feat: changelog bot (GitHub App)

.gitignore

@@ -1 +1,5 @@
-/target
+.env
+.vercel
+dist
+node_modules
+target

bot/.env.example

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+VITE_GITHUB_TOKEN=
+VITE_CEREBRAS_API_KEY=

bot/.gitignore

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+node_modules
+dist
+.dev.vars
+.env
+.wrangler

bot/api/webhook.ts

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+import * as AI from "../lib/ai.js";
+import * as Comment from "../lib/comment.js";
+import * as GitHub from "../lib/github.js";
+
+type Env = {
+  CEREBRAS_API_KEY?: string;
+  GITHUB_APP_ID: string;
+  GITHUB_PRIVATE_KEY: string;
+  GITHUB_WEBHOOK_SECRET: string;
+};
+
+export default {
+  async fetch(request: Request, env: Env): Promise<Response> {
+    if (request.method !== "POST") {
+      return Response.json({ error: "Method not allowed" }, { status: 405 });
+    }
+
+    const signature = request.headers.get("x-hub-signature-256");
+    const event = request.headers.get("x-github-event");
+    const body = await request.text();
+
+    if (
+      !(await GitHub.verifySignature({
+        payload: body,
+        signature,
+        secret: env.GITHUB_WEBHOOK_SECRET,
+      }))
+    ) {
+      return Response.json({ error: "Invalid signature" }, { status: 401 });
+    }
+
+    if (event !== "pull_request") {
+      return Response.json({ ok: true, skipped: true });
+    }
+
+    const payload = JSON.parse(body);
+    const action: string = payload.action;
+
+    if (action !== "opened" && action !== "synchronize") {
+      return Response.json({ ok: true, skipped: true });
+    }
+
+    const pr = payload.pull_request;
+    const repo = payload.repository;
+    const installationId: number = payload.installation.id;
+    const owner: string = repo.owner.login;
+    const repoName: string = repo.name;
+    const prNumber: number = pr.number;
+    const headRef: string = pr.head.ref;
+    const headSha: string = pr.head.sha;
+    const fullRepo = `${owner}/${repoName}`;
+
+    try {
+      const octokit = await GitHub.createOctokit({
+        appId: env.GITHUB_APP_ID,
+        privateKey: env.GITHUB_PRIVATE_KEY,
+        installationId,
+      });
+
+      const [changelogFiles, existingComment] = await Promise.all([
+        GitHub.getChangelogFiles({ octokit, owner, repo: repoName, prNumber }),
+        GitHub.findBotComment({ octokit, owner, repo: repoName, prNumber }),
+      ]);
+
+      let commentBody: string;
+
+      if (changelogFiles.length > 0) {
+        commentBody = Comment.found({
+          repo: fullRepo,
+          headRef,
+          changelogFile: changelogFiles[0],
+        });
+      } else {
+        let aiContent: string | null = null;
+
+        const changedPackages = await GitHub.getChangedPackages({
+          octokit,
+          owner,
+          repo: repoName,
+          ref: headSha,
+          prNumber,
+        });
+
+        const apiKey = env.CEREBRAS_API_KEY;
+        if (apiKey) {
+          const diff = await GitHub.getPRDiff({
+            octokit,
+            owner,
+            repo: repoName,
+            prNumber,
+          });
+          aiContent = await AI.generateChangelog({
+            apiKey,
+            diff,
+            packageNames: changedPackages,
+          });
+        }
+
+        commentBody = Comment.notFound({
+          repo: fullRepo,
+          headRef,
+          aiContent,
+          changedPackages,
+        });
+      }
+
+      await GitHub.upsertComment({
+        octokit,
+        owner,
+        repo: repoName,
+        prNumber,
+        body: commentBody,
+        existingCommentId: existingComment?.id ?? null,
+      });
+
+      return Response.json({ ok: true });
+    } catch (error) {
+      console.error("Error processing webhook:", error);
+      return Response.json({ error: "Internal server error" }, { status: 500 });
+    }
+  },
+};

bot/biome.json

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+{
+  "$schema": "https://biomejs.dev/schemas/2.3.14/schema.json",
+  "formatter": {
+    "indentStyle": "space",
+    "indentWidth": 2,
+    "lineWidth": 80
+  },
+  "linter": {
+    "rules": {
+      "recommended": true
+    }
+  }
+}

bot/lib/ai.test.ts

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+import { describe, expect, test } from "vitest";
+import * as AI from "./ai.js";
+
+const sampleDiff = `diff --git a/src/lib.rs b/src/lib.rs
+index abc1234..def5678 100644
+--- a/src/lib.rs
++++ b/src/lib.rs
+@@ -10,6 +10,10 @@ pub fn parse(input: &str) -> Result<()> {
+     let tokens = tokenize(input)?;
+     let ast = build_ast(tokens)?;
++    // Validate before processing
++    validate(&ast)?;
+     process(ast)
+ }
++
++fn validate(ast: &Ast) -> Result<()> {
++    Ok(())
++}`;
+
+describe("AI.generateChangelog", () => {
+  test("generates a valid changelog entry", async () => {
+    const result = await AI.generateChangelog({
+      apiKey: import.meta.env.VITE_CEREBRAS_API_KEY,
+      diff: sampleDiff,
+      packageNames: ["changelogs"],
+    });
+    expect(result).not.toBeNull();
+    expect(result).toContain("---");
+    expect(result).toMatch(/changelogs:\s*(patch|minor|major)/);
+  });
+
+  test("returns null without apiKey", async () => {
+    const result = await AI.generateChangelog({
+      apiKey: undefined,
+      diff: sampleDiff,
+      packageNames: ["changelogs"],
+    });
+    expect(result).toBeNull();
+  });
+});

