diff --git a/src/main/git/repo.ts b/src/main/git/repo.ts
index c3c4d7420..a8536765d 100644
--- a/src/main/git/repo.ts
+++ b/src/main/git/repo.ts
@@ -152,8 +152,14 @@ function hasGitRef(path: string, ref: string): boolean {
 /**
  * Resolve the default base ref for new worktrees.
  * Prefer the remote primary branch over a potentially stale local branch.
+ *
+ * Why: returns `null` when no candidate ref is resolvable. Previously this
+ * fell through to a hardcoded `'origin/main'` even when that ref did not
+ * exist, which silently handed `git worktree add` a bad ref and produced
+ * an opaque git error. Callers now fail loudly with a useful message, or
+ * degrade gracefully for non-creation uses (e.g. hosted URL building).
  */
-export function getDefaultBaseRef(path: string): string {
+export function getDefaultBaseRef(path: string): string | null {
   try {
     const ref = gitExecFileSync(['symbolic-ref', '--quiet', 'refs/remotes/origin/HEAD'], {
       cwd: path
@@ -179,14 +185,14 @@ export function getDefaultBaseRef(path: string): string {
     return 'master'
   }
 
-  return 'origin/main'
+  return null
 }
 
-export async function getBaseRefDefault(path: string): Promise<string> {
+export async function getBaseRefDefault(path: string): Promise<string | null> {
   return getDefaultBaseRefAsync(path)
 }
 
-async function getDefaultBaseRefAsync(path: string): Promise<string> {
+async function getDefaultBaseRefAsync(path: string): Promise<string | null> {
   try {
     const { stdout } = await gitExecFileAsync(
       ['symbolic-ref', '--quiet', 'refs/remotes/origin/HEAD'],
@@ -213,7 +219,7 @@ async function getDefaultBaseRefAsync(path: string): Promise<string> {
     return 'master'
   }
 
-  return 'origin/main'
+  return null
 }
 
 export async function searchBaseRefs(path: string, query: string, limit = 25): Promise<string[]> {
@@ -318,7 +324,11 @@ export function getRemoteFileUrl(
     return null
   }
 
-  const defaultBranch = getDefaultBaseRef(repoPath).replace(/^origin\//, '')
+  const defaultBaseRef = getDefaultBaseRef(repoPath)
+  if (!defaultBaseRef) {
+    return null
+  }
+  const defaultBranch = defaultBaseRef.replace(/^origin\//, '')
   const browseUrl = info.browseFile(relativePath, { committish: defaultBranch })
   if (!browseUrl) {
     return null
diff --git a/src/main/ipc/repos.ts b/src/main/ipc/repos.ts
index da965eb3c..2785bd5cd 100644
--- a/src/main/ipc/repos.ts
+++ b/src/main/ipc/repos.ts
@@ -49,32 +49,38 @@ export function registerRepoHandlers(mainWindow: BrowserWindow, store: Store): v
     return store.getRepos()
   })
 
-  ipcMain.handle('repos:add', async (_event, args: { path: string; kind?: 'git' | 'folder' }): Promise<{ repo: Repo } | { error: string }> => {
-    const repoKind = args.kind === 'folder' ? 'folder' : 'git'
-    if (repoKind === 'git' && !isGitRepo(args.path)) {
-      return { error: `Not a valid git repository: ${args.path}` }
-    }
+  ipcMain.handle(
+    'repos:add',
+    async (
+      _event,
+      args: { path: string; kind?: 'git' | 'folder' }
+    ): Promise<{ repo: Repo } | { error: string }> => {
+      const repoKind = args.kind === 'folder' ? 'folder' : 'git'
+      if (repoKind === 'git' && !isGitRepo(args.path)) {
+        return { error: `Not a valid git repository: ${args.path}` }
+      }
 
-    // Check if already added
-    const existing = store.getRepos().find((r) => r.path === args.path)
-    if (existing) {
-      return { repo: existing }
-    }
+      // Check if already added
+      const existing = store.getRepos().find((r) => r.path === args.path)
+      if (existing) {
+        return { repo: existing }
+      }
 
-    const repo: Repo = {
-      id: randomUUID(),
-      path: args.path,
-      displayName: getRepoName(args.path),
-      badgeColor: REPO_COLORS[store.getRepos().length % REPO_COLORS.length],
-      addedAt: Date.now(),
-      kind: repoKind
-    }
+      const repo: Repo = {
+        id: randomUUID(),
+        path: args.path,
+        displayName: getRepoName(args.path),
+        badgeColor: REPO_COLORS[store.getRepos().length % REPO_COLORS.length],
+        addedAt: Date.now(),
+        kind: repoKind
+      }
 
