diff --git a/dotnet/EcencyApi.Tests/FavoriteTagsPathTests.cs b/dotnet/EcencyApi.Tests/FavoriteTagsPathTests.cs
new file mode 100644
index 00000000..62d22431
--- /dev/null
+++ b/dotnet/EcencyApi.Tests/FavoriteTagsPathTests.cs
@@ -0,0 +1,119 @@
+using System.Text.Json.Nodes;
+using EcencyApi.Handlers;
+using Xunit;
+
+namespace EcencyApi.Tests;
+
+/// <summary>
+/// The favorite-tag check and delete handlers interpolate a body-supplied tag into
+/// an upstream path. A tag is an arbitrary string, so anything structural left
+/// unescaped is re-parsed when the string becomes a Uri, and the upstream call
+/// carries this service's credentials. Same reasoning as NotificationsPathTests.
+/// </summary>
+public class FavoriteTagsPathTests
+{
+    [Fact]
+    public void RealRequestsAreUnchanged()
+    {
+        // Hive names and tags are unreserved characters; escaping must be a no-op for
+        // them or this would change every live request.
+        Assert.Equal(
+            "isfavoritetag/good-karma/photography",
+            PrivateApi.FavoriteTagPath("isfavoritetag", "good-karma", "photography"));
+        Assert.Equal(
+            "favoriteTag/user.name/contest-2026",
+            PrivateApi.FavoriteTagPath("favoriteTag", "user.name", "contest-2026"));
+    }
+
+    [Fact]
+    public void LeadingHashIsEscapedNotDropped()
+    {
+        // Raw, `#` would truncate the path at a fragment. Escaped, it reaches the
+        // upstream, which normalises the tag and strips one leading hash itself.
+        Assert.Equal(
+            "isfavoritetag/good-karma/%23photography",
+            PrivateApi.FavoriteTagPath("isfavoritetag", "good-karma", "#photography"));
+    }
+
+    [Theory]
+    // A missing tag must not become the literal "undefined" the way a missing
+    // account does in the favorites handlers: "undefined", "null", "true" and "123"
+    // are all valid tag names upstream, so templating would check or delete a follow
+    // the caller never named.
+    [InlineData("{}")]
+    [InlineData("{\"tag\": null}")]
+    [InlineData("{\"tag\": true}")]
+    [InlineData("{\"tag\": 123}")]
+    [InlineData("{\"tag\": \"\"}")]
+    [InlineData("{\"tag\": [\"photography\"]}")]
+    [InlineData("{\"tag\": {\"name\": \"photography\"}}")]
+    public void AbsentOrNonStringTagIsRejected(string json)
+    {
+        var body = (JsonObject)JsonNode.Parse(json)!;
+
+        Assert.Null(PrivateApi.FavoriteTagField(body));
+    }
+
+    [Fact]
+    public void StringTagIsReadAsIs()
+    {
+        // Read leniently and passed through untouched: normalising is the upstream's job.
+        var body = (JsonObject)JsonNode.Parse("{\"tag\": \"#Photography\"}")!;
+        Assert.Equal("#Photography", PrivateApi.FavoriteTagField(body));
+
+        // Lone surrogates are valid JSON string content and must not be dropped.
+        var lone = (JsonObject)JsonNode.Parse("{\"tag\": \"\\ud800\"}")!;
+        Assert.Equal("\ud800", PrivateApi.FavoriteTagField(lone));
+    }
+
+    [Theory]
+    // A slash would add path segments and address a different resource.
+    [InlineData("a", "b/c")]
+    [InlineData("a/b", "c")]
+    // A question mark would truncate the path and turn the rest into a query.
+    [InlineData("a", "b?x=1")]
+    // A hash would truncate the path at a fragment.
+    [InlineData("a", "b#f")]
+    // Whitespace and non-ASCII must not leak into the request line either.
+    [InlineData("a", "b c")]
+    [InlineData("a", "caf\u00e9")]
+    public void StructuralCharactersCannotEscapeTheirSegment(string username, string tag)
+    {
+        var path = PrivateApi.FavoriteTagPath("isfavoritetag", username, tag);
+
+        Assert.NotNull(path);
+        Assert.DoesNotContain("?x=1", path);
+        Assert.DoesNotContain("#f", path);
+        Assert.DoesNotContain(" ", path);
+        Assert.True(path!.All(c => c < 128));
+        // The only separators left are the two this builder wrote itself.
+        Assert.Equal(2, path.Split('/').Length - 1);
+    }
+
+    [Fact]
+    public void LoneSurrogateIsEncodedNotThrown()
+    {
+        // JSON strings are arbitrary UTF-16, so a lone surrogate reaches the helper as
+        // a real value (see TryGetStringLenient). On the runtime this service targets,
+        // EscapeDataString encodes it as U+FFFD rather than throwing, so the request
+        // stays on the handler's own path instead of falling through to the 500 page.
