diff --git a/app/[owner]/[repo]/[postNumber]/not-found.tsx b/app/[owner]/[repo]/[postNumber]/not-found.tsx
new file mode 100644
index 0000000..095de18
--- /dev/null
+++ b/app/[owner]/[repo]/[postNumber]/not-found.tsx
@@ -0,0 +1,38 @@
+"use client"
+
+import Link from "next/link"
+import { usePathname } from "next/navigation"
+import { Container } from "@/components/container"
+import { Title } from "@/components/typography"
+
+export default function PostNotFound() {
+  const pathname = usePathname()
+  const parts = pathname.split("/").filter(Boolean)
+  const owner = parts[0]
+  const repo = parts[1]
+
+  const repoUrl = owner && repo ? `/${owner}/${repo}` : "/"
+
+  return (
+    <Container>
+      <div className="py-12">
+        <Title>404 — Post Not Found</Title>
+        <p className="mt-4 text-muted">
+          This post doesn't exist in{" "}
+          <span className="font-medium text-dim">
+            {owner}/{repo}
+          </span>
+          .
+        </p>
+        <p className="mt-6 flex items-center gap-4 text-sm">
+          <Link className="text-accent hover:underline" href={repoUrl}>
+            View all posts in {owner}/{repo}
+          </Link>
+          <Link className="text-accent hover:underline" href="/">
+            Go home
+          </Link>
+        </p>
+      </div>
+    </Container>
+  )
+}
diff --git a/app/[owner]/[repo]/[postNumber]/page.tsx b/app/[owner]/[repo]/[postNumber]/page.tsx
index 29caf1e..8450d34 100644
--- a/app/[owner]/[repo]/[postNumber]/page.tsx
+++ b/app/[owner]/[repo]/[postNumber]/page.tsx
@@ -121,8 +121,6 @@ export default async function PostPage({
 }: {
   params: Promise<{ owner: string; repo: string; postNumber: string }>
 }) {
-  "use cache"
-
   const { postNumber: postNumberStr, owner, repo } = await params
 
   if (postNumberStr.endsWith(".md") || postNumberStr.endsWith(".txt")) {
@@ -136,6 +134,26 @@ export default async function PostPage({
     notFound()
   }
 
+  // Check post exists before cache boundary
+  const post = await getPostByNumber(owner, repo, postNumber)
+  if (!post) {
+    notFound()
+  }
+
+  return <PostPageContent owner={owner} postNumber={postNumber} repo={repo} />
+}
+
+async function PostPageContent({
+  owner,
+  repo,
+  postNumber,
+}: {
+  owner: string
+  repo: string
+  postNumber: number
+}) {
+  "use cache"
+
   const [
     postWithCategory,
     allLlmUsers,
@@ -224,11 +242,8 @@ export default async function PostPage({
       .where(and(eq(categories.owner, owner), eq(categories.repo, repo))),
   ])
 
-  if (!postWithCategory) {
-    notFound()
-  }
-
-  const { category, gitContexts, ...post } = postWithCategory
+  // Post existence already verified before cache boundary
+  const { category, gitContexts, ...post } = postWithCategory!
   const gitContext = gitContexts?.[0] ?? null
 
   cacheTag(`post:${post.id}`)
diff --git a/app/[owner]/[repo]/category/[categorySlug]/not-found.tsx b/app/[owner]/[repo]/category/[categorySlug]/not-found.tsx
new file mode 100644
index 0000000..f403bbc
--- /dev/null
+++ b/app/[owner]/[repo]/category/[categorySlug]/not-found.tsx
@@ -0,0 +1,39 @@
+"use client"
+
+import { ExternalLinkIcon } from "lucide-react"
+import Link from "next/link"
+import { usePathname } from "next/navigation"
+import { Container } from "@/components/container"
+import { Title } from "@/components/typography"
+
+export default function CategoryNotFound() {
+  const pathname = usePathname()
+  const parts = pathname.split("/").filter(Boolean)
+  const owner = parts[0]
+  const repo = parts[1]
+
+  const repoUrl = owner && repo ? `/${owner}/${repo}` : "/"
+
+  return (
+    <Container>
+      <div className="py-12">
+        <Title>404 — Category Not Found</Title>
+        <p className="mt-4 text-muted">
+          This category doesn't exist in{" "}
+          <span className="font-medium text-dim">
+            {owner}/{repo}
+          </span>
+          .
+        </p>
+        <p className="mt-6 flex items-center gap-4 text-sm">
+          <Link className="text-accent hover:underline" href={repoUrl}>
+            View all posts in {owner}/{repo}
+          </Link>
+          <Link className="text-accent hover:underline" href="/">
+            Go home
+          </Link>
+        </p>
+      </div>
+    </Container>
+  )
+}
diff --git a/app/[owner]/[repo]/category/[categorySlug]/page.tsx b/app/[owner]/[repo]/category/[categorySlug]/page.tsx
index 70680fc..791bc7b 100644
--- a/app/[owner]/[repo]/category/[categorySlug]/page.tsx
+++ b/app/[owner]/[repo]/category/[categorySlug]/page.tsx
@@ -65,26 +65,45 @@ export default async function CategoryPage({
 }: {
   params: Promise<{ owner: string; repo: string; categorySlug: string }>
 }) {
-  "use cache"
-
   const { owner, repo, categorySlug } = await params
-  cacheTag(`category:${categorySlug}`)
 
-  const [category, allLlmUsers, repoData] = await Promise.all([
+  // Check existence before cache boundary
+  const [category, repoData] = await Promise.all([
     getCategoryBySlug(owner, repo, categorySlug),
-    getModelsForPicker(),
     getGithubRepo(owner, repo),
   ])
 
-  if (!category) {
-    return notFound()
+  if (!category || !repoData) {
+    notFound()
   }
 
-  if (!repoData) {
-    return notFound()
-  }
+  return (
+    <CategoryPageContent
+      category={category}
+      categorySlug={categorySlug}
+      owner={owner}
+      repo={repo}
+    />
+  )
+}
+
+async function CategoryPageContent({
+  owner,
+  repo,
+  categorySlug,
+  category,
+}: {
+  owner: string
+  repo: string
+  categorySlug: string
+  category: NonNullable<Awaited<ReturnType<typeof getCategoryBySlug>>>
+}) {
+  "use cache"
+  cacheTag(`category:${categorySlug}`)
 
-  const categoryPosts = await db
+  const [allLlmUsers, categoryPosts] = await Promise.all([
+    getModelsForPicker(),
+    db
     .select({
       id: posts.id,
       number: posts.number,
@@ -111,7 +130,8 @@ export default async function CategoryPage({
         eq(posts.categoryId, category.id)
       )
     )
-    .orderBy(desc(posts.createdAt))
+    .orderBy(desc(posts.createdAt)),
+  ])
 
   const categoriesById = { [category.id]: category }
 
diff --git a/app/[owner]/[repo]/not-found.tsx b/app/[owner]/[repo]/not-found.tsx
new file mode 100644
index 0000000..823b20c
--- /dev/null
+++ b/app/[owner]/[repo]/not-found.tsx
@@ -0,0 +1,48 @@
+"use client"
+
+import { ExternalLinkIcon } from "lucide-react"
+import Link from "next/link"
+import { usePathname } from "next/navigation"
+import { Container } from "@/components/container"
+import { Title } from "@/components/typography"
+
+export default function RepoNotFound() {
+  const pathname = usePathname()
+  const parts = pathname.split("/").filter(Boolean)
+  const owner = parts[0]
+  const repo = parts[1]
+
+  const githubUrl =
+    owner && repo ? `https://github.com/${owner}/${repo}` : "https://github.com"
+
+  return (
+    <Container>
+      <div className="py-12">
+        <Title>404 — Repository Not Found</Title>
+        <p className="mt-4 text-muted">
+          We couldn't find a repository at{" "}
+          <span className="font-medium text-dim">
+            {owner}/{repo}
+          </span>
+          .
+        </p>
+        <p className="mt-6 flex items-center gap-2 text-sm">
+          <span className="text-faint">Does this repo exist on GitHub?</span>
+          <Link
+            className="flex items-center gap-1 text-accent hover:underline"
+            href={githubUrl}
+            target="_blank"
+          >
+            Check on GitHub
+            <ExternalLinkIcon className="h-3.5 w-3.5" />
+          </Link>
+        </p>
+        <p className="mt-6">
+          <Link className="text-accent hover:underline" href="/">
+            Go back home
+          </Link>
+        </p>
+      </div>
+    </Container>
+  )
+}
diff --git a/app/[owner]/[repo]/page.tsx b/app/[owner]/[repo]/page.tsx
index 56f3774..d084da3 100644
--- a/app/[owner]/[repo]/page.tsx
+++ b/app/[owner]/[repo]/page.tsx
@@ -58,12 +58,30 @@ export default async function RepoPage({
 }: {
   params: Promise<{ owner: string; repo: string }>
 }) {
-  "use cache"
-
   const { owner, repo } = await params
+
+  // Check repo exists before cache boundary
+  const repoData = await getGithubRepo(owner, repo)
+  if (!repoData) {
+    notFound()
+  }
+
+  return <RepoPageContent owner={owner} repo={repo} repoData={repoData} />
+}
+
+async function RepoPageContent({
+  owner,
+  repo,
+  repoData,
+}: {
+  owner: string
+  repo: string
+  repoData: NonNullable<Awaited<ReturnType<typeof getGithubRepo>>>
+}) {
+  "use cache"
   cacheTag(`repo:${owner}:${repo}`)
 
-  const [repoPosts, repoCategories, allLlmUsers, repoData] = await Promise.all([
+  const [repoPosts, repoCategories, allLlmUsers] = await Promise.all([
     db
       .select({
         id: posts.id,
@@ -91,13 +109,8 @@ export default async function RepoPage({
       .from(categories)
       .where(and(eq(categories.owner, owner), eq(categories.repo, repo))),
     getModelsForPicker(),
-    getGithubRepo(owner, repo),
   ])
 
-  if (!repoData) {
-    return notFound()
-  }
-
   const categoriesById = Object.fromEntries(
     repoCategories.map((c) => [c.id, c])
   )
diff --git a/app/not-found.tsx b/app/not-found.tsx
new file mode 100644
index 0000000..bedc231
--- /dev/null
+++ b/app/not-found.tsx
@@ -0,0 +1,21 @@
+import Link from "next/link"
+import { Container } from "@/components/container"
+import { Title } from "@/components/typography"
+
+export default function NotFound() {
+  return (
+    <Container>
+      <div className="py-12">
+        <Title>404 — Page Not Found</Title>
+        <p className="mt-4 text-muted">
+          The page you're looking for doesn't exist.
+        </p>
+        <p className="mt-6">
+          <Link className="text-accent hover:underline" href="/">
+            Go back home
+          </Link>
+        </p>
+      </div>
+    </Container>
+  )
+}