diff --git a/docs/sexp-architecture-plan.md b/docs/sexp-architecture-plan.md
index 3ab21e8..4b22deb 100644
--- a/docs/sexp-architecture-plan.md
+++ b/docs/sexp-architecture-plan.md
@@ -687,6 +687,27 @@ Each phase is independently deployable. The end state: a platform where the appl
- 27 tests: passthrough rendering (4), I/O collection (8), fragment resolution (3), query resolution (2), parallel I/O (1), request context (4), error handling (2), mixed content (3)
- **199 total tests across all 4 files, all passing**
+### Phase 4: Jinja Bridge — COMPLETE
+
+**Branch:** `sexpression`
+
+**Delivered** (`shared/sexp/`):
+- `jinja_bridge.py` — Two-way bridge between Jinja and s-expressions:
+ - `sexp(source, **kwargs)` — sync render for Jinja templates: `{{ sexp('(~card :title "Hi")') | safe }}`
+ - `sexp_async(source, **kwargs)` — async render with I/O resolution
+ - `register_components(sexp_source)` — load component definitions at startup
+ - `get_component_env()` — access the shared component registry
+ - `setup_sexp_bridge(app)` — register `sexp` and `sexp_async` as Jinja globals
+ - `_get_request_context()` — auto-builds RequestContext from Quart request
+
+**Integration point**: Call `setup_sexp_bridge(app)` after `setup_jinja(app)` in app factories. Components registered via `register_components()` are available globally across all templates.
+
+**First migration target: `~link-card`** — unified component replacing 5 separate Jinja templates (`blog/fragments/link_card.html`, `market/fragments/link_card.html`, `events/fragments/link_card.html`, `federation/fragments/link_card.html`, `artdag/l1/app/templates/fragments/link_card.html`). The s-expression component handles image/no-image, brand, and is usable from both Jinja templates and s-expression trees.
+
+**Tests** (`shared/sexp/tests/test_jinja_bridge.py`):
+- 19 tests: sexp() rendering (5), component registration (6), link-card migration (5), sexp_async (3)
+- **218 total tests across all 5 files, all passing**
+
### Test Infrastructure — COMPLETE
**Delivered:**
diff --git a/shared/sexp/jinja_bridge.py b/shared/sexp/jinja_bridge.py
new file mode 100644
index 0000000..4857821
--- /dev/null
+++ b/shared/sexp/jinja_bridge.py
@@ -0,0 +1,138 @@
+"""
+Jinja ↔ s-expression bridge.
+
+Provides two-way integration so s-expression components and Jinja templates
+can coexist during incremental migration:
+
+**Jinja → s-expression** (use s-expression components inside Jinja templates)::
+
+ {{ sexp('(~link-card :slug "apple" :title "Apple")') | safe }}
+
+**S-expression → Jinja** (embed Jinja output inside s-expressions)::
+
+ (raw! (jinja "fragments/link_card.html" :slug "apple" :title "Apple"))
+
+Setup::
+
+ from shared.sexp.jinja_bridge import setup_sexp_bridge
+ setup_sexp_bridge(app) # call after setup_jinja(app)
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from .types import NIL, Symbol
+from .parser import parse
+from .html import render as html_render
+
+
+# ---------------------------------------------------------------------------
+# Shared component environment
+# ---------------------------------------------------------------------------
+
+# Global component registry — populated at app startup by loading component
+# definition files or calling register_components().
+_COMPONENT_ENV: dict[str, Any] = {}
+
+
+def get_component_env() -> dict[str, Any]:
+ """Return the shared component environment."""
+ return _COMPONENT_ENV
+
+
+def register_components(sexp_source: str) -> None:
+ """Parse and evaluate s-expression component definitions into the
+ shared environment.
+
+ Typically called at app startup::
+
+ register_components('''
+ (defcomp ~link-card (&key link title image icon)
+ (a :href link :class "block rounded ..."
+ (div :class "flex ..."
+ (if image
+ (img :src image :class "...")
