diff --git a/lib/sandbox/__tests__/createSandboxHandler.test.ts b/lib/sandbox/__tests__/createSandboxHandler.test.ts
index c48b0cb5..269163eb 100644
--- a/lib/sandbox/__tests__/createSandboxHandler.test.ts
+++ b/lib/sandbox/__tests__/createSandboxHandler.test.ts
@@ -96,6 +96,56 @@ describe("createSandboxHandler", () => {
     });
   });
 
+  it("forwards `branch` (existing branch) to the source as branch when isNewBranch is false", async () => {
+    vi.mocked(validateCreateSandboxBody).mockResolvedValueOnce({
+      body: {
+        repoUrl: "https://github.com/o/r",
+        sessionId: "sess-1",
+        branch: "develop",
+        isNewBranch: false,
+      },
+      auth: { accountId: ACCOUNT_ID, orgId: null, authToken: "k" },
+    });
+
+    await createSandboxHandler(makeReq());
+
+    const args = vi.mocked(connectSandbox).mock.calls[0]?.[0] as {
+      state: { source?: { repo: string; branch?: string; newBranch?: string } };
+    };
+    expect(args.state.source).toMatchObject({ repo: "https://github.com/o/r", branch: "develop" });
+    expect(args.state.source?.newBranch).toBeUndefined();
+  });
+
+  it("forwards `branch` to the source as newBranch when isNewBranch is true", async () => {
+    vi.mocked(validateCreateSandboxBody).mockResolvedValueOnce({
+      body: {
+        repoUrl: "https://github.com/o/r",
+        sessionId: "sess-1",
+        branch: "xy/abcd1234",
+        isNewBranch: true,
+      },
+      auth: { accountId: ACCOUNT_ID, orgId: null, authToken: "k" },
+    });
+
+    await createSandboxHandler(makeReq());
+
+    const args = vi.mocked(connectSandbox).mock.calls[0]?.[0] as {
+      state: { source?: { repo: string; branch?: string; newBranch?: string } };
+    };
+    expect(args.state.source?.newBranch).toBe("xy/abcd1234");
+    expect(args.state.source?.branch).toBeUndefined();
+  });
+
+  it("omits both branch and newBranch when neither is provided (current default)", async () => {
+    await createSandboxHandler(makeReq());
+
+    const args = vi.mocked(connectSandbox).mock.calls[0]?.[0] as {
+      state: { source?: { repo: string; branch?: string; newBranch?: string } };
+    };
+    expect(args.state.source?.branch).toBeUndefined();
+    expect(args.state.source?.newBranch).toBeUndefined();
+  });
+
   it("short-circuits with the validator's response on validation failure", async () => {
     const fail = NextResponse.json({ status: "error", error: "bad" }, { status: 400 });
     vi.mocked(validateCreateSandboxBody).mockResolvedValueOnce(fail);
diff --git a/lib/sandbox/__tests__/validateCreateSandboxBody.test.ts b/lib/sandbox/__tests__/validateCreateSandboxBody.test.ts
index e3894f2e..37f18b3a 100644
--- a/lib/sandbox/__tests__/validateCreateSandboxBody.test.ts
+++ b/lib/sandbox/__tests__/validateCreateSandboxBody.test.ts
@@ -102,16 +102,31 @@ describe("validateCreateSandboxBody", () => {
     expect(result.body.sessionId).toBe("sess-1");
   });
 
-  it("strips an unknown branch input from the validated body", async () => {
+  it("preserves `branch` and `isNewBranch` on the validated body", async () => {
     const result = await validateCreateSandboxBody(
       makeReq({
         repoUrl: "https://github.com/o/r",
         branch: "feat/x",
+        isNewBranch: true,
       }),
     );
 
     expect(result).not.toBeInstanceOf(NextResponse);
     if (result instanceof NextResponse) return;
-    expect((result.body as Record<string, unknown>).branch).toBeUndefined();
+    expect(result.body.branch).toBe("feat/x");
+    expect(result.body.isNewBranch).toBe(true);
+  });
+
+  it("rejects a non-boolean isNewBranch", async () => {
+    const result = await validateCreateSandboxBody(
+      makeReq({
+        repoUrl: "https://github.com/o/r",
+        isNewBranch: "yes",
+      }),
+    );
+
+    expect(result).toBeInstanceOf(NextResponse);
+    if (!(result instanceof NextResponse)) return;
+    expect(result.status).toBe(400);
   });
 });
diff --git a/lib/sandbox/createSandboxHandler.ts b/lib/sandbox/createSandboxHandler.ts
index 83922877..bb405cc6 100644
--- a/lib/sandbox/createSandboxHandler.ts
+++ b/lib/sandbox/createSandboxHandler.ts
@@ -101,7 +101,17 @@ export async function createSandboxHandler(request: NextRequest): Promise<NextRe
         // the snapshot. Without this flag, Vercel treats the snapshot as a
         // base image and tries to clone fresh on top — which often fails
         // for private repos and definitely defeats the warm-boot benefit.
-        source: { repo: body.repoUrl, prebuilt: !!orgSnapshotId },
+        //
+        // Branch routing: `isNewBranch` flips the body's `branch` between
+        // "check out this existing ref" (`branch`) and "cut a fresh ref
+        // off default" (`newBranch`). When neither is set, the runtime
+        // resolves to the repo's default branch.
+        source: {
+          repo: body.repoUrl,
+          ...(body.branch && !body.isNewBranch ? { branch: body.branch } : {}),
+          ...(body.branch && body.isNewBranch ? { newBranch: body.branch } : {}),
+          prebuilt: !!orgSnapshotId,
+        },
       },
       options: {
         timeout: DEFAULT_TIMEOUT_MS,
diff --git a/lib/sandbox/validateCreateSandboxBody.ts b/lib/sandbox/validateCreateSandboxBody.ts
index c0f6d4ec..7332581a 100644
--- a/lib/sandbox/validateCreateSandboxBody.ts
+++ b/lib/sandbox/validateCreateSandboxBody.ts
@@ -14,6 +14,14 @@ export const createSandboxBodySchema = z.object({
       message: "repoUrl must be a valid GitHub repository URL",
     }),
   sessionId: z.string().optional(),
+  // Branch routing matches the open-agents contract: when
+  // `isNewBranch` is false (or omitted), `branch` names an existing
+  // ref to check out; when `isNewBranch` is true, `branch` names the
+  // *new* ref to create off the repo's default. The handler passes
+  // these to `connectSandbox.state.source` as `branch` / `newBranch`
+  // respectively.
+  branch: z.string().optional(),
+  isNewBranch: z.boolean().optional(),
 });
 
 export type CreateSandboxBody = z.infer<typeof createSandboxBodySchema>;