feat: custom branch name templates (#571)

* Add branch-name-template config option

* Logging

* Use branch name template

* Add label to template variables

* Add description template variable

* More concise description for branch_name_template

* Remove more granular time template variables

* Only fetch first label

* Add check for empty template-generated name

* Clean up comments, docstrings

* Merge createBranchTemplateVariables into generateBranchName

* Still replace undefined values

* Fall back to default on duplicate branch

* Parameterize description wordcount

* Remove some over-explanatory comments

* NUM_DESCRIPTION_WORDS: 3 -> 5

src/github/api/queries/github.ts

@@ -18,6 +18,11 @@ export const PR_QUERY = `
         additions
         deletions
         state
+        labels(first: 1) {
+          nodes {
+            name
+          }
+        }
         commits(first: 100) {
           totalCount
           nodes {
@@ -101,6 +106,11 @@ export const ISSUE_QUERY = `
         updatedAt
         lastEditedAt
         state
+        labels(first: 1) {
+          nodes {
+            name
+          }
+        }
         comments(first: 100) {
           nodes {
             id

src/github/context.ts

@@ -88,6 +88,7 @@ type BaseContext = {
     labelTrigger: string;
     baseBranch?: string;
     branchPrefix: string;
+    branchNameTemplate?: string;
     useStickyComment: boolean;
     useCommitSigning: boolean;
     sshSigningKey: string;
@@ -145,6 +146,7 @@ export function parseGitHubContext(): GitHubContext {
       labelTrigger: process.env.LABEL_TRIGGER ?? "",
       baseBranch: process.env.BASE_BRANCH,
       branchPrefix: process.env.BRANCH_PREFIX ?? "claude/",
+      branchNameTemplate: process.env.BRANCH_NAME_TEMPLATE,
       useStickyComment: process.env.USE_STICKY_COMMENT === "true",
       useCommitSigning: process.env.USE_COMMIT_SIGNING === "true",
       sshSigningKey: process.env.SSH_SIGNING_KEY || "",

src/github/operations/branch.ts

@@ -12,6 +12,15 @@ import type { ParsedGitHubContext } from "../context";
 import type { GitHubPullRequest } from "../types";
 import type { Octokits } from "../api/client";
 import type { FetchDataResult } from "../data/fetcher";
+import { generateBranchName } from "../../utils/branch-template";
+
+/**
+ * Extracts the first label from GitHub data, or returns undefined if no labels exist
+ */
+function extractFirstLabel(githubData: FetchDataResult): string | undefined {
+  const labels = githubData.contextData.labels?.nodes;
+  return labels && labels.length > 0 ? labels[0]?.name : undefined;
+}
 
 /**
  * Validates a git branch name against a strict whitelist pattern.
@@ -125,7 +134,7 @@ export async function setupBranch(
 ): Promise<BranchInfo> {
   const { owner, repo } = context.repository;
   const entityNumber = context.entityNumber;
-  const { baseBranch, branchPrefix } = context.inputs;
+  const { baseBranch, branchPrefix, branchNameTemplate } = context.inputs;
   const isPR = context.isPR;
 
   if (isPR) {
@@ -191,17 +200,8 @@ export async function setupBranch(
   // Generate branch name for either an issue or closed/merged PR
   const entityType = isPR ? "pr" : "issue";
 
-  // Create Kubernetes-compatible timestamp: lowercase, hyphens only, shorter format
-  const now = new Date();
-  const timestamp = `${now.getFullYear()}${String(now.getMonth() + 1).padStart(2, "0")}${String(now.getDate()).padStart(2, "0")}-${String(now.getHours()).padStart(2, "0")}${String(now.getMinutes()).padStart(2, "0")}`;
-
-  // Ensure branch name is Kubernetes-compatible:
-  // - Lowercase only
-  // - Alphanumeric with hyphens
-  // - No underscores
-  // - Max 50 chars (to allow for prefixes)
-  const branchName = `${branchPrefix}${entityType}-${entityNumber}-${timestamp}`;
-  const newBranch = branchName.toLowerCase().substring(0, 50);
+  // Get the SHA of the source branch to use in template
+  let sourceSHA: string | undefined;
 
   try {
     // Get the SHA of the source branch to verify it exists
@@ -211,8 +211,46 @@ export async function setupBranch(
       ref: `heads/${sourceBranch}`,
     });
 