+        Assert.Equal(
+            "isfavoritetag/good-karma/%EF%BF%BD",
+            PrivateApi.FavoriteTagPath("isfavoritetag", "good-karma", "\ud800"));
+        Assert.Equal(
+            "favoriteTag/good-karma/a%EF%BF%BDb",
+            PrivateApi.FavoriteTagPath("favoriteTag", "good-karma", "a\udc00b"));
+    }
+
+    [Theory]
+    // Dot segments cannot be fixed by escaping: Uri decodes %2E back to `.` before it
+    // removes dot segments, so they have to be rejected outright.
+    [InlineData(".", "photography")]
+    [InlineData("..", "photography")]
+    [InlineData("good-karma", ".")]
+    [InlineData("good-karma", "..")]
+    public void DotSegmentsAreRejected(string username, string tag)
+    {
+        Assert.Null(PrivateApi.FavoriteTagPath("isfavoritetag", username, tag));
+    }
+}
diff --git a/dotnet/EcencyApi/Handlers/PrivateApi.UserData2.cs b/dotnet/EcencyApi/Handlers/PrivateApi.UserData2.cs
index 2bf92ee8..0f77d930 100644
--- a/dotnet/EcencyApi/Handlers/PrivateApi.UserData2.cs
+++ b/dotnet/EcencyApi/Handlers/PrivateApi.UserData2.cs
@@ -129,6 +129,118 @@ public static async Task FavoritesDelete(HttpContext ctx)
         await Upstream.Pipe(ApiClient.ApiRequest($"favoriteUser/{username}/{account}", HttpMethod.Delete), ctx);
     }
 
+    /// <summary>
+    /// The body's `tag`, or null when it is absent, not a string, or empty.
+    ///
+    /// The favorites handlers template a missing `account` into the literal
+    /// "undefined". A tag must not go that way: "undefined", "null", "true" and
+    /// "123" are all valid tag names upstream, so a request with no tag, or a
+    /// non-string one, would check or delete a follow the caller never named.
+    /// String extraction is lenient on purpose: JSON strings are arbitrary UTF-16.
+    /// </summary>
+    public static string? FavoriteTagField(JsonObject body)
+    {
+        if (!body.TryGetPropertyValue("tag", out var node) || node is not JsonValue value)
+        {
+            return null;
+        }
+
+        return JsVal.TryGetStringLenient(value, out var tag) && tag.Length > 0 ? tag : null;
+    }
+
+    /// <summary>
+    /// Upstream path for a followed-tag check or removal, or null when a value cannot
+    /// be expressed as a single path segment.
+    ///
+    /// The tag is a body string, so one carrying `/`, `?` or `#` would otherwise be
+    /// re-parsed as URL structure once this string becomes a Uri, addressing a
+    /// different upstream resource with this service's credentials attached. Same
+    /// reasoning as NotificationsPath. Real tags are lowercase words with digits and
+    /// hyphens, which EscapeDataString leaves byte-identical. A leading `#` a client
+    /// may send is escaped rather than dropped: normalising the tag is the upstream's
+    /// job, and it strips one.
+    /// </summary>
+    public static string? FavoriteTagPath(string action, string username, string tag)
+    {
+        if (IsDotSegment(username) || IsDotSegment(tag))
+        {
+            return null;
+        }
+
+        return $"{action}/{Uri.EscapeDataString(username)}/{Uri.EscapeDataString(tag)}";
+    }
+
+    public static async Task FavoriteTags(HttpContext ctx)
+    {
+        var body = await ctx.ReadBody();
+        var username = await ValidateCode(body);
+        if (username == null)
+        {
+            await ctx.SendText(401, "Unauthorized");
+            return;
+        }
+        await Upstream.Pipe(
+            ApiClient.ApiRequest($"favorite-tags/{username}", HttpMethod.Get, null, null, UserData2Js.Query(ctx)),
+            ctx);
+    }
+
+    public static async Task FavoriteTagsCheck(HttpContext ctx)
+    {
+        var body = await ctx.ReadBody();
+        var username = await ValidateCode(body);
+        if (username == null)
+        {
+            await ctx.SendText(401, "Unauthorized");
+            return;
+        }
+        var tag = FavoriteTagField(body);
+        var path = tag == null ? null : FavoriteTagPath("isfavoritetag", username, tag);
+        if (path == null)
+        {
+            await ctx.SendText(400, "Invalid tag");
+            return;
+        }
+        await Upstream.Pipe(ApiClient.ApiRequest(path, HttpMethod.Get), ctx);
+    }
+
+    public static async Task FavoriteTagsAdd(HttpContext ctx)
+    {
+        var body = await ctx.ReadBody();
+        var username = await ValidateCode(body);
+        if (username == null)
+        {
+            await ctx.SendText(401, "Unauthorized");
+            return;
+        }
+        var tag = FavoriteTagField(body);
+        if (tag == null)
+        {
+            await ctx.SendText(400, "Invalid tag");
+            return;
+        }
+        var data = new JsonObject { ["username"] = username, ["tag"] = tag };
