diff --git a/openlibrary/asgi_app.py b/openlibrary/asgi_app.py
index f79de38f0ad..4fd3f8cdcda 100644
--- a/openlibrary/asgi_app.py
+++ b/openlibrary/asgi_app.py
@@ -16,6 +16,7 @@
 from uvicorn.middleware.proxy_headers import ProxyHeadersMiddleware
 
 import infogami
+from openlibrary.fastapi.middleware.experiments import ABTestingMiddleware
 from openlibrary.utils.request_context import set_context_from_fastapi
 from openlibrary.utils.sentry import Sentry, init_sentry
 
@@ -179,6 +180,8 @@ def create_app() -> FastAPI | None:
     # Needed for the staging nginx proxy
     app.add_middleware(ProxyHeadersMiddleware, trusted_hosts="*")
 
+    app.add_middleware(ABTestingMiddleware)
+
     @app.middleware("http")
     async def add_fastapi_header(request: Request, call_next):
         """Middleware to add a header indicating the response came from FastAPI."""
diff --git a/openlibrary/core/experiments.py b/openlibrary/core/experiments.py
new file mode 100644
index 00000000000..e6dec9e72ce
--- /dev/null
+++ b/openlibrary/core/experiments.py
@@ -0,0 +1,105 @@
+import hashlib
+from typing import Final, Literal, TypedDict
+
+
+class Audience:
+    """Constants for experiment audience targeting."""
+
+    ALL: Final = "all"
+    LOGGED_IN: Final = "logged_in"
+    LOGGED_OUT: Final = "logged_out"
+
+
+class ExperimentConfig(TypedDict, total=False):
+    """Type schema for autocomplete and static analysis."""
+
+    variants: dict[str, int]
+    audience: Literal["all", "logged_in", "logged_out"]
+
+
+ACTIVE_EXPERIMENTS: dict[str, ExperimentConfig] = {
+    "Sample_Experiment_1": {
+        "variants": {"control": 30, "a": 35, "b": 35},
+        "audience": Audience.ALL,
+    },
+    "Sample_Experiment_2": {
+        "variants": {"control": 50, "treatment": 50},
+        "audience": Audience.LOGGED_OUT,
+    },
+    "Sample_Experiment_3": {
+        "variants": {"control": 50, "treatment": 50},
+        "audience": Audience.LOGGED_IN,
+    },
+}
+
+
+def get_variant(experiment_name: str, user_identifier: str | None, is_logged_in: bool = False) -> str:
+    """Assigns a user to a variant based on configured weights and audience rules."""
+    if not user_identifier or experiment_name not in ACTIVE_EXPERIMENTS:
+        return "control"
+
+    config = ACTIVE_EXPERIMENTS[experiment_name]
+    audience = config.get("audience", Audience.ALL)
+
+    if audience == Audience.LOGGED_IN and not is_logged_in:
+        return "control"
+    if audience == Audience.LOGGED_OUT and is_logged_in:
+        return "control"
+
+    hash_key = f"{experiment_name}-{user_identifier}".encode()
+    bucket = int(hashlib.md5(hash_key).hexdigest()[:8], 16) % 100
+
+    cumulative_weight = 0
+    for variant, weight in config.get("variants", {}).items():
+        cumulative_weight += weight
+        if bucket < cumulative_weight:
+            return variant
+
+    return "control"
+
+
+def get_user_experiments(
+    user_identifier: str | None,
+    overrides: dict[str, str] | None = None,
+    is_logged_in: bool = False,
+) -> dict[str, str]:
+    """Evaluates all active experiments for a user, handling optional overrides."""
+    overrides = overrides or {}
+    results = {}
+
+    for exp_name, config in ACTIVE_EXPERIMENTS.items():
+        override_val = overrides.get(f"experiment_{exp_name}")
+
+        if override_val in config.get("variants", {}):
+            results[exp_name] = override_val
+        else:
+            results[exp_name] = get_variant(exp_name, user_identifier, is_logged_in)
+
+    return results
+
+
+def evaluate_experiments_for_request(
+    session_value: str | None,
+    forwarded_for: str | None,
+    client_ip: str | None,
+    query_params: dict[str, str],
+) -> dict[str, str]:
+    """Resolves active experiments for a request based on session, IP, and query params."""
+    from openlibrary.accounts.model import verify_session_cookie
+
+    is_authenticated = False
+    user_id = None
+
+    if session_value and session_value.startswith("/people/") and verify_session_cookie(session_value):
+        is_authenticated = True
+        user_id = session_value.split(",")[0]
+
+    if not user_id:
+        if forwarded_for:
+            user_id = forwarded_for.split(",")[0].strip()
+        else:
+            user_id = client_ip or "127.0.0.1"
+
+    query_overrides = {k: v for k, v in query_params.items() if k.startswith("experiment_")}
+
+    return get_user_experiments(user_id, overrides=query_overrides, is_logged_in=is_authenticated)
diff --git a/openlibrary/core/helpers.py b/openlibrary/core/helpers.py
index f01ab6fa566..abfb96a89da 100644
--- a/openlibrary/core/helpers.py
+++ b/openlibrary/core/helpers.py
@@ -105,12 +105,12 @@ def default(self, obj):
 
