diff --git a/apps/pi-extension/server/serverReview.ts b/apps/pi-extension/server/serverReview.ts
index 6ca1e35b..e7ff8b70 100644
--- a/apps/pi-extension/server/serverReview.ts
+++ b/apps/pi-extension/server/serverReview.ts
@@ -26,6 +26,7 @@ import {
 	type DiffType,
 	type GitCommandResult,
 	type GitContext,
+	detectRemoteDefaultBranch,
 	getFileContentsForDiff as getFileContentsForDiffCore,
 	getGitContext as getGitContextCore,
 	gitAddFile as gitAddFileCore,
@@ -106,11 +107,12 @@ export interface ReviewServerResult {
 export const reviewRuntime: ReviewGitRuntime = {
 	async runGit(
 		args: string[],
-		options?: { cwd?: string },
+		options?: { cwd?: string; timeoutMs?: number },
 	): Promise<GitCommandResult> {
 		const result = spawnSync("git", args, {
 			cwd: options?.cwd,
 			encoding: "utf-8",
+			...(options?.timeoutMs ? { timeout: options.timeoutMs } : {}),
 		});
 		return {
 			stdout: result.stdout ?? "",
@@ -207,6 +209,14 @@ export async function startReviewServer(options: {
 	// the reviewer is currently looking at. Honors an explicit initialBase from
 	// the caller — e.g. programmatic Pi callers can request a non-detected base.
 	let currentBase = options.initialBase || options.gitContext?.defaultBranch || "main";
+	let baseEverSwitched = false;
+
+	// Fire-and-forget: query the remote for its actual default branch.
+	if (options.gitContext && !options.initialBase && !isPRMode) {
+		detectRemoteDefaultBranch(reviewRuntime, options.gitContext.cwd).then((remote) => {
+			if (remote && !baseEverSwitched) currentBase = remote;
+		});
+	}
 
 	// Agent jobs — background process manager (late-binds serverUrl via getter)
 	let serverUrl = "";
@@ -547,6 +557,7 @@ export async function startReviewServer(options: {
 				currentGitRef = result.label;
 				currentDiffType = newType;
 				currentBase = base;
+				baseEverSwitched = true;
 				currentError = result.error;
 
 				// Recompute gitContext for the effective cwd so the client's
diff --git a/packages/server/git.ts b/packages/server/git.ts
index 864c46ba..3ceacc44 100644
--- a/packages/server/git.ts
+++ b/packages/server/git.ts
@@ -36,7 +36,7 @@ export type {
 
 async function runGit(
   args: string[],
-  options?: { cwd?: string },
+  options?: { cwd?: string; timeoutMs?: number },
 ): Promise<GitCommandResult> {
   const proc = Bun.spawn(["git", ...args], {
     cwd: options?.cwd,
@@ -44,12 +44,19 @@ async function runGit(
     stderr: "pipe",
   });
 
+  let timer: ReturnType<typeof setTimeout> | undefined;
+  if (options?.timeoutMs) {
+    timer = setTimeout(() => proc.kill(), options.timeoutMs);
+  }
+
   const [stdout, stderr, exitCode] = await Promise.all([
     new Response(proc.stdout).text(),
     new Response(proc.stderr).text(),
     proc.exited,
   ]);
 
+  if (timer) clearTimeout(timer);
+
   return { stdout, stderr, exitCode };
 }
 
diff --git a/packages/server/review.ts b/packages/server/review.ts
index f14306c0..4be67b09 100644
--- a/packages/server/review.ts
+++ b/packages/server/review.ts
@@ -11,10 +11,9 @@
 
