diff --git a/web/src/hash_util.ts b/web/src/hash_util.ts
index b3a67d3a81..38850b14aa 100644
--- a/web/src/hash_util.ts
+++ b/web/src/hash_util.ts
@@ -67,7 +67,13 @@ export function decode_operand(operator: string, operand: string): string {
     operand = internal_url.decodeHashComponent(operand);
 
     if (util.canonicalize_stream_synonyms(operator) === "stream") {
-        return stream_data.slug_to_name(operand);
+        const stream_id = stream_data.slug_to_stream_id(operand);
+        if (stream_id) {
+            const stream = stream_data.get_sub_by_id(stream_id);
+            if (stream) {
+                return stream.name;
+            }
+        }
     }
 
     return operand;
diff --git a/web/src/stream_data.ts b/web/src/stream_data.ts
index 9f8cbb40e6..3e5f5ba6d5 100644
--- a/web/src/stream_data.ts
+++ b/web/src/stream_data.ts
@@ -236,7 +236,7 @@ export function name_to_slug(name: string): string {
     return `${stream_id}-${name}`;
 }
 
-export function slug_to_name(slug: string): string {
+export function slug_to_stream_id(slug: string): number | undefined {
     /*
     Modern stream slugs look like this, where 42
     is a stream id:
@@ -244,6 +244,12 @@ export function slug_to_name(slug: string): string {
         42
         42-stream-name
 
+    The ID might point to a stream that's hidden from our user (perhaps
+    doesn't exist). If so, most likely the user doesn't have permission to
+    see the stream's existence -- like with a guest user for any stream
+    they're not in, or any non-admin with a private stream they're not in.
+    Could be that whoever wrote the link just made something up.
+
     We have legacy slugs that are just the name
     of the stream:
 
@@ -259,29 +265,29 @@ export function slug_to_name(slug: string): string {
 
     If there is any ambiguity about whether a stream slug
     is old or modern, we prefer modern, as long as the integer
-    prefix matches a real stream id.  Eventually we will
-    stop supporting the legacy slugs, which only matter now
-    because people have linked to Zulip threads in things like
-    GitHub conversations.  We migrated to modern slugs in
-    early 2018.
+    prefix matches a real stream id. We return undefined if the
+    operand has an unexpected shape, or has the old shape (stream
+    name but no ID) and we don't know of a stream by the given name.
     */
-    const m = /^(\d+)(-.*)?$/.exec(slug);
-    if (m) {
-        const stream_id = Number.parseInt(m[1]!, 10);
-        const sub = sub_store.get(stream_id);
-        if (sub) {
-            return sub.name;
-        }
-        // if nothing was found above, we try to match on the stream
-        // name in the somewhat unlikely event they had a historical
-        // link to a stream like 4-horsemen
+
+    // "New" (2018) format: ${stream_id}-${stream_name} .
+    const match = /^(\d+)(-.*)?$/.exec(slug);
+    const newFormatStreamId = match ? Number.parseInt(match[1]!, 10) : null;
+    if (newFormatStreamId && stream_info.get(newFormatStreamId)) {
+        return newFormatStreamId;
     }
 
-    /*
-    We are dealing with a pre-2018 slug that doesn't have the
-    stream id as a prefix.
-    */
-    return slug;
+    // Old format: just stream name.  This case is relevant indefinitely,
+    // so that links in old conversations (including off-platform like GitHub)
+    // continue to work.
+    const stream = get_sub_by_name(slug);
+    if (stream) {
+        return stream.stream_id;
+    }
+
+    // Unexpected shape, or the old shape and we don't know of a stream with
+    // the given name.
+    return undefined;
 }
 
 export function delete_sub(stream_id: number): void {
diff --git a/web/tests/stream_data.test.js b/web/tests/stream_data.test.js
index dfa4e885a3..41f88419fa 100644
--- a/web/tests/stream_data.test.js
+++ b/web/tests/stream_data.test.js
@@ -166,16 +166,21 @@ test("basics", () => {
     assert.ok(!stream_data.is_default_stream_id(social.stream_id));
     assert.ok(!stream_data.is_default_stream_id(999999));
 
-    assert.equal(stream_data.slug_to_name("2-social"), "social");
-    assert.equal(stream_data.slug_to_name("2-whatever"), "social");
-    assert.equal(stream_data.slug_to_name("2"), "social");
-
+    // "new" correct url formats
+    assert.equal(stream_data.slug_to_stream_id("2-social"), 2);
+    assert.equal(stream_data.slug_to_stream_id("2"), 2);
+    // we still get 2 because it's a valid stream id
+    assert.equal(stream_data.slug_to_stream_id("2-whatever"), 2);
+    // invalid stream id
+    assert.equal(stream_data.slug_to_stream_id("999-social"), undefined);
     // legacy
-    assert.equal(stream_data.slug_to_name("25-or-6-to-4"), "25-or-6-to-4");
-    assert.equal(stream_data.slug_to_name("2something"), "2something");
+    assert.equal(stream_data.slug_to_stream_id("social"), 2);
 
-    assert.equal(stream_data.slug_to_name("99-whatever"), "99-whatever");
-    assert.equal(stream_data.slug_to_name("99whatever"), "99whatever");
+    // invalid formats
+    assert.equal(stream_data.slug_to_stream_id("25-or-6-to-4"), undefined);
+    assert.equal(stream_data.slug_to_stream_id("2something"), undefined);
+    assert.equal(stream_data.slug_to_stream_id("99-whatever"), undefined);
+    assert.equal(stream_data.slug_to_stream_id("99whatever"), undefined);
 
     // sub_store
     assert.equal(sub_store.get(-3), undefined);