create_pull_request should reuse existing branch when preserve-branch-name is enabled

[mrjf]

Problem

When preserve-branch-name: true is set and the target branch already exists on the remote, create_pull_request refuses to push:

Git push failed: Remote branch "autoloop/build-tsb-pandas-typescript-migration" already exists 
and preserve-branch-name is enabled. Refusing to silently rename the branch.

This blocks workflows like autoloop where:

1. The agent can't push directly (git credentials are cleaned before the agent step)
2. The branch is long-lived and intentionally reused across iterations
3. There's no open PR (previous one was merged), so the agent correctly calls create_pull_request rather than push_to_pull_request_branch

The handler falls back to creating an issue instead of a PR, which defeats the purpose.

Expected behavior

When preserve-branch-name is enabled and the named branch already exists on the remote, create_pull_request should force-push the patch to the existing branch and create the PR from it. The branch name matching is the point of preserve-branch-name — reusing the branch is the intended behavior, not a conflict.

Context

Autoloop maintains one long-lived branch per program (autoloop/{program-name}). Each iteration:

1. Checks out the branch, merges main, commits new changes
2. Needs to push to the branch (but can't — no git credentials during agent step)
3. Calls create_pull_request to push via safe-outputs and create a PR

This works on the first iteration (branch doesn't exist yet). On subsequent iterations after a PR merge, the branch still exists on the remote but has no open PR. The agent calls create_pull_request again, which should push to the existing branch and open a new PR.

Example

• Failing run: https://github.com/githubnext/tsessebe/actions/runs/25117554579
• Program: build-tsb-pandas-typescript-migration
• Branch autoloop/build-tsb-pandas-typescript-migration exists (0 ahead, 99 behind main — fully merged)
• No open PR
• Agent calls create_pull_request with branch: autoloop/build-tsb-pandas-typescript-migration
• Handler refuses because branch exists → falls back to issue Parse agentic engine logs to extract exact Bash commands, tool usage etc. #242

Workaround

Manually delete the stale remote branches before each iteration. Not viable for automated loops.

Related

• create_pull_request patch uses stale origin/branchName instead of merge-base with default branch #28955 — stale patch base ref (fixed in v0.71.2)
• Count unique files in create_pull_request patch limit and add max-patch-files config #28472 — max-patch-files config (shipped in v0.71.2)