feat: add base_branch input to specify source branch for new Claude branches (#72)

* feat: add base_branch input to specify source branch for new Claude branches

- Add base_branch input parameter to action.yml allowing users to specify which branch to use as source
- Update setupBranch function to accept and use the base branch parameter  
- Defaults to repository default branch if no base branch is specified
- Addresses issue #62 for better branch control

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* perf: optimize setupBranch to avoid unnecessary default branch fetch

Only fetch repository default branch when actually needed:
- Skip initial fetch when baseBranch is provided
- Fetch default branch at end only for return value and GitHub Actions output
- Eliminates unnecessary API call when users specify base branch

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* fix: properly handle base branch throughout the action workflow

- Fix TypeScript error where defaultBranch was used before being assigned
- Replace DEFAULT_BRANCH with BASE_BRANCH in subsequent workflow steps
- Update PR creation and branch comparison to use the actual base branch
- Ensure custom base_branch input is respected in all operations

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* refactor: move BASE_BRANCH env reading into parseGitHubContext

- Move BASE_BRANCH environment variable reading into parseGitHubContext for consistency
- Update setupBranch to use context.inputs.baseBranch instead of process.env
- Fix test descriptions to correctly reference BASE_BRANCH instead of DEFAULT_BRANCH
- Update test environment setup to use BASE_BRANCH

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

src/github/context.ts

@@ -32,6 +32,7 @@ export type ParsedGitHubContext = {
     disallowedTools: string;
     customInstructions: string;
     directPrompt: string;
+    baseBranch?: string;
   };
 };
 
@@ -55,6 +56,7 @@ export function parseGitHubContext(): ParsedGitHubContext {
       disallowedTools: process.env.DISALLOWED_TOOLS ?? "",
       customInstructions: process.env.CUSTOM_INSTRUCTIONS ?? "",
       directPrompt: process.env.DIRECT_PROMPT ?? "",
+      baseBranch: process.env.BASE_BRANCH,
     },
   };
 

src/github/operations/branch-cleanup.ts

@@ -6,7 +6,7 @@ export async function checkAndDeleteEmptyBranch(
   owner: string,
   repo: string,
   claudeBranch: string | undefined,
-  defaultBranch: string,
+  baseBranch: string,
 ): Promise<{ shouldDeleteBranch: boolean; branchLink: string }> {
   let branchLink = "";
   let shouldDeleteBranch = false;
@@ -18,7 +18,7 @@ export async function checkAndDeleteEmptyBranch(
         await octokit.rest.repos.compareCommitsWithBasehead({
           owner,
           repo,
-          basehead: `${defaultBranch}...${claudeBranch}`,
+          basehead: `${baseBranch}...${claudeBranch}`,
         });
 
       // If there are no commits, mark branch for deletion

src/github/operations/branch.ts

@@ -14,7 +14,7 @@ import type { Octokits } from "../api/client";
 import type { FetchDataResult } from "../data/fetcher";
 
 export type BranchInfo = {
-  defaultBranch: string;
+  baseBranch: string;
   claudeBranch?: string;
   currentBranch: string;
 };
@@ -26,15 +26,9 @@ export async function setupBranch(
 ): Promise<BranchInfo> {
   const { owner, repo } = context.repository;
   const entityNumber = context.entityNumber;
+  const { baseBranch } = context.inputs;
   const isPR = context.isPR;
 
-  // Get the default branch first
-  const repoResponse = await octokits.rest.repos.get({
-    owner,
-    repo,
-  });
-  const defaultBranch = repoResponse.data.default_branch;
-
   if (isPR) {
     const prData = githubData.contextData as GitHubPullRequest;
     const prState = prData.state;
@@ -42,7 +36,7 @@ export async function setupBranch(
     // Check if PR is closed or merged
     if (prState === "CLOSED" || prState === "MERGED") {
       console.log(
-        `PR #${entityNumber} is ${prState}, creating new branch from default...`,
+        `PR #${entityNumber} is ${prState}, creating new branch from source...`,
       );
       // Fall through to create a new branch like we do for issues
     } else {
@@ -58,17 +52,36 @@ export async function setupBranch(
 
       console.log(`Successfully checked out PR branch for PR #${entityNumber}`);
 
-      // For open PRs, return branch info
+      // For open PRs, we need to get the base branch of the PR
+      const baseBranch = prData.baseRefName;
+
       return {
-        defaultBranch,
+        baseBranch,
         currentBranch: branchName,
       };
     }
   }
 
+  // Determine source branch - use baseBranch if provided, otherwise fetch default
+  let sourceBranch: string;
+
+  if (baseBranch) {
+    // Use provided base branch for source
+    sourceBranch = baseBranch;
+  } else {
+    // No base branch provided, fetch the default branch to use as source
+    const repoResponse = await octokits.rest.repos.get({
+      owner,
+      repo,
+    });
+    sourceBranch = repoResponse.data.default_branch;
+  }
+
   // Creating a new branch for either an issue or closed/merged PR
   const entityType = isPR ? "pr" : "issue";
-  console.log(`Creating new branch for ${entityType} #${entityNumber}...`);
+  console.log(
+    `Creating new branch for ${entityType} #${entityNumber} from source branch: ${sourceBranch}...`,
+  );
 
   const timestamp = new Date()
     .toISOString()
@@ -80,14 +93,14 @@ export async function setupBranch(
   const newBranch = `claude/${entityType}-${entityNumber}-${timestamp}`;
 
   try {
-    // Get the SHA of the default branch
-    const defaultBranchRef = await octokits.rest.git.getRef({
+    // Get the SHA of the source branch
+    const sourceBranchRef = await octokits.rest.git.getRef({
       owner,
       repo,
-      ref: `heads/${defaultBranch}`,
+      ref: `heads/${sourceBranch}`,
     });
 
-    const currentSHA = defaultBranchRef.data.object.sha;
+    const currentSHA = sourceBranchRef.data.object.sha;
 
     console.log(`Current SHA: ${currentSHA}`);
 
@@ -109,9 +122,9 @@ export async function setupBranch(
 
     // Set outputs for GitHub Actions
     core.setOutput("CLAUDE_BRANCH", newBranch);
-    core.setOutput("DEFAULT_BRANCH", defaultBranch);
+    core.setOutput("BASE_BRANCH", sourceBranch);
     return {
-      defaultBranch,
+      baseBranch: sourceBranch,
       claudeBranch: newBranch,
       currentBranch: newBranch,
     };