 
 def json_encode(d, **kw) -> str:
-    """Calls json.dumps on the given data d.
+    """Calls json.dumps on the given data d and escapes HTML special characters to prevent XSS.
     If d is a Nothing object, passes an empty list to json.dumps.
 
     Returns the json.dumps results.
     """
-    return json.dumps([] if isinstance(d, Nothing) else d, **kw)
+    return json.dumps([] if isinstance(d, Nothing) else d, **kw).replace("<", "\\u003c").replace(">", "\\u003e").replace("&", "\\u0026")
 
 
 def safesort(iterable: Iterable, key: Callable | None = None, reverse: bool = False) -> list:
diff --git a/openlibrary/fastapi/middleware/experiments.py b/openlibrary/fastapi/middleware/experiments.py
new file mode 100644
index 00000000000..ace039f4635
--- /dev/null
+++ b/openlibrary/fastapi/middleware/experiments.py
@@ -0,0 +1,37 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from fastapi import Request
+
+from infogami import config
+from openlibrary.core.experiments import evaluate_experiments_for_request
+
+if TYPE_CHECKING:
+    from starlette.types import ASGIApp, Receive, Scope, Send
+
+
+class ABTestingMiddleware:
+    def __init__(self, app: ASGIApp) -> None:
+        self.app = app
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if scope["type"] != "http":
+            await self.app(scope, receive, send)
+            return
+
+        request = Request(scope, receive=receive)
+
+        session_value = request.cookies.get(config.get("login_cookie_name", "session"))
+        forwarded_for = request.headers.get("X-Forwarded-For")
+        client_ip = request.client.host if request.client else "127.0.0.1"
+        query_params = dict(request.query_params)
+
+        request.state.experiments = evaluate_experiments_for_request(
+            session_value=session_value,
+            forwarded_for=forwarded_for,
+            client_ip=client_ip,
+            query_params=query_params,
+        )
+
+        await self.app(scope, receive, send)
diff --git a/openlibrary/plugins/openlibrary/code.py b/openlibrary/plugins/openlibrary/code.py
index cdf398523e8..5065d54eba9 100644
--- a/openlibrary/plugins/openlibrary/code.py
+++ b/openlibrary/plugins/openlibrary/code.py
@@ -84,6 +84,7 @@ def setup_contextvars(handler):
     )
 )
 delegate.app.add_processor(processors.PreferenceProcessor())
+delegate.app.add_processor(processors.ExperimentsProcessor())
 # Refer to https://github.com/internetarchive/openlibrary/pull/10005 to force patron's to login
 # delegate.app.add_processor(processors.RequireLogoutProcessor())
 # IMPORTANT: setup_contextvars must run AFTER other processors but BEFORE the handler
@@ -1137,7 +1138,6 @@ def get_supported_languages():
             "is_bot": is_bot,
             "time": time,
             "input": web.input,
-            "dumps": json.dumps,
         }
     )
 
