diff --git a/apps/desktop/src/lib/error/knownErrors.ts b/apps/desktop/src/lib/error/knownErrors.ts
index 68dbf55576..15c09286f0 100644
--- a/apps/desktop/src/lib/error/knownErrors.ts
+++ b/apps/desktop/src/lib/error/knownErrors.ts
@@ -8,7 +8,8 @@ export enum Code {
 	ProjectMissing = 'errors.projects.missing',
 	SecretKeychainNotFound = 'errors.secret.keychain_notfound',
 	MissingLoginKeychain = 'errors.secret.missing_login_keychain',
-	GitHubTokenExpired = 'errors.github.expired_token'
+	GitHubTokenExpired = 'errors.github.expired_token',
+	GitHubStackedPrFork = 'errors.github.stacked_pr_fork'
 }
 
 export const KNOWN_ERRORS: Record<string, string> = {
@@ -34,5 +35,10 @@ With \`seahorse\` or equivalent, create a \`Login\` password store, right click
 	`,
 	[Code.GitHubTokenExpired]: `
 Your GitHub token appears expired, please check your settings!
+	`,
+	[Code.GitHubStackedPrFork]: `
+Stacked pull requests across forks are not supported by GitHub.
+
+The base branch you specified doesn't exist in your fork. When creating a stacked PR, the base branch must exist in the same repository.
 	`
 };
diff --git a/apps/desktop/src/lib/forge/github/ghQuery.ts b/apps/desktop/src/lib/forge/github/ghQuery.ts
index 6cb5678e0f..a59e8c3b9e 100644
--- a/apps/desktop/src/lib/forge/github/ghQuery.ts
+++ b/apps/desktop/src/lib/forge/github/ghQuery.ts
@@ -86,7 +86,26 @@ export async function ghQuery<
 			: 'GitHub API error';
 
 		const message = isErrorlike(err) ? err.message : String(err);
-		const code = message.startsWith('Not Found -') ? Code.GitHubTokenExpired : undefined;
+
+		// Check for stacked PR across forks error (base field invalid)
+		let code: string | undefined;
+		if (isGitHubError(err)) {
+			const errors = err.response.data.errors;
+			if (errors instanceof Array) {
+				const hasInvalidBaseError = errors.some(
+					(error) =>
+						error.resource === 'PullRequest' && error.field === 'base' && error.code === 'invalid'
+				);
+				if (hasInvalidBaseError) {
+					code = Code.GitHubStackedPrFork;
+				}
+			}
+		}
+
+		// Check for expired token
+		if (!code && message.startsWith('Not Found -')) {
+			code = Code.GitHubTokenExpired;
+		}
 
 		return { error: { name: title, message, code } };
 	}
@@ -137,6 +156,35 @@ function extractDomainAndAction<
 	return undefined;
 }
 
+/**
+ * Type for GitHub API error response structure.
+ */
+interface GitHubErrorResponse {
+	response: {
+		data: {
+			errors?: Array<{
+				resource?: string;
+				field?: string;
+				code?: string;
+			}>;
+		};
+	};
+}
+
+/**
+ * Typeguard for checking if an error has the GitHub error response structure.
+ */
+function isGitHubError(err: unknown): err is GitHubErrorResponse {
+	return (
+		isErrorlike(err) &&
+		'response' in err &&
+		typeof (err as any).response === 'object' &&
+		(err as any).response !== null &&
+		'data' in (err as any).response &&
+		typeof (err as any).response.data === 'object'
+	);
+}
+
 /**
  * Typeguard for accessing injected `GitHubClient` dependency safely.
  */
diff --git a/apps/desktop/src/lib/forge/github/githubPrService.test.ts b/apps/desktop/src/lib/forge/github/githubPrService.test.ts
index 122f931b94..56bb3ec192 100644
--- a/apps/desktop/src/lib/forge/github/githubPrService.test.ts
+++ b/apps/desktop/src/lib/forge/github/githubPrService.test.ts
@@ -1,3 +1,4 @@
+import { Code } from '$lib/error/knownErrors';
 import { GitHub } from '$lib/forge/github/github';
 import { setupMockGitHubApi } from '$lib/testing/mockGitHubApi.svelte';
 import { type RestEndpointMethodTypes } from '@octokit/rest';
@@ -36,4 +37,75 @@ describe('GitHubPrService', () => {
 		const pr = await service?.fetch(123);
 		expect(pr?.title).equal(title);
 	});
+
+	test('should detect stacked PR across forks error', async () => {
+		const mockError = {
+			message: 'Validation Failed',
+			response: {
+				data: {
+					message: 'Validation Failed',
+					errors: [
+						{
+							resource: 'PullRequest',
+							field: 'base',
+							code: 'invalid'
+						}
+					]
+				}
+			}
+		};
+
+		vi.spyOn(octokit.pulls, 'create').mockRejectedValue(mockError);
+
+		try {
+			await service?.createPr({
+				title: 'Test PR',
+				body: 'Test body',
+				draft: false,
+				baseBranchName: 'feature-branch',
+				upstreamName: 'my-branch'
+			});
+			expect.fail('Should have thrown an error');
+		} catch (err: any) {
+			expect(err.code).toBe(Code.GitHubStackedPrFork);
+		}
+	});
+
+	test('should detect stacked PR error among multiple validation errors', async () => {
+		const mockError = {
+			message: 'Validation Failed',
+			response: {
+				data: {
+					message: 'Validation Failed',
+					errors: [
+						{
+							resource: 'Issue',
+							field: 'title',
+							code: 'missing'
+						},
+						{
+							resource: 'PullRequest',
+							field: 'base',
+							code: 'invalid'
+						}
+					]
+				}
+			}
+		};
+
+		vi.spyOn(octokit.pulls, 'create').mockRejectedValue(mockError);
+
+		try {
+			await service?.createPr({
+				title: 'Test PR',
+				body: 'Test body',
+				draft: false,
+				baseBranchName: 'feature-branch',
+				upstreamName: 'my-branch'
+			});
+			expect.fail('Should have thrown an error');
+		} catch (err: any) {
+			expect(err.code).toBe(Code.GitHubStackedPrFork);
+		}
+	});
 });
diff --git a/packages/shared/src/lib/branches/Minimap.svelte b/packages/shared/src/lib/branches/Minimap.svelte
index 0bd6a65cfb..bb67e198ab 100644
--- a/packages/shared/src/lib/branches/Minimap.svelte
+++ b/packages/shared/src/lib/branches/Minimap.svelte
@@ -2,11 +2,11 @@
 	import { goto } from '$app/navigation';
 	import ChangeStatus from '$lib/patches/ChangeStatus.svelte';
 	import { WEB_ROUTES_SERVICE } from '$lib/routing/webRoutes.svelte';
+	import { inject } from '@gitbutler/core/context';
 	import { getBranchReview } from '@gitbutler/shared/branches/branchesPreview.svelte';
 	import { isFound, map } from '@gitbutler/shared/network/loadable';
 	import { getPatch } from '@gitbutler/shared/patches/patchCommitsPreview.svelte';
 	import { reactive } from '@gitbutler/shared/reactiveUtils.svelte';
-	import { inject } from '@gitbutler/core/context';
 	import { CommitStatusBadge } from '@gitbutler/ui';
 	import {
 		EXTERNAL_LINK_SERVICE,