diff --git a/packages/ui/src/components/views/git/baseBranch.test.ts b/packages/ui/src/components/views/git/baseBranch.test.ts
new file mode 100644
index 00000000..18aaffb8
--- /dev/null
+++ b/packages/ui/src/components/views/git/baseBranch.test.ts
@@ -0,0 +1,30 @@
+import { describe, expect, test } from 'bun:test';
+import { deriveBaseBranch, hasResolvableBaseBranch } from './baseBranch';
+
+describe('deriveBaseBranch', () => {
+  test('prefers the remote default branch hint over conventional fallbacks', () => {
+    expect(deriveBaseBranch({
+      remoteNames: new Set(['origin']),
+      localBranches: ['next'],
+      rootBranchHint: 'origin/react',
+    })).toBe('react');
+  });
+});
+
+describe('hasResolvableBaseBranch', () => {
+  test('rejects the main fallback when it does not exist', () => {
+    expect(hasResolvableBaseBranch({
+      baseBranch: 'main',
+      localBranches: ['next', 'react'],
+      remoteBranches: ['origin/next', 'origin/react'],
+    })).toBe(false);
+  });
+
+  test('accepts a base branch available through a remote-tracking ref', () => {
+    expect(hasResolvableBaseBranch({
+      baseBranch: 'main',
+      localBranches: ['next'],
+      remoteBranches: ['origin/main', 'origin/next'],
+    })).toBe(true);
+  });
+});
diff --git a/packages/ui/src/components/views/git/baseBranch.ts b/packages/ui/src/components/views/git/baseBranch.ts
index 19e22b59..6409597b 100644
--- a/packages/ui/src/components/views/git/baseBranch.ts
+++ b/packages/ui/src/components/views/git/baseBranch.ts
@@ -62,3 +62,18 @@ export const deriveBaseBranch = (options: {
   if (localBranches.includes('develop')) return 'develop';
   return 'main';
 };
+
+/**
+ * Whether a base branch can be resolved locally or through one of the active
+ * remote-tracking refs. Callers must not offer comparisons against the `main`
+ * fallback when that ref does not actually exist in the repository.
+ */
+export const hasResolvableBaseBranch = (options: {
+  baseBranch: string;
+  localBranches: readonly string[];
+  remoteBranches: readonly string[];
+}): boolean => {
+  const { baseBranch, localBranches, remoteBranches } = options;
+  return localBranches.includes(baseBranch)
+    || remoteBranches.some((branch) => branch.endsWith(`/${baseBranch}`));
+};
diff --git a/packages/ui/src/components/views/walkthrough/WalkthroughView.tsx b/packages/ui/src/components/views/walkthrough/WalkthroughView.tsx
index 246a818c..a2c2527b 100644
--- a/packages/ui/src/components/views/walkthrough/WalkthroughView.tsx
+++ b/packages/ui/src/components/views/walkthrough/WalkthroughView.tsx
@@ -14,10 +14,10 @@ import { useI18n, type Locale } from '@/lib/i18n';
 import { buildWalkthroughView } from '@/lib/walkthrough/model';
 import type { WalkthroughSource, WalkthroughWorkingTreeScope } from '@/lib/walkthrough/types';
 import { ModelSelector } from '@/components/sections/agents/ModelSelector';
-import { deriveBaseBranch } from '@/components/views/git/baseBranch';
+import { deriveBaseBranch, hasResolvableBaseBranch } from '@/components/views/git/baseBranch';
 import { runtimeFetch } from '@/lib/runtime-fetch';
 import { useConfigStore } from '@/stores/useConfigStore';
-import { useGitBranches, useGitStatus } from '@/stores/useGitStore';
+import { useGitBranches, useGitStatus, useGitStore } from '@/stores/useGitStore';
 import { useGitHubAuthStore } from '@/stores/useGitHubAuthStore';
 import {
   getFreshestPrStatusForBranch,
@@ -152,6 +152,12 @@ export const WalkthroughView = ({ directory }: WalkthroughViewProps) => {
 
   const status = useGitStatus(directory || null);
   const branches = useGitBranches(directory || null);
+  const ensureAll = useGitStore((state) => state.ensureAll);
+  const { github, git } = useRuntimeAPIs();
+
+  useEffect(() => {
+    if (directory) void ensureAll(directory, git);
+  }, [directory, ensureAll, git]);
 