diff --git a/openlibrary/plugins/openlibrary/js/experiments.js b/openlibrary/plugins/openlibrary/js/experiments.js
new file mode 100644
index 00000000000..3f0fcde1e42
--- /dev/null
+++ b/openlibrary/plugins/openlibrary/js/experiments.js
@@ -0,0 +1,10 @@
+export function getExperiment(experimentName) {
+    if (typeof window === 'undefined') {
+        return 'control';
+    }
+    return window.OL_EXPERIMENTS?.[experimentName] || 'control';
+}
+
+if (typeof window !== 'undefined') {
+    window.getExperiment = getExperiment;
+}
diff --git a/openlibrary/plugins/openlibrary/js/index.js b/openlibrary/plugins/openlibrary/js/index.js
index 2ef67c01142..3a1a37a4480 100644
--- a/openlibrary/plugins/openlibrary/js/index.js
+++ b/openlibrary/plugins/openlibrary/js/index.js
@@ -3,6 +3,7 @@ import { exposeGlobally } from './jsdef';
 import initAnalytics from './ol.analytics';
 import init from './ol.js';
 import initServiceWorker from './service-worker-init.js';
+import './experiments.js';
 import '../../../../static/css/js-all.css';
 // polyfill Promise support for IE11
 import Promise from 'promise-polyfill';
diff --git a/openlibrary/plugins/openlibrary/processors.py b/openlibrary/plugins/openlibrary/processors.py
index 4ab78f93355..d7d818a471d 100644
--- a/openlibrary/plugins/openlibrary/processors.py
+++ b/openlibrary/plugins/openlibrary/processors.py
@@ -7,8 +7,10 @@
 import web
 
 from infogami import config
+from infogami.utils.context import context
 from openlibrary.accounts import get_current_user
 from openlibrary.core import helpers as h
+from openlibrary.core.experiments import evaluate_experiments_for_request
 from openlibrary.core.processors import (
     ReadableUrlProcessor,  # noqa: F401 side effects may be needed
 )
@@ -156,6 +158,34 @@ def __call__(self, handler):
         return handler()
 
 
+class ExperimentsProcessor:
+    """Processor to evaluate A/B testing feature flags and inject them
+    into the global web.ctx so they are available to macros and templates.
+    """
+
+    def __call__(self, handler):
+        session_value = web.cookies().get(config.get("login_cookie_name", "session"))
+        forwarded_for = web.ctx.env.get("HTTP_X_FORWARDED_FOR")
+        client_ip = web.ctx.ip
+
+        try:
+            query_params = dict(web.input(_method="GET"))
+        except AttributeError, KeyError, ValueError:
+            query_params = {}
+
+        experiments = evaluate_experiments_for_request(
+            session_value=session_value,
+            forwarded_for=forwarded_for,
+            client_ip=client_ip,
+            query_params=query_params,
+        )
+
+        web.ctx["experiments"] = experiments
+        context["experiments"] = experiments
+
+        return handler()
+
+
 if __name__ == "__main__":
     import doctest
 
diff --git a/openlibrary/plugins/upstream/utils.py b/openlibrary/plugins/upstream/utils.py
index 3fe9e756754..80a8285878c 100644
--- a/openlibrary/plugins/upstream/utils.py
+++ b/openlibrary/plugins/upstream/utils.py
@@ -301,11 +301,6 @@ def commify_list(items: Iterable[Any]) -> str:
     return commified_list
 
