feat(review): custom base branch for code review (#599)

apps/pi-extension/server/serverReview.ts

@@ -31,6 +31,7 @@ import {
 	gitAddFile as gitAddFileCore,
 	gitResetFile as gitResetFileCore,
 	parseWorktreeDiffType,
+	resolveBaseBranch,
 	type ReviewGitRuntime,
 	runGitDiff as runGitDiffCore,
 	validateFilePath,
@@ -192,6 +193,10 @@ export async function startReviewServer(options: {
 	let currentGitRef = options.gitRef;
 	let currentDiffType: DiffType = options.diffType || "uncommitted";
 	let currentError = options.error;
+	// Tracks the base branch the user picked from the UI. Agent review prompts
+	// read this (not gitContext.defaultBranch) so they analyze the same diff
+	// the reviewer is currently looking at.
+	let currentBase = options.gitContext?.defaultBranch || "main";
 
 	// Agent jobs — background process manager (late-binds serverUrl via getter)
 	let serverUrl = "";
@@ -213,7 +218,7 @@ export async function startReviewServer(options: {
 		async buildCommand(provider, config) {
 			const cwd = resolveAgentCwd();
 			const hasAgentLocalAccess = !!options.agentCwd || !!options.gitContext;
-			const userMessageOptions = { defaultBranch: options.gitContext?.defaultBranch, hasLocalAccess: hasAgentLocalAccess };
+			const userMessageOptions = { defaultBranch: currentBase, hasLocalAccess: hasAgentLocalAccess };
 
 			if (provider === "tour") {
 				return tour.buildCommand({
@@ -503,12 +508,18 @@ export async function startReviewServer(options: {
 					json(res, { error: "Missing diffType" }, 400);
 					return;
 				}
-				const defaultBranch = options.gitContext?.defaultBranch || "main";
+				const detectedBase = options.gitContext?.defaultBranch || "main";
+				const base = resolveBaseBranch(
+					typeof body.base === "string" ? body.base : undefined,
+					options.gitContext?.availableBranches,
+					detectedBase,
+				);
 				const defaultCwd = options.gitContext?.cwd;
-				const result = await runGitDiff(newType, defaultBranch, defaultCwd);
+				const result = await runGitDiff(newType, base, defaultCwd);
 				currentPatch = result.patch;
 				currentGitRef = result.label;
 				currentDiffType = newType;
+				currentBase = base;
 				currentError = result.error;
 				json(res, {
 					rawPatch: currentPatch,
@@ -613,12 +624,17 @@ export async function startReviewServer(options: {
 
 			// Local mode first (matches Bun server priority)
 			if (hasLocalAccess && !isPRMode) {
-				const defaultBranch = options.gitContext?.defaultBranch || "main";
+				const detectedBase = options.gitContext?.defaultBranch || "main";
+				const base = resolveBaseBranch(
+					url.searchParams.get("base") ?? undefined,
+					options.gitContext?.availableBranches,
+					detectedBase,
+				);
 				const defaultCwd = options.gitContext?.cwd;
 				const result = await getFileContentsForDiffCore(
 					reviewRuntime,
 					currentDiffType,
-					defaultBranch,
+					base,
 					filePath,
 					oldPath,
 					defaultCwd,