   // The branch source reviews everything on this branch that is not on its
   // base. Three-dot semantics server-side mean merges from the base are
@@ -162,22 +168,28 @@ export const WalkthroughView = ({ directory }: WalkthroughViewProps) => {
     if (!headRef) return null;
     const all = branches?.all ?? [];
     const localBranches = all.filter((name) => !name.startsWith('remotes/'));
+    const remoteBranches = all
+      .filter((name) => name.startsWith('remotes/'))
+      .map((name) => name.slice('remotes/'.length));
     const remoteNames = new Set(
-      all
-        .filter((name) => name.startsWith('remotes/'))
-        .map((name) => name.slice('remotes/'.length).split('/')[0])
+      remoteBranches
+        .map((name) => name.split('/')[0])
         .filter(Boolean)
     );
-    const baseRef = deriveBaseBranch({ remoteNames, localBranches });
-    if (!baseRef || baseRef === headRef) return null;
+    const trackingRemote = status?.tracking?.split('/')[0];
+    const rootBranchHint = (trackingRemote && branches?.defaultBranches?.[trackingRemote])
+      ?? branches?.defaultBranches?.origin;
+    const baseRef = deriveBaseBranch({ remoteNames, localBranches, rootBranchHint });
+    if (!baseRef || baseRef === headRef || !hasResolvableBaseBranch({ baseBranch: baseRef, localBranches, remoteBranches })) {
+      return null;
+    }
     return { kind: 'branch', baseRef, headRef };
-  }, [branches, currentBranch]);
+  }, [branches, currentBranch, status?.tracking]);
 
   // The pull request for this branch used to appear only after visiting the PR
   // panel, because nothing else asked GitHub about it. Ask here too: the status
   // store already dedupes by signature and throttles by TTL, so several panels
   // wanting the same answer produce one request.
-  const { github } = useRuntimeAPIs();
   const githubConnected = useGitHubAuthStore((state) => state.status?.connected ?? false);
   const githubAuthChecked = useGitHubAuthStore((state) => state.hasChecked);
   const ensurePrStatusEntry = useGitHubPrStatusStore((state) => state.ensureEntry);
diff --git a/packages/ui/src/lib/api/types.ts b/packages/ui/src/lib/api/types.ts
index 9b1ae0b7..9c5859af 100644
--- a/packages/ui/src/lib/api/types.ts
+++ b/packages/ui/src/lib/api/types.ts
@@ -183,6 +183,7 @@ export interface GitBranch {
   all: string[];
   current: string;
   branches: Record<string, GitBranchDetails>;
+  defaultBranches?: Record<string, string>;
 }
 
 interface GitCommitSummary {
diff --git a/packages/web/server/lib/git/DOCUMENTATION.md b/packages/web/server/lib/git/DOCUMENTATION.md
index 7dbd31ce..8bcbb964 100644
--- a/packages/web/server/lib/git/DOCUMENTATION.md
+++ b/packages/web/server/lib/git/DOCUMENTATION.md
@@ -105,6 +105,7 @@ The following functions are internal helpers used by exported functions:
 - `ahead`: Number of commits ahead of upstream.
 - `behind`: Number of commits behind upstream.
 - `upstreamComparison`: Optional comparison against `upstream/<current-branch>`, with `{ remote, branch, ahead, behind }`.
+- `defaultBranches`: Remote default branches derived from local symbolic refs such as `remotes/origin/HEAD -> origin/main`, keyed by remote name. Omitted by runtimes that do not provide this Git metadata.
 - `files`: Array of file objects with `path`, `index`, `working_dir` status codes.
 - `isClean`: Boolean indicating if working tree is clean.
 - `diffStats`: Object mapping file paths to `{ insertions, deletions }`.
diff --git a/packages/web/server/lib/git/service.js b/packages/web/server/lib/git/service.js
index a3a99f62..ed64b7c5 100644
--- a/packages/web/server/lib/git/service.js
+++ b/packages/web/server/lib/git/service.js
@@ -3367,6 +3367,7 @@ export async function getBranches(directory) {
     const allBranches = result.all;
     const remoteBranches = allBranches.filter(branch => branch.startsWith('remotes/'));
     const activeRemoteBranches = await filterActiveRemoteBranches(git, remoteBranches);
+    const defaultBranches = await getRemoteDefaultBranches(git);
 
     const filteredAll = [
       ...allBranches.filter(branch => !branch.startsWith('remotes/')),
@@ -3376,7 +3377,8 @@ export async function getBranches(directory) {
     return {
       all: filteredAll,
       current: result.current,
-      branches: result.branches
+      branches: result.branches,
+      defaultBranches,
     };
   } catch (error) {
     console.error('Failed to get branches:', error);
@@ -3384,6 +3386,28 @@ export async function getBranches(directory) {
   }
 }
 