 
-@public
-def json_encode(d, indent=0) -> str:
-    return json.dumps(d, indent=indent)
-
-
 @public
 def is_feature_enabled(feature_name: str) -> bool:
     return features.is_enabled(feature_name)
diff --git a/openlibrary/templates/account/readinglog_stats.html b/openlibrary/templates/account/readinglog_stats.html
index 0cd29f8e773..dea2efd03e4 100644
--- a/openlibrary/templates/account/readinglog_stats.html
+++ b/openlibrary/templates/account/readinglog_stats.html
@@ -121,7 +121,7 @@ <h1>$_('My %(shelf_name)s Stats', shelf_name=shelf_name)</h1>
     <p>$:_('Displaying stats about <strong>%(works_count)d</strong> books. Note all charts show only the top 20 bars. Note reading log stats are private.', works_count=works_count)</p>
 </div>
 
-<div class="readinglog-charts" data-config="$dumps(reading_log_stats_config)" style="padding: 12px;">
+<div class="readinglog-charts" data-config="$json_encode(reading_log_stats_config)" style="padding: 12px;">
 
     <h2>$_("Author Stats")</h2>
 
@@ -137,7 +137,7 @@ <h2>$_("Author Stats")</h2>
         $for chart in charts:
             <section class="readinglog-chart-container">
                 <h3>$chart['title']</h3>
-                <div class="readinglog-stats-chart" data-config="$dumps(chart)"></div>
+                <div class="readinglog-stats-chart" data-config="$json_encode(chart)"></div>
             </section>
     </section>
 
@@ -158,7 +158,7 @@ <h2>$_("Work Stats")</h2>
         $for chart in charts:
             <section class="readinglog-chart-container">
                 <h3>$chart['title']</h3>
-                <div class="readinglog-stats-chart" data-config="$dumps(chart)"></div>
+                <div class="readinglog-stats-chart" data-config="$json_encode(chart)"></div>
             </section>
     </section>
 </div>
diff --git a/openlibrary/templates/admin/attach_debugger.html b/openlibrary/templates/admin/attach_debugger.html
index 0f236438440..4d5f5887083 100644
--- a/openlibrary/templates/admin/attach_debugger.html
+++ b/openlibrary/templates/admin/attach_debugger.html
@@ -15,7 +15,7 @@ <h1>$_('Attach Debugger on Python %(version_number)s', version_number=python_ver
     <form
         method="POST"
         class="olform"
-        onsubmit="this._start.disabled = true; this._start.textContent = $dumps(_('Waiting for debugger to attach...'));"
+        onsubmit="this._start.disabled = true; this._start.textContent = $json_encode(_('Waiting for debugger to attach...'));"
     >
         <div class="formElement">
             <div class="formElement collapse">
diff --git a/openlibrary/templates/books/author-autocomplete.html b/openlibrary/templates/books/author-autocomplete.html
index 61f5a6be093..7a9932a8c30 100644
--- a/openlibrary/templates/books/author-autocomplete.html
+++ b/openlibrary/templates/books/author-autocomplete.html
@@ -24,7 +24,7 @@
     </div>
 
