pr-base-branch-merge-base

name	pr-base-branch-merge-base
description	Resolve PRBaseBranch for VS Code command: `drm-copilot: Collect PR Context` (command ID: `drmCopilotExtension.collectPrContext`) using merge-base ancestry. Use when orchestrators or review workflows need the correct comparison base branch and must select the branch with the most recent common ancestor commit with HEAD.

PR Base Branch (Merge-Base)

Deterministic branch-selection rules for PR context collection.

When to Use This Skill

Use this skill when:

• running via MCP server drmCopilotExtension tool collect_pr_context,
• delegating post-implementation review that depends on PRBaseBranch,
• constructing PR context artifacts where the base must not be hard-coded.

Selection Contract

PRBaseBranch MUST be resolved from git ancestry, not guessed.

Definition of correct base:

• choose the candidate branch whose merge-base with HEAD has the most recent commit timestamp,
• this implements “find the branch with the most recent commit in common.”

Deterministic Procedure

1. Enumerate candidate branches from local and remotes, excluding HEAD and detached refs.
2. For each candidate B, compute merge-base commit M = git merge-base HEAD B.
3. Compute merge_base_epoch(B) from git show -s --format=%ct M.
4. Select branch with maximum merge_base_epoch(B).
5. Tie-breakers (in order):
   • development
   • main
   • master
   • lexical ascending branch name

Guardrails

• Do not default to main unless merge-base resolution fails for all candidates.
• If all candidates fail, surface explicit error context and use repository default branch only as last-resort fallback.
• Persist chosen PRBaseBranch in orchestration state and reuse it within the same run.

Collector Invocation Rule

When invoking PR context collection, pass the resolved base explicitly:

• per pr-context-artifacts via MCP server drmCopilotExtension tool collect_pr_context with base: ${resolved-PRBaseBranch}

Evidence Recommendation

For auditability, include in logs/checkpoint:

• selected branch name,
• selected merge-base SHA,
• selected merge-base timestamp,
• top competing candidates with timestamps when available.