diff --git a/app/api/share/route.ts b/app/api/share/route.ts
index 79e1561..166a382 100644
--- a/app/api/share/route.ts
+++ b/app/api/share/route.ts
@@ -1,11 +1,13 @@
-import { timingSafeEqual } from "node:crypto";
 import { cdnUrl } from "@/lib/cdn-images";
 import { ensureDiscordUser } from "@/lib/auth/discord-user";
 import { getCanonicalUrl, getShareBotSecret } from "@/lib/config/server";
+import { getDiscordSenderProfile } from "@/lib/discord/discord";
 import {
-  DiscordProfileLookupError,
-  getDiscordSenderProfile,
-} from "@/lib/discord/discord";
+  buildBotShareResponse,
+  hasValidShareBotBearer,
+  rejectSuppliedBotProfile,
+  resolveBotSenderProfile,
+} from "@/lib/share/bot";
 import { createShare } from "@/lib/share/create";
 import { ShareHttpError, shareErrorResponse } from "@/lib/share/http-error";
 import { enforceShareRateLimit } from "@/lib/share/rate-limit";
@@ -19,19 +21,13 @@ export const dynamic = "force-dynamic";
 
 function hasValidBearerToken(request: Request) {
   const authorization = request.headers.get("authorization");
-  if (!authorization?.startsWith("Bearer ")) return false;
-
   let expected: string;
   try {
     expected = getShareBotSecret();
   } catch {
     throw new ShareHttpError("Share bot API is not configured", 503);
   }
-  const supplied = authorization.slice("Bearer ".length);
-  const suppliedBytes = Buffer.from(supplied);
-  const expectedBytes = Buffer.from(expected);
-  return suppliedBytes.length === expectedBytes.length &&
-    timingSafeEqual(suppliedBytes, expectedBytes);
+  return hasValidShareBotBearer(authorization, expected);
 }
 
 export async function POST(request: Request) {
@@ -53,19 +49,7 @@ export async function POST(request: Request) {
     } catch {
       throw new ShareHttpError("Malformed multipart form data", 400);
     }
-    for (const field of [
-      "displayName",
-      "avatarUrl",
-      "authorName",
-      "authorAvatarUrl",
-    ]) {
-      if (formData.has(field)) {
-        throw new ShareHttpError(
-          `Profile field ${field} must not be supplied`,
-          400,
-        );
-      }
-    }
+    rejectSuppliedBotProfile(formData);
     const file = formData.get("file");
     const senderDiscordId = formData.get("senderDiscordId");
     if (!(file instanceof File)) {
@@ -83,21 +67,10 @@ export async function POST(request: Request) {
       validateOptionalImage(formData.get("image")),
     ]);
     const description = validateShareDescription(formData.get("description"));
-    let profile;
-    try {
-      profile = await getDiscordSenderProfile(senderDiscordId);
-    } catch (error) {
-      if (error instanceof DiscordProfileLookupError) {
-        throw new ShareHttpError("Discord profile lookup is unavailable", 503);
-      }
-      throw error;
-    }
-    if (!profile) {
-      throw new ShareHttpError(
-        "Discord sender was not found in the configured guild",
-        422
-      );
-    }
+    const profile = await resolveBotSenderProfile(
+      senderDiscordId,
+      getDiscordSenderProfile,
+    );
 
     await enforceShareRateLimit({
       key: `sender:${senderDiscordId}`,
@@ -123,24 +96,14 @@ export async function POST(request: Request) {
         avatarUrl: profile.avatarUrl,
       },
     });
-    const url = `${baseUrl}/share/${share.id}`;
-
     return Response.json(
-      {
-        success: true,
-        share: {
-          id: share.id,
-          url,
-          author: {
-            displayName,
-            avatarUrl: profile.avatarUrl,
-          },
-          imageUrl: share.imageCdnId
-            ? `${baseUrl}${cdnUrl(share.imageCdnId)}`
-            : null,
-          createdAt: share.createdAt.toISOString(),
-        },
-      },
+      buildBotShareResponse({
+        baseUrl,
+        share,
+        displayName,
+        avatarUrl: profile.avatarUrl,
+        imagePath: share.imageCdnId ? cdnUrl(share.imageCdnId) : null,
+      }),
       { status: 201 }
     );
   } catch (error) {
diff --git a/lib/share/bot.test.ts b/lib/share/bot.test.ts
new file mode 100644
index 0000000..ac2278c
--- /dev/null
+++ b/lib/share/bot.test.ts
@@ -0,0 +1,60 @@
+import { describe, expect, test } from "bun:test";
+import {
+  buildBotShareResponse,
+  hasValidShareBotBearer,
+  rejectSuppliedBotProfile,
+  resolveBotSenderProfile,
+} from "@/lib/share/bot";
+
+describe("share bot contract", () => {
+  test("requires an exact bearer secret", () => {
+    expect(hasValidShareBotBearer(null, "secret")).toBeFalse();
+    expect(hasValidShareBotBearer("Basic secret", "secret")).toBeFalse();
+    expect(hasValidShareBotBearer("Bearer wrong", "secret")).toBeFalse();
+    expect(hasValidShareBotBearer("Bearer secret", "secret")).toBeTrue();
+  });
+
+  test("rejects request-provided identity snapshots", () => {
+    const formData = new FormData();
+    formData.set("displayName", "Untrusted name");
+    expect(() => rejectSuppliedBotProfile(formData)).toThrow("must not be supplied");
+  });
+
+  test("distinguishes an invalid guild sender from lookup downtime", async () => {
+    await expect(
+      resolveBotSenderProfile("1", async () => null),
+    ).rejects.toMatchObject({ status: 422 });
+    await expect(
+      resolveBotSenderProfile("1", async () => {
+        throw new Error("Discord unavailable");
+      }),
+    ).rejects.toMatchObject({ status: 503 });
+  });
+
+  test("returns the documented successful response shape", () => {
+    expect(
+      buildBotShareResponse({
+        baseUrl: "https://example.com",
+        share: {
+          id: "a1b2c3d4",
+          createdAt: new Date("2026-08-16T00:00:00.000Z"),
+        },
+        displayName: "Example User",
+        avatarUrl: "https://cdn.discordapp.com/avatar.png",
+        imagePath: "/api/cdn/image-1",
+      }),
+    ).toEqual({
+      success: true,
+      share: {
+        id: "a1b2c3d4",
+        url: "https://example.com/share/a1b2c3d4",
+        author: {
+          displayName: "Example User",
+          avatarUrl: "https://cdn.discordapp.com/avatar.png",
+        },
+        imageUrl: "https://example.com/api/cdn/image-1",
+        createdAt: "2026-08-16T00:00:00.000Z",
+      },
+    });
+  });
+});
diff --git a/lib/share/bot.ts b/lib/share/bot.ts
new file mode 100644
index 0000000..fb44aa4
--- /dev/null
+++ b/lib/share/bot.ts
@@ -0,0 +1,82 @@
+import { timingSafeEqual } from "node:crypto";
+import { ShareHttpError } from "@/lib/share/http-error";
+
+export type BotSenderProfile = {
+  id: string;
+  username: string;
+  globalName: string | null;
+  avatarUrl: string | null;
+};
+
+const FORBIDDEN_PROFILE_FIELDS = [
+  "displayName",
+  "avatarUrl",
+  "authorName",
+  "authorAvatarUrl",
+] as const;
+
+export function hasValidShareBotBearer(
+  authorization: string | null,
+  expected: string,
+) {
+  if (!authorization?.startsWith("Bearer ")) return false;
+  const suppliedBytes = Buffer.from(authorization.slice("Bearer ".length));
+  const expectedBytes = Buffer.from(expected);
+  return suppliedBytes.length === expectedBytes.length &&
+    timingSafeEqual(suppliedBytes, expectedBytes);
+}
+
+export function rejectSuppliedBotProfile(formData: FormData) {
+  for (const field of FORBIDDEN_PROFILE_FIELDS) {
+    if (formData.has(field)) {
+      throw new ShareHttpError(
+        `Profile field ${field} must not be supplied`,
+        400,
+      );
+    }
+  }
+}
+
+export async function resolveBotSenderProfile(
+  discordId: string,
+  lookup: (discordId: string) => Promise<BotSenderProfile | null>,
+) {
+  let profile: BotSenderProfile | null;
+  try {
+    profile = await lookup(discordId);
+  } catch {
+    throw new ShareHttpError("Discord profile lookup is unavailable", 503);
+  }
+  if (!profile) {
+    throw new ShareHttpError(
+      "Discord sender was not found in the configured guild",
+      422,
+    );
+  }
+  return profile;
+}
+
+export function buildBotShareResponse({
+  baseUrl,
+  share,
+  displayName,
+  avatarUrl,
+  imagePath,
+}: {
+  baseUrl: string;
+  share: { id: string; createdAt: Date };
+  displayName: string;
+  avatarUrl: string | null;
+  imagePath: string | null;
+}) {
+  return {
+    success: true as const,
+    share: {
+      id: share.id,
+      url: `${baseUrl}/share/${share.id}`,
+      author: { displayName, avatarUrl },
+      imageUrl: imagePath ? `${baseUrl}${imagePath}` : null,
+      createdAt: share.createdAt.toISOString(),
+    },
+  };
+}