 $ config = {'name_path': name_path, 'dict_path': dict_path}
-<div class="multi-input-autocomplete multi-input-autocomplete--author" data-config="$dumps(config)">
+<div class="multi-input-autocomplete multi-input-autocomplete--author" data-config="$json_encode(config)">
     $for author in (authors or [storage(name="", key="")]):
         $:render_author(name_path, dict_path, readonly, loop.index0, author)
     <a href="javascript:;" class="mia__add">$_("Add another author?")</a>
diff --git a/openlibrary/templates/books/edit/about.html b/openlibrary/templates/books/edit/about.html
index 3b1f6adfab4..8481569870f 100644
--- a/openlibrary/templates/books/edit/about.html
+++ b/openlibrary/templates/books/edit/about.html
@@ -35,7 +35,7 @@
             <span class="tip"><span class="highlight">$_("Please separate with commas.")</span> $:_('For example: <i><a href="/subjects/cheese">cheese</a>, <a href="/subjects/roman_empire">Roman Empire</a>, <a href="/subjects/psychology">psychology</a></i>')</span>
         </div>
         $ config = {'name': 'subjects', 'facet': 'subject', 'data': work.get_subjects() if len(work.get_subjects()) else []}
-        <div class="csv-autocomplete--subjects" data-config="$dumps(config)">
+        <div class="csv-autocomplete--subjects" data-config="$json_encode(config)">
             $:render_subject_field(config['name'], config['data'])
         </div>
     </div>
@@ -46,7 +46,7 @@
             <span class="tip">$:_('For example: <i><a href="/subjects/person:Theodore_Roosevelt">Theodore Roosevelt</a>, <a href="/subjects/person:Julian_of_Norwich">Julian of Norwich</a>, <a href="/subjects/person:Tintin">Tintin</a></i>')</span>
         </div>
         $ config = {'name': 'subject_people', 'facet': 'person', 'data': work.subject_people if len(work.subject_people) else []}
-        <div class="csv-autocomplete--subjects" data-config="$dumps(config)">
+        <div class="csv-autocomplete--subjects" data-config="$json_encode(config)">
             $:render_subject_field(config['name'], config['data'])
         </div>
     </div>
@@ -57,7 +57,7 @@
             <span class="tip">$:_('For example: <i><a href="/subjects/place:London">London</a>, <a href="/subjects/place:Atlantis">Atlantis</a>, <a href="/subjects/place:Omaha">Omaha</a></i>')</span>
         </div>
         $ config = {'name': 'subject_places', 'facet': 'place', 'data': work.subject_places if len(work.subject_places) else []}
-        <div class="csv-autocomplete--subjects" data-config="$dumps(config)">
+        <div class="csv-autocomplete--subjects" data-config="$json_encode(config)">
             $:render_subject_field(config['name'], config['data'])
         </div>
     </div>
@@ -68,7 +68,7 @@
             <span class="tip">$:_('For example: <i><a href="/subjects/time:1984">1984</a>, <a href="/subjects/time:the_middle_ages">The Middle Ages</a>, <a href="/subjects/time:1810-1890">1810-1890</a></i>')</span>
         </div>
         $ config = {'name': 'subject_times', 'facet': 'time', 'data': work.subject_times if len(work.subject_times) else []}
-        <div class="csv-autocomplete--subjects" data-config="$dumps(config)">
+        <div class="csv-autocomplete--subjects" data-config="$json_encode(config)">
             $:render_subject_field(config['name'], config['data'])
         </div>
     </div>
diff --git a/openlibrary/templates/books/edit/edition.html b/openlibrary/templates/books/edit/edition.html
index 3ca6970b796..43eab3b8a61 100644
--- a/openlibrary/templates/books/edit/edition.html
+++ b/openlibrary/templates/books/edit/edition.html
@@ -182,7 +182,7 @@
         <fieldset class="minor formBackLeft">
             <legend>$_("Contributors")</legend>
 
-            <div class="formElement" id="roles" data-config="$dumps({'Please select a role.': _('Please select a role.'), 'You need to give this ROLE a name.': _('You need to give this ROLE a name.')})">
+            <div class="formElement" id="roles" data-config="$json_encode({'Please select a role.': _('Please select a role.'), 'You need to give this ROLE a name.': _('You need to give this ROLE a name.')})">
                 <div class="label">
                     <label for="select-role">$_("List the people involved")</label>
                     <span class="tip"></span>
@@ -397,7 +397,7 @@
                 </span>
             </div>
             $ config = {'addnew': is_librarian, 'new_name': _('-- Move to a new work')}
-            <div class="multi-input-autocomplete multi-input-autocomplete--works" data-config="$dumps(config)">
+            <div class="multi-input-autocomplete multi-input-autocomplete--works" data-config="$json_encode(config)">
                 $if book.works:
                     $for i, work in enumerate(book.works):
                         $:render_work_field(i, work)
@@ -429,7 +429,7 @@
     $ 'ID must be exactly 13 digits [0-9]. For example: 978-1-56619-909-4': _('ID must be exactly 13 digits [0-9]. For example: 978-1-56619-909-4'),
     $ 'Invalid ID format': _('Invalid ID format')
 $ })
-<fieldset class="major" id="identifiers" data-config="$dumps(config)">
+<fieldset class="major" id="identifiers" data-config="$json_encode(config)">
     <legend>$_("ID Numbers")</legend>
     <div class="formBack">
 
