+    The compatibility engine keeps Jinja rendering but prioritizes legacy
+    template suffixes so old `.httkweb.html` files resolve first.
+    """
+
+    def __init__(self, template_dir: Path) -> None:
+        super().__init__(template_dir, template_suffixes=LEGACY_TEMPLATE_SUFFIXES)
diff --git a/src/httk/web/templating/jinja2_engine.py b/src/httk/web/templating/jinja2_engine.py
index 6679187..9bc0a55 100644
--- a/src/httk/web/templating/jinja2_engine.py
+++ b/src/httk/web/templating/jinja2_engine.py
@@ -15,8 +15,14 @@
 
 
 class JinjaTemplateEngine:
-    def __init__(self, template_dir: Path) -> None:
+    def __init__(
+        self,
+        template_dir: Path,
+        *,
+        template_suffixes: tuple[str, ...] = TEMPLATE_SUFFIXES,
+    ) -> None:
         self.template_dir = template_dir
+        self.template_suffixes = template_suffixes
         self._environment = Environment(
             loader=FileSystemLoader(str(template_dir)),
             autoescape=True,
@@ -69,7 +75,7 @@ def _resolve_template(self, name: str | None) -> str | None:
         if path_candidate.suffix:
             return None
 
-        for suffix in TEMPLATE_SUFFIXES:
+        for suffix in self.template_suffixes:
             with_suffix = self.template_dir / f"{candidate}{suffix}"
             if with_suffix.exists() and with_suffix.is_file():
                 return f"{candidate}{suffix}"
@@ -135,7 +141,7 @@ def _resolve_fragment_template(self, name: str) -> str | None:
         # For bare names, probe the same suffix families as standard template
         # resolution plus legacy html-style names used by old function metadata.
         if not path_candidate.suffix:
-            for suffix in TEMPLATE_SUFFIXES:
+            for suffix in self.template_suffixes:
                 template_key = self._resolve_template(f"{candidate}{suffix}")
                 if template_key is not None:
                     return template_key
diff --git a/tests/test_api.py b/tests/test_api.py
index e99572c..33540ba 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -34,3 +34,41 @@ def test_publish_writes_html_output(tmp_path: Path) -> None:
     index_out = out / "index.html"
     assert index_out.exists()
     assert any(path == index_out for path in report.written_files)
+
+
+def test_create_asgi_app_compatibility_mode_prefers_httkweb_templates(tmp_path: Path) -> None:
+    src = tmp_path / "src"
+    (src / "content").mkdir(parents=True)
+    (src / "static").mkdir(parents=True)
+    (src / "templates").mkdir(parents=True)
+
+    (src / "content" / "index.md").write_text("---\ntemplate: default\n---\n\nhello", encoding="utf-8")
+    (src / "templates" / "default.html.j2").write_text("modern={{ content }}", encoding="utf-8")
+    (src / "templates" / "default.httkweb.html").write_text("legacy={{ content }}", encoding="utf-8")
+
+    app = create_asgi_app(src, compatibility_mode=True)
+
+    with TestClient(app) as client:
+        response = client.get("/")
+
+    assert response.status_code == 200
+    assert "legacy=" in response.text
+    assert "modern=" not in response.text
+
+
+def test_publish_compatibility_mode_prefers_httkweb_templates(tmp_path: Path) -> None:
+    src = tmp_path / "src"
+    out = tmp_path / "public"
+    (src / "content").mkdir(parents=True)
+    (src / "static").mkdir(parents=True)
+    (src / "templates").mkdir(parents=True)
+
+    (src / "content" / "index.md").write_text("---\ntemplate: default\n---\n\nhello", encoding="utf-8")
+    (src / "templates" / "default.html.j2").write_text("modern={{ content }}", encoding="utf-8")
+    (src / "templates" / "default.httkweb.html").write_text("legacy={{ content }}", encoding="utf-8")
+
+    publish(src, out, "http://localhost/", compatibility_mode=True)
+
+    rendered = (out / "index.html").read_text(encoding="utf-8")
+    assert "legacy=" in rendered
+    assert "modern=" not in rendered