-    store.addRepo(repo)
-    await rebuildAuthorizedRootsCache(store)
-    notifyReposChanged(mainWindow)
-    return { repo }
-  })
+      store.addRepo(repo)
+      await rebuildAuthorizedRootsCache(store)
+      notifyReposChanged(mainWindow)
+      return { repo }
+    }
+  )
 
   ipcMain.handle(
     'repos:addRemote',
@@ -347,14 +353,17 @@ export function registerRepoHandlers(mainWindow: BrowserWindow, store: Store): v
   ipcMain.handle('repos:getBaseRefDefault', async (_event, args: { repoId: string }) => {
     const repo = store.getRepo(args.repoId)
     if (!repo || isFolderRepo(repo)) {
-      return 'origin/main'
+      // Why: folder-mode repos have no git state to resolve a base ref from.
+      // Return null so the renderer can decline to use a fabricated default
+      // (e.g. avoid running a branch compare against a ref that doesn't exist).
+      return null
     }
     // Why: remote repos need the relay to resolve symbolic-ref on the
     // remote host where the git data lives.
     if (repo.connectionId) {
       const provider = getSshGitProvider(repo.connectionId)
       if (!provider) {
-        return 'origin/main'
+        return null
       }
       try {
         const result = await provider.exec(
@@ -366,9 +375,11 @@ export function registerRepoHandlers(mainWindow: BrowserWindow, store: Store): v
           return ref.replace(/^refs\/remotes\//, '')
         }
       } catch {
-        // Fall through to default
+        // Fall through — no symbolic-ref on the remote.
       }
-      return 'origin/main'
+      // Why: don't fabricate 'origin/main'. Let the renderer surface "no
+      // default" and prompt the user to pick a base branch.
+      return null
     }
     return getBaseRefDefault(repo.path)
   })
diff --git a/src/main/ipc/worktree-remote.ts b/src/main/ipc/worktree-remote.ts
index 27bb922e5..7ff55ce1b 100644
--- a/src/main/ipc/worktree-remote.ts
+++ b/src/main/ipc/worktree-remote.ts
@@ -79,6 +79,11 @@ export async function createRemoteWorktree(
   const remotePath = `${repo.path}/../${sanitizedName}`
 
   // Determine base branch
+  // Why: previously fell back to a hardcoded 'origin/main' when
+  // symbolic-ref failed. That silently handed addWorktree a ref that may
+  // not exist on the remote (e.g. repos whose primary branch is master or
+  // develop), producing an opaque git error. Fail here with a clear
+  // message so the UI can surface it and prompt the user to pick a base.
   let baseBranch = args.baseBranch || repo.worktreeBaseRef
   if (!baseBranch) {
     try {
@@ -88,9 +93,14 @@ export async function createRemoteWorktree(
       )
       baseBranch = stdout.trim()
     } catch {
-      baseBranch = 'origin/main'
+      // Fall through — baseBranch stays unset.
     }
   }
+  if (!baseBranch) {
+    throw new Error(
+      'Could not resolve a default base ref for this repo. Pick a base branch explicitly and try again.'
+    )
+  }
 
   // Fetch latest
   const remote = baseBranch.includes('/') ? baseBranch.split('/')[0] : 'origin'
@@ -226,8 +236,20 @@ export async function createLocalWorktree(
     )
   }
 
-  // Determine base branch
+  // Determine base branch.
+  //
+  // Why: getDefaultBaseRef may return null when none of origin/HEAD,
+  // origin/main, origin/master, local main, or local master exist. In that
+  // case we must not fall back to a hardcoded 'origin/main' — passing a
+  // non-existent ref to `git worktree add` produces an opaque error. Fail
+  // here with a clear message so the UI can prompt the user to pick a base
+  // branch explicitly.
   const baseBranch = args.baseBranch || repo.worktreeBaseRef || getDefaultBaseRef(repo.path)
+  if (!baseBranch) {
+    throw new Error(
+      'Could not resolve a default base ref for this repo. Pick a base branch explicitly and try again.'
+    )
+  }
   const setupScript = getEffectiveHooks(repo)?.scripts.setup
   // Why: `ask` is a pre-create choice gate, not a post-create side effect.
   // Resolve it before mutating git state so missing UI input cannot strand
diff --git a/src/main/ipc/worktrees.test.ts b/src/main/ipc/worktrees.test.ts
index 80432f019..f02e8c756 100644
--- a/src/main/ipc/worktrees.test.ts
+++ b/src/main/ipc/worktrees.test.ts
@@ -259,6 +259,31 @@ describe('registerWorktreeHandlers', () => {
     })
   })
 
