diff --git a/mars-cli/mars_lib/authentication.py b/mars-cli/mars_lib/authentication.py
index 563cc2b..5155873 100644
--- a/mars-cli/mars_lib/authentication.py
+++ b/mars-cli/mars_lib/authentication.py
@@ -1,13 +1,14 @@
+from typing import Optional
 import requests
 import json
 
 
 def get_webin_auth_token(
-    credentials_dict,
-    header={"Content-Type": "application/json"},
-    auth_base_url="https://wwwdev.ebi.ac.uk/ena/dev/submit/webin/auth/token",
-    token_expiration_time=1,
-):
+    credentials_dict: dict[str, str],
+    header: dict[str, str] = {"Content-Type": "application/json"},
+    auth_base_url: str = "https://wwwdev.ebi.ac.uk/ena/dev/submit/webin/auth/token",
+    token_expiration_time: int = 1,
+) -> Optional[str]:
     """
     Obtain Webin authentication token.
 
diff --git a/mars-cli/mars_lib/biosamples_external_references.py b/mars-cli/mars_lib/biosamples_external_references.py
index d57b195..c321695 100644
--- a/mars-cli/mars_lib/biosamples_external_references.py
+++ b/mars-cli/mars_lib/biosamples_external_references.py
@@ -6,7 +6,7 @@
 import os
 from jsonschema import validate
 from jsonschema.exceptions import ValidationError, SchemaError
-from typing import Union
+from typing import Union, Any, Optional, List
 
 # -- #
 # Hardcoded values
@@ -21,7 +21,7 @@
 # -- #
 # Code blocks
 # -- #
-def load_json_file(file):
+def load_json_file(file: str) -> Any:
     """
     Function to load a JSON file as a dictionary.
     Args:
@@ -46,7 +46,7 @@ def load_json_file(file):
         )
 
 
-def handle_input_dict(input):
+def handle_input_dict(input: dict[str, str]) -> Optional[dict[str, str]]:
     """
     Function to handle the input: assert that it's either a dictionary or
         the filepath to an existing file containing the dictionary
@@ -73,7 +73,7 @@ def handle_input_dict(input):
             raise ValueError(f"The file '{input}' is not a valid JSON file.")
 
 
-def get_header(token):
+def get_header(token: str) -> dict[str, str]:
     """
     Obtain the header using a token.
 
@@ -90,7 +90,7 @@ def get_header(token):
     }
 
 
-def validate_bs_accession(accession_str):
+def validate_bs_accession(accession_str: str) -> None:
     """
     Validates that the given accession string conforms to the specified regex format.
         See: https://registry.identifiers.org/registry/biosample
@@ -108,8 +108,8 @@ def validate_bs_accession(accession_str):
 
 
 def validate_json_against_schema(
-    json_doc: Union[dict, str], json_schema: Union[dict, str]
-):
+    json_doc: Union[dict[str, List[str]], str], json_schema: Union[dict[str, str], str]
+) -> Optional[bool]:
     """
     Validates a JSON document against a given JSON Schema.
 
@@ -150,7 +150,7 @@ class BiosamplesRecord:
     production: boolean indicating environment mode
     """
 
-    def __init__(self, bs_accession):
+    def __init__(self, bs_accession: str) -> None:
         """
         Initialize the BiosamplesRecord with provided arguments.
 
@@ -159,8 +159,11 @@ def __init__(self, bs_accession):
         """
         validate_bs_accession(bs_accession)
         self.bs_accession = bs_accession
+        self.biosamples_credentials: Optional[dict[str, str]] = None
+        self.biosamples_externalReferences: List[str] = []
+        self.production: bool = False
 
-    def display(self):
+    def display(self) -> None:
         """
         Display the attributes for demonstration purposes.
         """
@@ -168,7 +171,7 @@ def display(self):
         print("Biosamples External References:", self.biosamples_externalReferences)
         print("Production Mode:", self.production)
 
-    def fetch_bs_json(self, biosamples_endpoint):
+    def fetch_bs_json(self, biosamples_endpoint: str) -> Optional[dict[str, str]]:
         """
         Fetches the BioSample's record (JSON) of the accession.
 
