diff --git a/.github/workflows/formatting.yml b/.github/workflows/formatting.yml
index b913e54..0449e96 100644
--- a/.github/workflows/formatting.yml
+++ b/.github/workflows/formatting.yml
@@ -7,7 +7,7 @@ on:
     branches: [main]
 
 jobs:
-  poetry:
+  format:
     runs-on: ubuntu-latest
     strategy:
       matrix:
@@ -29,3 +29,23 @@ jobs:
         run: poetry run black --check .
       - name: Check isort
         run: poetry run isort --profile black --check-only .
+  mypy:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.9"]
+        poetry-version: ["1.2.1"]
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Set up Poetry ${{ matrix.poetry-version }}
+        uses: abatilo/actions-poetry@v2
+        with:
+          poetry-version: ${{ matrix.poetry-version }}
+      - name: Install packages
+        run: poetry install --with dev
+      - name: Mypy
+        run: poetry run mypy cashu --ignore-missing
diff --git a/cashu/lightning/base.py b/cashu/lightning/base.py
index e38b6d8..adde18b 100644
--- a/cashu/lightning/base.py
+++ b/cashu/lightning/base.py
@@ -79,9 +79,9 @@ class Wallet(ABC):
     ) -> Coroutine[None, None, PaymentStatus]:
         pass
 
-    @abstractmethod
-    def paid_invoices_stream(self) -> AsyncGenerator[str, None]:
-        pass
+    # @abstractmethod
+    # def paid_invoices_stream(self) -> AsyncGenerator[str, None]:
+    #     pass
 
 
 class Unsupported(Exception):
diff --git a/cashu/mint/router.py b/cashu/mint/router.py
index 084d1f0..1ade005 100644
--- a/cashu/mint/router.py
+++ b/cashu/mint/router.py
@@ -1,4 +1,4 @@
-from typing import Union
+from typing import Union, List, Dict
 
 from fastapi import APIRouter
 from secp256k1 import PublicKey
@@ -13,6 +13,7 @@ from cashu.core.base import (
     MintRequest,
     PostSplitResponse,
     SplitRequest,
+    BlindedSignature,
 )
 from cashu.core.errors import CashuError
 from cashu.mint import ledger
@@ -21,19 +22,19 @@ router: APIRouter = APIRouter()
 
 
 @router.get("/keys")
-def keys():
+def keys() -> dict[int, str]:
     """Get the public keys of the mint"""
     return ledger.get_keyset()
 
 
 @router.get("/keysets")
-def keysets():
+def keysets() -> dict[str, list[str]]:
     """Get all active keysets of the mint"""
     return {"keysets": ledger.keysets.get_ids()}
 
 
 @router.get("/mint")
-async def request_mint(amount: int = 0):
+async def request_mint(amount: int = 0) -> GetMintResponse:
     """
     Request minting of new tokens. The mint responds with a Lightning invoice.
     This endpoint can be used for a Lightning invoice UX flow.
@@ -50,7 +51,7 @@ async def request_mint(amount: int = 0):
 async def mint(
     payloads: MintRequest,
     payment_hash: Union[str, None] = None,
-):
+) -> Union[List[BlindedSignature], CashuError]:
     """
     Requests the minting of tokens belonging to a paid payment request.
 
@@ -69,7 +70,7 @@ async def mint(
 
 
 @router.post("/melt")
-async def melt(payload: MeltRequest):
+async def melt(payload: MeltRequest) -> GetMeltResponse:
     """
     Requests tokens to be destroyed and sent out via Lightning.
     """
@@ -79,13 +80,13 @@ async def melt(payload: MeltRequest):
 
 
 @router.post("/check")
-async def check_spendable(payload: CheckRequest):
+async def check_spendable(payload: CheckRequest) -> Dict[int, bool]:
     """Check whether a secret has been spent already or not."""
     return await ledger.check_spendable(payload.proofs)
 
 
 @router.post("/checkfees")
-async def check_fees(payload: CheckFeesRequest):
+async def check_fees(payload: CheckFeesRequest) -> CheckFeesResponse:
     """
     Responds with the fees necessary to pay a Lightning invoice.
     Used by wallets for figuring out the fees they need to supply.
@@ -96,7 +97,9 @@ async def check_fees(payload: CheckFeesRequest):
 
 
 @router.post("/split")
-async def split(payload: SplitRequest):
+async def split(
+    payload: SplitRequest,
+) -> Union[CashuError, PostSplitResponse]:
     """
     Requetst a set of tokens with amount "total" to be split into two
     newly minted sets with amount "split" and "total-split".
@@ -111,7 +114,7 @@ async def split(payload: SplitRequest):
     except Exception as exc:
         return CashuError(error=str(exc))
     if not split_return:
-        return {"error": "there was a problem with the split."}
+        return CashuError(error="there was an error with the split")
     frst_promises, scnd_promises = split_return
     resp = PostSplitResponse(fst=frst_promises, snd=scnd_promises)
     return resp