diff --git a/.contentrain/content/system/error-messages/en.json b/.contentrain/content/system/error-messages/en.json
index b71ebe39..2236d9ff 100644
--- a/.contentrain/content/system/error-messages/en.json
+++ b/.contentrain/content/system/error-messages/en.json
@@ -291,6 +291,7 @@
   "project.config_not_found": "Project configuration file (.contentrain/config.json) not found in the repository.",
   "project.config_validation_failed": "Project configuration is invalid: {errors}",
   "project.content_branch_failed": "Could not create the 'contentrain' branch in this repository. Check that the GitHub App still has write access, then try connecting again.",
+  "project.migration_not_merged": "Contentrain Migrate delivered this site on the '{branch}' branch. Merge it into '{base}' first, then connect the repository — connecting now would open the project without its content.",
   "project.not_found": "Project not found or you don't have access.",
   "project.not_found_in_workspace": "Project not found in this workspace. It may have been moved or deleted.",
   "project.review_workflow_upgrade": "Review workflow is available on {plans:workflow.review}. Enable it in project settings.",
diff --git a/server/api/workspaces/[workspaceId]/projects/index.post.ts b/server/api/workspaces/[workspaceId]/projects/index.post.ts
index 8f39fdcf..a1659dc4 100644
--- a/server/api/workspaces/[workspaceId]/projects/index.post.ts
+++ b/server/api/workspaces/[workspaceId]/projects/index.post.ts
@@ -1,3 +1,4 @@
+import { unmergedMigrationBranch } from '~~/server/utils/ensure-content-branch'
 import { syncMigrationHandoff } from '~~/server/utils/migration-handoff'
 
 export default defineEventHandler(async (event) => {
@@ -46,11 +47,21 @@ export default defineEventHandler(async (event) => {
 
   if (installationId) {
     const [owner = '', repo = ''] = body.repoFullName.split('/')
+    const git = useGitProvider({ installationId, owner, repo, contentRoot: body.contentRoot || '/' })
+    // A Migrate delivery waiting on its own branch: `contentrain` forked from the pre-migration
+    // default branch would open the project empty (and pass Migrate's "present" check). Wait for the merge.
+    // Fail closed: a GitHub error here must not let the connect fork a stale `contentrain`.
+    const waiting = await unmergedMigrationBranch(git, defaultBranch).catch(() => {
+      throw createError({ statusCode: 502, message: errorMessage('project.content_branch_failed') })
+    })
+    if (waiting) {
+      throw createError({
+        statusCode: 409,
+        message: errorMessage('project.migration_not_merged', { branch: waiting, base: defaultBranch }),
+      })
+    }
     try {
-      await ensureContentBranch(
-        useGitProvider({ installationId, owner, repo, contentRoot: body.contentRoot || '/' }),
-        defaultBranch,
-      )
+      await ensureContentBranch(git, defaultBranch)
     }
     catch {
       throw createError({
diff --git a/server/utils/ensure-content-branch.ts b/server/utils/ensure-content-branch.ts
index 66807eef..eb91e680 100644
--- a/server/utils/ensure-content-branch.ts
+++ b/server/utils/ensure-content-branch.ts
@@ -49,3 +49,56 @@ async function branchExists(git: ContentBranchOps): Promise<boolean> {
   const matches = await git.listBranches(CONTENTRAIN_BRANCH)
   return matches.some(branch => branch.name === CONTENTRAIN_BRANCH)
 }
+
+/** The content store file Migrate commits; present on the default branch once the customer has merged the delivery. */
+const CONTENT_STORE_CONFIG = '.contentrain/config.json'
+
+export interface MigrationMergeOps {
+  listBranches: (prefix?: string) => Promise<{ name: string }[]>
+  readFile: (path: string, ref?: string) => Promise<string>
+}
+
+/** A read that found nothing — the file or the ref is absent. Anything else (rate limit, network, 5xx) is not an answer. */
+function isMissing(error: unknown): boolean {
+  const status = (error as { status?: unknown })?.status
+  if (status === 404) return true
+  const message = error instanceof Error ? error.message : ''
+  return /\b404\b|not found/i.test(message)
+}
+
+/**
+ * The Migrate delivery branch (`migrate/<planHash>`) that has not reached the
+ * default branch yet, or null. Migrate delivers into a non-empty repository on
+ * a branch of its own; until the customer merges it the default branch has no
+ * content store. Creating `contentrain` from that default branch now would
+ * freeze it at the pre-migration tree: the project would open empty, and
+ * Migrate's later "branch present" check would take that stale branch for the
+ * migrated one. So connecting waits for the merge.
+ *
+ * Only a `migrate/…` branch that itself carries the content store counts: a
+ * team's own `migrate/db-v2` branch is not a delivery and never holds a connect.
+ * Only a repository with no `contentrain` branch yet is ever held.
+ *
+ * Fails closed: a read that errors for any reason other than "not there" throws,
+ * so the caller refuses the connect instead of guessing.
+ */
+export async function unmergedMigrationBranch(git: MigrationMergeOps, defaultBranch: string): Promise<string | null> {
+  if ((await git.listBranches(CONTENTRAIN_BRANCH)).some(branch => branch.name === CONTENTRAIN_BRANCH)) return null
+  const candidates = (await git.listBranches('migrate/')).filter(branch => branch.name.startsWith('migrate/'))
+  if (!candidates.length) return null
+  if (await readsStore(git, defaultBranch)) return null
+  for (const candidate of candidates) {
+    if (await readsStore(git, candidate.name)) return candidate.name
+  }
+  return null
+}
+
+async function readsStore(git: MigrationMergeOps, ref: string): Promise<boolean> {
+  try {
+    return Boolean(await git.readFile(CONTENT_STORE_CONFIG, ref))
+  }
+  catch (error) {
+    if (isMissing(error)) return false
+    throw error
+  }
+}
diff --git a/tests/unit/ensure-content-branch.test.ts b/tests/unit/ensure-content-branch.test.ts
index 051f92c8..482357d8 100644
--- a/tests/unit/ensure-content-branch.test.ts
+++ b/tests/unit/ensure-content-branch.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it, vi } from 'vitest'
-import { ensureContentBranch } from '../../server/utils/ensure-content-branch'
+import { ensureContentBranch, unmergedMigrationBranch } from '../../server/utils/ensure-content-branch'
 
