diff --git a/src/providers/anthropic.rs b/src/providers/anthropic.rs
index 153a777..d9d863c 100644
--- a/src/providers/anthropic.rs
+++ b/src/providers/anthropic.rs
@@ -1067,4 +1067,60 @@ mod tests {
         assert!(matches!(context_error, ProviderError::ContextWindowExceeded { .. }));
         assert!(context_error.to_string().contains("250000"));
     }
+
+    #[test]
+    fn test_handle_error_response_maps_known_types() {
+        let provider = AnthropicProvider::new(
+            "test-key",
+            "claude-sonnet-4-20250514",
+            "https://api.anthropic.com",
+            ProviderConfig::default(),
+        );
+
+        let auth = provider.handle_error_response(
+            401,
+            r#"{"error":{"type":"authentication_error","message":"invalid auth"}}"#,
+        );
+        assert!(matches!(auth, ProviderError::AuthError(msg) if msg == "invalid auth"));
+
+        let overloaded = provider.handle_error_response(
+            529,
+            r#"{"error":{"type":"overloaded_error","message":"busy"}}"#,
+        );
+        assert!(matches!(overloaded, ProviderError::RateLimited(msg) if msg == "API overloaded"));
+
+        let model_not_found = provider.handle_error_response(
+            400,
+            r#"{"error":{"type":"invalid_request_error","message":"model does not exist"}}"#,
+        );
+        assert!(matches!(
+            model_not_found,
+            ProviderError::ModelNotFound(msg) if msg == "model does not exist"
+        ));
+    }
+
+    #[test]
+    fn test_handle_error_response_falls_back_for_unrecognized_or_plain_body() {
+        let provider = AnthropicProvider::new(
+            "test-key",
+            "claude-sonnet-4-20250514",
+            "https://api.anthropic.com",
+            ProviderConfig::default(),
+        );
+
+        let unknown = provider.handle_error_response(
+            422,
+            r#"{"error":{"type":"new_error_type","message":"unprocessable payload"}}"#,
+        );
+        assert!(matches!(
+            unknown,
+            ProviderError::ApiError { message, status_code: Some(422) } if message == "unprocessable payload"
+        ));
+
+        let plain = provider.handle_error_response(503, "service unavailable");
+        assert!(matches!(
+            plain,
+            ProviderError::ApiError { message, status_code: Some(503) } if message == "service unavailable"
+        ));
+    }
 }
diff --git a/src/providers/openai.rs b/src/providers/openai.rs
index 15d14e0..b219e6f 100644
--- a/src/providers/openai.rs
+++ b/src/providers/openai.rs
@@ -1132,4 +1132,50 @@ mod tests {
         let context_error = ProviderError::ContextWindowExceeded { used: 200000, limit: 128000 };
         assert!(context_error.to_string().contains("200000"));
     }
+
+    #[test]
+    fn test_handle_error_response_maps_known_types() {
+        let provider = OpenAIProvider::openai("test-key", "gpt-4o");
+
+        let auth = provider.handle_error_response(
+            401,
+            r#"{"error":{"message":"bad key","type":"invalid_api_key"}}"#,
+        );
+        assert!(matches!(auth, ProviderError::AuthError(msg) if msg == "bad key"));
+
+        let rate = provider.handle_error_response(
+            429,
+            r#"{"error":{"message":"slow down","type":"rate_limit_exceeded"}}"#,
+        );
+        assert!(matches!(rate, ProviderError::RateLimited(msg) if msg == "slow down"));
+
+        let context = provider.handle_error_response(
+            400,
+            r#"{"error":{"message":"context too long","type":"context_length_exceeded"}}"#,
+        );
+        assert!(matches!(
+            context,
+            ProviderError::ContextWindowExceeded { used: 0, limit } if limit == 128_000
+        ));
+    }
+
+    #[test]
+    fn test_handle_error_response_falls_back_for_unknown_or_plain_body() {
+        let provider = OpenAIProvider::openai("test-key", "gpt-4o");
+
+        let unknown = provider.handle_error_response(
+            418,
+            r#"{"error":{"message":"teapot","type":"something_new"}}"#,
+        );
+        assert!(matches!(
+            unknown,
+            ProviderError::ApiError { message, status_code: Some(418) } if message == "teapot"
+        ));
+
+        let plain = provider.handle_error_response(503, "upstream unavailable");
+        assert!(matches!(
+            plain,
+            ProviderError::ApiError { message, status_code: Some(503) } if message == "upstream unavailable"
+        ));
+    }
 }
diff --git a/src/tools/handlers/read_file.rs b/src/tools/handlers/read_file.rs
index 590a080..413fd5d 100644
--- a/src/tools/handlers/read_file.rs
+++ b/src/tools/handlers/read_file.rs
@@ -345,6 +345,55 @@ mod tests {
         assert!(matches!(result.unwrap_err(), ToolError::InvalidInput(_)));
     }
 
+    #[tokio::test]
+    async fn test_read_file_offset_zero_rejected() {
+        let temp = NamedTempFile::new().unwrap();
+
+        let handler = ReadFileHandler;
+        let result = handler
+            .execute(serde_json::json!({
+                "file_path": temp.path().to_str().unwrap(),
+                "offset": 0
+            }))
+            .await;
+
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), ToolError::InvalidInput(_)));
+    }
+
+    #[tokio::test]
+    async fn test_read_file_limit_zero_rejected() {
+        let temp = NamedTempFile::new().unwrap();
+
+        let handler = ReadFileHandler;
+        let result = handler
+            .execute(serde_json::json!({
+                "file_path": temp.path().to_str().unwrap(),
+                "limit": 0
+            }))
+            .await;
+
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), ToolError::InvalidInput(_)));
+    }
+
+    #[tokio::test]
+    async fn test_read_file_offset_exceeds_length_rejected() {
+        let mut temp = NamedTempFile::new().unwrap();
+        writeln!(temp, "line1").unwrap();
+
+        let handler = ReadFileHandler;
+        let result = handler
+            .execute(serde_json::json!({
+                "file_path": temp.path().to_str().unwrap(),
+                "offset": 10
+            }))
+            .await;
+
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), ToolError::InvalidInput(_)));
+    }
+
     #[tokio::test]
     async fn test_read_file_crlf() {
         let mut temp = NamedTempFile::new().unwrap();
diff --git a/src/tools/handlers/write_file.rs b/src/tools/handlers/write_file.rs
index 50551bf..bce77e6 100644
--- a/src/tools/handlers/write_file.rs
+++ b/src/tools/handlers/write_file.rs
@@ -234,4 +234,37 @@ mod tests {
         assert!(result.is_err());
         // Could be IoError or PermissionDenied depending on the system
     }
+
+    #[tokio::test]
+    async fn test_write_file_missing_content_rejected() {
+        let temp = tempdir().unwrap();
+        let file = temp.path().join("missing-content.txt");
+
+        let handler = WriteFileHandler;
+        let result = handler
+            .execute(serde_json::json!({
+                "file_path": file.to_str().unwrap()
+            }))
+            .await;
+
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), ToolError::InvalidInput(_)));
+    }
+
+    #[tokio::test]
+    async fn test_write_file_invalid_content_type_rejected() {
+        let temp = tempdir().unwrap();
+        let file = temp.path().join("bad-content-type.txt");
+
+        let handler = WriteFileHandler;
+        let result = handler
+            .execute(serde_json::json!({
+                "file_path": file.to_str().unwrap(),
+                "content": 123
+            }))
+            .await;
+
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), ToolError::InvalidInput(_)));
+    }
 }