 import { isRemoteSession, getServerHostname, getServerPort } from "./remote";
 import type { Origin } from "@plannotator/shared/agents";
-import { type DiffType, type GitContext, runVcsDiff, getVcsFileContentsForDiff, canStageFiles, stageFile, unstageFile, resolveVcsCwd, validateFilePath, getVcsContext } from "./vcs";
-import { parseWorktreeDiffType } from "@plannotator/shared/review-core";
+import { type DiffType, type GitContext, runVcsDiff, getVcsFileContentsForDiff, canStageFiles, stageFile, unstageFile, resolveVcsCwd, validateFilePath, getVcsContext, gitRuntime } from "./vcs";
+import { parseWorktreeDiffType, detectRemoteDefaultBranch, resolveBaseBranch } from "@plannotator/shared/review-core";
 import type { AgentJobInfo } from "@plannotator/shared/agent-jobs";
-import { resolveBaseBranch } from "@plannotator/shared/review-core";
 import { getRepoInfo } from "./repo";
 import { handleImage, handleUpload, handleAgents, handleServerReady, handleDraftSave, handleDraftLoad, handleDraftDelete, handleFavicon, type OpencodeClient } from "./shared-handlers";
 import { contentHash, deleteDraft } from "./draft";
@@ -144,6 +143,17 @@ export async function startReviewServer(
   // the reviewer is currently looking at. Honors an explicit initialBase from
   // the caller — e.g. programmatic Pi callers can request a non-detected base.
   let currentBase = options.initialBase || gitContext?.defaultBranch || "main";
+  let baseEverSwitched = false;
+
+  // Fire-and-forget: query the remote for its actual default branch. If it
+  // arrives before the user interacts, quietly upgrade currentBase from the
+  // local fallback (e.g. "main") to the upstream ref (e.g. "origin/main").
+  // Non-blocking — the server is already listening by the time this resolves.
+  if (gitContext && !options.initialBase && !isPRMode) {
+    detectRemoteDefaultBranch(gitRuntime, gitContext.cwd).then((remote) => {
+      if (remote && !baseEverSwitched) currentBase = remote;
+    });
+  }
 
   // Agent jobs — background process manager (late-binds serverUrl via getter)
   let serverUrl = "";
@@ -496,6 +506,7 @@ export async function startReviewServer(
               currentGitRef = result.label;
               currentDiffType = newDiffType;
               currentBase = base;
+              baseEverSwitched = true;
               currentError = result.error;
 
               // Recompute gitContext for the effective cwd so the client's
diff --git a/packages/shared/review-core.ts b/packages/shared/review-core.ts
index c9b4c3a5..b0fc6be7 100644
--- a/packages/shared/review-core.ts
+++ b/packages/shared/review-core.ts
@@ -59,7 +59,7 @@ export interface GitCommandResult {
 export interface ReviewGitRuntime {
   runGit: (
     args: string[],
-    options?: { cwd?: string },
+    options?: { cwd?: string; timeoutMs?: number },
   ) => Promise<GitCommandResult>;
   readTextFile: (path: string) => Promise<string | null>;
 }
@@ -111,6 +111,38 @@ export async function getDefaultBranch(
   return "master";
 }
 
+/**
+ * Query the remote for its default branch via `ls-remote --symref`. Returns
+ * `origin/<name>` if the remote answers and the tracking ref exists locally,
+ * otherwise `null`. Designed to run in the background at server startup — the
+ * caller fires it with `.then()` and uses the result if/when it arrives.
+ *
+ * Timeout-guarded: if the network is slow or absent, the promise resolves
+ * (with `null`) once the timeout fires. Never throws.
+ */
+export async function detectRemoteDefaultBranch(
+  runtime: ReviewGitRuntime,
+  cwd?: string,
+): Promise<string | null> {
+  try {
+    const lsRemote = await runtime.runGit(
+      ["ls-remote", "--symref", "origin", "HEAD"],
+      { cwd, timeoutMs: 5000 },
+    );
+    if (lsRemote.exitCode !== 0) return null;
+    const match = lsRemote.stdout.match(/^ref:\s+refs\/heads\/(\S+)\s+HEAD/m);
+    if (!match) return null;
+    const remoteBranch = `origin/${match[1]}`;
+    const refExists = await runtime.runGit(
+      ["show-ref", "--verify", "--quiet", `refs/remotes/${remoteBranch}`],
+      { cwd },
+    );
+    return refExists.exitCode === 0 ? remoteBranch : null;
+  } catch {
+    return null;
+  }
+}
+
 export async function listBranches(
   runtime: ReviewGitRuntime,
   cwd?: string,