+  it('throws a clear error when no default base ref can be resolved', async () => {
+    // Why: guard against regressing to a silent 'origin/main' fallback. When
+    // getDefaultBaseRef returns null (e.g. a fresh repo with no origin/HEAD,
+    // no origin/main, no origin/master, and no local main/master), we must
+    // fail loudly with a message that prompts the user to pick a base
+    // branch, not hand a non-existent ref to `git worktree add`.
+    getDefaultBaseRefMock.mockReturnValue(null)
+    store.getRepo.mockReturnValue({
+      id: 'repo-1',
+      path: '/workspace/repo',
+      displayName: 'repo',
+      badgeColor: '#000',
+      addedAt: 0,
+      worktreeBaseRef: null
+    })
+
+    await expect(
+      handlers['worktrees:create'](null, {
+        repoId: 'repo-1',
+        name: 'improve-dashboard'
+      })
+    ).rejects.toThrow(/Could not resolve a default base ref/)
+    expect(addWorktreeMock).not.toHaveBeenCalled()
+  })
+
   it('creates an issue-command runner for an existing repo/worktree pair', async () => {
     const result = await handlers['hooks:createIssueCommandRunner'](null, {
       repoId: 'repo-1',
diff --git a/src/main/runtime/orca-runtime.test.ts b/src/main/runtime/orca-runtime.test.ts
index 39fa74659..d5a4ce73d 100644
--- a/src/main/runtime/orca-runtime.test.ts
+++ b/src/main/runtime/orca-runtime.test.ts
@@ -51,6 +51,21 @@ vi.mock('../ipc/filesystem-auth', () => ({
   invalidateAuthorizedRootsCache: invalidateAuthorizedRootsCacheMock
 }))
 
+// Why: the CLI create-worktree path calls getDefaultBaseRef to resolve a
+// fallback base branch. Real resolution shells out to `git` against the
+// test's fabricated repo path, which has no refs, so we stub it to a
+// predictable 'origin/main'. The runtime no longer silently fabricates this
+// default, so tests that want the legacy behavior must express it via the mock.
+vi.mock('../git/repo', async (importOriginal) => {
+  const actual = (await importOriginal()) as Record<string, unknown>
+  return {
+    ...actual,
+    getDefaultBaseRef: vi.fn().mockReturnValue('origin/main'),
+    getBranchConflictKind: vi.fn().mockResolvedValue(null),
+    getGitUsername: vi.fn().mockReturnValue('')
+  }
+})
+
 afterEach(() => {
   vi.mocked(listWorktrees).mockResolvedValue(MOCK_GIT_WORKTREES)
   vi.mocked(addWorktree).mockReset()
diff --git a/src/main/runtime/orca-runtime.ts b/src/main/runtime/orca-runtime.ts
index adafb4a10..3ad0588f5 100644
--- a/src/main/runtime/orca-runtime.ts
+++ b/src/main/runtime/orca-runtime.ts
@@ -676,6 +676,15 @@ export class OrcaRuntimeService {
     const workspaceRoot = wslHome ? join(wslHome, 'orca', 'workspaces') : settings.workspaceDir
     worktreePath = ensurePathWithinWorkspace(worktreePath, workspaceRoot)
     const baseBranch = args.baseBranch || repo.worktreeBaseRef || getDefaultBaseRef(repo.path)
+    if (!baseBranch) {
+      // Why: getDefaultBaseRef returns null when no suitable ref exists.
+      // Don't fabricate 'origin/main' — passing it to addWorktree would
+      // produce an opaque git failure. Surface a clear error so the CLI
+      // caller can pick an explicit --base ref.
+      throw new Error(
+        'Could not resolve a default base ref for this repo. Pass an explicit --base and try again.'
+      )
+    }
 
     const remote = baseBranch.includes('/') ? baseBranch.split('/')[0] : 'origin'
     try {
diff --git a/src/preload/api-types.d.ts b/src/preload/api-types.d.ts
index cd2e43a5e..734a18e0a 100644
--- a/src/preload/api-types.d.ts
+++ b/src/preload/api-types.d.ts
@@ -294,7 +294,7 @@ export type PreloadApi = {
     }) => Promise<{ repo: Repo } | { error: string }>
     onCloneProgress: (callback: (data: { phase: string; percent: number }) => void) => () => void
     getGitUsername: (args: { repoId: string }) => Promise<string>
-    getBaseRefDefault: (args: { repoId: string }) => Promise<string>
+    getBaseRefDefault: (args: { repoId: string }) => Promise<string | null>
     searchBaseRefs: (args: { repoId: string; query: string; limit?: number }) => Promise<string[]>
     onChanged: (callback: () => void) => () => void
   }
diff --git a/src/preload/index.d.ts b/src/preload/index.d.ts
index b01d8cf05..5d7fa8049 100644
--- a/src/preload/index.d.ts
+++ b/src/preload/index.d.ts
@@ -37,7 +37,7 @@ type ReposApi = {
   cloneAbort: () => Promise<void>
   onCloneProgress: (callback: (data: { phase: string; percent: number }) => void) => () => void
   getGitUsername: (args: { repoId: string }) => Promise<string>
-  getBaseRefDefault: (args: { repoId: string }) => Promise<string>
+  getBaseRefDefault: (args: { repoId: string }) => Promise<string | null>
   searchBaseRefs: (args: { repoId: string; query: string; limit?: number }) => Promise<string[]>
   onChanged: (callback: () => void) => () => void
 }
diff --git a/src/preload/index.ts b/src/preload/index.ts
index 52b204f56..e295ba8ef 100644
--- a/src/preload/index.ts
+++ b/src/preload/index.ts
@@ -210,7 +210,7 @@ const api = {
     getGitUsername: (args: { repoId: string }): Promise<string> =>
       ipcRenderer.invoke('repos:getGitUsername', args),
 