@@ -451,7 +451,7 @@
     </div>
 </fieldset>
 
-<fieldset class="major" id="classifications" data-config="$dumps({'Please select a classification.': _('Please select a classification.'), 'You need to give a value to CLASS.': _('You need to give a value to CLASS.')})">
+<fieldset class="major" id="classifications" data-config="$json_encode({'Please select a classification.': _('Please select a classification.'), 'You need to give a value to CLASS.': _('You need to give a value to CLASS.')})">
     <legend>$_("Classifications")</legend>
     <div class="formBack">
 
diff --git a/openlibrary/templates/books/series-autocomplete.html b/openlibrary/templates/books/series-autocomplete.html
index 7c0904a31a4..dd9a53e48b2 100644
--- a/openlibrary/templates/books/series-autocomplete.html
+++ b/openlibrary/templates/books/series-autocomplete.html
@@ -45,7 +45,7 @@
     </div>
 
 $ config = {'name_path': name_path, 'dict_path': dict_path}
-<div class="multi-input-autocomplete multi-input-autocomplete--series" data-config="$dumps(config)">
+<div class="multi-input-autocomplete multi-input-autocomplete--series" data-config="$json_encode(config)">
     $for series in (series or [storage(name="", key="", position="")]):
         $:render_series(name_path, dict_path, readonly, loop.index0, series)
     <a href="javascript:;" class="mia__add">$_("Add another series?")</a>
diff --git a/openlibrary/templates/covers/change.html b/openlibrary/templates/covers/change.html
index c8f6ea75f25..b53c794560d 100644
--- a/openlibrary/templates/covers/change.html
+++ b/openlibrary/templates/covers/change.html
@@ -30,7 +30,7 @@
 <!-- The below style is inlined to avoid repaints -->
 <div class="$size sansserif manageCoversContainer" style="position: absolute; bottom: 15px">
   <a aria-controls="addImage-$imagesId" class="coverPop dialog--open" href="$(manage_url if doc.get_cover() else add_url)">
-    <div class="manageCovers" data-config="$dumps({'key': doc.type.key, 'url': get_coverstore_public_url(), 'selector': cover_selector, 'add_url': add_url, 'manage_url': manage_url})">$change</div>
+    <div class="manageCovers" data-config="$json_encode({'key': doc.type.key, 'url': get_coverstore_public_url(), 'selector': cover_selector, 'add_url': add_url, 'manage_url': manage_url})">$change</div>
   </a>
 </div>
 
diff --git a/openlibrary/templates/search/work_search_facets.html b/openlibrary/templates/search/work_search_facets.html
index d0e0f1a013b..7dd1624d707 100644
--- a/openlibrary/templates/search/work_search_facets.html
+++ b/openlibrary/templates/search/work_search_facets.html
@@ -71,7 +71,7 @@ <h4 class="facetHead">$(label)$:(magic_wand_markup)</h4>
             <ol-chip size="small" href="$facet_url" title="$link_title" data-ol-link-track="$add_track(header)" count="$commify(count)">$display</ol-chip>
     </div>
 