 function ops(overrides: Partial<{
   branches: { name: string }[][]
@@ -57,3 +57,43 @@ describe('ensureContentBranch', () => {
     await expect(ensureContentBranch(git, 'main')).rejects.toThrow('Resource not accessible')
   })
 })
+
+describe('unmergedMigrationBranch', () => {
+  /** `stores`: refs whose `.contentrain/config.json` reads; a ref in `broken` fails with a transient error. */
+  const git = (opts: { branches?: string[], stores?: string[], broken?: string[] }) => ({
+    listBranches: vi.fn(async (prefix?: string) =>
+      (opts.branches ?? []).filter(name => !prefix || name.startsWith(prefix)).map(name => ({ name }))),
+    readFile: vi.fn(async (_path: string, ref?: string) => {
+      if (opts.broken?.includes(ref ?? '')) throw Object.assign(new Error('rate limited'), { status: 403 })
+      if (!opts.stores?.includes(ref ?? '')) throw Object.assign(new Error('Not Found'), { status: 404 })
+      return '{}'
+    }),
+  })
+
+  it('holds the connect while a delivery branch (it carries the store) is unmerged and no content branch exists', async () => {
+    await expect(unmergedMigrationBranch(git({ branches: ['main', 'migrate/abc123'], stores: ['migrate/abc123'] }), 'main')).resolves.toBe('migrate/abc123')
+  })
+
+  it('does not hold on a team\'s own migrate/ branch: it carries no content store', async () => {
+    await expect(unmergedMigrationBranch(git({ branches: ['main', 'migrate/db-v2'] }), 'main')).resolves.toBeNull()
+  })
+
+  it('lets it through once the default branch carries the content store', async () => {
+    await expect(unmergedMigrationBranch(git({ branches: ['main', 'migrate/abc123'], stores: ['main', 'migrate/abc123'] }), 'main')).resolves.toBeNull()
+  })
+
+  it('does not hold a repository that already has a contentrain branch', async () => {
+    await expect(unmergedMigrationBranch(git({ branches: ['contentrain', 'migrate/abc123'], stores: ['migrate/abc123'] }), 'main')).resolves.toBeNull()
+  })
+
+  it('does not touch an ordinary repository (no migrate/ branch)', async () => {
+    const ops = git({ branches: ['main'] })
+    await expect(unmergedMigrationBranch(ops, 'main')).resolves.toBeNull()
+    expect(ops.readFile).not.toHaveBeenCalled()
+  })
+
+  it('fails closed: a transient read error is thrown, not read as "no delivery"', async () => {
+    await expect(unmergedMigrationBranch(git({ branches: ['main', 'migrate/abc123'], stores: ['migrate/abc123'], broken: ['main'] }), 'main')).rejects.toThrow('rate limited')
+    await expect(unmergedMigrationBranch(git({ branches: ['main', 'migrate/abc123'], broken: ['migrate/abc123'] }), 'main')).rejects.toThrow('rate limited')
+  })
+})