+async function getRemoteDefaultBranches(git) {
+  try {
+    const refs = await git.raw([
+      'for-each-ref',
+      '--format=%(refname) %(symref)',
+      'refs/remotes',
+    ]);
+    return Object.fromEntries(
+      refs.trim().split('\n').flatMap((line) => {
+        const [ref, symbolicRef] = line.split(' ');
+        const match = ref.match(/^refs\/remotes\/([^/]+)\/HEAD$/);
+        const prefix = match ? `refs/remotes/${match[1]}/` : '';
+        return match && typeof symbolicRef === 'string' && symbolicRef.startsWith(prefix)
+          ? [[match[1], symbolicRef.slice(prefix.length)]]
+          : [];
+      })
+    );
+  } catch {
+    return {};
+  }
+}
+
 async function filterActiveRemoteBranches(git, remoteBranches) {
   try {
     const remotes = await git.getRemotes();
diff --git a/packages/web/server/lib/git/service.test.js b/packages/web/server/lib/git/service.test.js
index e5110995..40f0c054 100644
--- a/packages/web/server/lib/git/service.test.js
+++ b/packages/web/server/lib/git/service.test.js
@@ -10,6 +10,7 @@ import {
   cherryPick,
   createWorktree,
   getWorktreeBootstrapStatus,
+  getBranches,
   getStatus,
   isGitRepository,
   populateWorktreeWithLockRecovery,
@@ -988,3 +989,25 @@ describe('hash validation', () => {
     ).rejects.not.toThrow('Invalid commit hash');
   });
 });
+
+describe.runIf(canRunGit())('getBranches', () => {
+  it('returns a remote default branch whose name is not a conventional fallback', async () => {
+    const remote = createTempDir();
+    const repository = createTempDir();
+    runGit(remote, ['init', '--bare', '--initial-branch=react']);
+    runGit(repository, ['init', '-b', 'next']);
+    runGit(repository, ['config', 'user.email', 'test@example.com']);
+    runGit(repository, ['config', 'user.name', 'Test']);
+    fs.writeFileSync(path.join(repository, 'README.md'), '# Test\n');
+    runGit(repository, ['add', 'README.md']);
+    runGit(repository, ['commit', '-m', 'init']);
+    runGit(repository, ['remote', 'add', 'origin', remote]);
+    runGit(repository, ['push', 'origin', 'HEAD:react']);
+    runGit(repository, ['fetch', 'origin']);
+    runGit(repository, ['remote', 'set-head', 'origin', '--auto']);
+
+    await expect(getBranches(repository)).resolves.toMatchObject({
+      defaultBranches: { origin: 'react' },
+    });
+  });
+});
diff --git a/packages/web/server/lib/walkthrough/DOCUMENTATION.md b/packages/web/server/lib/walkthrough/DOCUMENTATION.md
index 0f23fac5..b2d43ad1 100644
--- a/packages/web/server/lib/walkthrough/DOCUMENTATION.md
+++ b/packages/web/server/lib/walkthrough/DOCUMENTATION.md
@@ -55,6 +55,11 @@ written against staged code never silently re-anchors onto an unstaged edit.
 | `branch` | `branch` | `getRangeDiff` uses three-dot `base...head`, so work merged in from the base branch is excluded |
 | `pr` | `pr:<number>` | GitHub returns the merge-base diff, matching the branch semantics |
 
+For the current-branch source, the UI prefers the default branch of the current
+branch's tracking remote (from its local `remote/HEAD` symbolic ref), then uses
+the existing conventional-branch fallback. It does not offer the source when the
+chosen base cannot be resolved locally or through a remote-tracking ref.
+
 The panel offers the current branch's pull request on its own: it registers with
 the shared GitHub PR status store (`useGitHubPrStatusStore`) rather than waiting
 for the pull request panel to have been visited. That store already dedupes