-<div id="searchFacets" data-config="$dumps({'start_facet_count': start_facet_count, 'facet_inc': facet_inc})" data-param="$dumps(param)" data-async-load="$dumps(async_load)">
+<div id="searchFacets" data-config="$json_encode({'start_facet_count': start_facet_count, 'facet_inc': facet_inc})" data-param="$json_encode(param)" data-async-load="$json_encode(async_load)">
     <h3 class="collapse">$_("Zoom In")</h3>
     <div class="smallest lightgreen sansserif" style="margin-bottom:20px;">
         $:_('Focus your results using these <a href="/search/howto">filters</a>')
diff --git a/openlibrary/templates/site/head.html b/openlibrary/templates/site/head.html
index c9ba2f675df..fe7deaa645b 100644
--- a/openlibrary/templates/site/head.html
+++ b/openlibrary/templates/site/head.html
@@ -47,11 +47,16 @@
         }
       </style>
     </noscript>
+
+    <script>
+      window.OL_EXPERIMENTS = $:json_encode(ctx.get("experiments", {}));
+    </script>
+
  <script>
  $if not is_bot() and not ctx.get('disable_analytics'):
    // Dimension 1 (visit-scoped): "Days Since Registration" — ID must match Matomo admin config (issue #12144)
    var _paq = window._paq = window._paq || [];
-   _paq.push(['setCustomDimension', 1, $:dumps(get_days_registered(ctx.user))]);
+   _paq.push(['setCustomDimension', 1, $:json_encode(get_days_registered(ctx.user))]);
 
    var _mtm = window._mtm = window._mtm || [];
    _mtm.push({'mtm.startTime': (new Date().getTime()), 'event': 'mtm.Start'});
diff --git a/openlibrary/templates/type/author/view.html b/openlibrary/templates/type/author/view.html
index d5ba7c38aad..167a199db53 100644
--- a/openlibrary/templates/type/author/view.html
+++ b/openlibrary/templates/type/author/view.html
@@ -74,7 +74,7 @@ <h2 class="author collapse">
         $ data['olids'] = olids
 