+        await Upstream.Pipe(ApiClient.ApiRequest("favorite-tag", HttpMethod.Post, null, data), ctx);
+    }
+
+    public static async Task FavoriteTagsDelete(HttpContext ctx)
+    {
+        var body = await ctx.ReadBody();
+        var username = await ValidateCode(body);
+        if (username == null)
+        {
+            await ctx.SendText(401, "Unauthorized");
+            return;
+        }
+        var tag = FavoriteTagField(body);
+        var path = tag == null ? null : FavoriteTagPath("favoriteTag", username, tag);
+        if (path == null)
+        {
+            await ctx.SendText(400, "Invalid tag");
+            return;
+        }
+        await Upstream.Pipe(ApiClient.ApiRequest(path, HttpMethod.Delete), ctx);
+    }
+
     public static async Task Fragments(HttpContext ctx)
     {
         var body = await ctx.ReadBody();
diff --git a/dotnet/EcencyApi/Handlers/Routes.cs b/dotnet/EcencyApi/Handlers/Routes.cs
index dd74c8d6..9348c0a7 100644
--- a/dotnet/EcencyApi/Handlers/Routes.cs
+++ b/dotnet/EcencyApi/Handlers/Routes.cs
@@ -127,6 +127,10 @@ public static void Map(WebApplication app)
         app.MapPost("/private-api/favorites-check", PrivateApi.FavoritesCheck);
         app.MapPost("/private-api/favorites-add", PrivateApi.FavoritesAdd);
         app.MapPost("/private-api/favorites-delete", PrivateApi.FavoritesDelete);
+        app.MapPost("/private-api/favorite-tags", PrivateApi.FavoriteTags);
+        app.MapPost("/private-api/favorite-tags-check", PrivateApi.FavoriteTagsCheck);
+        app.MapPost("/private-api/favorite-tags-add", PrivateApi.FavoriteTagsAdd);
+        app.MapPost("/private-api/favorite-tags-delete", PrivateApi.FavoriteTagsDelete);
         app.MapPost("/private-api/fragments", PrivateApi.Fragments);
         app.MapPost("/private-api/fragments-add", PrivateApi.FragmentsAdd);
         app.MapPost("/private-api/fragments-update", PrivateApi.FragmentsUpdate);
diff --git a/dotnet/parity/driver.py b/dotnet/parity/driver.py
index c65239e3..2ee86618 100644
--- a/dotnet/parity/driver.py
+++ b/dotnet/parity/driver.py
@@ -62,6 +62,9 @@
     "/private-api/account-create-friend": {"username": "newuser123",
                                            "email": "x@example.com", "friend": True},
     "/private-api/subscribe": {"email": "x@example.com"},
+    "/private-api/favorite-tags-check": {"code": "invalid", "tag": "photography"},
+    "/private-api/favorite-tags-add": {"code": "invalid", "tag": "photography"},
+    "/private-api/favorite-tags-delete": {"code": "invalid", "tag": "photography"},
     "/private-api/notifications": {"code": "invalid", "filter": "", "since": 0,
                                    "limit": 50, "user": "good-karma"},
     "/private-api/report": {"type": "content", "data": {"a": 1}},
@@ -224,6 +227,13 @@ def norm_body(text):
     "::pop and ::badcode for every POST route, which is why all three appear here."
 )
 
+FAVORITE_TAGS_DIVERGENCE = (
+    "Followed-hashtag routes added after the port (the favorites passthroughs, keyed by "
+    "tag instead of account). The reference build has no such route and answers 404; "
+    "this one validates the signed code and answers 401 or proxies. Deterministic and "
+    "additive -- no behavior the reference ever had is changing."
+)
+
 AI_IMAGES_DIVERGENCE = (
     "Per-user AI image history route added after the port (ePoints users/<u>/ai-images). "
     "The reference build has no such route and answers 404; this one validates the "
@@ -272,6 +282,32 @@ def norm_body(text):
         AI_IMAGES_DIVERGENCE,
     "/private-api/ai-images::badcode":
         AI_IMAGES_DIVERGENCE,
+    # Followed-hashtag passthroughs, added after the port. Same shape as the entries
+    # above: additive routes the reference build never had.
+    "/private-api/favorite-tags::min":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags::pop":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags::badcode":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags-check::min":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags-check::pop":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags-check::badcode":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags-add::min":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags-add::pop":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags-add::badcode":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags-delete::min":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags-delete::pop":
+        FAVORITE_TAGS_DIVERGENCE,
+    "/private-api/favorite-tags-delete::badcode":
+        FAVORITE_TAGS_DIVERGENCE,
 }
 
 # Deliberately NOT listed above: /wallet-api/portfolio-v2::pop, whose HP action list