-    getBaseRefDefault: (args: { repoId: string }): Promise<string> =>
+    getBaseRefDefault: (args: { repoId: string }): Promise<string | null> =>
       ipcRenderer.invoke('repos:getBaseRefDefault', args),
 
     searchBaseRefs: (args: { repoId: string; query: string; limit?: number }): Promise<string[]> =>
diff --git a/src/renderer/src/components/right-sidebar/SourceControl.tsx b/src/renderer/src/components/right-sidebar/SourceControl.tsx
index 4f93e11d4..a733feead 100644
--- a/src/renderer/src/components/right-sidebar/SourceControl.tsx
+++ b/src/renderer/src/components/right-sidebar/SourceControl.tsx
@@ -173,7 +173,10 @@ function SourceControlInner(): React.JSX.Element {
   const [scope, setScope] = useState<SourceControlScope>('all')
   const [collapsedSections, setCollapsedSections] = useState<Set<string>>(new Set())
   const [baseRefDialogOpen, setBaseRefDialogOpen] = useState(false)
-  const [defaultBaseRef, setDefaultBaseRef] = useState<string | null>('origin/main')
+  // Why: start null rather than 'origin/main' so branch compare doesn't fire
+  // with a fabricated ref before the IPC resolves. effectiveBaseRef stays
+  // falsy until we have a real answer from the main process.
+  const [defaultBaseRef, setDefaultBaseRef] = useState<string | null>(null)
   const [filterQuery, setFilterQuery] = useState('')
   const filterInputRef = useRef<HTMLInputElement>(null)
 
@@ -238,8 +241,11 @@ function SourceControlInner(): React.JSX.Element {
         }
       })
       .catch(() => {
+        // Why: leave defaultBaseRef null on failure instead of fabricating
+        // 'origin/main'. effectiveBaseRef stays falsy, so branch compare and
+        // PR fetch skip running against a ref that may not exist.
         if (!stale) {
-          setDefaultBaseRef('origin/main')
+          setDefaultBaseRef(null)
         }
       })
 
diff --git a/src/renderer/src/components/settings/BaseRefPicker.tsx b/src/renderer/src/components/settings/BaseRefPicker.tsx
index ce2f0ea8c..a0246e8ff 100644
--- a/src/renderer/src/components/settings/BaseRefPicker.tsx
+++ b/src/renderer/src/components/settings/BaseRefPicker.tsx
@@ -16,7 +16,10 @@ export function BaseRefPicker({
   onSelect,
   onUsePrimary
 }: BaseRefPickerProps): React.JSX.Element {
-  const [defaultBaseRef, setDefaultBaseRef] = useState('origin/main')
+  // Why: null until the IPC resolves (or when the repo has no default base ref
+  // available). We avoid seeding with 'origin/main' because that would display
+  // a fabricated default in repos that don't actually have origin/main.
+  const [defaultBaseRef, setDefaultBaseRef] = useState<string | null>(null)
   const [baseRefQuery, setBaseRefQuery] = useState('')
   const [baseRefResults, setBaseRefResults] = useState<string[]>([])
   const [isSearchingBaseRefs, setIsSearchingBaseRefs] = useState(false)
@@ -32,7 +35,7 @@ export function BaseRefPicker({
         }
       } catch {
         if (!stale) {
-          setDefaultBaseRef('origin/main')
+          setDefaultBaseRef(null)
         }
       }
     }
@@ -93,11 +96,15 @@ export function BaseRefPicker({
     <div className="space-y-2.5">
       <div className="flex flex-wrap items-center justify-between gap-2">
         <div>
-          <div className="text-sm font-medium text-foreground">{effectiveBaseRef}</div>
+          <div className="text-sm font-medium text-foreground">
+            {effectiveBaseRef ?? 'No default base ref'}
+          </div>
           <p className="text-xs text-muted-foreground">
             {currentBaseRef
               ? 'Pinned for this repo'
-              : `Following primary branch (${defaultBaseRef})`}
+              : defaultBaseRef
+                ? `Following primary branch (${defaultBaseRef})`
+                : 'Pick a base branch below'}
           </p>
         </div>
         {onUsePrimary && (