         <div class="message" style="display: none;">
-            <div id="preMerge" style="display: none;" data-keys="$dumps(data)">
+            <div id="preMerge" style="display: none;" data-keys="$json_encode(data)">
                 <p class="larger collapse"><strong>$_('Merging Authors...')</strong></p>
                 <p class="collapse adjust"><img src="/static/images/ajax-loader-bar.gif" width="220" height="19" alt="$_('In progress...')"/></p>
                 <p class="smaller lightgreen collapse">$_('Duplicates')</p>
diff --git a/openlibrary/tests/core/test_experiments.py b/openlibrary/tests/core/test_experiments.py
new file mode 100644
index 00000000000..49c70f476f5
--- /dev/null
+++ b/openlibrary/tests/core/test_experiments.py
@@ -0,0 +1,142 @@
+import pytest
+
+from openlibrary.core.experiments import ACTIVE_EXPERIMENTS, Audience, get_user_experiments, get_variant
+
+
+@pytest.fixture(autouse=True)
+def _clean_active_experiments():
+    original = ACTIVE_EXPERIMENTS.copy()
+    yield
+    ACTIVE_EXPERIMENTS.clear()
+    ACTIVE_EXPERIMENTS.update(original)
+
+
+def test_get_variant_fallback():
+    # Empty or None user identifier should fall back to control
+    assert get_variant("Sample_Experiment_1", "") == "control"
+    assert get_variant("Sample_Experiment_1", None) == "control"
+
+    # Non-existent experiment should fall back to control
+    assert get_variant("Non_Existent", "user123") == "control"
+
+
+def test_get_variant_distribution():
+    # Set up a test experiment
+    ACTIVE_EXPERIMENTS["Test_Distribution"] = {
+        "variants": {
+            "control": 30,
+            "a": 35,
+            "b": 35,
+        },
+        "audience": Audience.ALL,
+    }
+    variants_seen = set()
+    for i in range(100):
+        variant = get_variant("Test_Distribution", f"user_{i}")
+        assert variant in ACTIVE_EXPERIMENTS["Test_Distribution"]["variants"]
+        variants_seen.add(variant)
+
+    # Ensure all variants are allocated at least once (probabilistically guaranteed for N=100)
+    assert len(variants_seen) > 1
+    assert "control" in variants_seen
+    assert "a" in variants_seen
+    assert "b" in variants_seen
+
+
+def test_get_user_experiments():
+    ACTIVE_EXPERIMENTS["Test_User_Exp"] = {
+        "variants": {
+            "control": 50,
+            "treatment": 50,
+        },
+        "audience": Audience.ALL,
+    }
+    experiments = get_user_experiments("user_456")
+    assert "Test_User_Exp" in experiments
+    assert experiments["Test_User_Exp"] in ACTIVE_EXPERIMENTS["Test_User_Exp"]["variants"]
+
+
+def test_get_user_experiments_overrides():
+    ACTIVE_EXPERIMENTS["Test_Overrides"] = {
+        "variants": {
+            "control": 50,
+            "treatment": 50,
+        },
+        "audience": Audience.ALL,
+    }
+    # Valid override should change the variant
+    overrides = {"experiment_Test_Overrides": "treatment"}
+    experiments = get_user_experiments("user_456", overrides=overrides)
+    assert experiments["Test_Overrides"] == "treatment"
+
+    # Invalid override group/variant should be ignored
+    overrides = {"experiment_Test_Overrides": "invalid_group"}
+    experiments = get_user_experiments("user_456", overrides=overrides)
+    assert experiments["Test_Overrides"] != "invalid_group"
+    assert experiments["Test_Overrides"] in ACTIVE_EXPERIMENTS["Test_Overrides"]["variants"]
+
+    # Invalid override key/name format should be ignored
+    overrides = {"Test_Overrides": "treatment"}
+    experiments = get_user_experiments("user_456", overrides=overrides)
+    assert experiments["Test_Overrides"] in ACTIVE_EXPERIMENTS["Test_Overrides"]["variants"]
+
+
+def test_audience_targeting_logged_in():
+    ACTIVE_EXPERIMENTS["Test_Logged_In"] = {
+        "variants": {
+            "control": 20,
+            "treatment": 80,
+        },
+        "audience": Audience.LOGGED_IN,
+    }
+    # If not logged in, should always fall back to control
+    for i in range(100):
+        assert get_variant("Test_Logged_In", f"user_{i}", is_logged_in=False) == "control"
+
+    # If logged in, should distribute into variants
+    variants_seen = set()
+    for i in range(100):
+        variant = get_variant("Test_Logged_In", f"user_{i}", is_logged_in=True)
+        assert variant in ["control", "treatment"]
+        variants_seen.add(variant)
+
+    assert len(variants_seen) > 1
+
+
+def test_audience_targeting_logged_out():
+    ACTIVE_EXPERIMENTS["Test_Logged_Out"] = {
+        "variants": {
+            "control": 20,
+            "treatment": 80,
+        },
+        "audience": Audience.LOGGED_OUT,
+    }
+    # If logged in, should always fall back to control
+    for i in range(100):
+        assert get_variant("Test_Logged_Out", f"user_{i}", is_logged_in=True) == "control"
+
+    # If not logged in, should distribute into variants
+    variants_seen = set()
+    for i in range(100):
+        variant = get_variant("Test_Logged_Out", f"user_{i}", is_logged_in=False)
+        assert variant in ["control", "treatment"]
+        variants_seen.add(variant)
+
+    assert len(variants_seen) > 1
+
+
+def test_get_user_experiments_audience():
+    ACTIVE_EXPERIMENTS["Test_User_Exp_Audience"] = {
+        "variants": {
+            "control": 20,
+            "treatment": 80,
+        },
+        "audience": Audience.LOGGED_IN,
+    }
+    # If not logged in, should get control
+    experiments = get_user_experiments("user_456", is_logged_in=False)
+    assert experiments["Test_User_Exp_Audience"] == "control"
+
+    # If logged in, should evaluate variant
+    experiments_logged_in = get_user_experiments("user_456", is_logged_in=True)
+    assert experiments_logged_in["Test_User_Exp_Audience"] in ["control", "treatment"]