add merge context (branch log, diff stats, per-file diffs) to improve conflict resolution quality

src/commands/merge.ts

@@ -29,13 +29,46 @@ export async function action(branch: string) {
   try {
     await vm.ensure((msg) => { spin.text = msg })
 
+    // Gather merge context
+    spin.text = "Gathering merge context"
+    const current = await git.currentBranch(root)
+    const base = await git.mergeBase("HEAD", branch, root)
+    const branchLog = await git.log(`${base}..${branch}`, root)
+    const branchDiffStat = await git.diffStat(`${base}..${branch}`, root)
+
     for (const file of conflicts) {
       spin.text = `Resolving ${file}`
+
+      const oursDiff = await git.fileDiff(base, "HEAD", file, root)
+      const theirsDiff = await git.fileDiff(base, branch, file, root)
       const content = await Bun.file(join(root, file)).text()
 
-      const resolved = await vm.claudePipe(
+      const context = [
+        `## Merge Context`,
+        `Merging branch \`${branch}\` into \`${current}\``,
+        ``,
+        `### Commits on \`${branch}\`:`,
+        branchLog || "(no commits)",
+        ``,
+        `### Overall changes on \`${branch}\`:`,
+        branchDiffStat || "(no changes)",
+        ``,
+        `---`,
+        `## Conflicted File: ${file}`,
+        ``,
+        `### Changes made on current branch (\`${current}\`):`,
+        oursDiff || "(no changes to this file)",
+        ``,
+        `### Changes made on incoming branch (\`${branch}\`):`,
+        theirsDiff || "(no changes to this file)",
+        ``,
+        `### File with conflict markers:`,
         content,
-        "resolve this merge conflict. output ONLY the resolved file content with no markdown fences, no explanation, no surrounding text.",
+      ].join("\n")
+
+      const resolved = await vm.claudePipe(
+        context,
+        "Resolve the merge conflicts in the file shown at the end. Use the diffs and commit history to understand the intent of each side. Preserve all non-conflicting changes from both sides. Output ONLY the resolved file content — no markdown fences, no explanation, no surrounding text.",
       )
 
       if (resolved.exitCode !== 0) {

src/git.ts

@@ -126,6 +126,33 @@ export async function isDirty(worktreePath: string): Promise<boolean> {
   return result.text().trim().length > 0
 }
 
+/** Find the merge base (common ancestor) between two refs. */
+export async function mergeBase(ref1: string, ref2: string, cwd: string): Promise<string> {
+  const result = await $`git merge-base ${ref1} ${ref2}`.cwd(cwd).nothrow().quiet()
+  if (result.exitCode !== 0) {
+    throw new Error(`Could not find merge base between "${ref1}" and "${ref2}"`)
+  }
+  return result.text().trim()
+}
+
+/** Get a one-line-per-commit log for a revision range. */
+export async function log(range: string, cwd: string): Promise<string> {
+  const result = await $`git log --oneline ${range}`.cwd(cwd).nothrow().quiet()
+  return result.text().trim()
+}
+
+/** Get a diff stat summary for a revision range. */
+export async function diffStat(range: string, cwd: string): Promise<string> {
+  const result = await $`git diff --stat ${range}`.cwd(cwd).nothrow().quiet()
+  return result.text().trim()
+}
+
+/** Get the diff for a specific file between two refs. */
+export async function fileDiff(ref1: string, ref2: string, file: string, cwd: string): Promise<string> {
+  const result = await $`git diff ${ref1} ${ref2} -- ${file}`.cwd(cwd).nothrow().quiet()
+  return result.text().trim()
+}
+
 /** Check if a branch has commits beyond main. */
 export async function hasNewCommits(worktreePath: string): Promise<boolean> {
   const main = await mainBranch(worktreePath)