restoreConfigFromBase fails on repos with non-"main" default branch

[elijahr]

Bug

When claude-code-action runs on a repository whose default branch is master (or anything other than main), the config restore step fails with:

fatal: couldn't find remote ref main
Error: Action failed with error: Command failed: git fetch origin main --depth=1

This happens because baseBranch falls back to hardcoded "main" in several places when no explicit base_branch input is provided.

Affected locations

File	Line(s)	Code
src/modes/agent/index.ts	~89, ~96	process.env.BASE_BRANCH || context.inputs.baseBranch || "main"
src/entrypoints/run.ts	~315	baseBranch || "main"
src/entrypoints/update-comment-link.ts	~256	process.env.BASE_BRANCH || "main"

Expected behavior

When base_branch is not provided, the action should detect the repository's actual default branch instead of assuming "main".

The fix already exists in the codebase

src/github/operations/branch.ts (~line 185) already implements the correct pattern:

const repoResponse = await octokits.rest.repos.get({ owner, repo });
sourceBranch = repoResponse.data.default_branch;

Alternatively, ${{ github.event.repository.default_branch }} is available in the Actions context and could be used as the fallback instead of hardcoding "main".

Workaround

Callers can pass base_branch explicitly:

- uses: anthropics/claude-code-action@v1
  with:
    base_branch: ${{ github.event.repository.default_branch }}

Reproduction

1. Use claude-code-action@v1 on a repo whose default branch is master
2. Trigger via PR comment
3. Action fails at the config restore step before Claude even runs

[CervEdin]

Worth noting that the hardcoded "main" fallback is also a potential security risk, not just a functional bug.

restoreConfigFromBase exists to prevent RCE via attacker-controlled .claude/ and .mcp.json from PR heads. But if a repo uses master as its default branch, the fallback restores config from main instead — a branch name any contributor can push to. An attacker could push a main branch with a malicious .claude/settings.json or .mcp.json and the action would trust it.

#1083 fixes this by reading repository.default_branch from the event payload.

[CervEdin]

another (less persistent) workaround is

git fetch origin && git branch main origin/HEAD && git push origin main

if you're affected but can't change the GH actions