+ (div :class "..." (i :class icon)))
+ (div :class "..." (div :class "..." title)))))
+ ''')
+ """
+ from .evaluator import _eval
+ from .parser import parse_all
+
+ exprs = parse_all(sexp_source)
+ for expr in exprs:
+ _eval(expr, _COMPONENT_ENV)
+
+
+# ---------------------------------------------------------------------------
+# sexp() — render s-expression from Jinja template
+# ---------------------------------------------------------------------------
+
+def sexp(source: str, **kwargs: Any) -> str:
+ """Render an s-expression string to HTML.
+
+ Keyword arguments are merged into the evaluation environment,
+ so Jinja context variables can be passed through::
+
+ {{ sexp('(~link-card :title title :slug slug)',
+ title=post.title, slug=post.slug) | safe }}
+
+ This is a synchronous function — suitable for Jinja globals.
+ For async resolution (with I/O primitives), use ``sexp_async()``.
+ """
+ env = dict(_COMPONENT_ENV)
+ env.update(kwargs)
+ expr = parse(source)
+ return html_render(expr, env)
+
+
+async def sexp_async(source: str, **kwargs: Any) -> str:
+ """Async version of ``sexp()`` — resolves I/O primitives (frag, query)
+ before rendering.
+
+ Use when the s-expression contains I/O nodes::
+
+ {{ sexp_async('(frag "blog" "card" :slug "apple")') | safe }}
+ """
+ from .resolver import resolve, RequestContext
+
+ env = dict(_COMPONENT_ENV)
+ env.update(kwargs)
+ expr = parse(source)
+
+ # Try to get request context from Quart
+ ctx = _get_request_context()
+ return await resolve(expr, ctx=ctx, env=env)
+
+
+def _get_request_context():
+ """Build RequestContext from current Quart request, if available."""
+ from .primitives_io import RequestContext
+ try:
+ from quart import g, request
+ user = getattr(g, "user", None)
+ is_htmx = bool(request.headers.get("HX-Request"))
+ return RequestContext(user=user, is_htmx=is_htmx)
+ except Exception:
+ return RequestContext()
+
+
+# ---------------------------------------------------------------------------
+# Quart integration
+# ---------------------------------------------------------------------------
+
+def setup_sexp_bridge(app: Any) -> None:
+ """Register s-expression helpers with a Quart app's Jinja environment.
+
+ Call this in your app factory after ``setup_jinja(app)``::
+
+ from shared.sexp.jinja_bridge import setup_sexp_bridge
+ setup_sexp_bridge(app)
+
+ This registers:
+ - ``sexp(source, **kwargs)`` — sync render (components, pure HTML)
+ - ``sexp_async(source, **kwargs)`` — async render (with I/O resolution)
+ """
+ app.jinja_env.globals["sexp"] = sexp
+ app.jinja_env.globals["sexp_async"] = sexp_async
diff --git a/shared/sexp/tests/test_jinja_bridge.py b/shared/sexp/tests/test_jinja_bridge.py
new file mode 100644
index 0000000..5e779e5
--- /dev/null
+++ b/shared/sexp/tests/test_jinja_bridge.py
@@ -0,0 +1,201 @@
+"""Tests for the Jinja ↔ s-expression bridge."""
+
+import asyncio
+
+from shared.sexp.jinja_bridge import (
+ sexp,
+ sexp_async,
+ register_components,
+ get_component_env,
+ _COMPONENT_ENV,
+)
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+def run(coro):
+ return asyncio.run(coro)
+
+
+def setup_function():
+ """Clear component env before each test."""
+ _COMPONENT_ENV.clear()
+
+
+# ---------------------------------------------------------------------------
+# sexp() — synchronous rendering
+# ---------------------------------------------------------------------------
+
+class TestSexp:
+ def test_simple_html(self):
+ assert sexp('(div "Hello")') == "Hello
"
+
+ def test_with_kwargs(self):
+ html = sexp('(p name)', name="Alice")
+ assert html == "Alice
"
+
+ def test_multiple_kwargs(self):
+ html = sexp('(a :href url title)', url="/about", title="About")
+ assert html == 'About'
+
+ def test_escaping(self):
+ html = sexp('(p text)', text="")
+ assert "<script>" in html
+ assert "