-    const currentSHA = sourceBranchRef.data.object.sha;
-    console.log(`Source branch SHA: ${currentSHA}`);
+    sourceSHA = sourceBranchRef.data.object.sha;
+    console.log(`Source branch SHA: ${sourceSHA}`);
+
+    // Extract first label from GitHub data
+    const firstLabel = extractFirstLabel(githubData);
+
+    // Extract title from GitHub data
+    const title = githubData.contextData.title;
+
+    // Generate branch name using template or default format
+    let newBranch = generateBranchName(
+      branchNameTemplate,
+      branchPrefix,
+      entityType,
+      entityNumber,
+      sourceSHA,
+      firstLabel,
+      title,
+    );
+
+    // Check if generated branch already exists on remote
+    try {
+      await $`git ls-remote --exit-code origin refs/heads/${newBranch}`.quiet();
+
+      // If we get here, branch exists (exit code 0)
+      console.log(
+        `Branch '${newBranch}' already exists, falling back to default format`,
+      );
+      newBranch = generateBranchName(
+        undefined, // Force default template
+        branchPrefix,
+        entityType,
+        entityNumber,
+        sourceSHA,
+        firstLabel,
+        title,
+      );
+    } catch {
+      // Branch doesn't exist (non-zero exit code), continue with generated name
+    }
 
     // For commit signing, defer branch creation to the file ops server
     if (context.inputs.useCommitSigning) {

src/github/types.ts

@@ -63,6 +63,11 @@ export type GitHubPullRequest = {
   additions: number;
   deletions: number;
   state: string;
+  labels: {
+    nodes: Array<{
+      name: string;
+    }>;
+  };
   commits: {
     totalCount: number;
     nodes: Array<{
@@ -88,6 +93,11 @@ export type GitHubIssue = {
   updatedAt?: string;
   lastEditedAt?: string;
   state: string;
+  labels: {
+    nodes: Array<{
+      name: string;
+    }>;
+  };
   comments: {
     nodes: GitHubComment[];
   };

src/utils/branch-template.ts

@@ -0,0 +1,96 @@
+#!/usr/bin/env bun
+
+/**
+ * Branch name template parsing and variable substitution utilities
+ */
+
+const NUM_DESCRIPTION_WORDS = 5;
+
+/**
+ * Extracts the first `numWords` words from a title and converts them to kebab-case
+ */
+function extractDescription(title: string, numWords: number = NUM_DESCRIPTION_WORDS): string {
+  if (!title || title.trim() === "") {
+    return "";
+  }
+
+  return title
+    .trim()
+    .split(/\s+/)
+    .slice(0, numWords) // Only first `numWords` words
+    .join("-")
+    .toLowerCase()
+    .replace(/[^a-z0-9-]/g, "") // Remove non-alphanumeric except hyphens
+    .replace(/-+/g, "-") // Replace multiple hyphens with single
+    .replace(/^-|-$/g, ""); // Remove leading/trailing hyphens
+}
+
+export interface BranchTemplateVariables {
+  prefix: string;
+  entityType: string;
+  entityNumber: number;
+  timestamp: string;
+  sha?: string;
+  label?: string;
+  description?: string;
+}
+
+/**
+ * Replaces template variables in a branch name template
+ * Template format: {{variableName}}
+ */
+export function applyBranchTemplate(
+  template: string,
+  variables: BranchTemplateVariables,
+): string {
+  let result = template;
+
+  // Replace each variable
+  Object.entries(variables).forEach(([key, value]) => {
+    const placeholder = `{{${key}}}`;
+    const replacement = value ? String(value) : "";
+    result = result.replaceAll(placeholder, replacement);
+  });
+
+  return result;
+}
+
+/**
+ * Generates a branch name from the provided `template` and set of `variables`. Uses a default format if the template is empty or produces an empty result.
+ */
+export function generateBranchName(
+  template: string | undefined,
+  branchPrefix: string,
+  entityType: string,
+  entityNumber: number,
+  sha?: string,
+  label?: string,
+  title?: string,
+): string {
+  const now = new Date();
+
+  const variables: BranchTemplateVariables = {
+    prefix: branchPrefix,
+    entityType,
+    entityNumber,
+    timestamp: `${now.getFullYear()}${String(now.getMonth() + 1).padStart(2, "0")}${String(now.getDate()).padStart(2, "0")}-${String(now.getHours()).padStart(2, "0")}${String(now.getMinutes()).padStart(2, "0")}`,
+    sha: sha?.substring(0, 8), // First 8 characters of SHA
+    label: label || entityType, // Fall back to entityType if no label
+    description: title ? extractDescription(title) : undefined,
+  };
+
+  if (template?.trim()) {
+    const branchName = applyBranchTemplate(template, variables);
+
+    // Some templates could produce empty results- validate
+    if (branchName.trim().length > 0) return branchName;
+
+    console.log(
+      `Branch template '${template}' generated empty result, falling back to default format`,
+    );
+  }
+
+  const branchName = `${branchPrefix}${entityType}-${entityNumber}-${variables.timestamp}`;
+  // Kubernetes compatible: lowercase, max 50 chars, alphanumeric and hyphens only
+  return branchName.toLowerCase().substring(0, 50);
+}