@@ -206,7 +209,9 @@ def fetch_bs_json(self, biosamples_endpoint):
         self.bs_json = response_json
         return self.bs_json
 
-    def load_bs_json(self, bs_json: Union[str, dict]):
+    def load_bs_json(
+        self, bs_json: Union[str, dict[str, str]]
+    ) -> Optional[dict[str, str]]:
         """
         Loads a given JSON, or the file containing it, as the BioSample's record (JSON) for this instance.
             It is an alternative to fetching it directly from BioSample.
@@ -226,21 +231,27 @@ def load_bs_json(self, bs_json: Union[str, dict]):
                 "Neither the file containing the Biosamples JSON nor the Biosamples JSON itself were given to load it into the instance."
             )
 
-    def pop_links(self):
+    def pop_links(self) -> dict[str, str]:
         """
         Removes "_links" array (which is added automatically after updating the biosamples on the BioSample's side).
         """
 
-        if "_links" not in self.bs_json:
-            return self.bs_json
+        if "_links" in self.bs_json:
+            self.bs_json.pop("_links")
 
-        self.bs_json.pop("_links")
         return self.bs_json
 
-    def extend_externalReferences(self, new_ext_refs_list):
+    def extend_externalReferences(
+        self, new_ext_refs_list: List[dict[str, str]]
+    ) -> dict[str, str]:
         """Extends the JSON of the BioSample's record with new externalReferences"""
         if not self.bs_json:
-            self.fetch_bs_json()
+            endpoint = (
+                biosamples_endpoints["prod"]
+                if self.production
+                else biosamples_endpoints["dev"]
+            )
+            self.fetch_bs_json(endpoint)
         self.pop_links()
 
         if "externalReferences" not in self.bs_json:
@@ -259,7 +270,9 @@ def extend_externalReferences(self, new_ext_refs_list):
         self.bs_json["externalReferences"] = ext_refs_list
         return self.bs_json
 
-    def update_remote_record(self, header, webin_auth="?authProvider=WEBIN"):
+    def update_remote_record(
+        self, header: dict[str, str], webin_auth: str = "?authProvider=WEBIN"
+    ) -> Optional[str]:
         """
         Updates the remote record of the BioSample's accession with the current sample JSON.
 
diff --git a/mars-cli/mars_lib/credential.py b/mars-cli/mars_lib/credential.py
index 333c4c1..e534b81 100644
--- a/mars-cli/mars_lib/credential.py
+++ b/mars-cli/mars_lib/credential.py
@@ -52,19 +52,23 @@
 
 
 class CredentialManager:
-    def __init__(self, service_name):
+    def __init__(self, service_name: str) -> None:
         self.service_name = service_name
 
-    def get_credential_env(self, username):
+    def get_credential_env(self, username: str) -> str:
         """
         Retrieves a credential from environment variables.
 
         :param username: The environment variable username.
         :return: The value of the environment variable or None if not found.
         """
-        return os.getenv(username)
+        result = os.getenv(username)
+        if result is None:
+            raise ValueError(f"Environment variable '{username}' not found.")
 
-    def prompt_for_password(self):
+        return result
+
+    def prompt_for_password(self) -> str:
         """
         Securely prompts the user to enter a password in the console.
 
@@ -72,7 +76,7 @@ def prompt_for_password(self):
         """
         return getpass.getpass(prompt="Enter your password: ")
 
-    def set_password_keyring(self, username, password):
+    def set_password_keyring(self, username: str, password: str) -> None:
         """
         Stores a password in the keyring under the given username.
 
@@ -81,16 +85,19 @@ def set_password_keyring(self, username, password):
         """
         keyring.set_password(self.service_name, username, password)
 
-    def get_password_keyring(self, username):
+    def get_password_keyring(self, username: str) -> str:
         """
         Retrieves a password from the keyring for the given username.
 