bot/lib/ai.ts

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+const cerebrasApiUrl = "https://api.cerebras.ai/v1/chat/completions";
+
+export async function generateChangelog(
+  parameters: generateChangelog.Parameters,
+): Promise<string | null> {
+  const { apiKey } = parameters;
+  if (!apiKey) return null;
+
+  const packages =
+    parameters.packageNames.length > 0
+      ? parameters.packageNames.join(", ")
+      : "<package-name>";
+
+  const prompt = `Generate a changelog entry for this git diff.
+
+Available packages: ${packages}
+
+Respond with ONLY a markdown file in this exact format (no explanation):
+
+---
+<package-name>: patch
+<another-package>: minor
+---
+
+Brief description of changes.
+
+Rules:
+- Replace <package-name> with actual package names from the list above
+- Include ALL packages that were modified in the frontmatter
+- Use "patch" for bug fixes, "minor" for features, "major" for breaking changes
+- Keep the summary concise (1-3 sentences)
+- Use past tense (e.g. "Added", "Fixed", "Removed")
+- Code fences are allowed in the summary when helpful
+- For breaking changes to public API, include a migration path. \`\`\`diff code fences are encouraged.
+
+Git diff:
+${parameters.diff}`;
+
+  try {
+    const response = await fetch(cerebrasApiUrl, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/json",
+        Authorization: `Bearer ${apiKey}`,
+      },
+      body: JSON.stringify({
+        model: "llama-3.3-70b",
+        messages: [{ role: "user", content: prompt }],
+        max_completion_tokens: 512,
+        temperature: 0.3,
+      }),
+    });
+
+    if (!response.ok) return null;
+
+    const data = (await response.json()) as {
+      choices: Array<{ message: { content: string } }>;
+    };
+    const content = data.choices?.[0]?.message?.content?.trim();
+    if (!content || !content.startsWith("---")) return null;
+
+    return content;
+  } catch {
+    return null;
+  }
+}
+
+export declare namespace generateChangelog {
+  type Parameters = {
+    apiKey: string | undefined;
+    diff: string;
+    packageNames: string[];
+  };
+}

bot/lib/comment.test.ts

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+import { afterEach, beforeEach, describe, expect, test, vi } from "vitest";
+import * as Comment from "./comment.js";
+
+beforeEach(() => {
+  let i = 0;
+  vi.spyOn(Math, "random").mockImplementation(() => [0.1, 0.2, 0.3][i++ % 3]);
+});
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("Comment.found", () => {
+  test("default", () => {
+    expect(
+      Comment.found({
+        repo: "wevm/viem",
+        headRef: "my-branch",
+        changelogFile: ".changelog/cool-cats-dance.md",
+      }),
+    ).toMatchInlineSnapshot(`
+      "### ✅ Changelog found on PR.
+
+      [Edit changelog](https://github.com/wevm/viem/edit/my-branch/.changelog/cool-cats-dance.md)"
+    `);
+  });
+});
+
+describe("Comment.notFound", () => {
+  test("without ai", () => {
+    expect(
+      Comment.notFound({
+        repo: "wevm/viem",
+        headRef: "my-branch",
+        aiContent: null,
+      }),
+    ).toMatchInlineSnapshot(`
+      "### ⚠️ Changelog not found.
+
+      A changelog entry is required before merging.
+
+      **[Add changelog](https://github.com/wevm/viem/new/my-branch?filename=.changelog/gentle-birds-bow.md&value=---%0A%3Cpackage-name%3E%3A%20patch%0A---%0A%0ABrief%20description%20of%20changes.)**"
+    `);
+  });
+
+  test("with ai content", () => {
+    const aiContent = `---
+my-crate: patch
+---
+
+Fixed a bug.`;
+    expect(
+      Comment.notFound({
+        repo: "wevm/viem",
+        headRef: "my-branch",
+        aiContent,
+      }),
+    ).toMatchInlineSnapshot(`
+      "### ⚠️ Changelog not found.
+
+      A changelog entry is required before merging. We've generated a suggested changelog based on your changes:
+
+      <details>
+      <summary>Preview</summary>
+
+      \`\`\`markdown
+      ---
+      my-crate: patch
+      ---
+
+      Fixed a bug.
+      \`\`\`
+
+      </details>
+
+      **[Add changelog](https://github.com/wevm/viem/new/my-branch?filename=.changelog/gentle-birds-bow.md&value=---%0Amy-crate%3A%20patch%0A---%0A%0AFixed%20a%20bug.)** to commit this to your branch."
+    `);
+  });
+
+  test("with changed packages", () => {
+    expect(
+      Comment.notFound({
+        repo: "wevm/viem",
+        headRef: "my-branch",
+        aiContent: null,
+        changedPackages: ["my-core", "my-utils"],
+      }),
+    ).toMatchInlineSnapshot(`
+      "### ⚠️ Changelog not found.
+
+      A changelog entry is required before merging.
+
+      **[Add changelog](https://github.com/wevm/viem/new/my-branch?filename=.changelog/gentle-birds-bow.md&value=---%0Amy-core%3A%20patch%0Amy-utils%3A%20patch%0A---%0A%0ABrief%20description%20of%20changes.)**"
+    `);
+  });
+});