diff --git a/.dockerignore b/.dockerignore
index 9ee9d00..9ae2faf 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -5,6 +5,10 @@
 !dockerfile
 !requirements.txt
 !server.py
+# The router seam server.py injects its singletons into (R3). Root-level Python
+# that ships in the image must be re-allowed explicitly — this file starts with
+# a blanket `*` exclusion. `routers/` needs the same treatment when it lands.
+!appstate.py
 !main.py
 !VERSION
 !tailwind.config.js
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 57b0516..5fcf5cc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,29 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- **`appstate.py` — the router seam (R3).** Route modules moving out of `server.py`
+  need `meta_db` and friends but must not `import server`, or the import graph goes
+  circular the moment `server` imports them back. So `server.py` keeps *constructing*
+  its singletons and now **injects** them once — `appstate.configure(meta_db=…,
+  audio_effect_mappings=…)` — and a router reads them back as module attributes at call
+  time (`import appstate; appstate.meta_db.…`). This is the Python analogue of the
+  frontend refactor's injected `configureX({…})` seams and of the plugin
+  `setup(app, context)` contract: dependencies flow one way, `server → routers →
+  appstate`. Two properties are load-bearing and pinned by `tests/test_appstate.py`:
+  (1) `import appstate` constructs nothing and touches no disk, so the ~49 test fixtures
+  that `sys.modules.pop("server")` + re-import (to rebuild `meta_db` under a patched
+  `CONFIG_DIR`) keep working untouched — a singleton *owned* by `appstate` would survive
+  that pop and go stale; (2) reads must be late-bound (`appstate.meta_db`, never
+  `from appstate import meta_db`), since a `from` import freezes the binding and defeats
+  both a later `configure()` and `monkeypatch.setattr` — the same read-only-binding trap
+  as ES `import`. `configure()` rejects an unknown slot rather than silently creating a
+  global nothing reads, and the suite asserts `server` actually calls it (a seam whose
+  wiring can no-op undetected is worse than no seam). Ships in the image via
+  `Dockerfile` `COPY appstate.py /app/` plus a `.dockerignore` allowlist entry — that
+  file opens with a blanket `*` exclusion, so root-level Python must be re-allowed
+  explicitly or the build fails.
+
 ### Changed
 - **`AudioEffectsMappingDB` moved out of `server.py` into `lib/audio_effects_db.py`
   (R3, move-only).** The core-owned song/tone → provider routing index follows
diff --git a/Dockerfile b/Dockerfile
index ebbd38f..0c08d0e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -206,6 +206,9 @@ COPY --from=tailwind-builder /build/static/tailwind.min.css /app/static/tailwind
 # when a plugin is installed at runtime (see update_manager on-install hook).
 COPY tailwind.config.js /app/tailwind.config.js
 COPY server.py /app/
+# The router seam server.py injects its singletons into (R3). Root-level, like
+# server.py, so `import appstate` resolves off PYTHONPATH=/app.
+COPY appstate.py /app/
 COPY main.py /app/
 COPY VERSION /app/
 # Built-in diagnostic sloppaks seeded into DLC_DIR/diagnostics-builtin/ at scan
diff --git a/appstate.py b/appstate.py
new file mode 100644
index 0000000..6c6c69e
--- /dev/null
+++ b/appstate.py
@@ -0,0 +1,72 @@
+"""Shared application state — the seam that lets route modules reach core
+singletons without importing ``server``.
+
+``server.py`` is the host: it owns the FastAPI ``app``, constructs the DB
+singletons, and runs the lifecycle. As routes move out into ``routers/`` (R3),
+those modules need ``meta_db`` and friends — but they must not ``import
+server``, or the import graph goes circular the moment ``server`` imports them
+back.
+
+So ``server`` **injects** its singletons here once, at the point it builds them::
+
+    # server.py
+    meta_db = MetadataDB(CONFIG_DIR)
+    appstate.configure(meta_db=meta_db, ...)
+
+and a router reads them back as **module attributes, at call time**::
+
+    # routers/artists.py
+    import appstate
+
+    @router.get("/api/artist/{name}/page")
+    def artist_page(name):
+        return appstate.meta_db.artist_page(name)
+
+This is the Python analogue of the injected `configureX({...})` seams the
+frontend refactor uses (stems' ``configureStreaming``, studio's
+``configureAudioGraph``, the editor's ``src/host.js``), and of the plugin
+``setup(app, context)`` contract in Principle III: dependencies flow one way,
+``server -> routers -> appstate``, and nothing imports back up.
+
+Two properties this shape buys, both load-bearing:
+
+* **``import appstate`` performs no IO and constructs nothing.** ``server``
+  still owns construction, so the ~49 test fixtures that do
+  ``sys.modules.pop("server")`` + re-import (to rebuild ``meta_db`` under a
+  patched ``CONFIG_DIR``) keep working untouched — a singleton *owned* here
+  would survive that pop and go stale.
+* **Reads are late-bound.** Routers must use ``appstate.meta_db``, never
+  ``from appstate import meta_db`` — a ``from`` import freezes the binding at
+  its current value, so a later ``configure()`` (or a
+  ``monkeypatch.setattr(appstate, "meta_db", fake)``) would not reach the
+  router. This is the same read-only-binding trap as ES ``import``.
+
+Defaults are ``None`` on purpose: they are inert but *type-honest*, so a router
+that runs before ``configure()`` fails loudly on ``NoneType`` instead of
+quietly operating on a stand-in.
+
+Slots are added here only when a router actually needs one — this is a seam,
+not a grab-bag for everything in ``server.py``.
+"""
+
+# The singletons routers may read. Every name here must also be a `_SLOTS` key.
+meta_db = None
+audio_effect_mappings = None
+
+# Declared up front so `configure()` can reject a typo'd or stale keyword
+# instead of silently creating a new global that nothing ever reads. A seam
+# whose wiring can no-op undetected is worse than no seam.
+_SLOTS = frozenset({"meta_db", "audio_effect_mappings"})
+
+
+def configure(**kwargs) -> None:
+    """Publish `server`'s singletons into this module. Called once per
+    `server` import (and again on re-import), so it must be idempotent."""
+    unknown = set(kwargs) - _SLOTS
+    if unknown:
+        raise TypeError(
+            f"appstate.configure() got unknown slot(s): {sorted(unknown)}. "
+            f"Known slots: {sorted(_SLOTS)}. Add the name to _SLOTS if a router "
+            f"genuinely needs it."
+        )
+    globals().update(kwargs)
diff --git a/docker-compose.yml b/docker-compose.yml
index 19d931e..8c8ad8e 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -11,6 +11,7 @@ services:
       # Mount source for live reload during development
       - ./static:/app/static
       - ./server.py:/app/server.py
+      - ./appstate.py:/app/appstate.py
       - ./VERSION:/app/VERSION
       - ./ug_browser.py:/app/ug_browser.py
       - ./lib:/app/lib
diff --git a/docs/size-exemptions.md b/docs/size-exemptions.md
index f0682d5..777d845 100644
--- a/docs/size-exemptions.md
+++ b/docs/size-exemptions.md
@@ -55,8 +55,8 @@ without a *signed* exemption" is unenforceable.
 ## Planned, NOT exempt (owned by split plans — listed so nothing falls between states)
 
 core `static/app.js` (11,852) · `static/highway.js` (4,168, whole file) · `server.py`
-(9,433 — was 14,037; ratcheted by the R3 `MetadataDB` + `AudioEffectsMappingDB`
-extractions) ·
+(9,445 — was 14,037; ratcheted by the R3 `MetadataDB` + `AudioEffectsMappingDB`
+extractions, plus 12 lines for the `appstate.py` seam) ·
 `lib/metadata_db.py` (4,373 — new in R3; the `MetadataDB` class alone is 4,018 lines
 and is a monolith in its own right, to be split per-table once the router train
 lands) · `static/v3/songs.js` (4,134) · `static/capabilities/audio-session.js`
diff --git a/server.py b/server.py
index 2c5d8b5..5b6bee2 100644
--- a/server.py
+++ b/server.py
@@ -66,6 +66,9 @@ from metadata_db import (
 # The audio-effect routing index. Same shape as metadata_db: the class lives in
 # its own module, the `audio_effect_mappings` singleton below stays here.
 from audio_effects_db import AudioEffectsMappingDB
+# The router seam. Imported as a module (never `from appstate import ...`) so
+# `appstate.configure(...)` below publishes into the same namespace routers read.
+import appstate
 import sloppak as sloppak_mod
 import drums as drums_mod
 import notation as notation_mod
@@ -353,6 +356,15 @@ _TUNING_GROUP_KEY_SQL = _tuning_group_key_sql("songs")
 meta_db = MetadataDB(CONFIG_DIR)
 audio_effect_mappings = AudioEffectsMappingDB(CONFIG_DIR)
 
+# Publish the singletons to the router seam. server.py stays their owner — a
+# `sys.modules.pop("server")` + re-import must keep rebuilding them under a
+# patched CONFIG_DIR — and `routers/` read them back as `appstate.<name>` at
+# call time. See appstate.py for why the reads must be late-bound.
+appstate.configure(
+    meta_db=meta_db,
+    audio_effect_mappings=audio_effect_mappings,
+)
+
 
 class LocalLibraryProvider:
     id = "local"
diff --git a/tests/test_appstate.py b/tests/test_appstate.py
new file mode 100644
index 0000000..d9fb5a5
--- /dev/null
+++ b/tests/test_appstate.py
@@ -0,0 +1,122 @@
+"""The router seam (`appstate.py`).
+
+The load-bearing assertion here is `test_server_wires_the_seam`: that `server`
+actually calls `appstate.configure(...)`. Every other test in this file would
+pass just fine against a seam nothing ever wires up — the same class of silent
+no-op that bit the frontend refactor twice when a scripted `setHostHooks` edit
+stopped matching its anchor. Unit tests cannot see wiring unless you make them
+look at it.
+"""
+
+import importlib
+import sys
+
+import pytest
+
+import appstate
+
+
+@pytest.fixture()
+def isolated_server(tmp_path, monkeypatch):
+    """A freshly imported `server` bound to a throwaway CONFIG_DIR.
+
+    Importing `server` constructs `MetadataDB` + `AudioEffectsMappingDB` at
+    module level, so it MUST be re-imported under a patched CONFIG_DIR — an
+    unguarded `import server` would create/mutate the developer's real
+    `~/.local/share/feedback` databases. Same idiom as the other ~49
+    server-importing suites.
+    """
+    monkeypatch.setenv("CONFIG_DIR", str(tmp_path))
+    sys.modules.pop("server", None)
+    mod = importlib.import_module("server")
+    yield mod
+    conn = getattr(getattr(mod, "meta_db", None), "conn", None)
+    if conn is not None:
+        getattr(mod, "_join_background_db_threads", lambda: None)()
+        conn.close()
+    ae_conn = getattr(getattr(mod, "audio_effect_mappings", None), "conn", None)
+    if ae_conn is not None:
+        ae_conn.close()
+    # Leave no half-torn-down `server` behind: the next fixture re-imports it.
+    sys.modules.pop("server", None)
+
+
+def test_import_is_side_effect_free():
+    """`import appstate` must construct nothing and touch no disk.
+
+    This is why the ~49 fixtures that `sys.modules.pop("server")` and re-import
+    (to rebuild `meta_db` under a patched CONFIG_DIR) keep working untouched:
+    server owns construction, appstate only mirrors it. A singleton *owned*
+    here would survive that pop and go stale.
+    """
+    sys.modules.pop("appstate", None)
+    fresh = importlib.import_module("appstate")
+    try:
+        assert fresh.meta_db is None
+        assert fresh.audio_effect_mappings is None
+    finally:
+        sys.modules["appstate"] = appstate
+
+
+def test_configure_publishes_known_slots():
+    sentinel = object()
+    original = appstate.meta_db
+    try:
+        appstate.configure(meta_db=sentinel)
+        assert appstate.meta_db is sentinel
+    finally:
+        appstate.configure(meta_db=original)
+
+
+def test_configure_is_idempotent():
+    """server re-imports call configure() again; the last write must win."""
+    original = appstate.meta_db
+    try:
+        appstate.configure(meta_db="first")
+        appstate.configure(meta_db="second")
+        assert appstate.meta_db == "second"
+    finally:
+        appstate.configure(meta_db=original)
+
+
+def test_configure_rejects_an_unknown_slot():
+    """A typo'd or stale keyword must raise, not silently create a global that
+    nothing reads. A seam whose wiring can no-op undetected is worse than none."""
+    with pytest.raises(TypeError, match="unknown slot"):
+        appstate.configure(met_db="typo")
+    assert not hasattr(appstate, "met_db")
+
+
+def test_late_bound_read_sees_a_later_configure():
+    """Routers must read `appstate.meta_db`, never `from appstate import meta_db`.
+    This pins the property that makes that rule work."""
+    def router_style_read():
+        return appstate.meta_db          # module attribute, resolved at call time
+
+    original = appstate.meta_db
+    try:
+        appstate.configure(meta_db="before")
+        assert router_style_read() == "before"
+        appstate.configure(meta_db="after")
+        assert router_style_read() == "after"
+    finally:
+        appstate.configure(meta_db=original)
+
+
+def test_server_wires_the_seam(isolated_server):
+    """The one that catches a dropped `appstate.configure(...)` call.
+
+    Identity, not truthiness, so a stray re-assignment or a half-applied edit
+    fails here rather than in some router months later.
+    """
+    assert appstate.meta_db is isolated_server.meta_db
+    assert appstate.audio_effect_mappings is isolated_server.audio_effect_mappings
+    assert appstate.meta_db is not None
+
+
+def test_reimporting_server_republishes_the_fresh_singletons(isolated_server, tmp_path):
+    """The 49-fixture contract: re-importing server under a new CONFIG_DIR must
+    leave the seam pointing at the NEW meta_db, without those fixtures having to
+    know appstate exists."""
+    assert appstate.meta_db is isolated_server.meta_db
+    assert str(tmp_path) in isolated_server.meta_db.db_path