diff --git a/src/lib/microcosm/constellation.ts b/src/lib/microcosm/constellation.ts
index db82b23..2c38feb 100644
--- a/src/lib/microcosm/constellation.ts
+++ b/src/lib/microcosm/constellation.ts
@@ -10,7 +10,7 @@
  * Response shapes here were verified against the live public instance.
  * @see constellation.openapi.yaml at the repo root
  */
-import {CONSTELLATION_URL, MICROCOSM_USER_AGENT} from '#/env'
+import {CONSTELLATION_URL} from '#/env'
 
 /** A target being linked to: a DID, an at-uri, or a plain URL. */
 export type Target = string
@@ -121,9 +121,11 @@ async function get<T>(
     }
   }
   const res = await fetch(url.toString(), {
+    // NOTE: do not set a User-Agent header. Browsers forbid setting it on fetch
+    // and including a forbidden header can break the request; the server doesn't
+    // require it. Native gets the default UA.
     headers: {
       Accept: 'application/json',
-      'User-Agent': MICROCOSM_USER_AGENT,
     },
     signal,
   })
diff --git a/src/lib/microcosm/hydrate.ts b/src/lib/microcosm/hydrate.ts
index bfff34f..ccdf1aa 100644
--- a/src/lib/microcosm/hydrate.ts
+++ b/src/lib/microcosm/hydrate.ts
@@ -611,9 +611,22 @@ export async function hydratePost(
     lite?: boolean
   },
 ): Promise<AppBskyFeedDefs.PostView | undefined> {
-  const parsed = parseAtUri(postUri)
+  let parsed = parseAtUri(postUri)
   if (!parsed) return undefined
 
+  // Constellation indexes by DID, and record reads expect a DID authority. If
+  // the uri came in with a handle authority (e.g. from a route param), resolve
+  // it to a DID first, otherwise every Constellation backlink lookup returns 0.
+  if (!parsed.did.startsWith('did:')) {
+    const did = await resolveMiniDoc(parsed.did, signal)
+      .then(m => m.did)
+      .catch(() => undefined)
+    if (did) {
+      postUri = `at://${did}/${parsed.collection}/${parsed.rkey}`
+      parsed = {did, collection: parsed.collection, rkey: parsed.rkey}
+    }
+  }
+
   const lite = opts?.lite ?? false
   const [recordRes, author, likes, reposts, replies, quotes, viewer] =
     await Promise.all([
diff --git a/src/lib/microcosm/slingshot.ts b/src/lib/microcosm/slingshot.ts
index 8ff15da..e64c3fd 100644
--- a/src/lib/microcosm/slingshot.ts
+++ b/src/lib/microcosm/slingshot.ts
@@ -6,7 +6,7 @@
  *
  * @see slingshot.json at the repo root
  */
-import {MICROCOSM_USER_AGENT, SLINGSHOT_URL} from '#/env'
+import {SLINGSHOT_URL} from '#/env'
 
 /** A record as returned by getRecord — `value` is the raw record JSON. */
 export type FoundRecord = {
@@ -79,9 +79,10 @@ async function get<T>(
     if (value !== undefined) url.searchParams.set(key, value)
   }
   const res = await fetch(url.toString(), {
+    // No User-Agent: browsers forbid setting it on fetch and a forbidden header
+    // can break the request; the server doesn't require it.
     headers: {
       Accept: 'application/json',
-      'User-Agent': MICROCOSM_USER_AGENT,
     },
     signal,
   })
@@ -147,7 +148,6 @@ export async function hydrateFeedSkeleton(
       headers: {
         'Content-Type': 'application/json',
         Accept: 'application/json',
-        'User-Agent': MICROCOSM_USER_AGENT,
       },
       body: JSON.stringify(payload),
       signal,
@@ -286,7 +286,7 @@ export async function listRecords(
   if (args.reverse != null)
     url.searchParams.set('reverse', String(args.reverse))
   const res = await fetch(url.toString(), {
-    headers: {Accept: 'application/json', 'User-Agent': MICROCOSM_USER_AGENT},
+    headers: {Accept: 'application/json'},
     signal,
   })
   if (!res.ok) {
@@ -321,7 +321,7 @@ export async function countRecords(
     url.searchParams.set('limit', '100')
     if (cursor) url.searchParams.set('cursor', cursor)
     const res = await fetch(url.toString(), {
-      headers: {Accept: 'application/json', 'User-Agent': MICROCOSM_USER_AGENT},
+      headers: {Accept: 'application/json'},
       signal,
     })
     if (!res.ok) break
diff --git a/src/lib/microcosm/thread.ts b/src/lib/microcosm/thread.ts
index a87f96a..ff3de96 100644
--- a/src/lib/microcosm/thread.ts
+++ b/src/lib/microcosm/thread.ts
@@ -23,6 +23,7 @@ import {
 
 import * as constellation from '#/lib/microcosm/constellation'
 import {hydratePost} from '#/lib/microcosm/hydrate'
+import {resolveMiniDoc} from '#/lib/microcosm/slingshot'
 
 type Sort = 'newest' | 'oldest' | 'top' | (string & {})
 
@@ -85,9 +86,12 @@ async function collectAncestors(
       truncated = true
       break
     }
-    const parent = await hydratePost(currentParentUri, viewerDid, signal, 1, {
-      lite: true,
-    }).catch(() => undefined)
+    const parent = await hydratePost(
+      currentParentUri,
+      viewerDid,
+      signal,
+      1,
+    ).catch(() => undefined)
     if (!parent) break
     posts.push(parent)
     currentParentUri = parentUriOf(parent)
@@ -123,10 +127,11 @@ async function fetchReplies(
   const uris = page.records.map(refToUri)
   const hydrated = (
     await Promise.all(
+      // Full hydration (not lite): thread replies show their own like/reply/
+      // repost counts and viewer state, same as the AppView. Reply volume is
+      // bounded by branchingFactor, so this stays affordable.
       uris.map(uri =>
-        hydratePost(uri, viewerDid, signal, 1, {lite: true}).catch(
-          () => undefined,
-        ),
+        hydratePost(uri, viewerDid, signal, 1).catch(() => undefined),
       ),
     )
   ).filter((p): p is AppBskyFeedDefs.PostView => !!p)
@@ -231,6 +236,16 @@ export async function buildPostThread(
   const sort = opts.sort ?? 'newest'
   const includeAbove = opts.above ?? true
 
+  // Constellation indexes by DID. The anchor uri often arrives with a handle
+  // authority (route param), which would make every reply backlink lookup
+  // return nothing - resolve it to a DID first.
+  if (!parsed.did.startsWith('did:')) {
+    const did = await resolveMiniDoc(parsed.did, signal)
+      .then(m => m.did)
+      .catch(() => undefined)
+    if (did) anchorUri = `at://${did}/app.bsky.feed.post/${parsed.rkey}`
+  }
+
   // Hydrate the anchor with full counts/viewer state - it's the focal post.
   const anchor = await hydratePost(
     anchorUri,