         :param username: The username whose password to retrieve.
         :return: The password or None if not found.
         """
-        return keyring.get_password(self.service_name, username)
+        pwd = keyring.get_password(self.service_name, username)
+        if pwd is None:
+            raise ValueError(f"Password not found for username '{username}'.")
+        return pwd
 
-    def delete_password_keyring(self, username):
+    def delete_password_keyring(self, username: str) -> None:
         """
         Deletes a password from the keyring for the given username.
 
diff --git a/mars-cli/mars_lib/models/isa_json.py b/mars-cli/mars_lib/models/isa_json.py
index 6b529c7..ead6b34 100644
--- a/mars-cli/mars_lib/models/isa_json.py
+++ b/mars-cli/mars_lib/models/isa_json.py
@@ -42,7 +42,7 @@ class Data(IsaBase):
     type: Optional[DataTypeEnum] = None
 
     @field_validator("type")
-    def apply_enum(cls, v):
+    def apply_enum(cls, v: str) -> str:
         if v not in [item.value for item in DataTypeEnum]:
             raise ValueError("Invalid material type")
         return v
@@ -151,7 +151,7 @@ class Material(IsaBase):
     derivesFrom: List[Material] = []
 
     @field_validator("type")
-    def apply_enum(cls, v):
+    def apply_enum(cls, v: str) -> str:
         if v not in [item.value for item in MaterialTypeEnum]:
             raise ValueError("Invalid material type")
         return v
@@ -193,7 +193,7 @@ class Assay(IsaBase):
     unitCategories: List[OntologyAnnotation] = []
 
     @field_validator("comments")
-    def detect_target_repo_comments(cls, v):
+    def detect_target_repo_comments(cls, v: List[Comment]) -> Optional[List[Comment]]:
         target_repo_comments = [
             comment for comment in v if comment.name == TARGET_REPO_KEY
         ]
diff --git a/mars-cli/mars_lib/models/repository_response.py b/mars-cli/mars_lib/models/repository_response.py
index 1ec38da..4526052 100644
--- a/mars-cli/mars_lib/models/repository_response.py
+++ b/mars-cli/mars_lib/models/repository_response.py
@@ -21,7 +21,7 @@ class Accession(BaseModel):
     value: str
 
     @field_validator("path")
-    def validate_path(cls, path):
+    def validate_path(cls, path: List[Path]) -> List[Path]:
         keys = [p.key for p in path]
         if len(keys) != len(set(keys)):
             raise ValueError("Duplicate keys found in path list")
@@ -49,20 +49,20 @@ class RepositoryResponse(BaseModel):
     info: List[Info] = []
 
     @field_validator("target_repository")
-    def validate_target_repository(cls, v):
+    def validate_target_repository(cls, v: str) -> str:
         if v not in [item.value for item in TargetRepository]:
             raise ValueError(f"Invalid 'target repository' value: '{v}'")
         return v
 
     @classmethod
-    def from_json_file(cls, json_file):
-        with open(json_file, "r") as file:
+    def from_json_file(cls, json_file_path: str) -> "RepositoryResponse":
+        with open(json_file_path, "r") as file:
             data = json.load(file)
 
         return cls.model_validate(data)
 
     @classmethod
-    def from_json(cls, json_string: str):
+    def from_json(cls, json_string: str) -> "RepositoryResponse":
         data = json.loads(json_string)
 
         return cls.model_validate(data)
diff --git a/mars-cli/mars_lib/submit.py b/mars-cli/mars_lib/submit.py
index 9be9ae6..bb56b43 100644
--- a/mars-cli/mars_lib/submit.py
+++ b/mars-cli/mars_lib/submit.py
@@ -10,12 +10,13 @@
 from mars_lib.isa_json import reduce_isa_json_for_target_repo
 from mars_lib.target_repo import TargetRepository
 import requests
+from typing import Any
 
 
 def submit_to_biosamples(
     investiagation: Investigation,
-    biosamples_credentials,
-    url,
+    biosamples_credentials: dict[str, str],
+    url: str,
 ) -> requests.Response:
     bs_input_investiagation = reduce_isa_json_for_target_repo(
         investiagation, TargetRepository.BIOSAMPLES
@@ -32,8 +33,10 @@ def submit_to_biosamples(
 
 
 def create_external_references(
-    biosamples_credentials, biosamples_externalReferences, production
-):
+    biosamples_credentials: dict[str, str],
+    biosamples_externalReferences: dict[str, Any],
+    production: bool,
+) -> None:
     """
     Main function to be executed when script is run.
 
@@ -51,6 +54,8 @@ def create_external_references(
         json_doc=biosamples_externalReferences, json_schema=input_json_schema_filepath
     )
     token = get_webin_auth_token(biosamples_credentials)
+    if not token:
+        raise ValueError("The token could not be generated.")
     header = get_header(token)
 
     for biosample_r in biosamples_externalReferences["biosampleExternalReferences"]: